Few could disagree with Kevin Trenberth's assertion, in this issue, that the uncommon El Niño of 1997 and 1998 was a principal cause of the record-setting run of severe weather events of the last few years. Why this El Niño came on so strong and stayed so long is less certain. But a likely reason, he explains, is that global greenhouse warming has now preconditioned the atmosphere and oceans and land surface to amplify more typical El Niño effects.
El Niños seem indeed to have adjusted the timing and duration of their visits in the last decade or two. But has the Earth warmed enough in that time to make that much difference in anything? In the last twenty years the mean surface temperature of the Earth has risen a little less than half a degree Centigrade, or one degree Fahrenheit: about the limit of what you and I can discern on a common window thermometer. Could so small an adjustment in the temperature unleash so great a change? Enough to alter world weather patterns, crop yields, disease vectors, and the extent of floods and fires and severe storms? What is it about the natural environment that allows minor changes in almost any part of it to reach so far, and so pervasively?
A part of the answer is surely the interconnectedness of everything. The natural world, like that of commerce and communications today, is irreversibly entwined and interconnected. As John Muir reminded us, a long time ago, when one pulls up any part of it, he finds its roots entangled with all the rest. It may not be possible to adjust the world's thermostat, up or down, however slightly, or surreptitiously, or unintentionally, without perturbing almost everything.The mystic poet Francis Thompson may well have said it best, in 1897: