With respect to the science of climate change, many experts regard the Intergovernmental Panel on Climate Change as the world's authoritative institution.

A draft summary of its forthcoming report was leaked last week. It describes the panel's growing confidence that climate change is real, that it is a result of human action, and that if the world continues on its current course, it will face exceedingly serious losses and threats (including a significant rise in sea levels by century's end).

While the draft report states these conclusions with unprecedented conviction, they are broadly consistent with the panel's judgments from the past two decades, which raises an obvious question: Why have so many nations (including China and the U.S., the world's leading greenhouse-gas emitters) not done more in response?

There are many answers. Sceptics say that the IPCC is biased and wrong. Companies whose economic interests are at stake continue to fight against regulatory controls.

The leaders of some nations think that if they acted unilaterally to reduce their emissions, they would impose significant costs on their citizens without doing much to reduce climate change. Especially in a difficult economic period, they don't think it makes sense to act on their own.

Advertisement

To this extent, the real challenge lies in producing an international agreement. It isn't easy to obtain a consensus on the timing and expense of reductions, especially because developing nations (including China) insist that developed nations (including the U.S.) are obliged to take the most costly steps toward reducing emissions.

Psychological barriers

All of these positions play a major role. But we should not disregard purely psychological factors. An understanding of what human beings fear - and what they do not - helps to explain why nations haven't insisted on more significant emissions reductions.

The first obstacle is that people tend to evaluate risks by way of “the availability heuristic,” which leads them to assess the probability of harm by asking whether a readily available example comes to mind.

An act of terrorism, for example, is likely to be both available and salient, and hence makes people fear that another such event will occur (whether it is likely to or not). So, too, a recent crime or accident can activate attention and significantly inflate people's assessment of risk.

By contrast, climate change is difficult to associate with any particular tragedy or disaster. To be sure, many scientists think that climate change makes extreme weather events, such as Hurricane Sandy, substantially more likely.

But it is hard to prove that climate change “caused” any particular event, and as a result, the association tends to be at best speculative in many people's minds.

Second, people tend to be especially focused