Seven viewpoints

Introduction

Katharine Hayhoe

Richard Alley

John Nielsen-Gammon

John Williams

Michael Notaro

Kent McGregor

Kevin Trenberth

Drought and searing heat in Texas: Is this the face of global warming?

Courtesy Eric Bruning, Texas Tech University Atmospheric Science

The cold front that blew through Lubbock, Texas on Oct. 17 raised a dust storm not seen since the 1930s Dust Bowl. The dust storm, seen in this movie, is called a “haboob,” an event more common to Saudi Arabia than Texas.

On Oct. 17, a cold front blowing through Lubbock, Tex. raised a red dust cloud that recalled the awesome Dust Bowl of the 1930s, an epoch of drought, enormous dust storms, poverty and social upheaval that depopulated the Great Plains.

The 2011 dust storm served as an exclamation point on a cruel Texan summer, with drought, wildfires, and temperature records that would not quit. On Oct. 19, the Lower Colorado River Authority, source of much water in the Southwest, warned customers that the drought was likely to force another 20 percent cut in water supplies.

On Oct. 18, Texas Lt. Gov. David Dewhurst instructed the state legislature to study drought-related problems like helping homeowners protect against fire, and ensuring that utilities would get enough water to cool their generators.

As far as we could tell, the multi-pronged assignment did not mention something that many observers think contributes to heat waves, fires and droughts: climate change.

Many recent “natural” disasters have raised the same question: Is the no-sense-denying-it-any-longer human-caused planetary warming intensifying devastating hurricanes, giant rainfalls and snowfalls, or the deadly heat waves in Europe (2003) or Russia (2010)?

Despite political skepticism in the United States, the scientific study of changing climates has grown exponentially for 20 years. In 2009, almost 14,000 research reports focused on climate change, and 20 scientific journals are devoted to the issue.

UPDATED NOV. 18: Today, the New York Times reported that a United Nations panel has concluded that “At least some of the weather extremes being seen around the world are consequences of human-induced climate change and can be expected to worsen in coming decades. It is likely that greenhouse gas emissions related to human activity have already led to more record-high temperatures and fewer record lows, as well as to greater coastal flooding and possibly to more extremes of precipitation, the report said.”

Enough introductory blather. Let’s ask some experts: Is the hot, dry weather in Texas a reflection of global warming? Or is it just proof that the essence of weather is its natural variability? The Why Files talked to seven climate scientists. Peruse their viewpoints in the box above.

El Niños, the global cycles of weather that are driven by a hot spot in the tropical Pacific Ocean, have been linked to drought, storms and famine in many parts of the tropics.

Today, a study in Nature finds that deadly conflicts have started twice as often during the el Niño years – but only in the many countries affected by el Niño.

Scientific interest in el Niño mushroomed during the 1980s, when climate experts began to correlate historic cycles of anchovy harvests along the west coast of South America with changes in weather thousands of kilometers distant, and eventually unraveled a planetary cycle driven by the appearance of huge pools of warm water in the western Pacific.

Because the warming seemed to coincide with Christmas, it was called el Niño, for the Christ Child.

El Niño is now recognized as the warm-water segment of the el Niño Southern Oscillation (ENSO), which also includes a cold-water counterpart called la Nina. Now acknowledged as an engine of global climate, el Niño is linked to prolonged droughts, heat waves and crop failures.

Previous efforts to study whether weather and global warming could affect war have related past environmental changes with conflict and the decline of civilizations, says Solomon Hsiang, who completed the new study as a graduate student at the Lamont Doherty Earth Observatory at Columbia University. But the studies tended to be case-by-case, he notes, and “even if every conflict or collapse happened at random, some would occur during a period of environmental change, so this isn’t compelling evidence.”

Looking carefully

To study the issue more systematically, Hsiang and collaborators Mark Cane and Kyle Meng:

The data showed that conflicts are twice as likely to start during an el Niño, says Hsiang, and that 21 percent of overall conflicts can be attributed to el Niño. The increase was only seen in countries strongly affected by el Niño.

Surprisingly, the average changes wrought by an el Niño are quite minor, Hsiang admits – about 0.05°C rise in temperature, and about 0.1 millimeter reduction in daily rainfall.

Small is … powerful?

How could such minor changes affect warfare?

A study that correlates data does not show why they are related, but there are many ways that seemingly small effects could change human behavior, says Hsiang, who is now at Princeton University, especially considering that averages can conceal major alterations in different locations:

However, Marshall Burke, who published an influential 2009 paper ¹ that found a significant increase in warfare during hot weather in sub-Saharan Africa, noted by email that the increase in conflict was seen only inside the el Niño region, and thus, “We might conclude that these global market mechanisms are not at work.”

Still, the new study adds something to the discussion, Burke says. “The [Hsiang] paper’s main innovation is in linking historical changes in the global climate to conflict risk, whereas past studies (including ours in PNAS) looked only at the effect of local weather variations on conflict.”

Burke, a Ph.D. candidate at UC Berkeley department of agricultural and resources economics, added, “They provide very convincing evidence that ENSO-related changes in the global climate are strong drivers of conflict risk in the regions whose weather is affected by ENSO.”

Looking at limits

Mark Cane, a climate scientist at Columbia’s Lamont-Doherty Earth Observatory and a co-author of the new study, said weather does not equal destiny. “No one should take this to say that climate is our fate. Rather, this is compelling evidence that it has a measurable influence on how much people fight overall.”

Ultimately, the motivation for the new study was to peer through the keyhole of time and anticipate a warmed world, Hsiang says, but he admits that the predictive power is limited. “In relationship to global warming, we want to be careful. El Niño is very different … in terms of its spatial pattern, the changes on the ground, and the rate of change. Until we have a much better grasp of these, it’s very hard to take these results and produce any kind of projection for future climate change.”

Still, he adds, “The debate until now has been whether there is any reason to believe that a shift in climate can produce conflict.” Now, “The question is not whether it’s possible, but how much global climate will influence conflict.”

– David J. Tenenbaum

Burning of the Alaskan tundra can release massive amounts of carbon dioxide, the major greenhouse gas, according to a study published in Nature this week. The Arctic is warming faster than the rest of the planet, causing scientists to wonder what will happen to the carbon that plants have stored in Arctic soils and plant matter, both living and dead.

The new study looked at the aftermath of the Anaktuvuk River wildfire, which burned more than 1,000 square kilometers of tundra on Alaska’s North Slope in 2007. Anaktuvuk burned for almost three months, and by itself, accounted for two-thirds of the total area burned in Alaskan tundra since 1950.

The immediate cause was lightning, but weather played a major role. Between July and September, 2007, the North Slope had the hottest weather in a 129-year record. When the fire was really roaring, daily highs were 5°C to 10°C above average. The Slope also received less than 20 percent of the average rainfall that summer, leaving the tundra abnormally arid.

In 2008, Michelle Mack, an associate professor of biology at the University of Florida and her colleagues visited the area and took samples from 1-square-meter quadrants both inside and outside the fire zone. Mack was in the field in Alaska, alas, and did not answer our emails, but her group calculated that the fire oxidized more than 2 million tons of carbon, which entered the atmosphere as carbon dioxide.

Accounting for carbon

The movement of carbon through soils, ecosystems, waters and the atmosphere is critical to the issue of global warming. Releasing carbon to the atmosphere as carbon dioxide speeds warming; and storing carbon compounds can slow or potentially reverse warming.

The moist acidic tundra under study covers as much as one-third of a billion square kilometers of the global Arctic – making it a major “sink” for carbon dioxide. The 2 million-ton release of carbon was equal to at least 50 percent of the amount of carbon stored annually in the Alaskan tundra, meaning this one fire almost cancelled the anti-warming benefit of photosynthesis in the region.

Chilling news about a burning issue

The link between global warming and fire also appeared in a new analysis of Yellowstone National Park. “Large, severe fires are normal for this ecosystem,” said Monica Turner, a Yellowstone expert and professor of ecology at the University of Wisconsin-Madison. Historically, the entire Yellowstone landscape has burned every 100 to 300 years, but Turner and company calculated that current trends toward hotter, drier summers, mean fires could consume the entire area every 30 years by 2050.

Wildfires are also becoming more common in the normally fire-resistant tundra of Alaska, and for reasons related to permafrost, reflectivity and feedback, the consequences could be dire:

And feedback moves us from the additive realm to the multiplicative one. In the Arctic, feedback also plays a central role related to the release of methane, which has even more warming potential than carbon dioxide. Many warming Arctic habitats have started releasing larger amounts of methane, which could warm the planet, feed back, and stimulate the release of yet more methane.

This feedback, like the one that may be affecting burning tundra, paints a darker picture of what could happen if we ignore the atmosphere and blithely assume that the future will be just like the present.

– David J. Tenenbaum

Uganda is looking for answers as about 20 students and a teacher were killed June 28 by lightning that struck their school in this highland nation in Eastern Africa. With dozens of children also injured by electricity, Ugandans wonder if the serious string of lightning strikes is related to climate changes, or are just the consequence of an unusually heavy stream of moist air coming from the Atlantic.

We can’t answer, but the tragedy did get us Why Filers to thinking about lightning. Although lightning bolts killed “only” an average of 39 Americans over a recent 10-year stretch, the injuries, which concentrate on the vulnerable nervous system, can be severe and lifelong.

Satellites tell us that 1.2 billion lightning flashes occur in the atmosphere each year — although not all reach Earth.

What is lightning? How does it injure and kill? And what has been learned in the past few years from the millions spent studying nature’s electricity?

2003, NASA Johnson Space Center

A string of lightning flashes are seen from space.

Boom-boom room

Thunder — the cracking or rumbling you often hear — is caused by thermal expansion and contraction. Lightning bolts can get far hotter than the sun’s surface — up to 20,000° Celsius. That heats the air, causing it to expand, and starting a shock wave that moves as sound waves — thunder.

If you’re close to the lightning bolt, you’ll hear a cracking; further away, you’ll hear rumbling because that sound has come from several parts of the bolt, and been reflected from buildings and hills.

And yes, if you start counting “one Mississippi,” when you see the flash, you can estimate the distance to the bolt: Light essentially reaches you instantly, but sound takes about five seconds to travel one mile. Divide the number of seconds by five to find miles, or by three for kilometers.

Silence is — mysterious

One of the many lightning mysteries is this: Sometimes you hear the thunder, and sometimes you don’t. For example, “heat lightning” is an eerie, silent flash that often lights clouds in thunderstorms.

The sound has been gobbled by an audio version of the visual mirages that cause trekkers to see water in stone-dry desert. These visual mirages are caused by heat that bends light waves. You look straight ahead, but you actually see the sky, shimmering like a tempting lake.

Similarly, in a thunderstorm, the sharp boundaries between warm and cool air can channel sound waves away from the observer, as you can see from the nifty applet, below.

Much the same phenomenon was noticed during the Civil War, when artillery was visible in the distance but audible only in some parts of the battlefield.

Go play with lightning.

Nature’s lighting foundry

We think of clouds as billowy places, couches for angels in Renaissance paintings. In thunderclouds, however, air and water – liquid, frozen and in between — may be whizzing up and down at a furious clip — up to 100 miles an hour.

That’s a place where angels fear to tread.

The motion in these cumulonimbus clouds is powered by convection, a force that separates fluids based on density. The dense, cold air falls while the warmer air rises. Smaller water droplets hitchhike up on the updrafts, which can’t support the larger droplets.

Because smaller particles tend to carry positive charges, the movement caused by temperature, humidity and density (which can include snow, ice, and water vapor) segregates electrical charges: The top of a cloud becomes positive and the bottom negative.

Regions of different charge can only exist if surrounded by an insulator — namely air. Insulators, however, eventually fail when they are overwhelmed by electric “pressure.” In a thunderstorm, that “failure” results in lightning.

Hangin’-motor blues

Having trouble envisioning this? Imagine a chain holding a greasy V-8 motor above a ’63 Ford Fairlane in a shade-tree auto mechanic’s backyard. If the engine is too heavy, or the chain too weak, the chain will snap as it is overwhelmed by the gravitational attraction between Earth and engine.

Thunk!

Substitute air’s insulation for the chain, and electrical attraction between positive and negative charges for gravity, and you have a greasy-fingered picture of how air can separate electrical charges during a thunderstorm.

To go further, we need one hunk of physical-science jargon: electrical potential is how fast charge changes with distance, and it’s measured in volts per meter. Electrical potential is the “pressure” that’s “trying” to start an electric current between areas of opposite charge.

(Opposite electrical charges are like young lovers: They will do anything to get together.)

Just as an overweight V-8 can snap a skimpy chain, excess electrical potential can “break” air’s insulation. When that happens, an electrical current — in the form of a lightning bolt — neutralizes the opposing charges.

Flash!

Diagram: Encyclopædia Britannica, Inc.

Lightning leaps between separate negative and positive regions during a storm. Most cloud-to-ground flashes originate in the cloud’s negative regions.

In a cloud-to-ground flash, the huge electrical potential — measured in millions of volts — eventually overcomes air’s electrical resistance, and a “streamer” or “leader” begins reaching, about 50 meters at a time, toward ground. The streamer makes an ionized (conducting) pathway of plasma, allowing current to flow.

Where am I safe?

As the current approaches the ground, its electrical potential can cause a surge of oppositely-charged particles to “reach” up toward it. Because this upward current often springs from tall objects, trees and other tall objects make lousy shelter during a storm.

For a 2001 Why File on lightning, David Rust, who was then director of forecast research and development at the National Severe Storms Laboratory, told us that the safety of a building is determined by the degree of grounding. A steel building that’s securely grounded, he said, will be safer than a wooden one that’s not, even if the steel building is taller. Steel and other conductive metals provide an easy pathway to ground for the lightning, and that translates into safety.

Once the ionized pathway is established, electric currents flow back and forth between ground and cloud so quickly that they appear as flickers rather than separate bolts. (More on lightning safety.)

We’ve heard that a big cloud-to-ground bolt carries one trillion watts of electricity. If that estimate is right, during its fraction-of-a-millisecond life, the flash carries about the same current as the total U.S. generating capability. (Watts measure the flow of electric current at any instant. The more familiar watt-hours measures an hour of flow of a given current; 1 kilowatt hour equals 1,000 watt hours.)

But nobody has figured out how to put this energy to work. Though we have heard one proposal, the currents are insanely high and the strikes are too brief and too unpredictable.

Keeping a close watch on lightning

Our understanding of lightning grows with improvements in technology, and a new instrument on trusty weather balloons has pointed to a surprising source for the electric charge. The process involves a small, spongy relative of hail called graupel, says Don MacGorman a physicist at NOAA’s National Severe Storms Laboratory.

“As graupel accumulates tiny, pristine ice particles, and then falls through liquid water, there can be some charge exchange in collisions where the tiny ice particles rebound,” MacGorman says. In the lab, this interaction seems powerful enough to be main source of electricity – and therefore lightning — in large areas of the storm.

Within a few years, a better understanding of lightning formation could improve predictions, MacGorman says. “We will not be able to say lightning will a hit particular location. Lightning is too random for that, but we are getting to the place where it may be possible to say that a storm will produce a little or lot of lightning, and that would be helpful for storm safety.”

Cloudy picture

The graupel explanation, however, raises a question: If the interaction of water and ice creates the electric charge, why is lightning found in dry sectors of the storm, including the large “anvil” structure that exhausts cold, dry air above the storm? “We have seen lightning initiated almost 100 kilometers from the heavy precipitation area, so something else must be going on in the anvil,” says MacGorman. “This does not accord with how we’d viewed anvils.”

Scientists are also probing cloud flashes, caused by the flow of current between regions of clouds with opposite charges and does not hit the ground. Formerly dissed because they don’t kill people, cloud flashes are getting some respect.

For one thing, they are the most common type of lightning, accounting for perhaps one-quarter of all lightning flashes. Adding cloud-to-ground and cloud-to-cloud lightning gives a better indicator of total storm intensity than ground flashes alone, “which have very little relationship to storm severity,” says MacGorman. “You can have huge ground flashes in a relatively innocuous storm, but total lightning is well related to things that affect severity and strength: the size of the updraft, the amount of ice in the clouds, and so it gives us clues as to how intense the storm is.”

Positively speaking

The biggest recent discovery on lightning, says MacGorman, concerns storms that produce a large amount of positively charged cloud-to-ground lightning rather than the usual negative currents. During a field research program called STEPS, in a lightning-rich region of the high plains, some storms contained negative charges in places that normally would be positive, and vice versa. In these conditions, instead of dropping the normal negative charge to the ground, the lightning bolts were positive.

The unusual phenomenon could arise in clouds containing a high concentration of liquid water, MacGorman says, and that would also raise the odds of large hail. “Hail typically forms because graupel or another seed particle starts collecting liquid water faster than it can freeze, and the water spreads over the surface, then freezes into a solid layer of ice.”

These dense particles are more likely to happen in an area with a lot of liquid water, and therefore, these positive lightning strikes could be a harbinger of large, destructive, hail.

The view from on high

For the next stage in lightning observations, scientists will go to GOES-R, a series of geostationary satellites scheduled for launch in 2015. These high-orbital spyglasses will carry an optical gadget that should “see” upwards of 90 percent of total lightning activity. “The viewing area will cover pretty much all of the continental United States, and parts of Africa and South America, and eventually, half of the Pacific Ocean,” says MacGorman. “This will allow us to detect thunderstorms over the oceans, which we have not had good way to see in the past.”

That should help airplanes dodge storms, but also aid weather prediction, MacGorman says, since thunderstorms can trigger other thunderstorms. They also add water vapor to the lower atmosphere, which also feeds storms.

Nothing light about lightning

Lightning gathers myths. Whether it’s Zeus throwing thunderbolts from the ancient Greek sky, or the moronic misconception that victims become untouchables because they retain an electric charge, these bolts spark the imagination.

But lightning can change your life, as Steven Marshburn, Sr., of Jacksonville, N.C., told us in 2001. Marshburn was struck in 1969 while working in a bank. Although the sky was blue and no storm was in sight, a bolt entered through a wire from the drive-up window.

Afterwards, Marshburn “suffered from severe headaches, chronic daily pain, grand mal [epileptic] seizures, dizziness, problems with my eyes going blurry. Many health problems persist. I have had 20 lightning-related surgeries…”

In 1989, in response to his brush with death, he formed Lightning Strike & Electric Shock Survivors International to investigate the medical aspects of lightning and to support victims and families. In 2001, he told us that members had talked 13 fellow survivors out of suicide.

A shock to the nervous system

Lightning usually kills by attacking the heart, which runs on electrical impulses. While high-voltage electrical injuries often cause severe burns, they are rare with lightning, likely because the bolts — lasting only 0.1 to 1 millisecond –- are too brief to cause severe burns.

Although burns may result if clothing ignites or sweat boils and steam is trapped under clothing, wet, sweaty clothing may actually conduct a heavy current outside the body and reduce the damage.

Raphael Lee, a professor of surgery and medicine at the University of Chicago, and an expert on the effects of lightning strike, told us that most of the initial current in a lightning strike does not pass through the body. However, two electromagnetic phenomena can produce a strong voltage drop across the body:

Strong electric fields are a problem for nerves and muscles, Lee says, because they “have been structured through evolution to be very sensitive to tiny electric fields.” That, combined with their physical length, which spans a large electrical gradient, “makes them very sensitive to lightning.”

Nerve cells can be a meter long, and by extending into different parts of an electric field, they are exposed to high voltages, Lee says. One focus of concern is the cell membrane which can die if strong voltages poke holes in it. Voltage can also wreak havoc in the pores in the membrane, which regulate the cell’s physiology by controlling how ions enter and leave the cell. Normally, for example, the potassium concentration is 1,000 times higher inside a cell, and damage to the pores can result in malfunction or cell death.

Lightning = thunder in the brain?

Although electricity is the natural focus of lightning damage, Lee suspects that an acoustic pulse, or shock wave, plays a major role, and perhaps a dominant one. A lightning bolt is surrounded by hot, ionized gas that arises in nanoseconds or microseconds and whose temperature may exceed 10,000 ° C. “When you heat something in a small area in such a short period, there are going to be shock waves,” he says.

The power of this acoustic wave is obvious when lightning hits and splits a tree, Lee adds. But inside the brain, the shock can trigger traumatic injuries similar to those caused by a roadside bomb or artillery shell.

Neurological injury: no passing matter

Lightning injury can be severe, long-lasting, and hard to treat, and it “may affect any or all parts of the nervous system,” according to Mary Ann Cooper, an emerita professor of emergency medicine at the University of Illinois-Chicago.

In a 2009 study of survivors of lightning and other electric shocks, 78 percent of the survivors had at least one psychiatric diagnosis; many of the troubles related to learning, memory and executive function.

In 2001, Cooper told The Why Files that confusion, caused by slowed information processing, is a hallmark of lightning injury. Symptoms include “difficulty in short-term memory, coding new information and accessing old information, multitasking, distractibility, irritability and personality change.”

Damage to the frontal lobe, the site of much higher thinking, is common, according to Cooper. “Many suffer personality changes because of frontal lobe damage and become quite irritable and easy to anger. The person who ‘wakes up’ after the injury often does not have the ability to express what is wrong with them…and cannot carry on a conversation, work at their previous job, or do the same activities that they used to handle. As a result, many self-isolate, withdrawing from church, friends, family and other activities.”

Cooper said some cell types continue suffering for weeks after the injury, and that nerve cells seem to “spend a long period trying to heal themselves, until finally the cell body is exhausted” and the cell dies. That process accounts for a delayed disability syndrome among survivors.

Help at hand?

Long-term neurological consequences are a major research area, Lee says, because they also occur in traumatic brain injury. “People are trying to sort out what is the best treatment, and understand why some people are more susceptible to delayed neurological problems. The body is very complicated and … the weight of evidence suggests there are genetic predispositions to complications after a blast causes traumatic injury to the brain, and lightning injury may be no different. Many people recover, but some don’t. What is different about the people who don’t?”

– David J. Tenenbaum

The death toll from the May 22, 2011 tornado in Joplin – now 122 — is the latest tragedy of a horrific year for tornadoes. On April 27, twisters in Alabama and nearby states killed 314, the fourth highest in U.S. history. The 480 deaths in 2011 are already the highest number since 1953, and tornado season continues through mid-August.

The Why Files asked Jonathan Martin, an expert on the large atmospheric disturbances that form tornadoes, some questions about tornado prediction. We edited the answers of Martin, a professor of atmospheric and oceanic sciences at the University of Wisconsin-Madison, after the interview.

The Why Files: What must we know to make a good tornado prediction?

The Why Files: Are predictions getting more accurate?

The Why Files: Why so much damage and death this year? Is this a result of climate change?

Skeptical about global warming? 2010 has just tied 2005, making these the two hottest years on record. And nine of the 10 warmest years on record have occurred since 2001.

Photo: williumbillium

Was New York’s epic blizzard last month related to climate change?

But temperature is only part of the story. After a year that saw epic floods in Pakistan and California, massive floods have swamped Brisbane, Australia, population 2 million. Russia was toasted by a record heat wave last summer. Europe and, of course, New York were smothered by giant snowstorms.

And we just read that 2010 had the heaviest precipitation on records that date to 1880.

So we have to ask: Is this normal weather, or is this climate change in action?

And as greenhouse gases continue to accumulate in the atmosphere, what will happen the day after tomorrow?

There is good theoretical reason to think that an accelerating greenhouse effect will affect weather: Add greenhouse gases like carbon dioxide and methane to the atmosphere, and they trap more heat. In hotter conditions, more water evaporates from the ocean, which eventually falls as precipitation. Heat is energy, and more energy in the ocean and atmosphere provides more power to drive intense storms.

These questions are devilishly difficult to answer. It’s a big planet, and assessing conditions during the past few decades, and making projections for the future, is a gnarly task. Climate models are better at getting the big picture than making regional forecasts for future weather. Data records are incomplete, especially as we delve further in the past.

Graph: Progress Report of the Interagency Climate Change Adaptation Task Force: Recommended Actions in Support of a National Climate Change Adaptation Strategy, October 5, 2010

If you doubt that warming temperatures have anything to do with carbon dioxide, the primary greenhouse gas, here’s something to think about. Horizontal divider shows average temperatures, 1901-2000.

Nevertheless, let’s ask our question about both recent weather data and future forecasts.

Record temperatures

As the climate warms, one easy prediction is that record warm days will become more common, and record colds will be less common. When Gerald Meehl, a senior scientist at the National Center for Atmospheric Research, compared the number of record daily highs to the number of record daily lows in the U.S., he found they were roughly equal in the 1950s.
Today, he says, “for every two record highs, there is only one record low. If there was no warming going on, the ratio would be one to one, so we are shifting the odds toward having a better chance for setting a record high versus a record low.”

Meehl says Australian data show the same thing.

Even though the climate has warmed by only about 0.6° C, he says, “This shows that even with a very small change in average temperature, about 1° Fahrenheit, we can get a pretty noticeable change in the extremes.”

Click here to view the embedded video.

At some point, we may look fondly upon today’s two-to-one ratio, as climate models suggest the ratio will reach 20 to 1 by year 2050 and 50 to 1 in 2100. Yet even then, when the U.S. average temperature may have risen by several degrees C, “We still get some daily record low temperatures,” Meehl says. “We still get extremely cold weather, although it will happen much less frequently.”

Today, he notes, “When there’s a cold snap, people ask, ‘What happened to global warming?’ But even with warming, it will still get cold, but not extremely cold, and not as often.”

Precipitous rise in precipitation?

Rain and snow are two ways that the atmosphere feeds life on the planet. A hotter atmosphere has the ability to hold more moisture because more water evaporates from the ocean, and warmer air can also store more moisture.

Already, says Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research, the water-vapor contained in an imaginary cylinder stretching from Earth to space has been rising 1.3 percent per decade since the 1970s.

And so warming means more potential for precipitation.

“When we review change in the hydrological cycle,” Trenberth says, “not just tropical cyclones [hurricanes and typhoons] but extra-tropical cyclones and individual thunderstorms, the evidence from around the world is that when it rains, it rains harder, when it snows, it snows harder. This is consistent with the understanding we have, the theory.”

That is also happening in the United States, where days with intense rain and snow have been increasing, says Meehl. “When it rains, it pours, we see this in observations, and models show an increase in the future.” For example, a summary published in 2007¹ found that, “Over the last century there was a 50% increase in the frequency of days with precipitation over 101.6 mm (four inches) in the upper Midwestern U.S.”

However, land use plays a role in some observed precipitation changes, says James O’Brien, emeritus professor of meteorology and oceanography at Florida State University. “We studied heavy rainfall over 62 years in Orlando, Fla., and did a simple thing: We divided the time into two periods of 32 years each, and looked at the probability of one or more two-inch rainfalls.”

In the recent period, during almost all non-summer months, Orlando had a big increase in heavy rain, but Gainesville, 40 miles away, did not. “The cause in Orlando is absolutely clear,” says O’Brien. “It’s Disney World. It’s all the roads, the concrete, which act as a heat sink. In winter, a cold fronts hits a bubble of heat caused by this heat island, and it kicks up a storm and you get more rain.”

Heavy rain = heavy drought?

Even if total precipitation does not change, there are consequences to the newer “when-it-rains-it-pours” precip pattern. Heavy rain runs off rather than percolating into the soil, so instead of feeding plants, it can cause soil erosion and floods. If, as some models suggest, extreme precipitation increases in springtime, when the ground is still frozen, “that has a significantly different impact than extreme rainfall during summer,” says Daniel Vimont, an assistant professor at the Center for Climatic Research at the University of Wisconsin-Madison, because the rain cannot enter the soil and must run off.

Heavy rain can also contribute to drought by drying the atmosphere, Meehl says. “We have to take into account the number of days between precipitation events. On a map of North America, almost everywhere intensity shows an increase to date, and a projected increase, but we also see dry days increasing, like in the southern tier of states and especially the Southwest. When it rains, it rains really hard, but there are more days between rainfalls. On average, you are getting less total precipitation, but the risk for floods has increased because of this intensity increase. Over long periods, we are seeing drier conditions, because the number of days between events is also increased.”

Facing a wave of drought

A trend toward drought is already under way, according to a 2004 study by Aiguo Dai of the National Center for Atmospheric Research, which found that the percentage of Earth’s land area stricken by serious drought had more than doubled between the 1970s and the early 2000s.

The future seems no more benign. Last October, Dai published a review, based on 22 computer climate models, that projected a major expansion of drought over the next 30 years. The affected area includes the breadbasket regions of North and South America, most of Africa and Australia, and parts of China and neighboring countries.

According to the study, the western two-thirds of the United States will be significantly drier in the 2030s, after which matters will only get worse.

In general, the only places that will see more precipitation are in the extreme north — Northern Russia, Scandinavia, Canada and Alaska.

So reindeer need raincoats…

But seriously, “We are facing the possibility of widespread drought in the coming decades, but this has yet to be fully recognized by both the public and the climate change research community,” Dai says. “If the projections in this study come even close to being realized, the consequences for society worldwide will be enormous.”

Cyclones, typhoons and hurricanes

In terms of extreme weather, nothing beats the tropical storms variously called typhoons, tropical cyclones or hurricanes — for their winds, high seas and astonishing rainfalls. So hurricanes are the natural focus of study on the past and future effects of global warming.

In 2005, Hurricane Katrina played the starring role in a series of powerful hurricanes that pounded the Gulf of Mexico and Caribbean, and we reported that hurricanes were packing more power in a warmed planet.

Then came a counter-rebellion: scientists began questioning whether hurricanes were really more powerful, and noted that they were not getting more common (although everybody agrees that increasing population and development along the coasts both contribute to greater storm damage).

The chief hindrances to finding real trends in the tropical cyclones are their long-term, natural variation in strength and frequency, and the wobbly nature of data on older cyclones. In the North Atlantic, home of the best hurricane data, the quality of the data jumped when airplanes began flying into hurricanes in 1944, and again when satellite tracking began around 1970. Data on older Pacific and Indian Ocean storms are even more questionable.

To explore how global warming will affect tropical cyclones, the World Meteorological Organization set up a team under the leadership of Thomas Knutson, of the Geophysical Fluid Dynamics Laboratory. Knutson’s group projected that hurricanes, globally, will become 6 percent to 34 percent less common by 2100, despite the warming trends².

The counterintuitive reduction may be due to wind. These storms need a warm ocean to provide energy, “but you also need an atmosphere that cooperates,” explains Charles Conrad, an associate professor of geography at the University of North Carolina and director of the Southeast Regional Climate Center. Wind shear, a change in wind velocity with altitude, can blow a developing storm apart. “Some global climate models suggest that more wind shear over the tropical and sub-tropical Atlantic may inhibit cyclones, so when you put that together with higher sea-surface temperatures, this suggests that when a system can develop, it will be stronger.”

A question of intensity

Given the rickety data on older storms, Knutson’s group concluded that “it remains uncertain whether past changes in tropical cyclone activity have exceeded the variability expected from natural causes.” According to team member Christopher Landsea, science and operations officer at the National Hurricane Center, “Every single paper in the peer reviewed literature, looking at the theoretical side of hurricanes and global warming, or the climate model simulations, says the same thing. The changes today are very, very tiny, maybe 1 percent stronger, due to manmade global warming.”

But another member of the team begs to disagree. “I think the evidence is fairly unequivocal that there has been an increase in intensity,” says Kerry Emanuel, professor of tropical meteorology and climate at Massachusetts Institute of Technology. To gauge intensity, Emanuel used wind speed, measured at six-hour intervals, to calculate a “power dissipation index,” fancy lingo for the amount of energy that enters the hurricane.

Graph: Weather and Climate Extremes in a Changing Climate³

Heat energy from the ocean powers hurricanes, and storm intensity closely follows changes in sea surface temperature in the North Atlantic. “Power dissipation” is a measure of the storm’s total power, based on a cube of maximum wind speed.

The index, he says, shows that recent hurricane intensity is “beautifully correlated with ocean temperature in the tropics,” and those warm seas, in turn, result from accelerating greenhouse warming. Changing levels of greenhouse gases and reflective aerosols in the atmosphere “are the cleanest explanation for what happened with hurricanes,” Emanuel says. “I think there is a strong [human-caused] signal in Atlantic hurricanes over the last 40 years.”

Tower of power

And what of the future? The Knutson team projected that average maximum winds would increase 2 percent to 11 percent by 2100, so “a substantial increase in the frequency of the most intense storms is more likely than not globally, although this may not occur in all tropical regions.”

Although the group wrote that intense tropical cyclones, “deserve particular attention, as these storms historically have accounted for an estimated 85 percent of U.S. hurricane damage,” Landsea said, “That’s a very small increase, a long ways in the future,” and it could be offset by a decreasing frequency of storms.

In the world of climate, it’s usually possible to find another voice, and last year, a modeling study⁴ projected that the number of category 4 and 5 storms will almost double by 2100. (Category 5 includes the strongest hurricanes.)

We asked James Kossin, a scientist with the National Oceanic and Atmospheric Administration, who has studied hurricanes since 1987, about those results, and he told us, “There is a lot of uncertainty in our understanding of how tropical cyclones respond to their environment and to changes in their environment.”

Linking changes in hurricanes to human-caused climate changes is “very challenging,” said Kossin. “I have medium confidence that climate change could lead to the strongest storms getting stronger” globally.

Emanuel, however, says the creators of these models “freely admit they will not model intense hurricanes, they don’t have the resolution. What does a 2 percent to 11 percent increase mean if the models are constitutionally incapable of having hurricanes? And this is what the models are telling us, but what does nature say? It tells us that hurricanes intensity is changing much more rapidly.”

Emanuel reminds us that storm destruction equates to at least the cube of wind speed, and therefore, a small increase in maximum wind can mask a much larger increase in intensity and damage.

From here, gentle reader, the arguments devolve from murky to truly obscure. We promise to report back in a few years, but we’re happy to note that this dispute, however contentious, is being fought in print by civil scientists who can cooperatively ponder on our climatic future.

Easy questions can be tough to answer

We’d love to know if warming is affecting wind, but the records do not support such a comparison, says Dan Vimont. In a study on climate change in Wisconsin, for example, “We started to look at wind, but there is not as much observational data. There are 200-odd temperature-precipitation gauges around Wisconsin reporting daily, but … it’s difficult to find a continuous record from a gauge that is monitored well.”

The reality is that as much as we’d like to attribute particular events like the floods in Pakistan and Australia to climate change, we may never know. “For any given event, it’s really hard to gauge how much climate change has contributed,” says Claudia Tebaldi, a climate statistician with the non-profit Climate Central. “Even for heat waves, where it’s obvious that as climate warms you would expect more intense heat waves, [you have to acknowledge that] a given heat wave may have happened anyway without climate change.”