NASA Study Links Extreme Summer Heat to Climate Change

A new statistical analysis from NASA reveals that the extreme summer heat felt by many Americans this year is the result of global warming. Hari Sreenivasan talks to Columbia University's James Hansen who says the greatest barrier to public recognition of human-made climate change is the natural variability of local climate.

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    And finally tonight — climate change and extreme weather.

    Scientists will now be looking very closely at this summer's data to try to better understand the latest drought. But, today, a new paper was published linking extreme heat episodes of recent years, including the Texas and Oklahoma drought of last year, with climate change.

    It came from one of the first scientists to call attention to the phenomenon, James Hansen, head of the NASA Goddard Institute for Space Studies. Hansen is also a professor of earth and environmental studies at Columbia University.

    He spoke earlier with Hari Sreenivasan.

    Dr. Hansen, thanks so much for joining us.

    JAMES HANSEN, NASA Goddard Institute for Space Studies: Sure.


    So, help us in a nutshell understand your findings.


    What we show is that there is a connection to these extreme weather events that we have been seeing.

    The probability of these unusually hot, hot spells, forest fires and extreme droughts has increased substantially over the last few decades. We had shown in — we had — I had shown dice in the 1980s when I testified to Congress about global warming. And I said that the frequency of unusually warm seasons was going to increase.

    So, I used one dice to — for typical or average climate that existed 50 years ago. There were equal chances of an average climate, which was represented by a white side of a dice, or an unusually warm season, which was red, or unusually cool season, which was blue. You had equal chances of those.

    But what we said was that, by the end of the century, we would get — we would have loaded dice, so that only one side would be normal, one side cooler than normal, and four sides would be unusually warm.

    Now, in fact, that is what has happened. However, what I didn't think about at the time in the 1980s was that, as we push the climate toward these hotter climates, these higher temperatures, the extremes, the highest temperatures will be very extreme.

    And we are now getting about 10 percent of the area in a given summer is covered by what we call three standard deviation anomalies. Those would have occurred only once or twice out of 1,000 times 50 years ago. But now they are occurring about 10 percent of the time.

    That's the wildfires, the extreme droughts, the heat waves. Those are now significant chance of having those. And those are the things that have big economic impacts.


    So, explain the — how we know that these events are happening and that they are not part of any sort of natural cycle?


    They are, in fact, a product of global warming, and we can see that very easily.

    One thing, I think that the public can appreciate, what we call the bell curve. You know when you have a random variable, they tend to fall on both sides of the average value, sometimes colder than normal, sometimes hotter than normal.

    And this is the way this bell curve looked in the 1980s. But, as time went on, as the planet gets warmer, it starts shifting toward the right, and you get more red and less blue.

    And, by now, this past decade, it has shifted so far that we have got about 10 percent of the cases out here in this extreme warm anomaly, more than three standard deviations.

    And, to understand, averaged over the whole summer, it is only a few degrees warmer, a few degrees Celsius warmer. But that is a big effect. To see how important that is, look at Moscow two years ago, Texas last year, the Midwest this year.


    So, your research shows that the temperature increases between the last 30 years and the 30 years before that have increased by about a degree Fahrenheit. Now, is that within the sampling error? Is that significant?


    Oh, it's significant.

    It's much larger than the error of measurement. And it's enough to change the frequency of these extreme events by a large amount. So, it's important. In fact, it's now driven our climate outside the range that has existed the last 10,000 years, this geologic epoch that we call the Holocene.


    So, one of the scientists that we asked for a comment on your paper hey, you know, you are paying attention partly because it's Dr. Hansen. The science — the IPCC report in 2007 has said some of the same things he is saying now.

    Is there some new science? Is there some new findings that you are talking about today?


    Yes, absolutely.

    They didn't show this sort of a shift in the probability distribution for the temperatures of summer season. And this is extremely important, because it's the large anomalies that have the practical impact. You can see what's happening in the Midwest this year.


    All right, Dr. James Hansen, thanks so much for joining us.