Forcing Agents Underlying Climate Change: Page 9 of 11
Submitted Testimony: 7. Benchmarks.
The alternative scenario sets a target (1 W/m2 added climate forcing in 50 years) that is much more ambitious than IPCC business-as-usual scenarios. Achievement of this scenario requires halting the growth of non-CO2 climate forcings and slightly declining CO2 emissions. Climate change is a long-term issue and strategies surely must be adjusted as evidence accumulates and our understanding improves. For that purpose it will be important to have quantitative measures of the climate forcings
Non-CO2 forcings. The reason commonly given for not including O3 and soot aerosols in the discussions about possible actions to slow climate change is the difficulty in quantifying their amounts and sources. That is a weak argument. These atmospheric constituents need to be measured in all countries for the sake of human health. The principal benchmark for these constituents would be their actual amounts. At the same time, we must develop improved understanding of all the sources of these gases and aerosols, which will help in devising the most cost-effective schemes for reducing the climate forcings and the health impacts
Methane, with an atmospheric lifetime of several years, presents a case that is intermediate between short-lived air pollutants and CO2. Measurements of atmospheric amount provide a means of gauging overall progress toward halting its growth, but individual sources must be identified better to allow optimum strategies. Improved source identification is practical. In some cases quantification of sources can be improved by regional atmospheric measurements in conjunction with global tracer transport modeling
Carbon Dioxide. Is it realistic to keep the CO2 growth rate from exceeding that of today? The single most important benchmark will be the annual change of CO2 emissions. The trend of CO2 emissions by the United States is particularly important for the reasons discussed above. Figure 11 shows the United States record in the 1990s. The requirement to achieve the "alternative scenario" for climate forcings is that these annual changes average zero or slightly negative. It is apparent that, despite much rhetoric about global warming in the 1990s, CO2 emissions grew at a rate that, if continued, would be inconsistent with the alternative scenario
We suggest in the discussion above that it is realistic to aim for a lower emission rate that is consistent with the alternative scenario. This particular benchmark should receive much closer scrutiny than it has heretofore. The climate simulations and rationale presented above suggest that, if air pollution is controlled, the trend of this CO2 benchmark, more than any other single quantity, can help make the difference between large climate change and moderate climate change