The same can be said of climate change policy. The world has suffered an extraordinary string of weather disasters over the past decade, ranging from crippling droughts and floods, to severe tornado and hail outbreaks, to highly destructive hurricanes. Insurance industry statistics reflect a substantial increase in damages from these events, but in only a few cases can scientists confidently attribute them to climate change. (For example, increased incidence of droughts, floods, and high category hurricanes may be partly pinned on climate change.)
But we know next to nothing about the relationship between climate change and other weather phenomena, such as tornadoes, and we have yet to establish a link to hybrid storms like Sandy. For all but a few of these phenomena, the scientifically correct conclusion is that we can't rule out the possibility that they were purely manifestations of natural variability. But from a public policy perspective, it would be prudent to assume that climate change might be behind some of these changes, given that it is manifestly changing the environment in which these events develop.
Consider the following thought experiment. Suppose we begin pumping sulfate aerosols into the stratosphere in an attempt to slow the pace of global warming. Suppose further, that over the next two years we suffer unprecedented drought, summer freezes, and a series of crippling blizzards. When confronted, scientists say that they need at least ten more years of data to establish with 95 percent confidence whether or not these phenomena were made substantially more likely by the sulfate aerosols. My guess is that most everyone, including scientists, would want the experiment terminated right away. A small chance that the signal is real justifies taking action, given the magnitude of the consequences.
The real experiment we are performing by increasing greenhouse gas concentrations in our atmosphere differs from the thought experiment in several crucial respects. First, it is accidental rather than intentional, thereby entailing a different moral culpability. Second, turning off the experiment would be costly, especially for many of the most profitable industries on the planet. And finally, we must terminate the experiment very soon to minimize risks that will continue for hundreds of years.
Yet the outcome asymmetry of global warming is real and must be accounted for in any rational assessment of its risks. The most likely outcomes would have serious but manageable consequences for our descendents. Somewhat less probable, but not impossible, are benign outcomes. On the far side of the probability distribution are dire consequences ranging from flooded coastal cities to global armed conflict brought about by natural disasters and chronic food and water shortages. Reasonable people will differ on how far we should go to mitigate these highly asymmetric risks. But the argument that there is no risk or that we should do nothing is both scientifically and morally indefensible.