Photograph by David McNew, Getty Images

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A runner braves the heat in Death Valley National Park in California as temperatures approach record-breaking highs.

Photograph by David McNew, Getty Images

What’s Behind the Heat Wave: Climate Change or Weather?

Will 2013 set another heat record?

The American Southwest is broiling in triple-digit temperatures for the fourth consecutive day as a result of a record-breaking heat wave that is smothering the region.

In Death Valley, California, the temperature reached 128° Fahrenheit (53° Celsius) on Sunday—just a few degrees shy of the July 10, 1913, record of 134° Fahrenheit (57° Celsius).

The heat wave has also been partly blamed for a wildfire that killed 19 firefighters on Sunday in Yarnell Hill, Arizona.

So what's behind the heat wave? Is global warming a factor? And how does it compare with past events? We talked to Martin Tingley, a climatologist at Harvard University, to find out.

In April, Tingley and his colleague Peter Huybers published a study in the journal Nature that concluded that the years 2005, 2007, 2010 and 2011 were warmer than any year going back 600 years, to 1400.

Do scientists know what's behind the current heat wave?

Looking at the meteorological charts, it looks to be a blocking event. That happens when there's a particular configuration of the jet stream that's quite stable. So there's a big high-pressure ridge on the West Coast and a low-pressure trough in the East Coast. That's why it's quite rainy here [in Cambridge, Massachusetts] and very hot on the West Coast.

Have you or other scientists had a chance to analyze this current heat wave and determine how it compares to past years?

No ... because 2013 is not over yet. One very hot week will have some signature on the seasonal average, but how large that signature will be depends on what happens for the rest of the summer.

Also, the study that Peter [Huybers] and I did made use of paleoclimate records—things like tree rings and ice cores. One of the limitations of that study is we can't really think of week-long heat waves like what's going on. We were limited to seasonal averages—hot summers versus cool summers—and we found that recent warm extremes in terms of summer average conditions at the high northern latitudes are unprecedented in the last 600 years.

You reached those conclusions after performing a statistical analysis on the paleoclimate data. Do you have an analogy that helps explain how that analysis works?

One imperfect analogy: Even though you and I have never met, I would bet that I'm taller than you because I'm 6-foot-4-inches (1.9 meters), and that puts me in a high percentile of the distribution of heights. But if I were to walk into a room of 1,000 people, I probably wouldn't be the tallest.

It's the same when we're addressing the question of was 2011 the hottest year in the last 600 years. It's a different statistical question to ask was it warmer than one particular year in the past—that's like me saying I'm taller than you—and it's a much different question to ask whether 2011 was the hottest year amongst all of the past 600 years.

To deal with this, what we do is instead of coming up with a single best estimate ... we use a simple statistical model to simulate 4,000 equally likely realizations of the climate [based on the paleoclimate data]. Then we can ask in how many of these 4,000 possible climate histories was 2011 the warmest year? So instead of having a single best estimate, we have 4,000 possible realizations.

Climate scientists often compare the effects of global warming to loaded dice: Not every roll of a loaded die will come up six—but sixes will occur more often than if the die had not been tampered with. Is that still the thinking?

Can we attribute this particular heat wave to an anthropogenic impact on the climate? The only safe answer is, well, probably not. It's like if I flip one coin and it comes up heads, that doesn't mean the coin is loaded.

But what we're seeing now, there seems to be a trend toward more hot extremes and fewer cold extremes. That's a pattern that's consistent with an anthropogenically-forced increase in temperatures.

What can people expect from future extreme heat waves that are affected by global warming? Will they be anything like the current heat wave affecting the West Coast?

I think that's a really big open question right now: Given rising mean temperatures, how will the extremes change, in both magnitude and frequency? It's actually one of the research projects we are tackling at the moment: How are extremes in temperature on daily timescales changing with respect to the mean temperature?

If the mean temperature goes up by half a degree, do the extremes simply track that half-degree increase? Or are the extremes being amplified in some sense so that they are becoming hotter with respect to the mean?

As a climate scientist, are you doing anything personally to prepare for the hotter summers that will result from global warming?

Well, I like to ski a lot in the winter, and I'm concerned that ski conditions are becoming more variable, especially on the East Coast.

That's not really something that I've done in my life to prepare, but it's something that's given me a lot of pause. Will the slopes have consistent snow cover for the next generation of skiers, and will the tradition of East Coast skiing survive a warming climate?