If anthropogenic global warming is acknowledged and related threats are taken seriously, what can be done about it? In broad terms, there are three paths that can be taken. The first two are by no means mutually exclusive and, in fact, should be pursued concurrently. One path involves mitigation measures. What can be done to reduce GHG emissions and stabilize atmospheric concentrations at levels of low risk? Globally, can CO2 emissions be reduced to 20% of current levels by 2050 and atmospheric CO2 stabilized at 450 ppm? And what if mitigation measures are insufficient to reduce atmospheric GHG concentrations to acceptable levels, a scenario that is becoming ever more likely?

The second path involves adaptation. Recognizing diminishing prospects for stabilizing GHG concentrations at acceptably low levels, what can be done to increase the resilience of human and natural systems to the effects of climate change? In this chapter we'll examine a range of mitigation and adaptation options. But should mitigation and adaptation measures both fall short of desired outcomes, what then? The chapter concludes with a discussion of measures falling under the rubric of geoengineering – a term used to characterize large-scale engineering endeavors designed to counteract the warming effects of GHG emissions.

We'll begin with mitigation. As shown in Figure 6.1, options for reducing GHG emissions fall into three categories. One category deals with increasing the efficiency of producing and using energy; the second deals with transitioning the world's energy portfolio from carbon-rich to carbon-free sources of energy; and the third deals with collecting and sequestering GHG gases before they are released to the atmosphere. In each case, the scale of implementation needed to achieve meaningful reductions is enormous, requiring large capital investments and global cooperation.