Two years ago there was a big snowstorm that stretched from the Virginia-North Carolina border, all the way to southern New England. Some locations set their record totals - a common occurrence somewhere in almost all big snows. But, because the big dump centered around Washington, DC, it became an Even More Important event of Global Significance.
At least that's what you would have concluded from the front cover of the January 22, 1996 issue of Newsweek. "Blizzards, Floods & Hurricanes: Blame Global Warming" festooned with a photo of some hapless (Washingtonian?) man struggling through the storm.
The United Nations Intergovernmental Panel on Climate Change (IPCC) is pretty equivocal. The new report, often cited as the "Consensus of Scientists," says that:
"Warmer temperatures will lead to ...more severe droughts and/or floods in some places and less severe droughts and/or floods in other places."
This statement looks a bit opaque, because it seems to indicate 1) that substantially above or below normal rain will be caused by global warming and 2) substantially normal (i.e. "more normal than normal") rain will be caused by global warming. Further, this will all happen in "some places" and/or "others." But another portion of the IPCC report says this: "Overall, there is no evidence that extreme weather events, or climate variability, has increased in a global sense, through the 20th century, although data and analyses are poor and not comprehensive."
Maybe it's time to check whether or not things are getting more variable after all.
First, maybe we can get rid of the "some places" and "others" problem by looking at the globe, or at least the portion for which there are adequate records for the last 50 years. Turns out this restricts us to about 50% of the planet--which tells how limited our "global" temperature histories turn out to be!
Let's stipulate that summer temperatures vary less between months and between years than winter ones. Thatıs pretty obvious from an appreciation of the normal range of daily high temperature in July (80-90° or so for here) compared to January (30-60° in Virginia). So, given the fact that the planet's temperature has warmed about 0.6°C in the 20th century, maybe temperatures should become less variable, i.e.- more "summerlike."
In a study soon to be published in the journal Climate Research, we divided up the available data - where records were nearly complete - into months and then measured the variability between months, totalled for the year. Then we compared each succeeding year in the record.
We only looked at data after 1945, as coverage is very spotty before then. Even so, we had to throw out just about everything north of 60° latitude and south of 45°. This latter number shows how poor "global" temperature records really arenearly 1/3 of the Southern Hemisphere is missing.
Our figure shows the time-history of variation within the year. The decrease over the last 50 years is highly significant.
We then replaced the time axis with the temperature departure from the mean, and again found a statistically significant decline. In other words, what "common sense" (an increasingly scarce commodity in the global warming story) would dictate, turns out to be true: the warmer it gets, the less temperature varies within and between years.
Figure: Annual temperature variability measured since 1945 shows a statistically significant decline.
Figure: When the variability is compared to mean global temperature, it is apparent that the warmer years tend to be those with less variability.
What about the U.N.'s peculiar statement that warmer temperatures will make "for more severe droughts and/or floods in some places" and less in "others." We think, on the average that the American translation of this bureaucratese (Tom Karl of the National Climatic Data Center informs us that the sentence "was written by a committee") is that things arenıt really going to change very much on the whole. Why the sentence was necessary is just one of those taxpayer mysteries that we think is related to international travel.
Sit down for this shocker. We found no change in the variability of rainfall. So we guess the U.N.ıs contention that things could get wetter, drier, or less wetter and drier adds up to a wash.
Figure: Same as in our first figure, except for precipitation. No change here.