Greenhouse Gases: Refining the Role of Carbon Dioxide
There has been growing interest in the global temperature of Earth. Recent headlines such as "1997 Hottest Year on Record" have generated a greater awareness that the global climate may be changing. Global warming is attributed to the steady increase of atmospheric trace gases produced largely by human activities, such as carbon dioxide, methane, nitrous oxides, and chlorofluorocarbons (CFC or "Freon"). These gases are commonly referred to as "greenhouse gases" because they let in most of the incoming solar radiation that heats Earth's surface, yet prevent part of the outgoing thermal radiation from escaping to space, thus trapping some of the surface heat energy. Water vapor is also a major natural greenhouse gas, but its volatility, i.e., readily evaporating and condensing in response to temperature changes, complicates its role. Increases in the amount of atmospheric water vapor, under warmer conditions, reinforces the heat absorption by the other greenhouse gases. On the other hand, more clouds may form, as a consequence of increasing amount of atmospheric water vapor. Clouds can provide either a positive or a negative feedback by trapping outgoing thermal radiation or increasing the amount of solar radiation reflected back to space, respectively. At present, roughly 30% of the incoming solar radiation is reflected back to space by the clouds, aerosols, and the surface of Earth. Without naturally occurring greenhouse gases, Earth's average temperature would be near 0°F (or -18°C) instead of the much warmer 59°F (15°C).
The concentration of greenhouse gases, especially carbon dioxide and methane, has fluctuated naturally over geological time scales. While the mechanisms responsible for these fluctuations are unclear, the temperature of Earth has responded to them by switching between ice age and interglacial conditions, i.e., periods of reduced and increased greenhouse warming. In addition to these slow natural variations, the atmospheric concentrations of these gases are being changed rapidly (on a geological time scale) by human activity as we burn fossil fuels, clear forests, and use gasoline-dependent transportation. In particular, the amount of carbon dioxide (CO2) has increased by 30% since pre-industrial times (from about 270 molecules of CO2 per million molecules of air in 1850 to the present 360 parts per million), and continues to rise over time, due primarily to the burning of fossil fuel.
Because carbon dioxide can drive climate change, it is important to be able to accurately determine its heat absorption characteristics. The spectrum of heat absorption by Earth's atmosphere contains hundreds of thousands of absorption "lines". For carbon dioxide alone there are over sixty thousand lines. In order to model the absorption spectrum of CO2, we need to know the spectral location (wavelength), the strength, and also the shape of each line. If we visualize the absorption line as an inverted bell-shaped curve, the depth (amplitude) of the curve is determined by the strength of the line and the amount of the absorbing gas present in the atmosphere, whereas the width of the line is determined by pressure and temperature. Near the center of the line, the "Lorentz profile", based on a simple assumption that collisions between molecules take place instantaneously, works well. However, as we move away from the center toward the wings of the line, this is no longer true. Here the details of collision processes play their roles in altering the line shape. Understanding this collision-broadened absorption and determining more accurate far-wing line shapes has been the focus of research conducted by Drs. Ma and Tipping.
The figure at right shows the amount of absorption (y-axis) beyond one of the CO2 bands as a function of spectral location (x-axis) calculated assuming a Lorentz line profile (dashed curve) compared to laboratory measurements (shown by +). This clearly shows that away from the band the Lorentz profile greatly overestimates the amount of absorption. Thus, this line shape is not applicable here. Meanwhile, the calculated absorption based on Ma and Tipping's line profile (solid curve) agrees well with the measurements. As a result of this study, a more accurate calculation of CO2 absorption can be made, permitting a more precise determination of greenhouse warming due to CO2.
Ma, Q. and R. H. Tipping 1998. The distribution of density matrices over potential-energy surfaces: Application to the calculation of the far-wing line shapes for CO2. J. Chem. Phys. 108, 3386-3399.
Please address all inquiries about this research to Dr. Qiancheng Ma.