3.0 Background
3.1 The Climate Change Problem
Global climate change caused by global warming is probably the most important environmental problem of the 21st century. The explanation for this can be found in an analysis of how solar radiation interacts with the earth’s atmosphere and surface. Around 50% of the radiant energy arriving from the sun at the top of the atmosphere (on average 342 Wm-2: watts/meter2, including at night when the solar radiation is 0) as shortwave radiation is absorbed by the earth’s surface (on average 168 Wm-2) and re-emitted skyward as longwave infrared or thermal radiation (9-11). Watts/meter2 or irradiance is the rate or flux at which radiant solar energy is incident on a surface per unit area (12).
Trace gases in the atmosphere (water vapor, carbon dioxide, methane, nitrous oxide, ozone and halocarbons) absorb the upwelling longwave radiation and re-radiate it back to the surface where it is absorbed again. This radiation is eventually radiated back into space at high altitudes (13). The net effect is to double the amount of infrared radiation in the lower atmosphere (creating an upward flux of 350 Wm-2) and raise the temperature of the earth by 60?F in a process analogous to that occurring in a greenhouse and commonly referred to as the “greenhouse effect” (14-17). In actual greenhouses, heat is mainly trapped by the inhibition of convection of air and not by radiative processes as it is in the atmosphere (18).
The atmospheric concentrations of these trace gases, also known as “greenhouse gases,” or GHGs, have increased significantly (more than 30% for carbon dioxide and nearly 250% for methane) since 1750 due to burning of fossil fuels and cultivation and clearing of land for food production and human habitation (13, 19). This has in turn caused additional infrared radiation to be retained in the atmosphere near the earth’s surface (about 2.5 Wm-2 to date) resulting in an enhanced greenhouse effect or radiative forcing that forces the temperature of the earth to be raised. Carbon dioxide emissions have been responsible for nearly two thirds of this forcing and thus carbon dioxide is the most important GHG (10).
Sulfate aerosols produced as a result of fossil fuel combustion, clouds formed from the increased evaporation of water and dark tree covered land made bare and lighter by deforestation and development all reflect sunlight, partly offsetting the effects of the GHGs, although the past and future contributions of these to radiative forcing are still uncertain.
The radiative forcing of these GHGs has caused the earth’s average temperature to increase by 0.5?C since 1850, part of a process known as global warming. Positive feedback mechanisms involving the evaporation of seawater and the melting of polar ice increase the overall heating by a factor of 2.4 times that due to the GHGs alone (17, 20).
The radiative forcing due to the GHGs and the feedback mechanisms has begun to change the overall climate of the earth. Some of these changes that have already been detected, include melting of mountain glaciers and Arctic permafrost, warmer nights and a rise in sea level due to thermal expansion of seawater (21).
Concerns about the warming of the earth led the United Nations Environment Programme and the World Meteorological Organization to establish the Intergovernmental Panel on Climate Change (IPCC) in 1988 to provide periodic consensus scientific reports on climate change. According to the most recent IPCC projections issued in 2001, GHG emissions in this century due to continued industrialization and population increases may add 2-4 more Wm-2 of radiative forcing, raising average temperatures an additional 1.4-5.8?C by 2100 and even more later on due to the slow absorption of heat by the oceans (22).
This increase in temperature is projected to cause loss of species, droughts in crop growing areas and spread of tropical diseases into temperate regions (21). Left unchecked, the warming of the earth could eventually bring about abrupt (over a few decades) and catastrophic shifts in key ocean currents that might return the earth to an ice age (21, 23-24) and possibly result in the extinction of humans, since an ice world could not support 10 billion people. If the human population were reduced by 99% due to famine and loss of habitable land due to reversion to an ice world, its long-term prospects would be poor. The surviving 100 million people might be reduced by another 99% within a few generations and ultimately to zero.
