Greenhouse Gases and Global Climate Change Frequently Asked Questions
What is the latest on the science of Global Climate Change and the need to cut greenhouse gas emissions?
Many of the undisputed facts have not changed over the last decade. For example, based on samples of air trapped in arctic ice, scientist have determined that, prior to the industrial revolution (which began in England in the mid 1800s), the concentration of carbon dioxide in the atmosphere had been stable at a level of around 288 parts per million (ppm). At that time (when people began to burn fossil fuels) the concentration of carbon dioxide in the atmosphere began to increase and is now at 370 ppm. This strong correlation indicates that increased concentrations of greenhouse gases in the atmosphere have likely increased the amount of heat from the sun that stays within the Earth’s ecosystem, and thus contributing to increased global temperatures.
Differences of opinion arise in (1) the extent to which any climate changes are caused by greenhouse gas emissions from human activity, and (2) how much and when the changes in the climate will disrupt agriculture, forestry, and other human activities as well as natural ecosystems beyond a level that can be easily adapted to. The Inter-governmental Panel on Climate Change (IPCC) is the leading scientific body charged with studying the effects of increased greenhouse gases in the atmosphere. The IPCC’s most recent report (2001) projects that, under a business as usual scenario, globally averaged surface temperature will increase by 2.5 to 10.4°F between 1990 and 2100. A 2.5°F increase in temperature would be a relatively mild outcome, but a 10.4°F increase in temperature would be severe. As a comparison, during the last ice age the average temperature was roughly 6°F lower than it is today. Earlier, in 1995, the IPCC projected temperature increase in the range of 1.8 – 6.3°F, so the latest report is a significant increase.
In order to stabilize the concentration of greenhouse gases in the atmosphere, global emissions of greenhouse gases would have to be reduced 60-90% below current levels over the next 50 years. The severity of reductions depends on the level one is willing to let the concentrations rise to, which is ultimately a policy decision based upon available scientific information. Recent trends do indicate a directional change in greenhouse gas emissions from human activity. Current annual U.S. greenhouse gas emissions are 12% higher than they were in 1992, and worldwide greenhouse gas emissions increased 14% over that period. Also, the atmospheric concentration of carbon dioxide increased by 6 ppm between 1997 and 2002.
What is the global warming potential of water vapor? Are the anthropogenic water vapor emissions significant?
Water vapor is a very important part of the Earth’s natural greenhouse gas effect and the chemical species that exerts the largest heat trapping effect. Water has the biggest heat trapping effect because of its large concentration compared to carbon dioxide and other greenhouse gases. Water vapor is present in the atmosphere in concentrations of 3-4% whereas carbon dioxide is at 387 ppm or 0.0386%. Clouds absorb a portion of the energy incident sunlight and water vapor absorbs reflected heat as well.
Combustion of fossils fuels produces water vapor in addition to carbon dioxide, but it is generally accepted that human activities have not increased the concentration of water vapor in the atmosphere. However an article written in 1995 indicates that water vapor concentrations are increasing. [S.J. Oltmans and D.J. Hoffman, Nature 374 (1995):146-149] Some researchers argue there is a positive correlation between water vapor in the air and global temperature. As with many climate issues, this one is still evolving.
How important are the other non-carbon dioxide greenhouse gases?
In reference to the above question, water vapor is not included as an anthropogenic greenhouse gas. Non-carbon dioxide greenhouse gases include methane, nitrous oxide (N2O), Hydrofluorocarbons, Perfluorocarbons, and Sulfur Hexafluoride.
In 2001 carbon dioxide accounted for 83.7% of the total U.S. greenhouse gas gas emissions. Methane accounted for 9.3% of total greenhouse gas emissions, N2O 5.2%, and the others combine for 1.7%. These percentages are based on 100-year global warming potentials (100 year GWPs standardize the heat-trapping effects of various greenhouse gas gases based on the average effects over 100 years. It is the methodology chosen by the IPCC).
Methane emissions “endure” in the atmosphere for a relatively short time (20 years) compared to carbon dioxide (varies but averages 100 years). Thus, if one compares the greenhouse gas emissions on a 20-year or even 1-year GWP basis, methane has a much more significant impact.
What percentage of the carbon dioxide in the atmosphere has been produced by human activities?
The answer to that question is complicated by the fact that human activity is not the only factor affecting the amount of carbon dioxide in the atmosphere. Plants, both on land and in the oceans, continually absorb and release large amounts of carbon dioxide. Plants absorb carbon dioxide when they grow and release carbon dioxide as they decay. Forest fires, volcanoes, droughts and other natural phenomena can affect the natural rate of carbon dioxide uptake and release. Separating out the impact of human activity from all the other factors is difficult.
However, if you consider that the atmospheric carbon dioxide concentration was stable at 288 pmm for a long time before the industrial revolution and has since increased 367 ppm, one might assume that all the difference between 288 ppm and 370 ppm is attributable to human activity. This would be a high end estimate of the human impact. Under that assumption human activity accounts for 22% of the carbon dioxide in the atmosphere.
What is the difference between global warming and global climate change?
Increased concentrations of greenhouse gases in the atmosphere result in increased heat trapping and a rise in the average global temperature. Thus, when people first started thinking about the effects of greenhouse gas emissions from human activity they called the effect "Global Warming." As the scientific understanding of the issue progressed, people recognize that increased global heat trapping caused many varied effects, including shifting of ocean currents and increased or decreased precipitation in certain areas. Scientists coined a new phrase “Global Climate Change” to represent this complexity.
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