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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.
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.
History of the Mauna Loa Record: Keeling Curve Lessons (Scripps CO2 Program)
The Carbon Cycle (Spark: NCAR & UCAR Science Education)
Earth's Greenhouse Gases (Windows to the Universe)