Forcing Agents Underlying Climate Change: Page 6 of 11
Submitted Testimony: 4. Scenarios for 2000-2050.
We extend our climate model simulations into the future for two climate forcing scenarios shown in Figure 5. In the popular "business-as-usual" scenario, which the media focuses upon, the climate forcing increases by almost 3 W/m2 in the next 50 years. This leads to additional global warming of about 1.5° C by 2050 and several degrees by 2100. Such a scenario, with exponential growth of the greenhouse forcing, leads to predictions of dramatic climate change and serious impacts on society.
The "alternative scenario" assumes that global use of fossil fuels will continue at about today's rate, with an increase of 75 ppm in airborne CO2 by 2050. Depending on the rate of CO2 uptake by the ocean and biosphere this may require a small downtrend in CO2 emissions, which would be a helpful trend for obtaining climate stabilization later in the century. The alternative scenario also assumes that there will be no net growth of the other forcings: in somewhat over-simplified terminology, "air pollution" is not allowed to get any worse that it is today. The added climate forcing in the alternative scenario is just over 1 W/m2 in the next 50 years.
The alternative scenario results in an additional global warming in the next 50 years of about 3/4 ° C, much less than for the business-as-usual scenario. In addition, the rate of stratospheric cooling declines in the alternative scenario (top panel of Figure 5), and in fact the lower stratospheric temperature would probably level out because of expected stratospheric ozone recovery (not included in this simulation). The planetary energy imbalance increases by only about 1/4 W/m2 in the alternative scenario, compared with almost 1 W/m2 in the business-as-usual scenario. In other words, our children will leave their children a debt (3/4 ° C additional warming in the pipeline) that is only slightly more than the amount of unrealized warming (1/2 ° C) hanging over our heads now.
Figure 6 is a cartoon summarizing the two parts of the alternative scenario. First, the scenario keeps the added CO2 forcing at about 1 W/m2, which requires that annual increases in atmospheric CO2 concentrations be similar to those in the past decade. The precise scenario that we employ has the CO2 growth rate declining slowly during these 50 years, thus making it more feasible to achieve still lower growth rates in the second half of the century and an eventual "soft landing" for climate change. Second, the net growth of other climate forcings is assumed to cease. The most important of these "other" forcings are methane, tropospheric ozone, and black carbon aerosols. Specific trace gas scenarios used in our global climate model simulations are shown in Figure 7.
In the following two sections we provide data that helps provide an indication of how difficult or easy it may be to achieve the elements of the alternative scenario