Global Warming & Climate Change - Frequently Asked Questions

We've assembled some of the questions we hear most often about Earth's climate and answered them based on the current state of scientific knowledge. We update this FAQ frequently with results of recent research and links to supporting material from NCAR scientists and colleagues at nationally and internationally recognized universities, laboratories, and research units.

A few definitions

Weather is what's happening in the atmosphere on any given day, in a specific place. Local or regional weather forecasts include temperature, humidity, winds, cloudiness, and prospects for storms or other changes over the next few days. (Learn more about how NCAR studies weather.)

Climate is the average of these weather ingredients over many years. Some meteorologists like the saying that "climate is what you expect; weather is what you get," memorable words variously attributed to Mark Twain, Robert Heinlein, and others.

In practical terms, the climate for a particular city, state, or region tells you whether to pack short-sleeved shirts and shorts or parkas and mittens before you visit, while the local weather forecast tells you if you'll want to wear the parka by itself or with an extra sweater today.

What's a normal climate?

Climate varies across space and time, so climate is studied on a variety of spatial and time scales.

To interpret today's atmospheric conditions, we need a reference period of average, or "normal," climate to compare it against. How long is long enough to define the average climate for a city, state, or region? The National Oceanic and Atmospheric Administration's National Weather Service calculates a 30-year average once a decade. The current "normals" (issued July 1, 2011) are based on data from 1981 to 2010. NOAA's FAQ helps put this metric in context. For example, it notes that "Normals were not designed to be metrics of climate change." 

When it comes to climate on a global scale, the "normal" reference period  depends on which climate components scientists want to study. For example, many scientists compare average global temperatures, precipitation, and other variables for the 20th and 21st centuries with the 30-year averages for 1870 to 1899, before major industrialization produced large quantities of greenhouse gas.

You can see how recent observations and future projections of warming and cooling compare to conditions at the end of the 19th century by watching a visualization of data from the NCAR-based Community Climate System Model in our Climate Change Multimedia Gallery.

Before thermometers

To understand how climate varies across time, scientists examine three kinds of climate data: observations, historical accounts, and environmental evidence locked up in fossils, ice cores, and other "proxy climate records."

Observations of temperature at Earth's surface date back as far as 350 years for some locations in England, but only about 100 to 150 years in most of the developed world. But even before the thermometer was invented, ancient civilizations kept records of droughts, floods, unusual hot or cold weather, and other climate indicators, including planting and harvest times.

While human accounts can take us back hundreds or thousands of years, we need other tools to understand how Earth's climate has varied during its much longer lifetime of about 4.5 billion years.

Paleoclimatology delves into the deep history of past climate variation through what are called "proxy records." Air bubbles trapped in ice cores, the composition of lake sediments, changes in tree rings, pollen fossils, and other parts of Earth's ancient environment have given scientists many clues to past temperature, precipitation, wind patterns, and the chemical composition of the atmosphere through time.

Observations, historic accounts, and paleoclimate data are used to test the reliability of computer models that simulate Earth's climate on time scales from decades, to centuries, to millennia. Studying prehistoric variations can also provide important clues about what to expect in a warmer world.

Related Links

In Depth: Weather on Steroids (AtmosNews: NCAR & UCAR news, views, analysis)

For researchers

Paleoclimate Research in the Climate Change Research Section, Climate and Global Dynamics Division, NCAR

For teachers and students

Learning Resources: Climate (Spark: UCAR Science Education)

What is Climate? (Living in the Greenhouse - Kids' Crossing, UCAR)

How Do We Investigate Climates of the Past? (Windows to the Universe)



Without the so-called greenhouse gases, including carbon dioxide, methane, nitrous oxide, and water vapor, Earth would be too cold to inhabit. These gases in Earth's atmosphere absorb and emit heat energy, creating the greenhouse effect that keeps our planet's temperature livable.

Water vapor is the most plentiful greenhouse gas on the planet, accounting for about 60% of the current greenhouse effect. Even ozone helps trap some of the heat that makes life on Earth possible, but the "ozone hole" is a separate issue not directly related to global warming.

Color visualization of the greenhouse effect
Watch a 1-minute animation on the greenhouse effect. Click here or on the image to launch in a new window (streaming video, ©UCAR.)

Too much of a good thing

Since the industrial revolution, people have burned vast amounts of coal, petroleum, and other fossil fuels to create heat and power. This releases carbon dioxide, the most plentiful human-produced greenhouse gas, into the atmosphere. The result: more heat is trapped in Earth's atmosphere instead of radiating out into space.

On average, carbon dioxide lasts more than a century in the atmosphere. As a result, CO2 is well mixed around the globe. Measurements collected atop Hawaii’s Mauna Loa and other locations show a steady rise in global carbon dioxide concentrations since 1958. These concentrations have increased by more than 35% since preindustrial times, according to the World Meteorological Organization. Other, less prevalent greenhouse gases have increased at different rates. Methane, for example, virtually leveled off after 1999 at 155% above its preindustrial level, but began climbing again in 2007.

The relationship between Earth's water cycle and global warming creates a well-known feedback loop. Warmer temperatures cause more water to evaporate from land and oceans into the atmosphere. The added water vapor then contributes to warmer temperatures, completing the feedback loop. This is just one of many feedbacks in the Earth system that climate scientists are studying to improve projections of future climate change.

Explore these topics in depth on Windows to the Universe: The Greenhouse Effect & Greenhouse Gases.


Two scientists are credited with the discovery more than 100 years ago that increasing carbon dioxide in the atmosphere warms the entire planet: French researcher Jean Baptiste Fourier and Swedish scientist Svante Arrhenius. Their identification of what came to be called the greenhouse effect applies to both natural and human-produced additions of CO2.

As measurements of atmospheric CO2 levels showed steady increases after World War II (see What is the average global temperature now?), Earth system scientists looked for a corresponding rise in global average temperatures, basing their studies on the physical laws governing the greenhouse effect. By the early 1980s, climate scientists were calling this atmospheric response global warming. Not every place on Earth was expected to warm at the same rate, and rising temperatures were not the only impacts anticipated.

Boosting Earth's temperature and adding more acidity to the oceans creates wide-ranging effects that are changing all the "normal" weather and climate conditions on which we've based our agriculture, industry, and social systems (see What's the difference between climate and weather?). So some researchers talk about global climate change to convey that the situation is far more complex than temperature alone.

To some ears, "climate change" sounds less ominous than "global warming." However, the phrase was introduced by researchers not to minimize the situation but to convey the full scope of disturbances that can occur in association with changes in global temperature, such as changes in patterns of flood and drought. 

Related Links

Climate change glossary (U.S. EPA)


There are a few connections between the two, but they are largely separate issues.

First, it's important to know that ozone plays two different roles in the atmosphere. At ground level, "bad ozone" is a pollutant caused by human activities; it's a major component of health-damaging smog. The same chemical occurs naturally in the stratosphere, and this "good ozone" acts as a shield, filtering out most of the ultraviolet light from the Sun that could otherwise prove deadly to people, animals, and plants. The Ennvironmental Protection Agency has a resource exploring these two roles: Ozone: Good Up High, Bad Nearby.

The ozone hole refers to the seasonal depletion of the ozone shield in the lower stratosphere above Antarctica. It occurs as sunlight returns each spring, triggering reactions that involve chlorofluorocarbons (CFCs) and related molecules produced by industrial processes. These reactions consume huge amounts of ozone over a few weeks' time. Later in the season, the ozone-depleted air mixes with surrounding air and the ozone layer over Antarctica recovers until the next spring. Other parts of the globe have experienced much smaller losses in stratospheric ozone.

Because of international agreements to limit CFCs and related emissions instituted with the Montreal Protocol, it's expected that the ozone hole will be slowly healing over the next few decades.

The ozone hole does not directly affect air temperatures in the troposphere, the layer of the atmosphere closest to the surface, although changes in circulation over Antarctica related to the ozone hole appear to be changing surface temperature patterns over that continent. Ozone is actually a greenhouse gas, and so are CFCs, meaning that their presence in the troposphere contributes slightly to the heightened greenhouse effect. The main greenhouse gas responsible for present-day and anticipated global warming, however, is carbon dioxide produced by burning of fossil fuels for electricity, heating, and transportation.

Higher up, the loss of stratospheric ozone has led to some cooling in that layer of the atmosphere. An even larger effect comes from carbon dioxide, which acts as a cooling agent in the stratosphere even though it warms the atmosphere closer to ground level. This paradox occurs because the atmosphere thins with height, changing the way carbon dioxide molecules absorb and release heat. Together, the increase in carbon dioxide and the loss of ozone have led to record-low temperatures recently in the stratosphere and still higher up in the thermosphere. Far from being a good thing, this cooling is another sign that increasing levels of carbon dioxide are changing our planet's climate.

Related Links

Introduction to Ozone (UCAR Project LEARN)

Repairing the Antarctic Ozone Hole (Windows to the Universe)

Ozone Depletion (U.S. EPA)

Current state of our atmosphere and our planet

Climatologists prefer to combine short-term weather records into long-term periods (typically 30 years) when they analyze climate, including global averages. Between 1961 and 1990, the annual average temperature for the globe was around 57.2°F (14.0°C), according to the World Meteorological Organization.

In addition, we can evaluate climate over longer periods of observation. For example, in 2014, the global temperature was 1.24°F (0.69°C) above the long-term average for the 20th century, according to NOAA's National Climatic Data Center. That number made 2014 the warmest year on record in the NOAA database, which goes back to 1880.

Why are global temperatures expressed as a departure from normal, instead of a simple global temperature? 

One reason is that there are several different techniques for coming up with a global average, depending on how one accounts for temperatures above the data-sparse oceans and other poorly sampled regions.

Since there is no universally accepted definition for Earth’s average temperature, several different groups around the world use slightly different methods for tracking the global average over time, including:

The important point is that the trends that emerge from year to year and decade to decade are remarkably similar—more so than the averages themselves. This is why global warming is usually described in terms of anomalies (variations above and below the average for a baseline set of years) rather than in absolute temperature. A website from NASA's Goddard Institute for Space Studies goes into more detail on the topic of The Elusive Absolute Surface Air Temperature.

Related FAQ

"Hasn't Earth been cooling since 1998?"

Graph showing rise in global mean temperature, esp. after 1975
Global average temperature since 1880. This graph from NOAA shows the annual trend in average global air temperature in degrees Celsius, through December 2013. For each year, the range of uncertainty is indicated by the gray vertical bars. The blue line tracks the changes in the trend over time. Click here or on the image to enlarge. (Image courtesy NOAA's National Climatic Data Center.)

Averaged over all land and ocean surfaces, temperatures warmed roughly 1.53°F (0.85ºC) from 1880 to 2012, according to the Intergovernmental Panel on Climate Change (see page 3 of the IPCC's Climate Change 2013: The Physical Science Basis, Summary for Policymakers  - PDF). Because oceans tend to warm and cool more slowly than land areas, continents have warmed the most. In the Northern Hemisphere, where most of Earth's land mass is located, the three decades from 1983 to 2012 were likely the warmest 30-year period of the last 1400 years, according to the IPCC.

The graph above clearly shows the variability of global temperature over various time intervals (such as year to year or between decades) as well as the long-term increase since 1880. The full 2013 Working Group 1 report addressed in detail the slowdown observed in the rate of warming since the late 1990s. (For more, see the FAQ entry: "Hasn't Earth been cooling since 1998?")

There are slight differences in global records between groups at NCDC, NASA, and the University of East Anglia. Each group calculates global temperature year by year, using slightly different techniques. However, analyses from all three groups point to the decade between 2000 and 2009 as the hottest since modern records began more than a century ago. Temperatures in the 2010s have been running slightly warmer still.

Contiguous U.S. Temperature, January-December, since 1895
Average temperature for the contiguous 48 U.S. states from 1895 through 2013. The red line is the 12-month average for each year. The green line is a calculation of the trend during each decade. The blue line shows the trend over the last century of records. The gray line is flat because it is a simple average for the entire period. Click here or on the image to enlarge. (Image courtesy NOAA's National Climatic Data Center.)

What about the United States?

The year 2013 tied with 1980 as the 34th warmest in 120 years of data for the contiguous 48 U.S. states, according to the National Climatic Data Center (NCDC). This comes two years after the record warmth of 2012, which was substantially warmer—a full degree Fahrenheit (0.6°C)—than any other year since national records began in 1895. The U.S. warming rate of about 1.3°F (0.72°C) per century (blue line in the graph at bottom right) is roughly comparable to the global rate of warming (see above).

Despite some cooler years from 2008 to 2010, the decade as a whole (2000–2009) was the nation's warmest on record, with an average temperature of 54.0°F. In contrast, the 1990s averaged 53.6°F, and the 1930s averaged 53.4°F.

The first four years of this decade (2010–2013) were slightly cooler on average than 2000–2009 for the United States, with an average of 53.5°F. The United States covers only about 2% of the globe's total surface, so there can be noticeable differences at times between U.S. and world temperature trends. Overall, global readings this decade are running warmer than in the 2000s.


Graph of monthly CO2 concentration with steady upward trend
Monthly carbon dioxide concentrations at NOAA's Mauna Loa Observatory. This graph shows an annual seasonal cycle and a steady upward trend since CO2 measurements began atop Mauna Loa, Hawaii, in 1958.
The seasonal cycle is due to the vast land mass of the Northern Hemisphere, which contains the majority of land-based vegetation. The result is a decrease in atmospheric carbon dioxide during northern spring and summer, when plants are absorbing CO2 as part of photosynthesis. The pattern reverses, with an increase in atmospheric carbon dioxide during northern fall and winter. The yearly spikes during the cold months occur as annual vegetation dies and leaves fall and decompose, which releases their carbon back into the air.
This graphic portrayal of rising CO2 levels is known as the Keeling curve in honor of the originator of these measurements, Charles David Keeling of the Scripps Institution of Oceanography. Click here or on the image to enlarge. (Image courtesy Scripps CO2 Program.)

One of the strongest pieces of evidence for human-induced climate change is the consistent rise in carbon dioxide (CO2) in modern times, as measured at NOAA's Mauna Loa Observatory in Hawaii, where CO2 has been observed since 1958. In early 2015, the seasonally adjusted concentration of CO2 in Earth’s atmosphere was close to 400 parts per million (ppm), with a recent growth rate of between 2 and 4 ppm per year.

Around this seasonally adjusted average, the concentrations rise during northern spring and summer and drop during autumn and winter (see graph at right). The weekly average at Mauna Loa first rose above 400 ppm early in 2013 before falling back later in the year. The last time Earth's atmosphere held this much carbon dioxide was at least 3 million years ago.

Because CO2 stays in the air so long, it becomes very well mixed throughout the global atmosphere. This makes the Mauna Loa record an excellent indication of long-term trends.

Current atmospheric concentrations of CO2 are more than 35% higher than they were at the dawn of the industrial revolution. According to the Scripps Institution of Oceanography, ice core reconstructions going back roughly 800,000 years show concentrations of around 200 ppm during the ice ages. The concentrations were about 280 ppm during the last several interglacial periods and from the most recent ice age to the mid-18th century, as the industrial revolution was getting under way. In other words, our current CO2 levels are higher than they've been in at least the last 800 millennia. See the Scripps Web site for a graphic illustrating this trend.

Almost a quarter of the carbon dioxide emitted by human activities is absorbed by land areas, though this varies from year to year depending on large-scale drought and other factors. Another quarter is absorbed by the ocean. The remainder stays in the atmosphere for a century or longer.

Other gases

Carbon dioxide accounts for more than half of the human-produced enhancement to Earth’s greenhouse effect.  Among the other gases involved is methane, which has increased dramatically over the last century. Methane concentrations rose about 1% a year in the 1980s. The concentrations leveled off beginning about 2000, and then began increasing again in 2007. The reasons for the slowdown and resurgence are still being investigated, as discussed here.

Methane stays in the atmosphere for much less time than carbon dioxide (around a decade) and there is much less of it, but molecule for molecule, it is a far more powerful greenhouse gas. Calculating the global average is challenging, because methane concentrations vary by season and location, as illustrated by this informal analysis from 2011. According to a 2013 IPCC report, the concentration of methane in Earth’s atmosphere was about 1803 parts per billion as of 2011. Recent values measured at stations in Ireland and Australia are available from the Oak Ridge National Laboratory, and several historical perspectives on methane trends are depicted by the U.S. Environmental Protection Agency.

Other important greenhouse gases include nitrous oxide and near-surface ozone. Water vapor is actually the most prevalent greenhouse gas, but human activity has not directly increased its concentration in the atmosphere, unlike the other chemicals above. However, as global temperatures increase, more water vapor is released by oceans and lakes, and this in turn helps to increase temperatures further. This is one of many feedback loops that help to reinforce and intensify climate change.

Related Links

History of the Mauna Loa Record: Keeling Curve Lessons (Scripps CO2 Program)

The Carbon Cycle (UCARCenter for Science Education)

Earth's Greenhouse Gases (Windows to the Universe)


Graph of monthly CO2 concentration with steady upward trend
Monthly carbon dioxide concentrations at NOAA's Mauna Loa Observatory. This graph shows an annual seasonal cycle and a steady upward trend since CO2 measurements began atop Mauna Loa, Hawaii, in 1958.

The seasonal cycle is due to the vast land mass of the Northern Hemisphere, which contains the majority of land-based vegetation. The result is a decrease in atmospheric carbon dioxide during northern spring and summer, when plants are absorbing CO2 as part of photosynthesis. The pattern reverses, with an increase in atmospheric carbon dioxide during northern fall and winter. The yearly spikes during the cold months occur as annual vegetation dies and leaves fall and decompose, which releases their carbon back into the air.

This graphic portrayal of rising CO2 levels is known as the Keeling curve in honor of the originator of these measurements, Charles David Keeling of the Scripps Institution of Oceanography. Click here or on the image to enlarge. (Image courtesy Scripps CO2 Program.)

People don’t always produce more CO2 from one year to the next. When the global economy weakens, emissions from human activities can actually drop slightly for a year or two, as they did in 2009. When the economy rebounds, so can emissions. Yet in either case, the accumulation of CO2 in the atmosphere continues to rise over time, as shown in the graph to the right. It’s a bit like a savings account: even if your contributions get smaller in a tight budget year, the total in your account still goes up.

Vegetation also makes a difference, because growing plants absorb CO2. Large-scale atmospheric patterns such as El Niño and La Niña bring varying amounts of flooding, drought, and fires to different regions at different times, which affects global plant growth. Thus, the amount of human-produced CO2 emissions absorbed by plants varies from as little as 30% to as much as 80% from year to year. Over the long term, just over half of the CO2 we add to the atmosphere remains there for as long as a century or more. About 25% is absorbed by oceans, and the rest by plants. This "balance sheet" is known as the global carbon budget.

It’s not yet clear which forests absorb the most CO2. Because the answer will influence global planning and diplomatic agreements on climate, scientists are working hard to measure how CO2 varies by latitude, altitude, and season. One such study is HIPPO, a field project led by NCAR and colleagues from 2009 to 2011 to take pole-to-pole measurements aboard an airborne laboratory, the NSF/NCAR Gulfstream V jet. Satellites such as Japan's GOSAT and others on the drawing board at NASA will help fill in more carbon-budget details.


The next major assessment report about the impacts of climate change on the environment and society is scheduled to be released by Working Group II of the Intergovernmental Panel on Climate Change (IPCC) on March 29, 2014.

Highlights from the IPCC Working Group II, Summary for Policymakers, Climate Change 2007: Impacts, Adaptation and Vulnerability

“Some adaptation is occurring now, to observed and projected future climate change, but on a limited basis.”

“Adaptation will be necessary to address impacts resulting from the warming which is already unavoidable due to past emissions.”

“However, adaptation alone is not expected to cope with all the projected effects of climate change, and especially not over the long run as most impacts increase in magnitude.”

“A wide array of adaptation options is available, but more extensive adaptation than is currently occurring is required to reduce vulnerability to future climate change. There are barriers, limits and costs, but these are not fully understood.”

“Vulnerability to climate change can be exacerbated by the presence of other stresses.”

“. . .[T]he projected impacts of climate change can vary greatly due to the development pathway assumed. For example, there may be large differences in regional population, income and technological development under alternative scenarios, which are often a strong determinant of the level of vulnerability to climate change.”

“Sustainable development can reduce vulnerability to climate change by enhancing adaptive capacity and increasing resilience. At present, however, few plans for promoting sustainability have explicitly included either adapting to climate change impacts, or promoting adaptive capacity.”

“A portfolio of adaptation and mitigation measures can further diminish the risks associated with climate change.”

“This Assessment makes it clear that the impacts of future climate change will be mixed across regions. For increases in global mean temperature of less than 1 to 3°C [1.8 to 5.4°F] above 1990 levels, some impacts are projected to produce benefits in some places and some sectors, and produce costs in other places and other sectors. It is, however, projected that some low latitude and polar regions will experience net costs even for small increases in temperature. It is very likely that all regions will experience either declines in net benefits or increases in net costs for increases in temperature greater than about 2 to 3°C [3.6 to 5.4°F].”


But what about . . .

Warming by the Sun and other variations in natural systems cannot explain global warming. Experiments using computer models confirm the importance of human-produced emissions in the temperature trends of recent decades. This graphic depicts global average temperature since 1890 as reproduced by the NCAR/DOE Parallel Climate Model.

Color graph in red and blue
(Image courtesy Gerald Meehl, NCAR.)
  • The black line plots human observations of Earth's temperature.
  • The blue line summarizes simulations performed using only natural influences on climate (volcanoes and solar variations). 
  • The red line, from a set of simulations that includes sulfate aerosol pollution and greenhouse gases.
  • Shading in pale blue and pale pink shows the range of results (the model uncertainty) for each group of simulations, or ensemble.

The simulations that include only natural variability, including changes in the Sun and eruptions of volcanoes, show that we should have seen a decrease in the global average temperature in the last several decades.

The simulations that most closely resemble the observed record are the ones that take the cooling effect of air pollution and the warming effect of greenhouse gases into account.

Related links

Learn more about climate (NCAR)

What tools are used to study climate? (video explaining the complexity and reliability of the observations, prehistoric records, and climate models used to study past, present, and future climate; part 2 of Climate Future: Voices of Science)


El Niño events can raise the average global temperature by a few tenths of a degree Fahrenheit for a year or two. Likewise, La Niña events can produce a comparable cooling effect. Neither of these short-lived cyclic phenomena explain the longer-term warming observed in the last century and especially in the last 30 to 40 years. (These effects have been investigated for some time. For an in-depth report updated in 2001, see Children of the Tropics: El Niño and La Niña.)




The Sun’s energy output rises and falls in a regular cycle, with peaks every 11 to 12 years. Data since the 1950s show that the difference in output from each peak to valley is about 0.1%, which has little effect on Earth’s temperature. The last solar cycle ended with a minimum in 2008–09, when sunspot activity dropped to its lowest level since the 1910s.

The Sun also goes through longer, more irregular periods of greater or lesser activity. These include the Maunder Minimum, when sunspots nearly vanished from 1645 to 1715. Parts of the Northern Hemisphere were significantly cooler during this time, which occurred within a longer period called the Little Ice Age. Other factors were also involved in the cooling, including powerful volcanic eruptions.

Since 2010 , the Sun has been building toward the next peak in its 11-year cycle, which typically boosts sunspot activity and space-weather events. However, the peak may have occurred already, judging from monthly plots of sunspot activity published by NOAA. Some data now indicate that the Sun may be entering a longer period of relatively low activity. If this were to extend over several decades, creating what’s known as a “grand minimum,” then the usual peaks in the Sun’s 11-year cycle could weaken or even disappear. The amount of solar energy reaching Earth—the total solar irradiance (TSI)—could drop to the levels seen in a typical 11-year minimum, or perhaps further, and remain there for decades.

Because many solar variables have been measured only for a few decades, and because the Sun includes both predictable and naturally chaotic behavior, it is not possible to pin down the likelihood of a grand minimum. Its effect on Earth’s temperature would depend on exactly how much the TSI dropped and for how long. The TSI has only been measured directly for about 30 years, but proxy data from lake sediments, ice cores, tree rings, and other sources suggest that the total solar energy reaching Earth dropped by as little as 0.1% during the Maunder Minimum.

So could a lengthy drop in solar output be enough to counteract human-caused climate change? Recent studies at NCAR and elsewhere have estimated that the total global cooling effect to be expected from reduced TSI during a grand minimum such as Maunder might be in the range of 0.1° to 0.3° Celsius (0.18° to 0.54° Fahrenheit). A 2013 study confirms the findings. This compares to an expected warming effect of 3.0°C (5.4°F) or more by 2100 due to greenhouse gas emissions. In other words, even a grand solar minimum might only be enough to offset one decade of global warming. Moreover, since greenhouse gases linger in the atmosphere, the impacts of those added gases would continue after the end of any grand minimum.

During the solar cycle, the Sun's ultraviolet (UV) output varies much more than TSI does, including sharp sudden increases associated with solar storms. UV variations affect Earth’s upper atmosphere and may also influence weather and climate, particularly by way of the stratosphere, through their effects on ozone and related processes.

Related links

Don't changes in the Sun, like sunspots, explain global warming? (Climate FAQ)

Turning down the heat (AtmosNews - Research Brief)

Could a weaker Sun avert global warming? (AtmosNews - Perspective)

Learn more about the Sun (NCAR)


After rising in the early 20th century, global surface temperatures cooled slightly from just after World War II (the mid-1940s) into the 1970s. These temperature drops were focused in the Northern Hemisphere.

Scientists already knew that carbon dioxide was accumulating in the atmosphere and that it could lead to eventual global warming. In 1975, Wallace Broecker (Lamont-Doherty Earth Observatory) published the first major study with "global warming" in the title.

A few researchers believed that pollution from burgeoning postwar industry in North America and Eurasia was shielding sunlight and shading the planet, causing the observed cooldown. Some even theorized that a "snow blitz" could accelerate the cooling and bring on the next ice age. Their statements got major play in the media. But the majority of scientists publishing in peer-reviewed journals were concerned that greenhouse gases would play a more dominant, warming role that would overtake the cooling of sulfate aerosol pollution in the coming decades. The state of climate science knowledge in the 1970s was summarized in a 2008 article on "The Myth of the 1970s Global Cooling Scientific Consensus" in the Bulletin of the American Meteorological Society (abstract). 

Starting in the 1970s, new clean-air laws began to reduce sulfates and other sunlight-blocking pollutants from U.S. and European sources, while greenhouse gases continued to accumulate unchecked. Global temperatures began to warm sharply in the 1980s and have continued rising since then.

Increasingly detailed models suggest that the more recent warmup can be attributed to greenhouse gases overpowering the effect of sunlight-shielding pollution.  Computer simulations also suggest that today's atmosphere would be even warmer still, were it not for that air pollution.

Color graph of mean temperatures in red and blue
We've zoomed in for a closeup of temperatures over the last three decades, taken from a longer timeline discussed here. This closeup shows the annual trend in average global air temperature, in degrees Celsius, from 1975 to 2012. For each year, the range of uncertainty is indicated by the vertical bars. The blue line tracks the changes in the trend over time. Click here or on the image to see the full graph.
(Image courtesy NOAA's National Climatic Data Center.)

Thanks in large part to the record-setting El Niño of 1997–98, the year 1998 was the warmest year, globally, in the 20th century. Since 2001 the global trend has been relatively flat (see graph). However, temperatures continue to run warmer than in previous decades. The global average from 2000–09 exceeds the average for 1990–99, which in turn was warmer than 1980–89. And the average for 2010–12 topped the 2000–09 average.

Although scientists are confident that global temperatures will rise further in the coming decades, there could still be occasional "pauses" in warming that last a few years, like the one we've been seeing since the late 1990s. The Intergovernmental Panel on Climate Change (IPCC) devoted a section of its 2013 physical science assessment to analysis of the 1998–2012 warming hiatus. Among the possible factors involved:

  • A series of small volcanoes since 2000 that have spewed sunlight-blocking ash skyward

  • The natural 11-year solar cycle, whose declining phase lasted longer than usual: from 2000 to 2009

  • Variations in the exchange of heat between the ocean and atmosphere (one aspect of internal climate variability). Simulations by NCAR researchers suggest that periods when more heat gets stored deep in the ocean could be an important factor in slowing down atmospheric warming for a decade or more.

Two of the main vehicles for ocean-atmosphere heat exchange are El Niño (which tends to warm the global atmosphere) and La Niña (which tends to cool it). La Niña events have predominated over El Niños since 2006, which helps explain the lack of a new global record in the decade thus far.

As the IPCC notes, natural variations can either amplify or mute longer-term warming trends over periods of 10 to 15 years. This is one reason why 30 years is the typical reference period in determining climate. In your local weather forecast, for example, the average daily temperatures and precipitation posted alongside today's measurements were calculated based on the period from 1981 to 2010. This was a warmer period for most U.S. cities than the previous 30-year range, from 1971 to 2000.

Meanwhile, decision makers and planners often need guidance on shorter time scales than three decades. Researchers are responding by experimenting with outlooks for global temperature in the 10- to 15-year range. The most recent decadal outlook from the UK Met Office calls for a good chance that the upcoming five years (2013–17) will average slightly warmer globally than the record year of 1998. The IPCC expects that, for periods without a major volcanic eruption, most future 15-year periods will show a stronger warming trend than 1998–2012.

Color map of the world for surface temperatures
Temperature anomalies (variations above and below average, in degrees Celsius) for Wednesday, January 17, 2007. Despite the cold across Texas and New England, most of the Northern Hemisphere was running well above average that day. Click here or on the image to enlarge. (Image courtesy NOAA/ESRL PSD Map Room.)

Climate change doesn't mean that winters will disappear or that summers will be uniformly hot. There are always cold spells and warm spells going on in one place or another. But even where weather is cold, what's considered "typical" can change. For example, the heavy snow that struck Colorado and Kansas at the end of 2006 was actually more characteristic of that area's autumn or spring weather than a typical December. Time will tell whether and where such individual cases recur often enough to be considered a trend.

To examine long-term warming, climate scientists often look at larger areas and longer time periods. Globally, Earth's natural processes don't follow a linear pattern, so the global average temperature may be slightly cooler or warmer from one year to the next. Different parts of Earth's ecosystem also respond to the greenhouse effect in different ways. The oceans, for example, hold more heat and respond to atmospheric changes more slowly than land masses do. Average temperatures of the land, oceans, and atmosphere also vary from year to year as well as from each other.

Just as a baseball player on steroids will occasionally strike out, a climate warmed by extra greenhouse gases will still produce unusually cold weather at times. It's also important to remember that there are multiple factors contributing to every weather event—which is why you'll often hear forecasters and researchers pointing out that no particular weather feature can be entirely "blamed" on climate change.

To learn more about the links between weather events and global climate change, see our in-depth report Weather on Steroids.


Four differenet maps of the US with different resolution for climate model
The amount of detail in climate models has continued to increase in recent years, largely because of the calculation power provided by newer supercomputers. In the 1990s, high-resolution global climate models operated on the T42 resolution scheme (upper left). At this resolution, temperature, moisture, and other features were tracked in grid boxes that each spanned about 200 by 300 kilometers at midlatitudes (120 x 180 miles), an area roughly as large as West Virginia.

In more recent modeling that led up to the 2007 IPCC Working Group I report, the NCAR-based Community Climate System Model (CCSM) routinely operated at T85 resolution (upper right), with midlatitude grid points of about 100 by 150 km (60 x 90 miles)—the size of Connecticut.

Better resolution not only provides a more true-to-life depiction of atmospheric processes, but also allows for more realistic topography, which makes regional climate projections more accurate. For example, the highest Rocky Mountains appear as two coarse grid points at T42 but as a more diverse assortment of high peaks at T170 (lower left). Enhancements in computing power will help scientists explore the use of higher resolutions, such as T170 and T340 (lower right). Click here or on the image to enlarge. (©UCAR. Illustration courtesy Warren Washington, NCAR. This image is freely available for media and nonprofit use*)

Global climate models—the software packages that simulate the past, present, and future of our atmosphere—have grown in complexity and quality over the last 10 to 20 years, and the most sophisticated models agree on the big picture of climate change. This includes the rough amount of warming expected and the idea that poles will warm faster than lower latitudes. As models have continued to improve, increasing agreement on regional details has emerged, such as the likelihood of more precipitation in the northern subpolar latitudes and a northward expansion of the hot, dry subtropics around 30°N. Climate models are not perfect, but the main ingredients are well understood and tested, and scientists are making progress in areas that remain a challenge, such as the behavior of tropical oceans and the evolution of cloud patterns.

It’s important to note that models are not the only reason why scientists are concerned about climate change.  For more than a century—long before many recent advances in science, and long before computer models—we’ve known that increased greenhouse gases could produce a global temperature increase.  Observations of the last century of climate, including those from instruments and from the behavior of ice and plants, concur that the planet is warming.  As greenhouse gases continue to increase, it stands to reason that more warming can be expected.

Even the earliest models of the 1960s, which were quite crude by today’s standards, showed that a doubling of carbon dioxide in the atmosphere could increase global temperature by around 5°F (3°C). That projection remains close to the modern consensus, and temperatures over the last 30 years have risen at a rate consistent with this early estimate.

Far more information is available from today’s models, such as the NCAR-based Community Earth System Model, because they now include many more aspects of the Earth system, including ice sheets, vegetation, cloud areas, and soil moisture.

Research conducted for the 2007 IPCC Working Group 1 assessment compared the output from major models at research centers around the world. While these models are far from perfect, scientists are confident that they capture the key processes that drive climate. For example, models now replicate the ups and downs of 20th-century global temperature quite accurately.

As in other areas of science, rigorous testing and continual improvement are part and parcel of climate modeling. Researchers can test models against reality, identify and correct flaws, and compare their models with others. 


flow chart showing increasing complexity of climate models
The complexity of global climate models has increased enormously over the last four decades, as shown in this graphic. The most powerful models, such as the Community Earth System Model (developed by scientists at the Department of Energy and NCAR with colleagues at other organizations), now have the capability of simulating a broad range of atmospheric processes, such as the impact of marine ecosystems on the atmosphere. Click here or on the image to enlarge. (©UCAR. This image is freely available for media and nonprofit use*)
3-D view of column of air rising through layers of atmosphere
Computer models reach high into a virtual atmosphere and deep into the ocean. They simulate climate by dividing the world into into 3-dimensional grid boxes, measuring physical processes such as temperature at each grid point. Such models can be used to simulate changes in climate over years, decades, or even centuries. Click here or on the image to enlarge. (©UCAR. This image is freely available for media and nonprofit use*)
3-D view of Earth system, from ocean floor to troposphere
The NCAR-based Community Earth System Model (CESM) is one of the world’s most sophisticated models of global climate. Created by scientists at NCAR, the Department of Energy, and collaborators, this powerful model simulates the many processes in our climate system, ranging from clouds and atmospheric chemicals to ice to marine ecosystems. Click here or on the image to enlarge. (©UCAR. This image is freely available for media and nonprofit use*)


What's happening to glaciers and sea ice?

Glacial periods have occurred about every 100,000 years, in sync with well-understood cycles of change in the way Earth orbits the Sun. These "Milankovitch cycles" affect where sunlight hits the planet, which can speed up or slow down the accumulation of ice across high latitudes. 

It has been about 10,000 years since the last glacial period ended. All else being equal, another glacial period would be expected to arrive in the next several tens of thousands of years. However, the exact process is not fully understood, so we don’t know exactly when this will occur.

In contrast, the increase in human-produced greenhouse gases is proceeding rapidly, and it is already having effects that could intensify significantly in the next few decades. One analysis, published in the journal Science in 2013, shows that the warming of the last 100 years has occurred far more quickly than the gradual rise and fall of temperatures over the preceding 11,200 years. We cannot count on the arrival of the next ice age to offset the potentially catastrophic effects of human-triggered climate change over the next several centuries.



Ice melts each summer and refreezes each winter in the Arctic Ocean. Summer sunshine only melts part of the sea ice, but the amount that melts has increased dramatically over the last few years, especially since 2007. The extent of sea ice at its summer minimum (which usually occurs in September) is now little more than half of where it was in the 1980s, and the minimum ice volume (which considers both extent and thickness) has dropped by more than 60%.

Experts differ on how quickly Arctic sea ice will continue to melt, with some projections leading to summer periods that are virtually free of ice by the 2020s or even sooner. Ice will continue to refreeze each winter, but the increased areas of open ocean will affect human activities and wildlife and may influence regional climate in ways now being studied. Also, the dark ocean can absorb more heat than brighter, more reflective sea ice, which helps foster a vicious cycle of more melting and more warming.

Unlike ice that’s melting on land, the loss of ice floating in the ocean doesn’t raise sea level, since the ice will take up less space once it melts (just as ice melting in a glass of lemonade doesn’t cause the glass to overflow).

The Intergovernmental Panel on Climate Change

The Intergovernmental Panel on Climate Change was formed in 1988 by two United Nations organizations, the United Nations Environment Programme and the World Meteorological Organization, to assess the state of scientific knowledge about the human role in climate change.

To accomplish its mission, the IPCC coordinates the efforts of more than 2,000 scientists from 154 countries. Together, they represent a vast array of climate specialties, from physics, to chemistry, to interactions with Earth's surface, to the role of human behavior. Their reports take years of critical assessment and review before they are issued to the public. The scientists who participate volunteer their time to IPCC activities, assisted by a small number of paid staff.

Because each chapter is subjected to more extensive review than perhaps any other scientific report, and because the authors are assessing multiple studies, many of the findings reported by the IPCC are considered more cautious or conservative than the outlooks provided by any single experiment or analysis.

Because different types of expertise are required to assess different aspects of climate change, the IPCC is divided into three working groups.

  • Working Group I reviews the physical science, including observations and computer modeling of the past, present, and future
  • Working Group II examines the likely impacts on people and the environment.
  • Working Group III explores policy options for lessening the likelihood of climate change.

Each working group prepares a lengthy report and a much briefer "Summary for Policymakers." In addition to the three working groups, the IPCC Task Force on National Greenhouse Gas Inventories was created in 1991 to help participating countries calculate and report their production and elimination of greenhouse gases.

In addition to reviews by individual scientists and scientist panels, each chapter within an assessment is also scrutinized by representatives of the governments participating in the IPCC process. While governments negotiate on how the findings are worded, the final product is based on a scientific, not a political, consensus.

After years of planning, collecting, writing, and responding to multiple reviews, each assessment report reflects the scientific consensus on what is known and what is still uncertain about the environmental and societal consequences of continuing to add greenhouse gases to Earth's atmosphere.

In late 2007, the IPCC shared the Nobel Peace Prize with former U.S. vice president Al Gore for its work in having "created an ever-broader informed consensus about the connection between human activities and global warming." (The prize was awarded to the panel rather than to individual participants.)

The IPCC has published major assessments in 1990, 1996, 2001, and 2007, as well as special interim reports on topics such as aviation, land use, assessment methods, or emissions scenarios. All of the major and interim reports are available in the six official languages of the United Nations and may be downloaded from the IPCC Publications and Data page.

The next major IPCC assessment will be released in several stages in late 2013 and 2014.

Related Links

Intergovernmental Panel on Climate Change

IPCC History (United Nations Foundation/

Politics and opinion

The unique structure of the IPCC includes both scientific and governmental review, but the input of diplomats to the final report is designed to be distinct and different from that of scientists.

Scientists who are experts in their subject matter prepare the chapters that go into the full assessment reports. Those chapters are scrutinized by individual scientists and scientist panels, whose questions must be addressed before a chapter can be approved for inclusion.  The chapters making up the 2007 draft report from IPCC Working Group 1 report run to over 1,600 pages.

It is only the Summary for Policymakers, typically around 30 pages, that receives a word-by-word review, during the final plenary session, by diplomats from almost every nation in the world. The lead authors of the report are on hand at the plenary to make sure that any changes are scientifically valid. The diplomats have a say in how the Summary for Policymakers is worded, but the scientists have the last word on what is said.

The result of the IPCC process is a report that carries the weight of formal approval by the world's governments as well as the authority of hundreds of participating scientists. For more about the process, see the IPCC Fact Sheet: How does the IPCC review process work? (PDF).

The scientific method is built on debate among scientists, who test a question, or hypothesis, and then submit their results to the scrutiny of other experts in their field. That scrutiny, known as "peer review," includes examining the scientists' data, experiment and/or analysis methods, and findings (read more about peer review here).

The spirited debate around remaining uncertainties in climate science is a healthy indicator that the scientific method is alive and well. But the fundamental elements of climate change are not in dispute. To take just a few examples, we understand

The questions on this page represent many of those raised by debaters who are not actively engaged in climate research. These questions have been answered here and elsewhere with evidence from research that has been tested by the scientific method.

Science is a human activity, and no human is infallible. Science is also a community activity, and scientists rely on each other to question, challenge, and improve one another's work. When corrections are made, this is not a sign that the system is broken but rather that it's working as designed.

The science reported by researchers at NCAR and our collaborating institutions around the world is built on decades of investigation and represents the current state of our knowledge on climate change.

Our understanding of the particulars of climate change continues to evolve, and predictions of specific impacts may be revised upward or downward. However, the majority of climate scientists who specialize in understanding the complex interactions of our atmosphere, Earth, and Sun have concluded that:

Global warming is unequivocal and primarily human-induced (U.S. Global Change Research Program, citing its 2009 report on Global Climate Change Impacts in the United States).

Changes in the atmosphere, the oceans, and glaciers and ice caps now show unequivocally that the world is warming due to human activities (United Nations, citing the IPCC 2007 report)

Here's what several major scientific organizations say about global warming and climate change, including the uncertainties climate scientists continue to examine (each link opens a new window):



A handful of errors in the 2007 IPCC Working Group II report were discovered in early 2010. They represent a tiny fraction of thousands of pages of findings in the complete report. To date, none of these errors has directly challenged the physical basis for detecting a human influence on Earth's climate.

The IPCC responded to these issues, as they report on their website. In March 2010, the United Nations and the IPCC asked the InterAcademy Council "to conduct an independent review of the IPCC’s processes and procedures to further strengthen the quality of the Panel’s reports on climate change." (IPCC, 2010 - PDF) The IAC is the umbrella organization for national academies of science around the world.

The IAC's report, released in August 2010, called the IPCC's process for producing assessments "successful overall," while recommending changes in management structure and procedures going forward, including greater transparency and communication. Many of these proposals have been put into place for the 2013–14 IPCC assessment.

Other concerns were raised in late 2009 when thousands of personal e-mail messages among several climate scientists were obtained without consent and posted on the Internet. UCAR and many other scientific organizations around the world responded to these concerns.

While the e-mail hacking incident raised a variety of issues, none of them directly challenged the major findings reported in climate change assessments, including the 2007 IPCC report.

What can we do?

This FAQ addresses global warming—the rise in the average temperature of our planet as a whole—and climate change—the complex ways Earth's ecosystems respond. The evidence-based conclusion of scientists around the world is that global warming due to human activity is happening. The mounting data from real-world observations and the projections of future changes from reality-tested climate models have provoked a variety of actions and reactions from all levels of society. These actions boil down to three basic responses.

Mitigation is reducing the emissions of greenhouse gases responsible for climate change, so that less change occurs.

Adaptation is dealing with the consequences of warming and other aspects of climate change, such as changes in extreme weather events.

"Business as usual" is also a response. This option saves expenditures for mitigation in the near term, but risks higher adaptation costs to wildlife, human populations, infrastructure, and economies later on. It also increases the odds of unforeseen consequences from unchecked climate change.

Because some amount of climate change has already occurred, and more change is inevitable based on the greenhouse gases we have already added to the atmosphere and oceans, society will need to adapt. Yet in order to prevent even more-extreme climate change from happening, mitigation will be required.

Democratic societies around the world are examining these options, including how much attention and which resources to devote to each one, and how to find a balance between mitigation, adaptation, and competing economic and social concerns.

Renewable sources of energy

Fossil fuels pour carbon dioxide and other greenhouse gases into the atmosphere whenever we burn these fuels to power our transportation, heat and light our homes, and keep our industries and other businesses running. Alternatives to coal, gasoline, heating oil, and other fossil fuels are being explored, improved, and put to use by the private sector and governments around the world, across a wide range of options.

In the United States, the National Renewable Energy Laboratory serves as a hub for research and development across the renewable energy spectrum, including

fuel from

  • biomass
  • photovoltaics
  • wind

energy savings from

  • building construction and retrofitting
  • electric infrastructure systems
  • vehicle fuel-use technology

A recent search on a popular online search engine produced 23,900,000 results for "renewable energy"—just one indication that the global hunt for solutions to the fossil fuel problem is in full swing.

Numerous colleges and universities are also focusing research and course offerings on renewable energy questions, including many of UCAR's member, affiliate, and international affiliate universities. Since 2009, NCAR researchers have been developing methods to produce highly detailed, localized weather forecasts that are helping electric utilities pinpoint when wind power will be available.

Businesses, local government, schools, neighborhoods

People around the world are noticing and responding to impacts on natural systems; human adaptation is already happening. Farmers are exploring drought-resistant plants and modifying planting and harvesting schedules. Insurance companies are accounting for climate change as they set rates and examine policies. In the developed world, air conditioning is becoming more widespread in places it was never needed before. Some communities on small islands are planning how they will relocate as sea levels rise around them. The fate of plants and animals that cannot readily adapt is being discussed.

Mitigation is also happening on the local and personal level. Many U.S. cities and states have committed to reducing their output of greenhouse gases over the coming decades. Individuals are choosing fuel-thrifty or hybrid vehicles, for instance, or installing energy-saving light bulbs, and a growing number of businesses and organizations have pledged to become carbon neutral.

Volunteer "citizen scientists" are recording their observations to provide information about our climate over time. Some researchers are tapping a rich historical record of bird migration and seeking new student and backyard volunteers to report migration arrivals and departures. A collaboration between public and private agencies encourages volunteers of all ages to report the timing of budburst in spring. Participants record when dormant plants produce leaves and their flower buds first open in response to climate signals.

National and international policy


The United States joined with many other nations in signing a treaty in 1994 known as the United Nations Framework Convention on Climate Change. The UNFCCC, which has been ratified by 192 countries, recognizes that the climate system has no boundaries and that international cooperation is needed to seek solutions to the problems posed by rising greenhouse gases.

Considered a first step in a long diplomatic process, the Kyoto Protocol was an early and well-known agreement that emerged from the UNFCCC process. The protocol, which set modest targets for reducing greenhouse gas emissions, was adopted in 1997 and ratified by most countries in the world, though not the United States. Canada withdrew from the protocol in 2011.

The targets have been in force for over 180 signatory nations since early 2005. Before this point, more than 50 industrialized countries (referred to as “Annex I”) had agreed to reduce emissions by an average of 5.2% for the period 2008–12 compared to 1990. The signatories then dropped the rate to 4.2% in 2005 because the United States had not ratified the treaty.

While final numbers have not yet been calculated, it appears that the Annex I emissions have dropped by more than 10% (see PDF). However, emissions outside Annex I have risen dramatically. China’s alone have more than doubled, now comprising about 30% of all global emissions. As a result, total global emissions have continued to rise, apart from short-lived drops related to the 2008 economic crisis and recession.

The process of creating a post-Kyoto agreement has been challenging. In the most recent annual meeting of the Conference of the Parties to the UNFCCC, which took place in Doha, Qatar, in late 2012, many nations agreed to continue their participation in the Kyoto Protocol. But getting a majority of nations around the world to agree on anything new takes time. The next global protocol, based on the Durban Platform from the 2011 Conference of the Parties meeting, is expected to take shape in negotiations extending from now through 2015.

Policy-relevant news from NCAR & UCAR

Climate change in cities: Vulnerabilities and strengths (July 18, 2012)

Climate change poses major risks for unprepared cities (April 06, 2011)

U.S. must take steps to adapt to climate change (September 29, 2010)

Climate in 2050 crucial to impacts in 2100 (January 11, 2010)

Cuts in Emissions Would Save Arctic Ice (April 14, 2009)

Related Links

What You Can Do

Environmental Protection Agency

At Home, At the Office, On the Road, At School

State and Local Governments

A Student's Guide to Global Climate Change: Be Part of the Solution


IGLO: International Action on Global Warming

IGLO Toolkit of educational resources on Earth's polar regions


Adaptation, Mitigation, and Other Policy Issues

In the United States 

America's Climate Choices (National Academy of Sciences)


Around the World

Climate Change (European Commission)
Climate Change (Government of Australia)
Canada's Action on Climate Change (Government of Canada)
UK Climate Impacts Programme (United Kingdom)

At the United Nations

UN Framework Convention on Climate Change


Research Partnerships - Examples

Global Climate & Energy Project (Stanford University)
Carbon Mitigation Initiative (Princeton University)
Carbon Capture & Storage Research (Department of Energy)


Updated: October 2013

How can I learn more?


Here are some additional science-based resources to investigate

Research news

Research programs at NCAR

And a few additional science-based FAQs