Expanding tropics pushing high altitude clouds towards poles, NASA study finds

The Hadley cells describe how air moves through the tropics on either side of the equator. They are two of six major air circulation cells on Earth. Credit: NASA

from phys.org, May 5, 2016

A new NASA analysis of 30 years of satellite data suggests that a previously observed trend of high altitude clouds in the mid-latitudes shifting toward the poles is caused primarily by the expansion of the tropics.

Clouds are among the most important mediators of heat reaching Earth's surface. Where clouds are absent, darker surfaces like the ocean or vegetated land absorb heat, but where clouds occur their white tops reflect incoming sunlight away, which can cause a cooling effect on Earth's surface. Where and how the distribution of cloud patterns change strongly affects Earth's climate. Understanding the underlying causes of cloud migration will allow researchers to better predict how they may affect Earth's climate in the future.

George Tselioudis, a climate scientist at NASA's Goddard Institute for Space Studies and Columbia University in New York City, was interested in which air currents were shifting clouds at high altitude - between about three and a half and six miles high - toward the poles.

The previous suggested reason was that climate change was shifting storms and the powerful air currents known as the jet streams - including the one that traverses the United States - toward the poles, which in turn were driving the movement of the clouds.

To see if that was the case, Tselioudis and his colleagues analyzed the International Satellite Cloud Climatology Project data set, which combines cloud data from operational weather satellites, including those run by the National Oceanic and Atmospheric Administration, to provide a 30-year record of detailed cloud observations. They combined the cloud data with a computer re-creation of Earth's air currents for the same period driven by multiple surface observations and satellite data sets.

What they discovered was that the poleward shift of the clouds, which occurs in both the Northern and Southern Hemispheres, connected more strongly with the expansion of the tropics, defined by the general circulation Hadley cell, than with the movement of the jets.

The Hadley cell is one of the major ways air is moved around the planet. Existing in both hemispheres, it starts when air in the tropics, which is heated at the surface by intense sunlight, warms and rises. At high altitudes it is pushed away from the equator towards the mid-latitudes to the north and south, then it begins to sink back to Earth's surface, closing the loop.

"What we find, and other people have found it as well, is that the sinking branch of the Hadley cell, as the climate warms, tends to be moving poleward," said Tselioudis. "It's like you're making the tropical region bigger." And that expansion causes the tropical air currents to blow into the high altitude clouds, pushing them toward the poles, he said. The results were published in Geophysical Research Letters, a journal of the American Geophysical Union.

Scientists are working to understand exactly why the tropics are expanding, which they believe is related to a warming climate.

The poleward shift of high altitude clouds affects how much sunlight reaches Earth's surface because when they move, they reveal what's below.

"It's like pulling a curtain," said Tselioudis. And what tends to be revealed depends on location - which in turn affects whether the surface below warms or not.

"Sometimes when that curtain is pulled, as in the case over the North Atlantic ocean in the winter months, this reduces the overall cloud cover" in the lower mid-latitudes, the temperate regions outside of the tropics, Tselioudis said. The high altitude clouds clear to reveal dark ocean below - which absorbs incoming sunlight and causes a warming effect.

However, in the Southern Ocean around Antarctica, the high altitude clouds usually clear out of the way to reveal lower altitude clouds below - which continue to reflect sunlight from their white tops, causing little effect on the solar radiation reaching the surface.

When the results are taken together, the bottom line is that the cloud interactions with atmospheric circulation and solar radiation are complicated, and the tropical circulation appears to play a dominant role, said Tselioudis.

That information is a new insight that will likely be used by the climate modeling community, including the scientists who contribute modeling expertise to the Intergovernmental Panel on Climate Change, said Lazaros Oreopoulos, a cloud and radiation budget researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study. Climate modelers aim for their computer simulations to correspond as closely to reality as possible in order to reliably predict Earth's future climate.

"If current behavior is not well simulated, then confidence in predicted future behavior will be lower," Oreopoulos said. "I anticipate this study to be looked at carefully and affect thinking on these matters."

Read more at: http://phys.org/news/2016-05-tropics-high-altitude-clouds-poles.html

Hurricanes May Threaten Northern Cities Like Never Before as Tropics Expand, Study Finds

by Andrew Freedman, Mashable, May 15, 2014

In this December 5, 2012, file photo, men walk past a house damaged during Superstorm Sandy in the Belle Harbor section of the Queens borough of New York. IMAGE: KATHY WILLENS/ASSOCIATED PRESS

BY ANDREW FREEDMAN1 DAY AGO

Hurricanes and tropical storms are reaching their peak intensity closer to the poles, migrating at about 30 miles per decade, according to a new study published Wednesday. If this shift continues, it could have major consequences for places like New York City, Tokyo, Japan and Brisbane, Australia, as well as other high latitude areas that don't normally see intense hurricanes.

The study, published in the journal Nature, is the first to detect this trend, and in doing so it reveals a somewhat indirect but tangible link between human activities and Mother Nature's most powerful storms.

SEE ALSO: What Irreversible Antarctic Ice Melt Could Mean for Some U.S. Cities

In order to reach their conclusions, the researchers overcame a formidable obstacle that has long hindered research on the links between hurricanes and global warming. Changes in how we detect and measure the intensity of tropical cyclones over the years makes finding statistically significant trends difficult to near impossible.

Typhoon Francisco and Super Typhoon Lekima seen from a satellite on October 23, 2013, as they tracked northwestward toward China and Japan. IMAGE: NOAA

The new study examines a metric known as a storm's "lifetime-maximum intensity" during the period from 1982 to 2012, which is a time frame that is not complicated by changes in storm observations. The metric refers to the point where storms max out in strength.

By examining storm data using this metric, the researchers found a strikingly apparent poleward shift in the locations where storms are reaching their peak intensity. Although the changes varied from ocean basin to ocean basin, with the greatest migration seen in the western North Pacific Ocean, which is the most active area for tropical cyclones, the shift was found in both the Northern and Southern Hemispheres.

In the Northern Hemisphere, the lifetime-maximum intensity point is moving north at 33 miles per decade, whereas in the Southern Hemisphere, that point is moving south at 39 miles per decade.

Such a change is enough to alter the risk that a coastal location will be hit by a major storm, said lead author James Kossin of the National Climatic Data Center.

"There’s no doubt that a signal like this introduces the potential for a change in the risk,” Kossin said in an interview with Mashable. There's certainly a potential for decreased risk in some areas and increased risk in others at higher latitudes.”

The study itself says that if the observed trends continue, there could be "potentially profound consequences for life and property."

In other words, the risk of damage and fatalities from tropical storms and hurricanes could increase in northern latitudes, while declining somewhat at lower latitudes.

Color-enhanced infrared satellite image of Typhoon Usagi as it moved northwestward toward Hong Kong while explosively intensifying to a Category-5 storm. IMAGE: NOAA/COOPERATIVE INSTITUTE FOR METEOROLOGICAL SATELLITE STUDIES, UNIVERSITY OF WISCONSIN-MADISON.

In addition, parts of the tropics that depend on rainfall from tropical storms and hurricanes (collectively referred to as tropical cyclones) to provide water resources may be at risk for lower water availability as the storms move away from them.

One caveat from the study is that there hasn't been a detectable poleward shift in the maximum intensity point of Atlantic tropical cyclones. However, this may be due to other factors that are masking this movement.

"The Atlantic is fairly unique in the last 30 years in how it’s been behaving,” Kossin says. "We kind of suspect that this global signal that we uncovered is just kind of getting muddled up with strong regional effects.

“The takeaway from that is just because we have not seen a trend there in the last 30 years doesn’t mean this effect is not present there, it’s just being masked by other things and it won’t necessarily be masked forever.”

Tracks of all tropical cyclones which formed worldwide from 1985 to 2005. IMAGE: WIKIMEDIA COMMONS

Chris Landsea, a meteorologist at the National Hurricane Center in Miami, Florida, who was not involved in the study, told Mashable: "This is an important, very well researched paper that uncovers something that was unknown previously."

He continued, "Such changes in where storms are peaking are somewhat unexpected (at least to me) but are apparently due to global scale changes in the atmosphere, perhaps tied to anthropogenic [man-made] global warming."

Expanding tropics

The study ties the migration of storms' peak intensity points to a gradual expansion of the tropics, which in turn has been linked to manmade factors. The expansion has to do with the widening of what is known as the Hadley Cell, which is a pattern of air circulation that causes air to converge near the Equator, sparking thunderstorms with heavy rain throughout the tropics. (This helps explain the typical location of rain forests worldwide.)

As air spills out of the Hadley cell and descends, it dries and warms, which is why the subtropics at the edges of the Hadley Cell are often home to deserts.

Studies of how emissions of planet-warming greenhouse gases will affect the tropics have consistently shown that the tropics will expand poleward in both hemispheres, which will push the dry subtropics poleward as well. This is part of the reason why the Southwest U.S. is poised to become drier as the climate warms, since the subtropics are starting to encroach on that area.

Diagram of tropical air circulation, including the Hadley Cell.

According to Kossin, the expansion of the tropics has also been tied to emissions of aerosols, which are small particles in the atmosphere, from factories and natural sources like desert dust, as well as manmade depletion of the stratospheric ozone layer. It is not yet clear which factor is the biggest contributor to the expansion of the tropics, however, Kossin said.

If greenhouse gases are driving the expansion of tropical atmospheric circulation, "then we won’t be seeing any stop to this in the near future," Kossin says.

However, if the main driver is the depletion of the ozone layer, the phaseout of ozone-depleting pollution should slow this expansion over time.

In addition to the growing tropics, two other environmental changes have taken place that affect storm intensity. The first concerns vertical wind shear. This refers to the change in wind speed or direction (or both) with height. Vertical wind shear has increased in the tropics, but decreased farther away from the tropics, closer to the poles. Vertical wind shear can make it difficult for tropical storms and hurricanes to form and intensify, since they tear apart the thunderstorms that comprise the inner core of such storms.

The second is that potential intensity, which is how strong a storm can get given sea surface temperatures and the temperature and moisture content of the upper atmosphere, is also changing. “In the tropics the potential intensity is decreasing, and it's increasing outside of the tropics,” Kossin said.

Hugh Willoughby, a hurricane researcher at Florida State University, told Mashable in an email that the study's results match his understanding of how air circulation is changing in and around the tropics. "My take on the situation is that shear attributable to the subtropical jet will increase as the planet warms; whereas the shear attributable to the middle latitude jet will decrease as high latitudes warm more than the tropics," he said. Willoughby was not involved in the new study.

The link between global warming and tropical cyclone behavior has been a source of controversy in the past, but this study appears to present clear, and unexpected, evidence that storms are already responding to the changing climate.

The big question is what it will mean for coastal residents around the world, but early hints are not encouraging for higher latitude cities.

http://mashable.com/2014/05/14/migrating-hurricane-intensity-tropics-expand-global-warming/

"The poleward migration of the location of tropical cyclone maximum intensity," by James P. Kossin, Kerry A. Emanuel & Gabriel A. Vecchi, Nature (2014); doi:

Nature, 509 (15 May 2014) 349–352; doi: 10.1038/nature13278

The poleward migration of the location of tropical cyclone maximum intensity

• James P. Kossin,
• Kerry A. Emanuel and
•  Gabriel A. Vecchi

Abstract

Temporally inconsistent and potentially unreliable global historical data hinder the detection of trends in tropical cyclone activity^1,2,3. This limits our confidence in evaluating proposed linkages between observed trends in tropical cyclones and in the environment^4,5. Here we mitigate this difficulty by focusing on a metric that is comparatively insensitive to past data uncertainty, and identify a pronounced poleward migration in the average latitude at which tropical cyclones have achieved their lifetime-maximum intensity over the past 30 years. The poleward trends are evident in the global historical data in both the Northern and the Southern Hemisphere, with rates of 53 and 62 kilometres per decade, respectively, and are statistically significant. When considered together, the trends in each hemisphere depict a global-average migration of tropical cyclone activity away from the tropics at a rate of about one degree of latitude per decade, which lies within the range of estimates of the observed expansion of the tropics over the same period⁶. The global migration remains evident and statistically significant under a formal data homogenization procedure³, and is unlikely to be a data artefact. The migration away from the tropics is apparently linked to marked changes in the mean meridional structure of environmental vertical wind shear and potential intensity, and can plausibly be linked to tropical expansion, which is thought to have anthropogenic contributions⁶.

http://www.nature.com/nature/journal/v509/n7500/full/nature13278.html

Changing Hadley and Walker circulations due to a warming Arctic

Comments from Neven's excellent Arctic Sea Ice News blog:

Werther,

re: NH cells This is, in my opinion, the largest effect of climate change. The weakening of the polar jet is indicating a gradual merge of the ferrell and polar cells. It is this global restructuring of global weather patterns that will produce the most immediate impacts on society and humanity's food production capabilities. This will occur due to increases in droughts in current food producing regions and floods in current population centers.

What we are also seeing in this regime of climate change is a gradual strengthening of the hadley cell as tropical evaporation increases. This is increasing desertification. The northward expansion of the 30'N desert belt will also significantly impact grain and livestock production.

Studies of these effects and predictions of their impacts are in the peer-reviewed record produced over the last 20 years. The fact that we have done nothing to act on the impacts that have been observed indicates that only a "breakthrough" event will produce real motivation for change. The longer that we wait, the more painful climate change will be and the more difficult it will be to fix it. In fact, we are approaching a point where it is becoming very likely that modernity will not survive the next 65 years.

With the arctic ice pack being the "canary in the coal mine" and the most effective messenger of these developing threats, it is important that we more fully understand and and are able to explain what is happening in the arctic today.

That is why I monitor these blogs, and sincerely appreciate the work that is being done here.

http://onlinelibrary.wiley.com/doi/10.1029/2010JD015197/abstract

"poleward shift of the subtropical dry zones (up to 2° decade−1 in June-July-August (JJA) in the Northern Hemisphere "

Posted by: Jai Mitchell | July 10, 2013 at 17:19

Werther 

In fact, what has been going on since autumn last year, is a restructuring of the three NH cells. The gradient between them is fading. The behavior of the Polar Jet Stream is directly related to that.

The geopotential difference is lower, meaning that the height of the atmospheric pressure zones, especially over the Arctic and Boreal zones have gone up.

Thanks Werther, but I'm not sure I understand this logic.
Why would the height of the atmospheric pressure zones, especially over the Arctic, go up if the gradient between the NH cells fades ?

What I understand is that in 2012 the gradient between the NH cells was low, (and the polar vortex was weak, and that is why we had a warm Arctic.

So why would 2013 show the opposite ?

Posted by: Rob Dekker | July 11, 2013 at 08:45

Rob Dekker wrote: July 11, 2013 at 08:45

Why would the height of the atmospheric pressure zones, especially over the Arctic, go up if the gradient between the NH cells fades?

Hi Rob,

I think the increased height of the troposphere is a direct consequence of more water vapour in the atmosphere due to a warmer earth, rather than the gradient between circulation cells. Wikipedia says:

As a rule, the "cells" of Earth's atmosphere shift polewards in warmer climates (e.g. interglacials compared to glacials)

So it seems the Hadley and Ferrel cells themselves are crowding the Polar cell, raising the height of the atmosphere in the Arctic. Or more exactly pushing the Polar cell further to the North.

However, as the jet stream weakens, one would also expect the loss of this natural barrier between cells to raise the average height of the polar cells.

Have you seen the polar jet stream this week? It's running from 70N to 83N in the CAA, and 75N to 85N over the Laptev sea right now (00z 11 Jul 2013).

Highly unusual, and very far North.

Cheers,
Lodger

Posted by: Artful Dodger | July 11, 2013 at 10:22

http://neven1.typepad.com/blog/2013/07/so-how-slow-was-this-start.html

Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data

Journal of Geophysical Research: Atmospheres, Vol. 16, No. D9 (16 May 2011); doi:10.1029/2010JD015197

Recent trends of the tropical hydrological cycle inferred from Global Precipitation Climatology Project and International Satellite Cloud Climatology Project data

Y P. Zhou, Kuan-Man Xu, Y. C. Sud and A. K. Betts

Abstract

[1]Scores of modeling studies have shown that increasing greenhouse gases in the atmosphere impact the global hydrologic cycle; however, disagreements on regional scales are large, and thus the simulated trends of such impacts, even for regions as large as the tropics, remain uncertain. The present investigation attempts to examine such trends in the observations using satellite data products comprising Global Precipitation Climatology Project precipitation and International Satellite Cloud Climatology Project cloud and radiation. Specifically, evolving trends of the tropical hydrological cycle over the last 20–30 years were identified and analyzed. The results show (1) intensification of tropical precipitation in the rising regions of the Walker and Hadley circulations and weakening over the sinking regions of the associated overturning circulation; (2) poleward shift of the subtropical dry zones (up to 2° decade⁻¹ in June-July-August (JJA) in the Northern Hemisphere and 0.3–0.7° decade⁻¹ in June-July-August and September-October-November in the Southern Hemisphere) consistent with an overall broadening of the Hadley circulation; and (3) significant poleward migration (0.9–1.7° decade⁻¹) of cloud boundaries of Hadley cell and plausible narrowing of the high cloudiness in the Intertropical Convergence Zone region in some seasons. These results support findings of some of the previous studies that showed strengthening of the tropical hydrological cycle and expansion of the Hadley cell that are potentially related to the recent global warming trends.

http://onlinelibrary.wiley.com/doi/10.1029/2010JD015197/abstract

SciA: Global Warming and the Science of Extreme Weather. How rising temperatures change weather and produce fiercer, more frequent storms. Second of a three-part series by John Carey

Global Warming and the Science of Extreme Weather

How rising temperatures change weather and produce fiercer, more frequent storms. Second of a three-part series

by John Carey, Scientific American, June 29, 2011

Editor's note: This article is the second of a three-part series by John Carey. Part 1, posted on June 28, 2011, is "Storm Warning: Extreme Weather Is a Product of Climate Change."

Extreme floods, prolonged droughts, searing heat waves, massive rainstorms and the like don't just seem like they've become the new normal in the last few years—they have become more common, according to data collected by reinsurance company Munich Re (see Part 1 of this series). But has this increase resulted from human-caused climate change or just from natural climatic variations? After all, recorded floods and droughts go back to the earliest days of mankind, before coal, oil and natural gas made the modern industrial world possible.

Until recently scientists had only been able to say that more extreme weather is "consistent" with climate change caused by greenhouse gases that humans are emitting into the atmosphere. Now, however, they can begin to say that the odds of having extreme weather have increased because of human-caused atmospheric changes—and that many individual events would not have happened in the same way without global warming. The reason: The signal of climate change is finally emerging from the "noise"—the huge amount of natural variability in weather.

Scientists compare the normal variation in weather with rolls of the dice. Adding greenhouse gases to the atmosphere loads the dice, increasing odds of such extreme weather events. It's not just that the weather dice are altered, however. As Steve Sherwood, co-director of the Climate Change Research Center at the University of New South Wales in Australia, puts it, "it is more like painting an extra spot on each face of one of the dice, so that it goes from 2 to 7 instead of 1 to 6. This increases the odds of rolling 11 or 12, but also makes it possible to roll 13."

Why? Basic physics is at work: The planet has already warmed roughly 1 degree Celsius since preindustrial times, thanks to CO2 and other greenhouse gases emitted into the atmosphere. And for every 1 °C (1.8 °F) rise in temperature, the amount of moisture that the atmosphere can contain rises by 7%, explains Peter Stott, head of climate monitoring and attribution at the U.K. Met Office's Hadley Center for Climate Change. "That's quite dramatic," he says. In some places, the increase has been much larger. Data gathered by Gene Takle, professor of meteorology at Iowa State University in Ames, show a 13% rise in summer moisture over the past 50 years in the state capital, Des Moines.

The physics of too much rain
The increased moisture in the atmosphere inevitably means more rain. That's obvious. But not just any kind of rain, the climate models predict. Because of the large-scale energy balance of the planet, "the upshot is that overall rainfall increases only 2-3% per degree of warming, whereas extreme rainfall increases 6-7%," Stott says. The reason again comes from physics. Rain happens when the atmosphere cools enough for water vapor to condense into liquid. "However, because of the increasing amount of greenhouse gases in the troposphere, the radiative cooling is less efficient, as less radiation can escape to space," Stott explains. "Therefore the global precipitation increases less, at about 2-3% per degree of warming." But because of the extra moisture, when precipitation does occur (in both rain and snow), it's more likely to be in bigger events.

Iowa is one of many places that fits the pattern. Takle documented a 3- to 7-fold increase in high rainfall events in the state, including the 500-year Mississippi River flood in 1993, the 2008 Cedar Rapids flood, as well as the 500-year event in 2010 in Ames, which inundated the Hilton Coliseum basketball court in 8 feet (2.5 meters) of water. "We can't say with confidence that the 2010 Ames flood was caused by climate change, but we can say that the dice are loaded to bring more of these events," Takle says.

And more events seem to be in the news every month, from unprecedented floods in Riyadh, Saudi Arabia, to massive snowstorms that crippled the U.S. Northeast in early 2011, to the November 2010 to January 2011 torrents in Australia that flooded an area the size of Germany and France. This "disaster of biblical proportions," as local Australian officials called it, even caused global economic shock waves: The flooding of the country's enormously productive coal mines sent world coal prices soaring.

More stormy weather
More moisture and energy in the atmosphere, along with warmer ocean temperatures also mean more intense hurricanes, many scientists say. In fact, 2010 was the first year in decades in which two simultaneous category 4 hurricanes, Igor and Julia, formed in the Atlantic Ocean. In addition, the changed conditions bring an increased likelihood of more powerful thunderstorms with violent updrafts, like a July 23, 2010, tempest in Vivian, S.D., that produced hailstones that punched softball-size holes through roofs—and created a behemoth ball of ice measured at a U.S. record 8 inches (20 centimeters) in diameter even after it had partially melted. "I've never seen a storm like that before—and hope I'll never go through anything like it," says Les Scott, the Vivian farmer and rancher who found the hailstone.

Warming the planet alters large-scale circulation patterns as well. Scientists know that the sun heats moist air at the equator, causing the air to rise. As it rises, the air cools and sheds most of its moisture as tropical rain. Once 6-10 miles (9.5-16 km) aloft, the now dry air travels toward the poles, descending when it reaches the subtropics, normally at the latitude of the Baja California peninsula. This circulation pattern, known as a Hadley cell, contributes to desertification, trade winds and the jet stream.

On a warmer planet, however, the dry air will travel farther north and south from the equator before it descends, climate models predict, making areas like the U.S. Southwest and the Mediterranean even drier. Such an expanded Hadley cell would also divert storms farther north. Are the models right? Richard Seager of Columbia University's Lamont–Doherty Earth Observatory has been looking for a climate change–induced drying trend in the Southwest, "and there seems to be some tentative evidence that it is beginning to happen," he says. "It gives us confidence in the models." In fact, other studies show that the Hadley cells have not only expanded, they've expanded more than the models predicted.

Such a change in atmospheric circulation could explain both the current 11-year drought in the Southwest and Minnesota's status as the number one U.S. state for tornadoes last year. On October 26, 2010, the Minneapolis area even experienced record low pressure in what Paul Douglas, founder and CEO of WeatherNation in Minnesota, dubbed a "landicane"—a hurricane-like storm that swept across the country. "I thought the windows of my home would blow in," Douglas recalls. "I've chased tornados and flown into hurricanes but never experienced anything like this before." Yet it makes sense in the context of climate change, he adds. "Every day, every week, another piece of the puzzle falls into place," he says. "More extreme weather seems to have become the rule, not just in the U.S. but in Europe and Asia."

The rise of climate attribution
Is humankind really responsible? That's where the burgeoning field of climate attribution, pioneered by Hadley's Peter Stott and other scientists, comes in. The idea is to look for trends in the temperature or precipitation data that provide evidence of overall changes in climate. When those trends exist, it then becomes possible to calculate how much climate change has contributed to extreme events. Or in more technical terms, the probability of a particular temperature or rainfall amount is shaped roughly like a bell curve. A change in climate shifts the whole curve. That, in turn, increases the likelihood of experiencing the more extreme weather at the tail end of the bell curve. Whereas day-to-day weather remains enormously variable, the underlying human-caused shift in climate increases the power and number of the events at the extreme. The National Oceanic and Atmospheric Administration's (NOAA) Deke Arndt puts it more colorfully: "Weather throws the punches, but climate trains the boxer," he says. By charting the overall shift, then, it's possible to calculate the increased chances of extreme events due to global warming.

This idea was already in the air in 2003 when Stott traveled though the worst heat wave in recorded European history on a wedding anniversary trip to Italy and Switzerland. One of the striking consequences he noticed was that the Swiss mountains were missing their usual melodious tinkling of cowbells. "There was no water in the mountains, and the farmers had to take all their cows down in the valley," he says. He decided to see if he could pin part of the blame on climate change after he returned to his office in Exeter, England. "I didn't expect to get a positive result," he says

But he did. In fact, the signal of a warming climate was quite clear in Europe, even using data up to only 2000. In a landmark paper in Nature, Stott and colleagues concluded that the chances of a heat wave like the 2003 event have more than doubled because of climate change. (Scientific American is part of Nature Publishing Group.) Data collected since then show that the odds are at least 4 times higher compared with pre-industrial days. "We are very aware of the risks of misattribution," Stott says. "We don't want to point to specific events and say that they are part of climate change when they really are due to natural variability. But for some events, like the 2003 heat wave, we have the robust evidence to back it up."

Case in point: Hurricane Katrina
Another event with a clear global warming component, says Kevin Trenberth, head of climate analysis at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., was Hurricane Katrina. Trenberth calculated that the combination of overall planetary warming, elevated moisture in the atmosphere, and higher sea-surface temperatures meant that "4 to 6 percent of the precipitation—an extra inch [2.5 centimeters] of rain—in Katrina was due to global warming," he says. "That may not sound like much, but it could be the straw that breaks the camel's back or causes a levee to fail." It was also a very conservative estimate. "The extra heat produced as moisture condenses can invigorate a storm, and at a certain point, the storm just takes off," he says. "That would certainly apply to Nashville." So climate change's contribution to Katrina could have been twice as high as his calculations show, he says. Add in higher winds to the extra energy, and it is easy to see how storms can become more damaging.

This science of attribution is not without controversies. Another case in point: the 2010 Russian heat wave, which wiped out 25% of the nation's wheat crop and darkened the skies of Moscow with smoke from fires. The actual meteorological cause is not in doubt. "There was a blocking of the atmospheric circulation," explains Martin Hoerling, a research meteorologist at the NOAA's Earth System Research Laboratory, also in Boulder. "The jet stream shifted north, bringing a longer period of high pressure and stagnant weather conditions." But what caused the blocking? Hoerling looked for an underlying long-term temperature trend in western Russia that might have increased the odds of a heat wave, as Stott had done for the 2003 European event. He found nothing. "The best explanation is a rogue black swan—something that came out of the blue," he says.

Wrong, retorts NCAR's Trenberth. He sees a clear expansion of the hot, dry Mediterranean climate into western Russia that is consistent with climate change predictions—and that also intensified the Pakistan monsoon. "I completely repudiate Marty—and it doesn't help to have him saying you can't attribute the heat wave to climate change," he says. "What we can say is that, as with Katrina, this would not have happened the same way without global warming."

Yet even this dispute is smaller than it first appears. What is not in doubt is that the Russian heat wave is a portent—a glimpse of the future predicted by climate models. Even Hoerling sees it as a preview of coming natural disasters. By 2080, such events are expected to happen, on average, once every five years, he says: "It's a good wake-up call. This type of phenomenon will become radically more common."

Tomorrow: Part 3: "Our Extreme Future: Predicting and Coping with a Changing Climate."

Reporting for this story was funded by Pew Center on Global Climate Change

ABOUT THE AUTHOR(S)

John Carey is a freelance science writer and editor. For two decades prior to 2010 he was a senior correspondent for Business Week magazine, covering a range of topics including energy and global warming and cholesterol-lowering drugs and the human genome. Previously, he was an editor at The Scientist and a reporter at Newsweek. His stories have won awards from the American Association for the Advancement of Science, the Wistar Institute and a number of other organizations. He was also a National Magazine Award finalist.

http://www.scientificamerican.com/article.cfm?id=global-warming-and-the-science-of-extreme-weather