Global Warming, British Snow, and Filtered Evidence
Neither I nor the poster who made that argument knows enough about the physics of climate to judge whether that claim is or is not true; I am not sure anyone does. Suppose, however, that it is. We then have a serious problem for the ordinary citizen who is trying to figure out from the information that reaches him how seriously to take worries about global warming, who to believe. To see why, consider a simple back of the envelope calculation.
There are at least two features of the weather likely to show up in news stories: temperature and rainfall. Under our assumption, either unusually cold or unusually hot weather, either unusually dry or unusually wet seasons, count as evidence for global warming. Casual evidence for global warming need not be, usually is not, global, nor need it concern an entire year. An unusually hot summer in Australia or North America makes the news, just as an unusually cold winter in Britain does.
So how many different chances are there, each year, to generate evidence in favor of global warming? We have two weather variables, each of which can go in two directions. We have winter, summer, spring, and fall in which it can happen—although stories about temperature in spring and fall will be less striking than winter or summer—and a single month might also generate its own story. For simplicity, let's say there are five relevant time periods in a year. For further simplicity, let's say there are twenty geographical regions sufficiently salient so that unusual weather in one of them will be noticed. Multiply it out and we have four hundred different opportunities each year for the weather to turn out—somewhere, sometime—in a way that will generate a news story seen as evidence in favor of global warming. Whether or not global warming is real and significant, weather is notoriously variable, so we can be pretty sure that some of those four hundred will happen, giving the casual observer reason to believe that global warming is affecting the weather.
What about evidence in the other direction? Under our assumption, unusual weather in any direction counts as evidence in favor, so for evidence against we need usual weather. There are lots of opportunities for that too. But usual weather is not newsworthy—I don't remember any stories last winter, or the winter before, reporting that Britain was having about the usual amount of snow. The news media, for obvious reasons, filter in favor of the unusual, of man bites dog not dog bites man. If all unusual weather counts as evidence on one side of the argument, that side is going to look much stronger than it is.
The problem is not limited to this particular controversy. For an older and arguably more important example, consider religion. If a mother prays for the recovery of her dying child and the child recovers, everyone she knows and many people she doesn't know hear about it. If she prays and the dying child dies, that is likely to get much less attention. Death, after all, is what usually happens to dying people. In this case too, the evidence is filtered in favor of the unusual—and the unusual, in almost any direction, is going to look like evidence in favor,of religion, evidence that something beyond us is intervening.