When he spoke for the President during his 8-year tenure as Science Advisor to President Obama and Director of the White House Office of Science Technology Policy, widely regarded Harvard Professor John Holdren was fond of emphasizing some variant of a concise summary of humanity’s options with regard to changes in climate-related risks born of dangerous human interference with the climate system, itself. "When it comes to responding to climate policies”, he would say, “we have three choices - abate, adapt, or suffer”.

Abatement is called mitigation by those involved in climate science and climate action – taking action designed to reduce emissions of greenhouse gases. Adaptation involves making investments that can ameliorate some of the impacts that persist under all feasible mitigation pathways made with the understanding that no adaptation will be effective 100% of the time. Suffering will therefore always occur as a residual to any combination of these two active policy actions; that is to say, humanity cannot escape all of the impacts of rising temperatures, especially the most extreme possibilities that are known unknowns. Suffering is the “accept a tolerable level of risk” option that informs the level of investment in the other two choices. To be precise, setting the level of what is and is not tolerable is really the third policy option.

Earlier in 2007, the Intergovernmental Panel on Climate Change had achieved consensus support for a finding that fundamentally changed the way that the science community and their clients in the decision-making realm viewed the climate change challenge: "Responding to climate change involves an iterative risk management process that includes both adaptation and mitigation and takes into account climate change damages, co-benefits, sustainability, equity, and attitudes to risk" (my emphasis; Summary for Policy Makers of the Synthesis Report for the Fourth Assessment Report in 2007).

These worlds provide us access to a productive way of organizing our thoughts and assessments with regard to the relative values (positive or negative) of various mixes of the three options because they make the concept of risk the connecting tissue. At its very core, risk is simply the product of likelihood and consequence. Mitigation holds the potential to reduce the likelihood of damaging events (or at least delay them). Well-informed adaptation can reduce the potentially dire consequences of the warming that remains after abatement has done what it can. And perhaps investments in resilience and recovery programs can ameliorate some of the residual suffering.

This collection of essays will be organized in sections that reflect Holdren’s three broad categories of climate policy options. All will apply a risk-based perspective to discussions and assessments of what we do and do not know and thus what we can or perhaps cannot really do. It follows that each essay will include, in their concluding remarks at least, some attempt to identify important gaps in our current understanding of the coupling of the climate and human systems on earth and, by continuation, elaborate on constraints on our confidence in relative efficacies that we project in our policy deliberations. Specifically, efficacy judgments will take account of at least one of the IPCC metrics: net climate change damages, co-benefits and costs of policies, measures of sustainability (of systems and policies), equity calibrated in various metrics of human welfare security, and the degree to which people, communities, sub-national governance bodies, nations, and international institutions are averse to risk.

A synthetic introductory essay will elaborate on this framing of the climate challenge and responses to it before offering an integrated “signpost” description of the essays’ contributions to this volume specifically and conventional wisdom more generally.