EARTH’S ENERGY BALANCE

Many factors, including carbon dioxide, aerosols, reflections from ice and snow (albedo), and clouds, affect the energy balance between incoming solar radia­tion and outgoing radiation from the earth-atmosphere system. A total of 342 watts per square meter (W/m2) of energy hit the earth’s atmosphere, which gets reflected (107 W/m2) or absorbed and re-radiated (235 W/m2 (Figure A.1).

The wavelength of re-radiated energy is longer than that of the incoming radia­tion, which allows it to be absorbed by gases such as carbon dioxide, methane, nitrous oxide, and water vapor. These gases are called “greenhouse gases” because they contribute to warming up the earth. If no greenhouse gases existed, the earth would be about 0°F (-18°C); because of greenhouse gases, the average tempera­ture is 59°F (15°C) (1). The problem occurs when the concentration of green­house gases gets too high and more of the surface radiation is trapped, heating up the earth (global warming).

The most abundant and important greenhouse gas is natural water vapor. Human activity has little direct effect on it, but the amount of water vapor increases when the earth warms. This provides a positive feedback that makes global warming worse when other greenhouse gases increase and cause the atmo­sphere to warm up. The anthropogenic greenhouse gases are those produced by human activity. These are the ones that we have control over and that are causing the concern for global warming.

The different greenhouse gases have different efficiencies for absorbing infra­red radiation, and they also have different lifetimes in the atmosphere. The actual value for CO2 is 1.4 x 10-5 W/m2/ppb. In relative terms, methane is 26 times as efficient and nitrous oxide is 216 times as efficient as CO2 in absorbing infrared radiation. However, the different gases have different lifetimes in the atmosphere. Carbon dioxide has a complex lifetime because it constantly cycles between the atmosphere and the oceans and biosphere. About 50% of a pulse of CO2 emitted into the atmosphere now will be gone in 30 years, another 15% will be gone in 100 years, but about 20% will still be present for thousands of years. Methane has a lifetime of only 12 years as it is rapidly converted to CO2 and water.

The Intergovernmental Panel on Climate Change (IPCC) uses a comparative concept called the global warming potential (GWP) to compare various green­house gases to CO2 for their potential to cause global warming. The GWP varies depending on the time frame. For example, methane has a GWP of 72 over a 20-year time frame and 25 over a 100-year time frame. This means that methane is 25 times as effective as CO2 in causing global warming over a 100-year time frame. The lifetime of nitrous oxide is 114 years, with a GWP of 289 over 20 years and 298 over 100 years (2). Since the concentration of CO2 in the atmosphere is much higher than that of methane or nitrous oxide, and we are adding it at a higher rate, it is the greenhouse gas of most concern.