We’ve been inviting readers to send us their questions about climate change, and seeking answers from climate scientists at UC Irvine. Michael Prather, who specializes in climate modeling and for years has been an author of reports from the United Nations’ Intergovernmental Panel on Climate Change, answers a question about greenhouse gases from Jim Craig (blog name Jack Kreg).
Craig, 59, is an aeronautical engineer who lives in Lake Forest. His question focuses on man-made vs. natural carbon dioxide, or CO2.
Please send us your questions about climate change by commenting on this blog post or e-mailing reporter Pat Brennan directly at firstname.lastname@example.org.
Question: We are led to believe that global warming is caused by man made CO2, yet CO2 and other greenhouse gases are also produced in nature. EPA is careful to skirt this issue by using the term “man-made greenhouse gas emissions.” They pass right over the fact that GH gases are naturally produced. It is said that GH gases are in fact produced and consumed in nature, like plant life converting CO2 into O2, e.g. CO2 consumption. Doesn’t the ocean absorb huge amounts of CO2?
The question is, why is all the emphasis on “man-made” GH gases, when they are also produced in massive quantities in nature? Further, if they are produced in larger quantities in nature, then how will a fractional reduction in “man made” GH gases, say 10%, have any effect, if natural sources far outweigh “man made” sources?
Answer: Dear Jack Kreg,
Ever since scientists have been able to measure the composition of the air with any precision, they have found an evolving atmosphere, and they have pursued the research to understand the cause of these changes. As you note, there are three major naturally occurring greenhouse gases that, once emitted, remain in the atmosphere for decades to centuries: carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). There are also greenhouse gases that are totally synthetic, being a product of industry: these include dichlorodifluoromethane (CFC-12), sulfur hexafluoride (SF6), and many others.
Carbon dioxide was first documented to be increasing in the atmosphere by C. David Keeling using measurements on Mauna Loa Hawaii. His research and that by many others following have identified the increase as being caused almost entirely by burning of coal, oil and natural gas. You correctly state that there are large natural sources and sinks for CO2 (and CH4 and N2O as well). A large amount of scientific detective work has gone into attributing the cause of the increase in these gases. While the annual exchange of CO2 between atmosphere and ocean and land is large, the fossil burning has pushed it out of balance. In effect we are burning the fossil fuel carbon at a rate that is more than 30 times faster than formation of the sediments that will become future fossil fuels. The figure 7.3 from the IPCC 2007 report summarizes the global carbon cycle and demonstrates the current imbalance due to fossil fuel consumption.
Over the last few billion years of Earth history, evidence points to much higher levels of CO2 during some periods than today. That evidence is strong but circumstantial. In contrast, over the last 1 million years of the ice age cycles, we have absolute evidence of the CO2 abundances from the air bubbles trapped in ice cores. That record shows that CO2 varied during the ice ages from 0.018% to 0.028%. Since the 18th century, we have observed CO2 increase steadily to more than 0.038%. [Note, CO2 abundance is most often reported as parts of CO2 per million parts of air (ppm), and thus 0.038% is usually written as 380 ppm.] Research on the carbon cycle has reached the nearly unanimous conclusion that the imbalance shown in the figure below (from pre-human influence in black to current in red) is caused by our burning of fossil fuels. The rapid rise in CO2 throughout the 20th century is also mirrored in CH4 and N2O, as shown in the figure SPM-1.
There is no viable scientific theory that can explain these increases in the “natural” greenhouse gases other than through human activities, which are known and measured to emit these gases.
In terms of understanding the observed atmospheric increases in the synthetic gases (CFC-12, SF6 and others), the evidence is simpler because there are no known natural sources. For example, the rise in CFC-12 was documented by Professor Sherry Rowland at UC Irvine in the early 1980s and attributed to the chemical industry. The industry fought this, claiming that there were other sources, but the weight of scientific evidence was clear and eventually, even DuPont acknowledged its role in causing atmospheric CFC-12 to increase, leading to ozone depletion as well as global warming.
The role of humans in the recent increase in CO2 is not controversial within the science community. However, there remain many serious uncertainties as to what will happen to CO2 over the coming century. Our predictions of 21st century CO2 cannot be based solely on projections of fossil fuel consumption but must include the effects of climate change on the natural systems, and the uncertainty here remains large.