Extreme weather events remind us of our vulnerability to the truly awesome forces of nature. They reveal the weaknesses in our societal infrastructure. Sometimes they provide valuable insights into the workings of the climate system. But the debate about whether this winter’s cold weather over the central and eastern U.S. is due to global warming has become a stumbling block in our public discourse on human induced climate change.
Like many of my colleagues in the climate dynamics community, I am not convinced that this winter’s extreme cold lies outside the range of internally generated variability of the climate system or that it was exacerbated by the recent reduction of summer Arctic sea ice coverage. The evidence linking Arctic amplification to the behavior of the wintertime polar vortex is not strong and it is not well supported by independent, peer-reviewed studies. I expressed similar reservations 50 years ago when my father asked me whether I thought that nuclear weapons tests were changing the weather.
In contrast to the situation with nuclear weapons testing, we know that global warming does affect the weather in some ways. For example, as the global mean temperature rises, all time high temperature records are being set more frequently than they would be in a constant climate. At many tropical stations, the mean temperature of the past few decades lies within the top quartile of the probability distribution of temperature at those same stations a century ago. But at sites in middle latitudes, the internal variability of the climate system swamps the human-induced global warming trend, especially during wintertime. That’s why low temperature records are still being set occasionally, even in a warming climate.