Human beings have notoriously bad memories about weather, like just about everything else. We tend to overemphasize extreme events and downplay the dull normal, which is why your Granddad’s stories about walking uphill in the snow, both ways, probably aren’t true.
But if you think that the daily weather has gotten weirder lately—more extremes and more sudden changes—it turns out you might actually be right. According to a recent study by Princeton University researchers published in the Journal of Climate, extreme sunny or extremely cloudy days are more common now than they were in the early 1980s, and the swing from heavy storms to dry days is also more intense than it was in the late 1990s. Altogether, a third of the planet is experiencing more extreme and erratic variations in day-to-day weather. The weather—not just big storms, but the daily stuff of the evening newscast—really is getting weirder.
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The researchers—led by David Medvigy, an assistant professor of geosciences at Princeton—used a computer program to examine climate data on a daily basis for the first time. (The researchers determined sunshine variation from data gathered by the International Satellite Cloud Climatology Project between 1984 and 2007, while they took rainfall from the Global Precipitation Climatology Project http://cics.umd.edu/~yin/GPCP/main.html between 1997 and 2007.) Most climate models use monthly averages, which is obviously less precise, as Medvigy said:
Monthly averages reflect a misty world that is a little rainy and cloudy every day. That is very different from the weather of our actual world, where some days are very sunny and dry.
Our work adds to what we know about climate change in the real world and places the whole problem of climate change in a new light. Nobody has looked for these daily changes on a global scale. We usually think of climate change as an increase in mean global temperature and potentially more extreme conditions — there’s practically no discussion of day-to-day variability.
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The most extreme weather variations tend to be found in the tropics—see the map below—but the study also notes that severe weather in one place can affect the atmosphere elsewhere, William Rossow, a professor of earth system science and environmental engineering at the City University of New York put it:
Storms are violent and significant events — while they are individually localized, their disturbance radiates. Wherever it’s raining heavily, especially, or variably is where the atmosphere is being punched. As soon as it is punched somewhere in the tropics it starts waves that go all the way around the planet.
The study only looks at existing weather variability, so the research doesn’t make any clear connections between rising carbon dioxide concentrations and stranger weather on a day-to-day basis. And less than 20 years of data is hardly long enough to draw definitive conclusions. But we know from the Intergovernmental Panel on Climate Change report released last week that global warming does have a measurable impact on some extreme weather, including heat waves, droughts and the intensity of storms. It could well be that climate change could be is mucking up daily weather—along with the occasional hurricane.