Katharine Hayhoe

Atmospheric methane and global change

Abstract Methane (CH4) is the most abundant organic trace gas in the atmosphere. In the distant past, variations in natural sources of methane were responsible for trends in atmospheric methane levels recorded in ice cores. Since the 1700s, rapidly growing human activities, particularly in the areas of agriculture, fossil fuel use, and waste disposal, have more than doubled methane emissions. Atmospheric methane concentrations have increased by a factor of 2–3 in response to this increase, and continue to rise. These increasing concentrations have raised concern due to their potential effects on atmospheric chemistry and climate. Methane is important to both tropospheric and stratospheric chemistry, significantly affecting levels of ozone, water vapor, the hydroxyl radical, and numerous other compounds. In addition, methane is currently the second most important greenhouse gas emitted from human activities. On a per molecule basis, it is much more effective a greenhouse gas than additional CO2. In this review, we examine past trends in the concentration of methane in the atmosphere, the sources and sinks that determine its growth rate, and the factors that will affect its growth rate in the future. We also present current understanding of the effects of methane on atmospheric chemistry, and examine the direct and indirect impacts of atmospheric methane on climate.

SUBSTITUTION OF NATURAL GAS FOR COAL: CLIMATIC EFFECTS OF UTILITY SECTOR EMISSIONS

Substitution of natural gas for coal is one means of reducing carbon dioxide (CO2) emissions. However, natural gas and coal use also results in emissions of other radiatively active substances including methane (CH4), sulfur dioxide (SO2), a sulfate aerosolprecursor, and black carbon (BC) particles. Will switching from coal to gas reduce the net impact of fossil fuel use on global climate? Using the electric utility sector as an example, changes in emissions of CO2, CH4,SO2 and BC resulting from the replacement of coal by natural gas are evaluated, and their modeled net effect on global mean-annual temperature calculated. Coal-to-gas substitution initially produces higher temperatures relative to continued coal use. This warming is due to reduced SO2 emissionsand possible increases in CH4 emissions, and can last from 1 to 30years, depending on the sulfur controls assumed. This is followed by a net decrease in temperature relative to continued coal use, resulting from lower emissions of CO2 and BC. The length of this period and the extent of the warming or cooling expected from coal-to-gas substitution is found to depend on key uncertainties and characteristics of the substitutions, especially those related to: (1) SO2 emissions and consequentsulphate aerosol forcing; and (2) the relative efficiencies of the power plantsinvolved in the switch.

Global change: state of the science

Only recently, within a few decades, have we realized that humanity significantly influences the global environment. In the early 1980s, atmospheric measurements confirmed basic concepts developed a decade earlier. These basic concepts showed that human activities were affecting the ozone layer. Later measurements and theoretical analyses have clearly connected observed changes in ozone to human-related increases of chlorine and bromine in the stratosphere. As a result of prompt international policy agreements, the combined abundances of ozone-depleting compounds peaked in 1994 and ozone is already beginning a slow path to recovery. A much more difficult problem confronting humanity is the impact of increasing levels of carbon dioxide and other greenhouse gases on global climate. The processes that connect greenhouse gas emissions to climate are very complex. This complexity has limited our ability to make a definitive projection of future climate change. Nevertheless, the range of projected climate change shows that global warming has the potential to severely impact human welfare and our planet as a whole. This paper evaluates the state of the scientific understanding of the global change issues, their potential impacts, and the relationships of scientific understanding to policy considerations.

Atmospheric Methane: Trends and Impacts

The concentration of methane (CH4), the most abundant organic trace gas in the atmosphere, has increased dramatically over the last few centuries, more than doubling its concentration. Increasing concentrations of methane are of special concern because of their effects on climate and atmospheric chemistry. On a per molecule basis, additional methane is much more effective as a greenhouse gas than additional CO2. Methane is also important to both tropospheric and stratospheric chemistry. Here, we examine past trends in the concentration of methane, the sources and sinks affecting its growth rate, and the factors that could affect its growth rate in the future. This study also examines the current understanding of the effects of methane on atmospheric chemistry and climate.

Methane in the Global Environment

The concentration of methane in the atmosphere has increased dramatically over the last few centuries, from 0.7 ppmv to more than 1.7 ppmv, and continues to increase. This increasing concentration of methane (CH4) in the atmosphere is of particular concern because of the potential effects that it can have on global atmospheric chemistry and climate. Given its relatively long atmospheric lifetime, methane emissions do not appear, in general, to have an appreciable effect on local or regional air pollution. However, methane chemistry does have an important influence on the global atmosphere, affecting the amount of ozone (03) in both the troposphere and stratosphere, the amount of hydroxyl (OH) in the troposphere, and the amount of water vapor (H2O) in the stratosphere. Methane oxidation is also an important source of atmospheric carbon monoxide (CO) and formaldehyde (CH2O). Methane is the most abundant reactive trace gas in the troposphere. In addition, methane is a greenhouse gas, and its increasing concentrations are of special interest to concerns about climate change.