Moderating Climate Change by Limiting Emissions of Both Short- and Long- Lived Greenhouse Gases

Michael C. MacCracken 1
Chief Scientist for Climate Change Programs, Climate Institute, Washington DC

As emissions continue to increase, both warming and the commitment to future warming are increasing at a rate of ~0.2 degree Celsius per decade, with projections that the rate of warming will further increase if emissions controls are not put in place. Such warming and the associated changes in other climate variables are very likely to cause severe impacts to key societal and environmental support systems, especially if the changes are abrupt or the present trend is increased. Present estimates are that limiting the increase in global average surface temperature to no more than 2-2.5 degree Celsius above its 1750 value will be required to avoid the most catastrophic, although certainly not all, consequences of climate change. Limiting peak warming and initiating a return to temperatures below present levels will require sharply reducing global greenhouse gas emissions by 2050 and even more by 2100. With fossil fuels providing over 80% of global energy, and increasing use apparently inevitable in many developing nations in order to raise the standard-of-living of their populations, reducing emissions sufficiently to limit climate change presents a very significant challenge, with neither developed nor developing nations yet ready to commit to an agreement without commensurate action by all nations.

Analyses of the warming influences of the set of greenhouse gases contributing to the warming suggests that the extent of action needed to limit the increase in global average temperature to near 2 degree Celsius above its preindustrial level will require: (1) developed nations to rapidly reduce their emissions of all greenhouse gases by order of 80% by 2050, and even further by later in the century; and (2) developing nations, in a first phase, to significantly improve their carbon efficiency and reverse deforestation while sharply limiting their non-CO2 GHG emissions (i.e., methane, soot, and pollutants contributing to tropospheric ozone), and then, in several decades when their per capita GDP has risen to levels near those of developed nations, to join the developed nations in sharply reducing their per capita CO2 emissions. Because aggressive, near-term reductions in non-CO2 emissions by developing nations would both improve the environmental well-being of their citizens and offset the warming influence of their ongoing CO2 emissions, this strategy would allow for their ongoing development while cost-effective CO2-free energy technologies are developed. Such a coordinated approach would demonstrate their commitment to limiting global climate change while recognizing the equity imbalance created by very different per capita emissions that characterize the situation at present. To further limit global warming, if that proves necessary, and to counteract the warming influence of declining emissions of sulfur dioxide, geoengineering likely also merits consideration, and perhaps even regional deployment, to moderate the seriousness of the most critical impacts.

My current research is focused in two major areas: (1) the important role that non-CO2 greenhouse gases and absorbing aerosols play in global warming; and (2) the potential for regionally focused geoengineering to moderate the most threatening impacts of climate change.