The University of Arizona

What’s normal is getting hotter too.

July 22, 2011
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It’s nice that the monsoon rains have come again to Arizona and the Southwest, although the monsoon season never lasts forever. Meanwhile, the bigger weather story is the American heat wave of 2011 that is making life miserable in the Midwest and further east. More than miserable, it’s actually contributing to deaths.  It’s been a rough year for climatic extremes in our country (drought, tornados, floods and now an unusually bad heat wave), and there is a lesson in what Mother Nature has been throwing at us: “if you don’t like this weather, do something about global warming.” From a climate science perspective, heat waves, drought and floods are all likely to become more common with continued global warming; we’re not so sure about how tornado statistics will change (although I did collaborate on a paper nearly 20 years ago suggesting climate change would bring more tornado damage).

Last Wednesday’s New York Times had a opinion piece (“Sizzle Factor for a Restless Climate”) by another climate scientist – Heidi Cullen – that is worth reading. Her piece is about the heat wave and the strong scientific consensus that our summer temperatures will just keep going up, and just keep breaking records.

And, don’t forget… back East, with all the humidity, 100°F feels a lot worse than it does in Arizona.

Dr. Cullen ends with a short paragraph: “The next time NOAA calculates its new temperature normals will be in 2021 – when there will be almost another billion people on the planet. Lady Gaga may no longer be hot. But the climate almost surely will be.”

What is she talking about? No one doubts that there will be a lot more people on Earth in 2021, and thus more greenhouse gas pollution potential. But the more subtle issue has to do with “new temperature normals.”

Most everyone probably knows that NOAA, the National Oceanic and Atmospheric Administration, is the U.S. agency that includes the Weather Service, all our weather satellites, all our weather stations and other observing systems (key for weather forecasting – all the weather forecasts you find on the web or TV rely on NOAA). It would be difficult for our country to function well without a strong NOAA.

NOAA, and indeed other national weather services around the globe (they all have to share data with each other to provide their own countries with the best possible forecasts), have a longstanding role of defining what our “normal,” or average, climate is. That is, the average weather conditions (temperature, rainfall, etc.) for a given time and place. These values allow your weather report to place a forecast in the context of what is “normal” for the day in question – for example: “you can expect temperatures 5 degrees above normal tomorrow.” This sounds straightforward, but the problem is that our average climate is always changing, particularly with a warming planet. “Normal” is not static, stationary or unchanging.

The tradition for most weather agencies, including NOAA, is to calculate climate normals based on a recent 30-year period, recalculated every 10 years. So, Dr. Cullen is referring to the fact that NOAA has just replaced our 1971-2000 normal period with a new 1981-2010 normal period. Sounds simple enough.

Or is it? The period 2001 to 2010 is the warmest decade the planet has seen since we had a global thermometer record. This means the new normal will be a good deal warmer (Figure 1), at least in most places, than the old normal. And thus, a really warm day, like those we’re getting across much of our nation as I type, won’t be as unusual, relative to “normal,” than it would have been just one year ago. All of a sudden, we’ll have fewer days “above normal.”

Temperatures across the U.S. from 1981-2010 are much warmer in many regions compared to  temperatures from 1971-2000. Image credit: NOAA Climate Services

Of course, this makes no difference to climate change or global warming, and we’ll still be breaking daily warm records far more often than daily cold records. The Earth will continue to warm, and our “normal” will just keep getting hotter, jumping up every ten years as the weather services around the globe try to keep up.