This year's late winter heat wave over much of the United States, dubbed "March Madness," has been cited as evidence that human-induced global warming is causing the climate system to stray far outside its normal range of variability. The thousands of all-time high temperature records shattered during last month's climate rampage have been likened to home-run records shattered by a baseball player on steroids.
It is true that the signature of human-induced global warming is clearly apparent in the increasing number of new high temperature records, which are currently outnumbering low temperature records by a factor of about 3 to 1. Just as a rising tide lifts all ships, a rise in global mean temperature is bound to raise the levels of the highest temperatures. But we are also being told that the burning of fossil fuels is responsible for the recent spate of extreme weather, and that's a conclusion that goes beyond the evidence.
The cause of last month's strange weather was an extraordinarily large and persistent meander of the jet stream that swept tropical air, with temperatures reaching into the 80s as far north as southern Canada.
Likening today's climate system to a muscle-bound, drugged athlete performing feats far beyond the capabilities of straight athletes would be appropriate if the extreme and persistent distortions of the jet stream we saw in March could be demonstrated to have been caused by global warming.
But let's remember where the burden of proof lies. In the world of sports, when an athlete is accused of relying on performance-enhancing drugs, it is the prosecutor who must prove the case. The same should apply to claims that the behavior of the jet stream is being profoundly altered by global warming. Thus far, such assertions are not well supported by scientific evidence.
In the absence of proof that the jet stream's variability is human-induced, we must consider the possibility that the apparent weirdness of the weather in March isn't all that weird if viewed in a larger historical context. In this respect, it's noteworthy that large areas of the U.S. were just about as warm in March 1910 as they were in March 2012. With weather, weird things happen every now and again.
Fortunately, the flora and fauna and the human inhabitants of temperate latitudes are accustomed to dealing with huge swings in wintertime temperatures, and so most of the effects of March Madness will be short-lived.
A much more serious concern is that the 1 degree Fahrenheit rise in temperature that's attributable to global warming could already be threatening tropical biodiversity and reducing crop yields in densely populated countries in Africa and southern Asia. In the tropics, plants and animals evolved in an environment with only small seasonal and year-to-year temperature changes, and both crops and native flora and fauna are experiencing temperatures higher than they are ideally suited for. In combination with the continuing growth of human population, the increasing demand for grains to feed livestock, depletion of reserves of fossil groundwater and increasing levels of water pollution, human-induced global warming poses a serious and growing threat to half the world's population.
The gradual warming of the tropics may not seem as weird as March Madness, but it has much more important implications for biodiversity, food security and the stability of world financial markets. If global warming continues as projected, the global consequences of deteriorating conditions in the tropics will soon be a lot more serious than a foretaste of summer weather in late winter.
John M. Wallace is a professor of atmospheric sciences at the University of Washington who studies global atmospheric circulation and climate variability. He is member of the National Academy of Sciences.
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