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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.
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.
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.
Averaged over all land and ocean surfaces, temperatures warmed roughly 1.53 degrees Fahrenheit (0.85 degrees Celsius) from 1880 to 2012, according to the Intergovernmental Panel on Climate Change (see page 3 of the 2013 summary report). 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 1,400 years, according to the IPCC.
One of the strongest pieces of evidence for human-induced climate change is the consistent rise in carbon dioxide in modern times, as measured at NOAA's Mauna Loa Observatory in Hawaii, where carbon dioxide has been tracked 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. 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.
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.
Since the Industrial Revolution began in the 18th century, 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.
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 40 percent since preindustrial times.
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 thousands of scientists from around the world. 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.
Climate scientists prefer to combine short-term weather records into long-term periods (typically 30 years) when they analyze climate, including global averages.
Today's global temperature is typically measured by how it compares to one of these past long-term periods. For example, the average annual temperature for the globe between 1951 and 1980 was around 57.2 degrees Fahrenheit (14 degrees Celsius). In 2015, the hottest year on record, the temperature was about 1.8 degrees F (1 degree C) warmer than the 1951–1980 base period.
That calculation comes from NASA and NOAA. Other agencies may come up with a slightly different number because there are several techniques for calculating 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.
There are a few connections between the two, but they are largely separate issues.
The ozone hole refers to the seasonal depletion of ozone molecules in the lower stratosphere above Antarctica. This layer of "good ozone" near the top of the atmosphere acts as a shield, filtering out most of the ultraviolet light from the Sun that could otherwise prove deadly to people, animals, and plants.
Damage to the ozone shield 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.
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 temperature at Earth's surface, but ozone is actually a greenhouse gas, and so are CFCs, meaning that their presence in the troposphere contributes slightly to the heightened greenhouse effect. But the main greenhouse gas responsible for present-day and anticipated global warming, is carbon dioxide produced by burning of fossil fuels for electricity, heating, and transportation.
To interpret today's atmospheric conditions, we need a reference period of average, or "normal," climate for comparison. How long is long enough to define the average climate for a city, state, or region? The National Oceanic and Atmospheric Administration's National Climatic Data Center calculates a 30-year average once a decade.
When it comes to climate on a global scale, the "normal" reference period depends on which climate components scientists are studying. 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.
To study what the climate was like before modern record keeping began, climate scientists rely on "proxy records." Air bubbles trapped in ice cores, the composition of 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.