From the ABC 7 Weather team

Does climate change mean more extreme, deadly weather and tornado outbreaks?

April 30, 2011 - 10:02 PM
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If we keep getting "extreme" weather events with this frequency, we'll have to start calling them "routine."

tornadoes for 2011 thru April 30 SPC/NWS

If we keep getting "extreme" weather events with this frequency, we'll have to start calling them "routine." Take a look back at April: It set a preliminary record of more than 850 tornadoes, including the April 14-16  "outbreak" that killed 34 and the April 27 "super outbreak" that has taken more than 350 lives. With each episode, the question comes up, "What is causing this?"  Is the climate changing? Is the uptick in extreme weather a function of global warming?

Before I discuss these subjects, let’s take a look at the extent of the record "super outbreak" of April 27. There have been other super tornado outbreaks throughout history, such as the one in April 1974 that resulted in 335 fatalities.  But the death toll from the terrible EF 4 and 5 tornadoes that had paths close to 100 miles long is now more than 350.  This now makes April 27, 2011, the second-deadliest day for tornadoes in history for the United States.  Unfortunately, there will be more massively deadly tornado outbreaks in the future. With populations growing in tornado-prone areas, there is an increasing risk of future outbreaks causing loss of life. Even with all the radars, precise tracking and advance warning we have now, April 2011will be No. 1. as the month with the greatest loss of life from tornadoes.  Now over 400 fatalities.

As the tornadoes in mid-April were sweeping across the South, snow was spreading across the upper plains, Texas was/is suffering a prolonged drought and the last of some record snows were melting in northern New England after another brutal winter. What is going on with the weather? This must be because of climate change – correct?

First of all, the climate is not static. There is a great saying among meteorologists that “Climate is what you expect, weather is what you get.” Look at the long-term global average temperatures over the last 100-plus years:


And here are the average temperatures for the past 1,000 years, as inferred from things such as ice-core samples and tree-ring measurements:


What do you notice? The global temperature changes. The “climate” has always been and will always be changing. And the weather, what we get day to day and year to year, also will always be changing.

There are many natural “drivers” of the climate, from solar variations to volcanoes to changes in ocean currents and land and ice composition. Even the variation of the earth’s orbit and the wobble of the planet on its axis matters. But there is also the climate “driver” we have introduced into the weather system: billions of tons of carbon dioxide released into the environment, as shown in the famous Keeling curve.


I’m not going to go into the politics and policy of climate and weather---that "wouldn’t be prudent.” But suffice it to say that the measurements and accumulating evidence does show that the chemistry of the air and ocean is changing and that that the human footprint on the environment is becoming more obvious. Look at this picture I have shown before of contrails from aircraft over western France. The weather that day was “mostly cloudy,” but not because of any natural dynamic:


So are we changing the weather? Sure. Cities are warmer than surrounding countryside, humidity is higher in Arizona and New Mexico due to new grass and trees being planted (and watered) that are releasing moisture into the air. There’s even some evidence that raindrops are different in highly polluted regions of the world. And while we can’t blame every, or even any, storm on climate change, there is some recent research that indicates that weather extremes or at least the probability of heavy-rain events may be increasing in a slowly warming world.

Kevin Trenberth, a researcher at the National Center for Atmospheric Research in Boulder, Colo., says that there is a climate-change element in all weather. (Here is a fascinating read on the subject.) Trenberth believes that a slowly warming world leads to warmer oceans and more moisture in the air, creating a higher probability of heavier rains and, if the air is cold, even more snow. Hello, New England and Washington of two winters ago.

Some climatologists also suspect that with the global north-to-south temperature gradient becoming less pronounced (the high latitudes are showing a greater positive temperature change than the tropics), our average seasonal jet stream or pattern of mid-latitude Westerlies is changing. That might mean that large cut-off or almost stationary upper-level whirls are becoming more likely. The Weather Channel’s Stu Ostro has some fascinating studies on this and has quite a presentation showing what sure feels like long-term and significant changes in the general pattern. You can see the warmer high latitudes in this NOAA map of temperature anomalies for 2010:


So my final answer, at least right now, is that no, not every big weather event, even the April 27 tornado outbreak, is caused by climate change. However, weather is part of climate and climate is part of weather; they are not two distinct things. We know from observations that the earth is now in a general warming period. Will that eventually mean a higher probability of heavy rain events? I think so. Will it mean a higher probability of heavier winter snows, even with a warming world? Sure. A higher probability of slow-moving, persistent patterns of heavy rains, longer droughts and more prolonged heat waves? Yes, I think that's likely.  More "super outbreaks" such as April 27?  We all sure hope not but because of a number of factors beyond climate and weather something like this is likely to happen again.

But we’ll know better in another 20 to 50 years. Join me then? That depends more on modern medicine than modern meteorology.

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