21 Apr 2011: Analysis

What’s With the Weather?
Is Climate Change to Blame?

One of the thorniest questions facing climate scientists is whether human-induced climate change is leading to more heat waves, floods, and extreme weather events. Now, employing increasingly sophisticated methods of studying weather extremes, climatologists say they are closer to answering that key question.

by alyson kenward

On July 29, 2010, temperatures in Moscow climbed above 100 degrees F for the first time in recorded history, soaring to 102 degrees F. During a 35-day run when the daytime temperatures reached at least 86 degrees F, more than 10,000 people died in Moscow alone because of the sweltering conditions, Russian officials said. Throughout the summer, wildfires burned more than 1.6 million acres in western Russia and the heat-induced drought in July and August destroyed 40 percent of the country’s annual grain yield.

Almost from the beginning, Kevin Trenberth’s phone began ringing off the hook. Trenberth leads the Climate Analysis Section at the National Center for Atmospheric Research in Boulder, Colorado, where he studies how different manifestations of global climate connect with each other. That makes him one of the usual suspects when extreme weather events — including floods, heat waves, and intense storms — strike and the public wants to know: Is this happening because of climate change?

“We get requests from the media as these events are unfolding,” says Trenberth, “but it isn’t necessarily dealt with properly by anyone in the [climate science] community.” What happens all too often, he says, is that scientists end up giving the same hand-waving explanation that while
Scientists say you need to compare the most extreme events to ‘ordinary’ extreme events.
climate change may well have contributed to whatever is going on, it can’t be definitively fingered as the sole cause.

Finding trends in extreme climate events is tricky because extreme events are rare, by definition. It’s instinctive to compare heat waves, droughts, or floods to average weather conditions. So for the Russian heat wave, say, you would look to compare it to an average summer month. But climate scientists who specialize in statistics say this doesn’t capture the true nature of an extreme event. Instead, they argue, you need to compare the most extreme events to “ordinary” extreme events.

“When you’re looking at extreme heat, for example, you need to compare the five hottest days in every year, or the highest overnight temperature in a month,” says Gabriele Hegerl, a professor of climate science at the University of Edinburgh in Scotland. She says this so-called “extreme value approach” is a better way to see the long-term variations in even the most unusual kind of event.

Using the extreme value approach, Hegerl and others have confirmed that the number of extremely hot nights around the world has been increasing over the past few decades. And when they’ve looked to computer models to see if the same sort of thing might have happened in an emissions-free world, they don’t see it. It seems, says Hegerl, that the increased frequency of heat waves may well have been caused by human behavior.

“It is really interesting because for years we’ve just laid back and said, ‘You can’t say anything about a single climate event,’” says Hegerl. “But now people are showing you can say something about the probability of that kind of event occurring.”

In a broad sense, this is what detection and attribution — D&A, to insiders — is all about. It involves searching for both changes in climate trends and for what, exactly, has caused these changes. Basically, D&A is broken down into two questions: Is the climate changing? And if so, are humans causing it?

The simplest and oldest question in D&A is whether rising atmospheric concentrations of carbon dioxide (CO2) caused by human activities, such as
When an extreme event occurs, people ‘find out what we are not adapted to,’ says one expert.
fossil-fuel burning and deforestation, are driving up Earth’s average temperature. The increase in CO2 was first confirmed in the 1960s, and rising temperatures in the late 1980s. But simply spotting the two trends wasn’t proof there was a cause-and-effect relationship. In order to show that, scientists had to rule out other possible explanations.

In recent years, climate scientists have run thousands of computerized simulations to compare the climate with only so-called natural “forcings” — from the sun, volcanic activity, and other non-human influences on temperatures — and also with greenhouse gas emissions exactly as they have been in real life. Time and again, the simulations run with added CO2 have shown a warming climate, which allowed the 2007 Fourth Assessment Report from the Intergovernmental Panel on Climate Change (IPCC) to conclude that humans are the overwhelming cause of increasing global temperatures.

Climate models also show that global temperatures should continue to rise as concentrations of CO2 and other greenhouse gases increase, and suggest that heat waves, severe floods, and powerful storms will also become more frequent. But most people want to know: Is it already happening?

“When an extreme climate event occurs, then people find out what we are not adapted to,” says Francis Zwiers, a statistician who directs the Pacific Climate Impacts Consortium in Victoria, British Columbia, and was the lead author of the chapter on climate attribution for the IPCC’s Fourth Assessment Report. “These events cause a lot of damage, so people are particularly interested in them.”

The best that 2007 report could do was to say that such extreme events as heat waves and heavy rainstorms had “likely” become more frequent over the past 50 years, and that extreme heat waves in particular were at best “more likely than not” to have been caused by human activity.

Because these kinds of events happen so infrequently, it’s been difficult for scientists to confirm any difference in their long-term trends. Extreme heat waves might hit a region only once every 10 or 20 years, and mega-droughts may only happen a couple of times a century.

This is where the extreme value approach becomes key. To confirm that temperature extremes have been rising globally over the past few decades,
It’s clear that what used to be highly unusual events are now becoming more common.
D&A scientists have used as a benchmark the hottest days and nights of every month and every year. To see if heat waves — defined as three or more days of excessively high temperatures — are increasing, they’ve looked for the hottest three-day periods for each year going back through the last century’s weather records. In both cases, it’s clear that what used to be highly unusual events are now becoming more common.

That was the detection part. For attribution to human influences, climate scientists have looked once again to climate models. And once again, models in which only natural forces are at work don’t show any clear trend, but those with human emissions match the rise in heat waves that scientists have detected.

More recently, Hegerl and Zwiers have also looked at how heavy-rainfall events are changing. It’s tougher here than it is with heat waves. That’s because, unlike heat events, really heavy rainfall — the kind that can lead to vicious floods — usually lasts for hours, not days or weeks, and often happens over just a few hundred square miles. So far, Zwiers says, scientists have some good evidence that precipitation extremes are also on the rise worldwide, but it’s too early to say anything with complete confidence. “We’re at the beginning stages on extremes and on figuring out what the human influence is,” says Zwiers.

In terms of finding a human fingerprint in more frequent extreme climate events, he says the detection and attribution community is at a similar place today as it was a few decades ago when scientists were trying to prove, beyond a reasonable doubt, that greenhouse gas emissions had led to global warming. “We are ready to make a cautious statement that humans have contributed to more climate extremes because most of the evidence suggests this,” says Zwiers. “But it’s not a sure thing yet.”

For most people, that’s not good enough. They don’t care about statistics; they care about actual weather. When events like last summer’s heat wave strike, researchers still have to field questions about what role global warming played in that particular situation.

Answering those questions is one way scientists think they can help people better understand how climate change will affect them in the future. It will always be true, as researchers have cautioned, that no single climate event can be caused entirely by climate change. “There have always been extreme events,” says Peter Stott, a climatologist from the UK’s Met Office. “Natural variability does play a role, but now so does climate change. It is about changing the odds of the event happening.”

What Stott and others have started to do, therefore, is to use climate models to compare how often specific extreme events having the characteristics of, say, the 2010 Russian heat wave, would happen without man-made CO2, and how often they should happen with it. Then they can
Recent reports suggest climate conditions relating to the Russian heat wave were primarily natural.
work backwards and apply the extreme value approach to the most unusual events the climate models have simulated.

For example, after the blistering 2003 summer heat wave in Europe, Stott’s group calculated that greenhouse gas emissions had more than doubled the likelihood that such a heat wave would have occurred. More recently, they found that man-made climate change had also increased the odds in favor of the kind of major flood that struck the United Kingdom in 2000. Their analysis doesn’t rule out the possibility that the events were strictly natural, but they do show that greenhouse gas emissions have made those particular extremes much more likely.

According to Trenberth, however, while the idea of calculating the changing odds is a good one, in practice the computer models aren’t good enough to capture how the entire global climate might influence a particular region.

“To get a really good answer, you need a perfect climate model,” he says. “And to the extent that climate models aren’t perfect, then where does the error go?” All too often, according to Trenberth, scientists err in the direction of saying extreme climate events are still largely caused by natural variations.

The question of how well global climate models can be used to zoom in on regional climate has already emerged as scientists search for the causes of last summer’s Russian heat wave. Recent reports from the National Oceanic and Atmospheric Administration (NOAA) suggest that the climate conditions leading to the heat wave were primarily natural and that any human influence was drowned out by climatic variations that are inherently part of the natural system, rather than triggered by human influences.


Can We Trust Climate Models?
Increasingly, the Answer is ‘Yes’

Can We Trust Climate Models? Increasingly, the Answer is ‘Yes’
Forecasting what the Earth’s climate might look like a century from now has long presented a huge challenge to climate scientists. But better understanding of the climate system, improved observations of the current climate, and rapidly improving computing power are slowly leading to more reliable methods, Michael D. Lemonick reports.
What concerns Trenberth is that NOAA’s analysis doesn’t take into account the abnormally high sea surface temperatures in the Indian Ocean at the time of the heat wave, and how that could have influenced what happened in Europe. Even though the Indian Ocean and Western Russia are thousands of miles apart, the climate systems over them are interwoven. The Indian Ocean temperatures can affect air pressure across Asia, for example, which can influence the flow of the jet stream — and the influx of warmer air masses from the south — across Europe. “They haven’t answered the question of what caused the sea surface temperatures to get so high,” he says, which means that scientists can’t really be certain yet if global warming was a major cause of last year’s extreme heat in Russia.

Nevertheless, Trenberth and others in the D&A community say that the science of climate extremes is vital. Tracking how extreme events are changing, and searching for a human fingerprint in them, is an important step in helping the public figure out what they can expect from future climate.

“For extreme events, the question isn’t, ‘Is it global warming or natural variability?’” says Trenberth. “It is always both. The question is just how much each is contributing.”

POSTED ON 21 Apr 2011 IN Climate Climate Oceans Policy & Politics Pollution & Health Science & Technology Antarctica and the Arctic North America 


Good post.

Already shift in seasons is felt. Rains are not occurring during specified season but erratically and summer is severe in some parts.

Climate model projections summarized in the 2007 IPCC report indicate that the global surface temperature is likely to rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the 21st century.

An increase in global temperature will cause sea levels to rise and will change the amount and pattern of precipitation, probably including expansion of subtropical deserts. Warming is expected to be strongest in the Arctic and would be associated with continuing retreat of glaciers, permafrost and sea ice. Other likely effects of the warming include more frequent and intense precipitation events, extreme weather events, species extinctions due to shifting isotherms, and changes in agricultural yields. Warming and related changes will vary from region to region around the globe, though the nature of these regional changes is uncertain. As a result of contemporary increases in atmospheric carbon dioxide, the oceans have become more acidic, a result that is predicted to continue.

Dr.A.Jagadeesh Nellore(AP),India

Posted by Dr.A.Jagadeesh on 21 Apr 2011

There are two points on this piece that stand out to me.

If scientists are determining that an increase to the globally averaged greenhouse effect due to human emission is making specific extreme events simply more likely, then those events would have to be accounted for under any scenario of human behavior. Therefore, because we will have to account for brutal heat wave, heavy storms, hurricanes and the like, resource allocation seems much better spent in adaption to such events rather than mitigation of an increase in the globally average greenhouse effect. We win under any warming scenario.

More importantly, there is a flip side to Trenberth's concern over the connection between the Indian Ocean and the Russian heat wave on 2010. If scientists don't have a firm grasp on how different components of the climate system interact with one another to the extent that we cannot reject the influence of an increase in the globally average greenhouse effect in the Russian heat wave, why do scientists believe that they know enough to attribute ANY extreme event to global warming? There is necessary lack of knowledge to not reject the influence of global warming, but such a lack of knowledge also influences are ability to detect a meaningful global warming signal. It's a double-edged sword, so to speak.

NOAA's statement above points out there is no long time series increase in the occurrence or intensity of blockage events that the one that caused the Russian heat wave. How an increase in the globally average greenhouse effect would somehow cause a bump in the interaction between the Indian Ocean and the jet stream seems an odd thing to pick on, but with nothing else anomalous happening at the time, maybe it's worth examining. With no model explaining such a relationship, however, it looks like grabbing at straws to the passing observer.

Posted by maxwell on 22 Apr 2011

Maybe defining a carbon based material located in the earth's lithosphere as "energy" is an extreme event. The Yale professor who said petroleum is a good thing to set on fire (1859) was such a silly man, or make that Silliman.

I would say, an "energy system" that melts the cryosphere is not really an energy system at all. It is fuelish contamination that while invisible (both the carbonic acid gas contaminant and it's radiative forcing) it is reverting the atmosphere to prehistoric conditions. And soon the biosphere, along with ourselves, may follow.

The good news for those who invest is that oil is making record profits, again. Don't we feel rich?

Posted by James Newberry on 28 Apr 2011

I will pose the same question to people here that I have everywhere else. At what point do we stop caring what they can concretely prove and admit that mankind is responsible, if not wholly, then as a major contributor. And why are we waiting until someone comes out and says "yup, its all our fault, we should probably try to fix this now!" If we don't start taking action soon there wont be an environment to save, much less live in!

At some point, governments and policymakers are going to have to be FORCED to take action or make change. As far as I'm concerned, the only fact that has been proven is that the governments and industries of the world don't care about or see anything other than profit.

They are going to destroy not only our future but our children's and grandchildren's as well. If the people of the world cannot see this happening around them every day, especially with all the changes currently taking place, then shame on them.

Now is the time for change, there may not be a later, do YOU want to take that chance?

Posted by Chet Misialek on 09 Jun 2011

If we don't soon begin to calculate the cumulative effects of man-driven heat sources and begin to regulate them, we face more summers and winters and "natural" disasters as in the past half decade.

By specific calculations we mean: heat given off by car engine blocks (multiplied by the millions of these heating units driven daily, monthly, yearly); the amount of btus given off by the millions of aircraft over the planet at any one time; btus of exchange heat (exhaust from electricity-generating units heat exchangers such as heat pumps and portable window air conditioning units).

These btus of heat do not simply "disappear," although they might be mitigated by such natural phenomena as polar and antarctic ice ranges.

One of my bright students once suggested that what we are going through is similar to a home losing electricity (and thus air conditioning) in a hot summer month. During the first few hours, the temperature climb within the house is relatively gradual. Then comes a tipping point when the interior temperature rises perhaps ten or fifteen degrees in a half hour or forty-five minutes.

She suggested that that "tipping point" may have been reached in the tiny layer of habitable gasses we call our atmosphere. She was aghast that no "remediation" or "conservation" practices have even been suggested as our staples for survival (air, small spectrum of temps for plant growth) are now being threatened. She suggested that even a one day per week ban on driving and flights would have a cumulative negative btu effect on climate. She suggested, and I agree, that although the doomsday clock is well past midnight, we continue our "business as usual."

Naive? Perhaps. But where are the specific calculations on indexes of atmospheric btu input that we CAN and DO control?

Lee Schultz
Prof. retired,
Stephen F. Austin U.
Nacogdoches, TX

Posted by Dr. Lee Schultz on 06 Aug 2011

First, foremost, and obviously, until a viable "humanly perfect" model of the interacting global atmospheric conditions is constructed, we're a long way away from making predictions about cause and effect. The extreme value approach is a grand step in the right direction, but nothing can be said about cause/effect without the global interaction taken into consideration.

Second, given the posture of the global economy, Kyoto Protocol is, at this point, not attainable. It might be wise to consider offering some other more economical alternatives. I presume that the community has considered interdisciplinary cooperation as a possible means to just what alternatives are actually practical.

Third, as a layperson, the doomsday rant really does nothing to promote solutions. Until you open your eyes to the world as it exists, accepting warts and all, you are probably in a state of "scientific denial". I would suggest putting that energy into thinking along the lines of tangible alternatives.

I am, however, extremely grateful for your work. Just thought I might add a different perspective.

C. Hurlburt
Mount Dora, FL

Posted by C Hurlburt on 30 Dec 2011

These extreme weather events could very well be cyclical. It's well documented and is used
frequently by non believers to explain global warming. But I would have great difficulty believing that emissions from a billion cars and trucks on the roads, and the factories and companies, and ships, dumping their toxic waste with immunity, and deforestation are not having an impact on the weather. 1,300 scientists from 70 countries agree we are having a tremendous effect on the weather.

Posted by frank nosella on 07 Jul 2012

Comments have been closed on this feature.
alyson kenwardABOUT THE AUTHOR
Alyson Kenward is a science journalist at Climate Central, an independent media organization whose mission is to communicate climate science to the public. Her articles have been published by Scientific American online, OnEarth and SolveClimate. She also co-hosts and produces the Climopedia podcast



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