Society has four general options for addressing human caused changes in climate. We could take no upfront preventative action. We could reduce our greenhouse gas emissions (mitigation). We could improve our ability to cope with climate impacts (invest in our adaptive capacity). Finally, we could deploy global changes that attempt to counteract the effects of our greenhouse gas emissions (geoengineer). Each of the four options has advantages and disadvantages. None of three proactive responses are mutually exclusive and selecting among them amounts to an exercise in risk management.

In selecting among policy options, it is important to realize that carbon dioxide concentrations in the atmosphere during the next century (and beyond) depend on factors both within human control (how much we emit) and outside of human control (how the natural carbon cycle responds). Recent experiments suggest that the release of carbon from the land surface in response to warming is sufficient to push atmospheric concentrations to levels found in the highest IPCC emission scenario (A1FI) even if the human contribution corresponds to the lowest IPCC emission scenario (B1). This indicates that there is a greater risk exposure to society from a given level of human emissions than we currently recognize. As a result, polices that more aggressively seek to reduce the risks of climate change become more favorable.