Predicting how the earth’s climate will change is a huge job, even with the help of the largest, most advanced computers. Here, we wish to give some idea of how the problem is being tackled. Each factor that can change the earth’s average tempera­ture ( T) is evaluated for its ability to change T. This ability, called a “forcing,” is expressed in watts per square meter (W/m2), as if sunshine intensity were increased by that many W/m2, all else staying the same. Forcings have to be computed using a model. For instance, to compute the forcing due to CO2, one has to take into account the amount of CO2 in the atmosphere and how long it stays, its rate of absorption and emission of radiation, and feedback effects such as the rise in T due to the increase in water vapor caused by the temperature rise that the CO2 caused initially. Obviously, the result is only as good as the computer model used to calcu­late it, but these models are carefully checked, and the uncertainties are clearly stated. More on this will come later. For the GHGs, the forcing is known within ±10% with 90% confidence, but other effects (the small ones) can have errors of

±100% or so. Figure 1.3a compares the major radiative forcings; that is, the effectiveness of the main agents that can change T by altering the absorption of solar radiation.

These forcing numbers seem very small, less than 2 W/m2, compared with the peak solar irradiance of about 1,300 W/m2, or even the 342 W/m2 averaged over a hemisphere or the 240 W/m2 that reaches the earth’s surface. But a small change in T can have catastrophic effects, as we shall see. The man-made forcings have both positive (warming) and negative (cooling) values. Let us see where these figures come from. The three main GHGs dominate the warming effects. CH4 has 26 times the warming potential of CO2 and N2O, 216 times; but their concentrations are much lower than CO2’s, and CO2 is dominant. The ozone-depleting chlorine-containing gases which were banned by the Montreal protocol are lumped under the rubric CFCs. That value comes from 60 different gases which were evaluated one by one in the IPCC report of 2007 [6]. The value for ozone does not depend on the state of the ozone hole, because high-altitude ozone has a small role here. The ozone that contributes to warming is in the lower atmosphere and is generated on the ground by natural processes such as rotting of biological matter. What we have called “dust” is the sum of all aerosols emitted by factories and volcanoes. Industrial aerosols are mainly sulfate and carbon particles of varying sizes and reflectivities. You would think that black carbon would absorb well, but remember that black not only absorbs well but also emits well. More importantly, particulate matter can seed cloud forma­tion, and clouds reflect sunlight efficiently. The net result is that aerosols have a large negative forcing and give a cooling effect. Albedo is the change in the reflectivity of the earth’s surface, and this small effect comes from the balance between two effects. Black dust on snow will reduce the albedo of the snow and cause warming.

Deforestation and other land modifications by man will replace trees with farms or buildings, thus increasing the albedo. In this case, land use wins, and changes in albedo are a negative forcing. The result is very uncertain, but it is small in any case.

The natural forcings come from volcanoes and solar variability. Volcano dust stays in the atmosphere only a few years, and eruptions are rare and unpredictable. On the other hand, solar variability follows the 11-year sunspot cycle closely, and this 8% effect is accurately predictable. However, what concerns us is not the 11-year cycle but the long-term trend. Changes in the earth’s orbit or the tilt of its axis occur over tens of thousands of years, so only a very small part of these changes could have occurred in modern times. Recently obtained data on solar irradiance from 1,750 to the present yield a forcing of +0.12 W/m2, with a 90% chance of the exact value’s being within 50% of this. Figure 1.3b compares the net anthropogenic forcing with the natural forcing caused by solar variability. The man­made part is 13 times larger. Skeptics1 who say that present global warming is a natural phenomenon would imply that climate scientists are wrong by over an order of magnitude. Even if that were true, it is irrelevant. The present rate of CO2 emis­sions by man is not conjectural, and their effect on temperature can be calculated with ± 10% accuracy.