There is growing worldwide momentum to address the problem of climate change, one of the widest-reaching challenges modern society has faced. But we did not reach our current level of global concern without bumps and bruises along the way.

The natural greenhouse effect and its intensification by human-induced (anthropogenic) emissions of greenhouse gases are well understood and solidly grounded in basic science. This conclusion is a robust finding of the mainstream climate-science community. Yet, despite the preponderance of evidence, a number of interest groups—and some scientists—still do not accept the well-established evidence of the last 40 years of...

Since the second half of the nineteenth century, global temperatures have been on the rise. The increase in global average surface temperature, as estimated by the IPCC, is around 0.75°C (~1.4°F). Twelve of the thirteen years leading up to 2009 are the twelve warmest years on record. There is now overwhelming scientific evidence of a human fingerprint on this global warming.

Many impacts of warming can be—and have been—observed: the melting of mountain glaciers, the Greenland ice sheets and parts of the West Antarctic ice sheets, and northern polar sea ice; rising and increasingly acidic seas; increasing severity...

Most of us think about climate in local terms. The Caribbean has great weather—warm days and cool nights, plenty of sunshine, blue skies. It’s much nicer than dreary London or parched Dubai. All of these local conditions, however, are the products of an enormously complex global system in which myriad variables contribute to a diverse set of climates and ecosystems. That diversity has been relatively stable for the past several thousand years—until humans dramatically expanded their population size and economic activities. Now, major alterations to land surfaces, chemical composition of soils, air, and water and accelerating changes in...

Assessing climate science, impacts, and policy issues rarely involves certainties. Instead, we consider risks—potential outcomes associated with different levels of climate change, and the range of future climate change that could be induced by different levels of future emissions. In other words, what are the consequences, and what are the chances that they will be realized?

Assessing risk is primarily a scientific enterprise, but deciding which risks to tolerate and which to try to avoid—“risk-management”—is primarily a value-laden, normative activity appropriate to the political process. The climate problem is filled with deep uncertainties, uncertainties in both likelihoods...

Even the most optimistic business-as-usual emissions pathway is projected to result in some dramatic, and potentially dangerous, climate impacts. Therefore, despite uncertainty over the future of climate change, we have to improve on the status quo. Faced with these grave risks, and great uncertainty, what should we do?

While we cannot know the precise temperature increase and impacts of a specific trajectory for future emissions, we do know a few things with confidence. We know that reducing emissions will reduce the level of temperature increase that would otherwise occur, and thus reduce climate-change risks: that is why mitigation is so...

In our final chapter, we highlightvulnerability assessmentas an important tool to inform the development of climate change policies, particularly adaptation strategies.

Vulnerability often is defined in terms of three components: exposure, sensitivity, and adaptive capacity.Exposurerefers to the degree to which a system experiences stress and the nature of those stresses: the frequency and intensity of heat waves in a given location, the level of the sea.Sensitivityrefers to the degree to which a system is affected or modified by that exposure, and varies across different regions, populations, and sectors: the elderly and those without air...