How much carbon dioxide (and other kinds of greenhouse gas) is already in the atmosphere?

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. At the beginning of 2013, the seasonally adjusted concentration of CO₂ in Earth’s atmosphere was about 395 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). Researchers expect the value to reach 400 ppm sometime 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 about 30% higher than they were at the dawn of the industrial revolution. According to the Scripps Institution of Oceanography, ice core reconstructions going back over 400,000 years show concentrations of around 200 ppm during the ice ages. The concentrations were about 280 ppm during the warm 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 400 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; 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. As of 2008, the concentration of methane in Earth’s atmosphere was about 1786 parts per billion. Since then, the rise has continued, as measured at several observing stations around the globe, including Mauna Loa. Recent values measured at stations in Ireland and Australia are available from the Oak Ridge National Laboratory.

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.