A Delicate Balance: Signs of Change in the Tropics
 

Climatologists working under the stark fluorescent lighting and controlled conditions of a laboratory rarely have the luxury of directly studying Mother Nature. They cannot bring a slice of El Niño back to the bench to observe its movements. Nor can they encase a natural ecosystem in Pyrex and heat it up to see what happens. Climatologists must study firsthand the ongoing changes in the Earth’s climate system in all its chaos and mystery.

 

Page 2

Arbiters of Energy

Part 1: The Iris Hypothesis
Part 2: A Delicate Balance

  Photograph of transparent cirrus clouds and opaque cumulus clouds

Possibly no part of the climate system is more unpredictable than the interaction between atmospheric water and radiation emitted by the Sun and the Earth. At any given time, a small sea of water floats above our heads in the form of clouds, ice, and water vapor. The form and position the water takes in the sky changes the way it interacts with solar and thermal radiation. Clouds, especially low-lying, thick clouds, reflect an enormous amount of sunlight back into space and keep it from overheating the Earth. High-flying, wispy clouds and water vapor absorb greater amounts of outgoing thermal (heat) radiation, which is generated by the surface of the Earth after it is warmed by the Sun. Along with greenhouse gases, such clouds and water vapor contribute to keeping the average temperature of the Earth’s surface from dropping to Arctic levels year round.

 

High, thin cirrus clouds are almost transparent, in contrast to thicker cumulus clouds, which are opaque to sunlight. Changes in the relative amounts of these clouds affect the amount of energy from the Sun that is absorbed by the Earth. (Photograph copyright Reto Stöckli)

 

 
Space Shuttle Photographs of Cirrus and Cumulus Clouds
 

 

Due to its delicate interplay with energy, atmospheric water has tremendous potential to impact the Earth’s climate and temperature. For years, researchers have been attempting to pin down the complex interactions involved. Specifically, they’d like to know in what way clouds and water vapor might shift if the surface of our planet were to warm further due to elevated levels of human-generated greenhouse gases. In attempts to forecast possible future scenarios, scientists have created numerous computer models to simulate the interactions between clouds and radiation. Particular attention has been given to the tropics because more sunlight hits these latitudes than anywhere else. Resulting theories and model results have run the gamut. Some researchers profess that in the future, thick, low-lying clouds will decrease and high-flying cirrus clouds will increase, making global warming much worse. Others espouse theories, such as the Iris Hypothesis, which state that as the Earth’s surface heats up, cirrus clouds will dissipate and allow more thermal energy to vent into space, countering the effects of global warming.

 

Seen from space, the difference between cumulus and cirrus clouds is striking. On the left are the bright white cumulus towers of thunderstorms. On the right, thin cirrus clouds are partially transparent. (Space Shuttle Photographs STS084-706-69 and STS032-88-69 courtesy NASA JSC Gateway to Astronaut Photography of the Earth)

  Diagram of the Effects of Cumulus and Cirrus Clouds on the Earth's Energy Balance
 

The problem, however, is that relatively little real-world data have been collected to test these theories or even to build exacting computer models. Since clouds are constantly shifting, separating, growing, and shrinking, the only way to study them and their effects on radiation has been through the use of remote sensing instruments and weather satellites. Such instruments have been around for about 25 years. Only recently have scientists acquired enough data to put together any meaningful conclusions.

Last year, a group of researchers led by Bruce Wielicki and Tak Wong, climate scientists at NASA’s Langley Research Center, compiled a couple decades worth of radiation data from the tropics. While the researchers approached the data with the idea of testing existing notions, they witnessed a phenomenon no one expected. Over the past 15 years, progressively more thermal radiation has been escaping the atmosphere above the tropics and progressively less sunlight has been reflecting off of the clouds. Though researchers now believe that wind currents throughout the tropics have been fluctuating and altering clouds and radiation patterns, the discovery has brought many theories and climate models into question.

next Venting Unexpectedly

The data used in this study are available in one or more of NASA's Earth Science Data Centers.

 

The Earth’s energy balance refers to the amount of energy received from the sun (yellow arrows) minus the energy reflected and emitted from the Earth (red arrows). Clouds play an important role in regulating this balance. Thin cirrus clouds permit sunlight to pass through them, while blocking a significant amount of the heat radiating from the surface. Thick cumulus clouds reflect most sunlight, and block the majority of heat radiating from the surface. (Illustration by Robert Simmon, Earth Observatory)

 

Venting Unexpectedly

   
 

Wielicki says that he and his team collected and merged data over a 22-year period from a number of satellites that measure both solar and thermal radiation reflected by and emitted from the atmosphere over the tropics. They brought together older data from Nimbus 7, which was one of the earliest satellites to measure radiation over the Earth, and the Earth Radiation Budget Satellite (ERBS), which has kept a continuous record of the Earth’s radiation since 1985. New data sets were also used from NASA’s Clouds and the Earth’s Radiant Energy System (CERES) instruments that fly aboard the Tropical Rainfall Measuring Mission (TRMM) as well as the newer Terra satellite (Wielicki et al. 2002).

“What we found was a 4-watt-per-square-meter change within the climate system that the climate models did not predict,” says Wielicki. Over the last 15 years, without anyone’s knowledge, the amount of thermal, long-wave radiation escaping the atmosphere above the tropics increased by 4 watts per square meter. At the same time the amount of reflected sunlight, which is mostly in the form of short-wave visible and near-visible light, decreased by 4 watts per square meter. The change appears to have occurred gradually over the past decade and a half and as such was likely completely independent from El Niño (Wielicki et al. 2002). Though 4 watts of energy is only a fraction of the 342 watts per square meter of solar energy that hits the Earth’s outer atmosphere, the Earth’s energy budget is usually extremely stable over the long term, and changes of more than a couple of watts are significant.
 

  Page 1Page 3

At first, Wielicki and the research community did not quite know how to interpret the results. They suspected that cloud cover must have been changing over large regions of the tropics (Wielicki et al. 2002). Fewer high, thin clouds, for instance, would likely result in more thermal energy leaving the Earth, and fewer low-lying, thick clouds would result in more solar energy reaching the oceans. In order to verify their suspicions, the scientists would need proof. “But observing the obvious cloud patterns spatially across the tropics couldn’t give us these long-term changes in long-wave (thermal) radiation,” says Wielicki.
 

 

Satellite-borne instruments have measured energy fluxes at the top of the atmosphere for more than 15 years over the tropics (from 20° North to 20° South). The top two graphs show monthly changes in the amount of heat emitted and sunlight reflected back to space, from 1985 through 1999, relative to the long-term average (gray bar). The graphs reveal that increasing amounts of heat energy escaped to space in the 1990s as the Earth was absorbing more sunlight. The bottom graph shows changes in the Earth’s total net radiation budget during this same period—notice the erratic up and down pattern in the 1990s as compared to the relative stability of the net energy fluxes through the 1980s. (Graphs by Robert Simmon, based on data from the CERES Science Team, NASA LRC)

 

Maps of Reflected Shortwave and Emitted Longwave Radiation

He explains that the relatively short-term El Niño and La Niña cycles that occur every three to seven years dominate cloud patterns across the tropics. During a typical El Niño year, the trade winds that travel from east to west across the Pacific slow down or even reverse during the winter months. Without these winds pushing surface waters west, warm water from the western Pacific moves east. Since the warm water generates rising air and thus clouds, the clouds shift eastward en masse with the water. During a La Niña, the opposite phenomenon occurs. Trade winds speed up and the warm waters and clouds bunch up in the western Pacific and Indian oceans during the winter. These massive swings in water temperature and air pressure in the Pacific impact the tropical latitudes around the globe, causing cloud cover to shift in over the Atlantic Ocean, the Indian Ocean, and the continents as well. Though this short-term reshuffling of cloud cover does not result in a net difference in the amount of clouds or outgoing thermal radiation over the tropics as a whole, it obscures any subtle, long-term fluctuation. Attempting to locate a 4-watt-per-square-meter difference in cloud cover over 15 years across the tropics would be akin to listening for a gradual build up at a violin concerto as a foghorn intermittently blasts away.

next Winds of Change
back A Delicate Balance

 

Clouds and the Earth’s Radiant Energy System (CERES) measurements show the reflected solar radiation (left) and emitted heat radiation (right) for January 1, 2002. In both images, the lightest areas represent thick clouds, which both reflect radiation from the Sun and block heat rising from the Earth’s surface. Notice the clouds above the western Pacific Ocean, where there is strong upwelling of air, and the relative lack of clouds north and south of the equator. The animations, created from daily data, show how rapidly these measurements change. (Image and animation by Robert Simmon and Reto Stöckli, based on data from the CERES Science Team, NASA LaRC)

animations:
  small (1.4 MB Quicktime)
  large (6.8 MB Quicktime)

 

Winds of Change

   
 

A solution finally came from Junye Chen, a graduate student working at NASA’s Goddard Institute for Space Studies in New York under the advisement of Anthony Del Genio and Barbara Carson. Chen realized that changes in large-scale atmospheric circulation had the potential to alter cloud cover and radiation over the entire tropical region. More specifically, air currents that move vertically up and down over the tropics may have fluctuated gradually over the past 15 years, leading to less water vapor and clouds over the tropics and altering outgoing thermal radiation (Chen et al. 2002).

  Page 2Page 4
 

Diagram of the Hadley-Walker Circulation

 

In the tropics, there are two big circulation patterns that control most of the airflow through the troposphere like giant conveyor belts. The Walker circulation transports air east and west along both sides of the equator. Air currents rise high above the warm pool over the far western Pacific and the Indian oceans and then flow east towards the coasts of the Americas. The air currents then descend and travel west, skimming the surface of the Pacific and creating the trade winds. An El Niño or La Niña occurs when this last leg of the Walker circulation breaks down or speeds up. The other major circulation, known as the Hadley circulation, transports air both north and south away (on average) from the equator. In both hemispheres, air in equatorial region rises due to the warmth emitted from the tropical waters and travels away from these regions towards the cooler mid-latitudes. When the air hits roughly 30 degrees latitude on either side of the equator, it cools, plunges back down toward the ocean, and then flows along the surface back toward the equator, thus completing the loop. Together, these two circulation patterns are known as the Hadley-Walker cell.

 

A large-scale wind pattern transports air and moisture from one side of the Pacific Ocean to the other. Known as the Hadley-Walker circulation, these winds drive both weather and the transport of energy along the equator. Water-laden air rises over the very warm waters of the Western Pacific Ocean. As it rises, the air cools, and the moisture evaporates out, forming clouds and rain. The cool, dry air then moves to the east, and falls to the north and south of the equator. It is likely that this circulation is strengthening, leading to thicker clouds over the regions of upwelling in the western Pacific, and thinner clouds over areas of downwelling. (Illustration by Robert Simmon)

 

Map of Ocean Winds in the Pacific Ocean

To prove his hypothesis and tie the fluctuations in the upwelling and downwelling legs of the Hadley-Walker cell to changes in radiation and cloud cover data, Chen still had to see past the disruptive, short-term spatial shifts in clouds and air currents that took place during the El Niños and La Niñas of the last 15 years. “I tried to look at the region some other way than spatially. I finally came to the conclusion that you could separate the tropical region into subsidence and convection (areas where downwelling and upwelling currents exist). And that this was a more comprehensive view of the region,” says Chen. He concluded that analyzing cloud patterns and other atmospheric parameters spatially across the tropics would not be entirely necessary. It would be much simpler to narrow the field of view and track thermal radiation and cloud cover over only those regions where various atmospheric conditions pointed to the presence of upwelling and downwelling air currents in the Hadley-Walker cell (Chen et al. 2002).

“When you look at the tropics in this way, El Niño is not dominant anymore, and the long-term decadal change in radiation is,” says Chen. During an El Niño or La Niña, the upwelling and downwelling currents of the Hadley-Walker cell don’t change in area or intensity much, but simply shift. Long-term variations in these currents become more apparent. In essence, analyzing cloud cover and radiation via Chen’s method would be akin to taking note of the change in the number and color of pieces during a checkers game as opposed to simply recording the changing spatial patterns of the pieces on the board.

The results Chen received from his analysis were as expected--on average the thermal radiation escaping the atmosphere seemed to increase markedly over the past 15 years above downwelling legs of the Hadley-Walker circulation. The increase was much greater than the average 4- watt-per-square-meter increase over the entire tropics, suggesting that the change in radiation was due to changes in circulation. To further verify these results, Chen pored over a patchwork of data on cloud amount, upper tropospheric humidity, and vertical wind velocity over the past 15 years in the tropics. In downwelling regions, he found cloud amount and humidity decreased, while in upwelling regions they increased. Vertical wind velocities intensified over both types of regions (Chen et al. 2002). “So we came to the conclusion that the Hadley-Walker circulation as a whole must have intensified over the past 15 years,” says Chen.

Though the data Chen used weren’t thorough enough to prove the hypothesis beyond a shadow of a doubt, the researchers believe that the Hadley-Walker circulation has slowly sped up over the past 15 years. The acceleration likely led to a small increase in cloud cover over upwelling regions and a relatively greater decrease in cloud cover and humidity over downwelling regions. The net effect was less humidity and clouds across the tropics. With diminished water vapor and fewer high-flying cirrus clouds in the upper troposphere, more thermal radiation from the surface was able to make it into space without being re-absorbed. At the same time, clouds overshadowed a smaller portion of the tropics, so more sunlight in the form of short-wave radiation was able to reach the Earth’s surface without being reflected back into space (Chen et al. 2002).

“We are likely seeing a decadal fluctuation here. Right now we are looking at what might lead to such a fluctuation,” says Chen. He believes that this is a natural climate anomaly much like El Niño, La Niña, or the North Atlantic Oscillation. It is a natural part of the rhythms of the Earth’s climate system. But unlike these other anomalies, the Hadley-Walker cell fluctuates over the course of decades instead of years. Chen feels that this phenomenon has no direct relation to global warming or any other hypothesis related to climate change, including the Iris Hypothesis. The Iris Hypothesis states that as sea surface temperature increases due to climate change, the increase will alter the extent of certain types of overlying clouds so that the excess heat is allowed to vent through the top of the atmosphere. Though the Iris Hypothesis may seem to jibe with what has occurred over the past 15 years in the tropics, Chen says that the thermal radiation leaving the top of the atmosphere has increased much too rapidly. Evidence has also shown that the clouds above the tropics are not changing in the ways predicted by the Iris Hypothesis.

Wielicki agrees with Chen’s assessment. He adds, “What we are seeing is that the climate system has multiple ways it can arrange itself and still accomplish a heat balance. In this case as the clouds change, the Earth absorbs more heat at the surface while it radiates more heat from the atmosphere.” Wielicki explains that even from the start, this phenomenon appeared to be a climate fluctuation unrelated to global warming or greenhouse gases. Human-generated greenhouse gases have thus far led to a 0.5-watts-per-square-meter increase in the solar energy absorbed into the atmosphere, while the tropical radiation changes were almost ten times as large.

next The Trouble with Models
back Venting Unexpectedly

 

The surface winds of the Hadley-Walker circulation appear as the dense white stripe in the center of the map at left. Winds in the eastern Pacific converge near the equator, and then form the trade winds that flow from east to west. Arrows indicate wind direction, while color represents wind speed. (Image courtesy SeaWinds Science Team and Air-Sea Interaction and Climate Team, NASA JPL)

 

The Trouble with Models

   
 

What is disturbing to Wielicki about these findings isn’t that they point to global warming, but that the climate models used to forecast the effects of global warming failed to pick up on this fluctuation in radiation over the tropics. The models were run in a “hindcast” mode to essentially see if they could forecast changes in climate that had already occurred. Previously measured parameters, such as ocean temperature and solar variability from past years, are plugged into the virtual atmosphere of the climate model. The model is then run forward through the past and into the present to predict changes in other atmospheric parameters like clouds and radiation balance. Ideally, the models should spit out the same values for clouds and radiation balance that are known to exist at present. It’s a way of testing the models and is about as close as one gets to a controlled laboratory type of experiment in climatology. But in this case, the models failed to predict the observed changes.
 

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Graph of Observed Energy Balance Data Compared to Model Data

“These results render the models suspect. This is exactly the kind of test you’d want these models to pass,” Wielicki explains. In this case, the models’ oversight was particularly worrisome since the magnitude of thermal radiation fluctuation in the tropics was of the same magnitude as the changes in thermal radiation predicted to be caused by greenhouse gases over the next 50 years, which is the primary driver for global warming predictions. Out of five climate models, only one showed a significant correlation with observations in the tropics over the 15-year record, and then only for the emitted thermal fluxes and not for the solar reflected flux. In fact, when it came to simulating the changes in thermal radiation over the past 15 years in the tropics, the difference in readings between models was as large as the actual 4-watt observed change (Wielicki et al. 2002).

Wielicki believes that the lack of consensus among the models has to do with their inability to handle the complex physics of clouds and humidity. In general, climate models have trouble with high-flying cirrus clouds and particularly low-lying clouds, such as the stratus clouds that often result in fog along the California coastline. Both types of clouds, however, may play an important role in future climate change. As humans continue to produce more and more greenhouse gases, the Earth’s surface is likely to warm several degrees. One theory is that as the Earth’s surface warms, low-lying clouds worldwide could evaporate during the daytime hours and more high-flying cirrus clouds could form as surface heat causes the air to rise. This would greatly enhance the warming started by the greenhouse gases. But without models that can reliably forecast the behavior of such clouds, scientists can only speculate.

As for Wielicki and his team, they want to know what is causing the Hadley-Walker circulation to speed up and precisely how the clouds and humidity above the tropics are changing and affecting radiation. “Most of the cloud data we have now are qualitative and not quantitative. We cannot tell how the clouds are changing from this analysis alone. We need to go back and update cloud data from the 1980s. Then we need to compare the cloud property changes and the radiation changes,” says Wielicki.

He explains that in order to understand what has been happening in the tropics, the researchers must look at actual cloud data over a long period of time and compare them to radiation data of the same region. Though there are newer satellites, such as NASA’s TRMM, Terra, and Aqua satellites, that can collect all manner of data on clouds and radiation, most of the data prior to the late 1990s are not reliable by modern standards. For now researchers are working to verify the accuracy of the older data and clean them up, so that they can be used in a continuous time series linked with the newer data.

“Generally, the longer the time series, the better chance we have of unscrambling what is going on and seeing if we have a decadal oscillation here. It will also become a good test for how well our models are agreeing,” says Wielicki. In the long run, the information may also help scientists understand more in general about how clouds behave. Scientists may be able to incorporate this new knowledge into their climate models to improve forecasts of future climate change.

References:

  1. Wielicki, B. A., T. Wong, R. P. Allan, A. Singo, J. T. Kiehl, B. J. Soden, C. T. Gordon, A. J. Miller, S.-K. Yang, D. A. Randall, F. Robertson, J. Susskind, and H. Jacobowitz, 2002: Evidence for large decadal variability in the tropical mean radiative energy budget, Science, 295, 841-844.
  2. Chen, J., B. E. Carlson, and A. D. Del Genio, 2002: Evidence for Strengthening of the Tropical General Circulation in the 1990s, Science, 295, 838-841.

back Winds of Change

 

These graphs show measurements of the flow of energy to and from the Earth (light lines), as well as energy flows predicted by computer models (dark lines). The measurements show an increase in heat escaping from the Earth which the models did not predict. Likewise, there was a measured decrease in reflected sunlight that the model results did not show. (Graphs by Robert Simmon, based on data from the CERES Science Team, NASA LaRC)

Climatologists rarely have the luxury of passing Mother Nature under the stark florescent lighting and controlled conditions of a laboratory. They cannot bring a slice of El Niño back to the bench to observe its movements. Nor can they encase a natural ecosystem in Pyrex and heat it up to see what happens. Climatologists must study firsthand the ongoing changes in the Earth’s climate system in all its chaos and mystery.

Possibly no part of the climate system is more unpredictable than the interaction between atmospheric water and radiation emitted by the Sun and the Earth. At any given time, a small sea of water floats above our heads in the form of clouds, ice, and water vapor. The form and position the water takes in the sky changes the way it interacts with solar and thermal radiation. Clouds, especially low-lying, thick clouds, reflect an enormous amount of sunlight back into space and keep it from overheating the Earth. High-flying, wispy clouds and water vapor absorb greater amounts of outgoing thermal (heat) radiation, which is generated by the surface of the Earth after it is warmed by the Sun. Along with greenhouse gases, such clouds and water vapor contribute to keeping the average temperature of the Earth’s surface from dropping to Arctic levels year round.

Due to its delicate interplay with energy, atmospheric water has tremendous potential to impact the Earth’s climate and temperature. For years, researchers have been attempting to pin down the complex interactions involved. Specifically, they’d like to know in what way clouds and water vapor might shift if the surface of our planet were to warm further due to elevated levels of human-generated greenhouse gases. In attempts to forecast possible future scenarios, scientists have created numerous computer models to simulate the interactions between clouds and radiation. Particular attention has been given to the tropics because more sunlight hits these latitudes than anywhere else. Resulting theories and model results have run the gamut. Some researchers profess that in the future, thick, low-lying clouds will decrease and high-flying cirrus clouds will increase, making global warming much worse. Others espouse theories, such as the Iris Hypothesis, which state that as the Earth’s surface heats up, cirrus clouds will dissipate and allow more thermal energy to vent into space, countering the effects of global warming.

The problem, however, is that relatively little real-world data have been collected to test these theories or even to build exacting computer models. Since clouds are constantly shifting, separating, growing, and shrinking, the only way to study them and their effects on radiation has been through the use of remote sensing instruments and weather satellites. Such instruments have been around for about 25 years. Only recently have scientists acquired enough data to put together any meaningful conclusions.

Last year, a group of researchers led by Bruce Wielicki and Tak Wong, climate scientists at NASA’s Langley Research Center, compiled a couple decades worth of radiation data from the tropics. While the researchers approached the data with the idea of testing existing notions, they witnessed a phenomenon no one expected. Over the past 15 years, progressively more thermal radiation has been escaping the atmosphere above the tropics and progressively less sunlight has been reflecting off of the clouds. Though researchers now believe that wind currents throughout the tropics have been fluctuating and altering clouds and radiation patterns, the discovery has brought many theories and climate models into question.

Venting Unexpectedly

Wielicki says that he and his team collected and merged data over a 22-year period from a number of satellites that measure both solar and thermal radiation reflected by and emitted from the atmosphere over the tropics. They brought together older data from Nimbus 7, which was one of the earliest satellites to measure radiation over the Earth, and the Earth Radiation Budget Satellite (ERBS), which has kept a continuous record of the Earth’s radiation since 1985. New data sets were also used from NASA’s Clouds and the Earth’s Radiant Energy System (CERES) instruments that fly aboard the Tropical Rainfall Measuring Mission (TRMM) as well as the newer Terra satellite (Wielicki et al. 2002).

“What we found was a 4-watt-per-square-meter change within the climate system that the climate models did not predict,” says Wielicki. Over the last 15 years, without anyone’s knowledge, the amount of thermal, long-wave radiation escaping the atmosphere above the tropics increased by 4 watts per square meter. At the same time the amount of reflected sunlight, which is mostly in the form of short-wave visible and near-visible light, decreased by 4 watts per square meter. The change appears to have occurred gradually over the past decade and a half and as such was likely completely independent from El Niño (Wielicki et al. 2002). Though 4 watts of energy is only a fraction of the 342 watts per square meter of solar energy that hits the Earth’s outer atmosphere, the Earth’s energy budget is usually extremely stable over the long term, and changes of more than a couple of watts are significant.

At first, Wielicki and the research community did not quite know how to interpret the results. They suspected that cloud cover must have been changing over large regions of the tropics (Wielicki et al. 2002). Fewer high, thin clouds, for instance, would likely result in more thermal energy leaving the Earth, and fewer low-lying, thick clouds would result in more solar energy reaching the oceans. In order to verify their suspicions, the scientists would need proof. “But observing the obvious cloud patterns spatially across the tropics couldn’t give us these long-term changes in long-wave (thermal) radiation,” says Wielicki.

He explains that the relatively short-term El Niño and La Niña cycles that occur every three to seven years dominate cloud patterns across the tropics. During a typical El Niño year, the trade winds that travel from east to west across the Pacific slow down or even reverse during the winter months. Without these winds pushing surface waters west, warm water from the western Pacific moves east. Since the warm water generates rising air and thus clouds, the clouds shift eastward en masse with the water. During a La Niña, the opposite phenomenon occurs. Trade winds speed up and the warm waters and clouds bunch up in the western Pacific and Indian oceans during the winter. These massive swings in water temperature and air pressure in the Pacific impact the tropical latitudes around the globe, causing cloud cover to shift in over the Atlantic Ocean, the Indian Ocean, and the continents as well. Though this short-term reshuffling of cloud cover does not result in a net difference in the amount of clouds or outgoing thermal radiation over the tropics as a whole, it obscures any subtle, long-term fluctuation. Attempting to locate a 4-watt-per-square-meter difference in cloud cover over 15 years across the tropics would be akin to listening for a gradual build up at a violin concerto as a foghorn intermittently blasts away.

Winds of Change

A solution finally came from Junye Chen, a graduate student working at NASA’s Goddard Institute for Space Studies in New York under the advisement of Anthony Del Genio and Barbara Carson. Chen realized that changes in large-scale atmospheric circulation had the potential to alter cloud cover and radiation over the entire tropical region. More specifically, air currents that move vertically up and down over the tropics may have fluctuated gradually over the past 15 years, leading to less water vapor and clouds over the tropics and altering outgoing thermal radiation (Chen et al. 2002).

In the tropics, there are two big circulation patterns that control most of the airflow through the troposphere like giant conveyor belts. The Walker circulation transports air east and west along both sides of the equator. Air currents rise high above the warm pool over the far western Pacific and the Indian oceans and then flow east towards the coasts of the Americas. The air currents then descend and travel west, skimming the surface of the Pacific and creating the trade winds. An El Niño or La Niña occurs when this last leg of the Walker circulation breaks down or speeds up. The other major circulation, known as the Hadley circulation, transports air both north and south away (on average) from the equator. In both hemispheres, air in equatorial region rises due to the warmth emitted from the tropical waters and travels away from these regions towards the cooler mid-latitudes. When the air hits roughly 30 degrees latitude on either side of the equator, it cools, plunges back down toward the ocean, and then flows along the surface back toward the equator, thus completing the loop. Together, these two circulation patterns are known as the Hadley-Walker cell.

To prove his hypothesis and tie the fluctuations in the upwelling and downwelling legs of the Hadley-Walker cell to changes in radiation and cloud cover data, Chen still had to see past the disruptive, short-term spatial shifts in clouds and air currents that took place during the El Niños and La Niñas of the last 15 years. “I tried to look at the region some other way than spatially. I finally came to the conclusion that you could separate the tropical region into subsidence and convection (areas where downwelling and upwelling currents exist). And that this was a more comprehensive view of the region,” says Chen. He concluded that analyzing cloud patterns and other atmospheric parameters spatially across the tropics would not be entirely necessary. It would be much simpler to narrow the field of view and track thermal radiation and cloud cover over only those regions where various atmospheric conditions pointed to the presence of upwelling and downwelling air currents in the Hadley-Walker cell (Chen et al. 2002).

“When you look at the tropics in this way, El Niño is not dominant anymore, and the long-term decadal change in radiation is,” says Chen. During an El Niño or La Niña, the upwelling and downwelling currents of the Hadley-Walker cell don’t change in area or intensity much, but simply shift. Long-term variations in these currents become more apparent. In essence, analyzing cloud cover and radiation via Chen’s method would be akin to taking note of the change in the number and color of pieces during a checkers game as opposed to simply recording the changing spatial patterns of the pieces on the board.

The results Chen received from his analysis were as expected–on average the thermal radiation escaping the atmosphere seemed to increase markedly over the past 15 years above downwelling legs of the Hadley-Walker circulation. The increase was much greater than the average 4- watt-per-square-meter increase over the entire tropics, suggesting that the change in radiation was due to changes in circulation. To further verify these results, Chen pored over a patchwork of data on cloud amount, upper tropospheric humidity, and vertical wind velocity over the past 15 years in the tropics. In downwelling regions, he found cloud amount and humidity decreased, while in upwelling regions they increased. Vertical wind velocities intensified over both types of regions (Chen et al. 2002). “So we came to the conclusion that the Hadley-Walker circulation as a whole must have intensified over the past 15 years,” says Chen.

Though the data Chen used weren’t thorough enough to prove the hypothesis beyond a shadow of a doubt, the researchers believe that the Hadley-Walker circulation has slowly sped up over the past 15 years. The acceleration likely led to a small increase in cloud cover over upwelling regions and a relatively greater decrease in cloud cover and humidity over downwelling regions. The net effect was less humidity and clouds across the tropics. With diminished water vapor and fewer high-flying cirrus clouds in the upper troposphere, more thermal radiation from the surface was able to make it into space without being re-absorbed. At the same time, clouds overshadowed a smaller portion of the tropics, so more sunlight in the form of short-wave radiation was able to reach the Earth’s surface without being reflected back into space (Chen et al. 2002).

“We are likely seeing a decadal fluctuation here. Right now we are looking at what might lead to such a fluctuation,” says Chen. He believes that this is a natural climate anomaly much like El Niño, La Niña, or the North Atlantic Oscillation. It is a natural part of the rhythms of the Earth’s climate system. But unlike these other anomalies, the Hadley-Walker cell fluctuates over the course of decades instead of years. Chen feels that this phenomenon has no direct relation to global warming or any other hypothesis related to climate change, including the Iris Hypothesis. The Iris Hypothesis states that as sea surface temperature increases due to climate change, the increase will alter the extent of certain types of overlying clouds so that the excess heat is allowed to vent through the top of the atmosphere. Though the Iris Hypothesis may seem to jibe with what has occurred over the past 15 years in the tropics, Chen says that the thermal radiation leaving the top of the atmosphere has increased much too rapidly. Evidence has also shown that the clouds above the tropics are not changing in the ways predicted by the Iris Hypothesis.

Wielicki agrees with Chen’s assessment. He adds, “What we are seeing is that the climate system has multiple ways it can arrange itself and still accomplish a heat balance. In this case as the clouds change, the Earth absorbs more heat at the surface while it radiates more heat from the atmosphere.” Wielicki explains that even from the start, this phenomenon appeared to be a climate fluctuation unrelated to global warming or greenhouse gases. Human-generated greenhouse gases have thus far led to a 0.5-watts-per-square-meter increase in the solar energy absorbed into the atmosphere, while the tropical radiation changes were almost ten times as large.

The Trouble with Models

What is disturbing to Wielicki about these findings isn’t that they point to global warming, but that the climate models used to forecast the effects of global warming failed to pick up on this fluctuation in radiation over the tropics. The models were run in a “hindcast” mode to essentially see if they could forecast changes in climate that had already occurred. Previously measured parameters, such as ocean temperature and solar variability from past years, are plugged into the virtual atmosphere of the climate model. The model is then run forward through the past and into the present to predict changes in other atmospheric parameters like clouds and radiation balance. Ideally, the models should spit out the same values for clouds and radiation balance that are known to exist at present. It’s a way of testing the models and is about as close as one gets to a controlled laboratory type of experiment in climatology. But in this case, the models failed to predict the observed changes.

“These results render the models suspect. This is exactly the kind of test you’d want these models to pass,” Wielicki explains. In this case, the models’ oversight was particularly worrisome since the magnitude of thermal radiation fluctuation in the tropics was of the same magnitude as the changes in thermal radiation predicted to be caused by greenhouse gases over the next 50 years, which is the primary driver for global warming predictions. Out of five climate models, only one showed a significant correlation with observations in the tropics over the 15-year record, and then only for the emitted thermal fluxes and not for the solar reflected flux. In fact, when it came to simulating the changes in thermal radiation over the past 15 years in the tropics, the difference in readings between models was as large as the actual 4-watt observed change (Wielicki et al. 2002).

Wielicki believes that the lack of consensus among the models has to do with their inability to handle the complex physics of clouds and humidity. In general, climate models have trouble with high-flying cirrus clouds and particularly low-lying clouds, such as the stratus clouds that often result in fog along the California coastline. Both types of clouds, however, may play an important role in future climate change. As humans continue to produce more and more greenhouse gases, the Earth’s surface is likely to warm several degrees. One theory is that as the Earth’s surface warms, low-lying clouds worldwide could evaporate during the daytime hours and more high-flying cirrus clouds could form as surface heat causes the air to rise. This would greatly enhance the warming started by the greenhouse gases. But without models that can reliably forecast the behavior of such clouds, scientists can only speculate.

As for Wielicki and his team, they want to know what is causing the Hadley-Walker circulation to speed up and precisely how the clouds and humidity above the tropics are changing and affecting radiation. “Most of the cloud data we have now are qualitative and not quantitative. We cannot tell how the clouds are changing from this analysis alone. We need to go back and update cloud data from the 1980s. Then we need to compare the cloud property changes and the radiation changes,” says Wielicki.

He explains that in order to understand what has been happening in the tropics, the researchers must look at actual cloud data over a long period of time and compare them to radiation data of the same region. Though there are newer satellites, such as NASA’s TRMM, Terra, and Aqua satellites, that can collect all manner of data on clouds and radiation, most of the data prior to the late 1990s are not reliable by modern standards. For now researchers are working to verify the accuracy of the older data and clean them up, so that they can be used in a continuous time series linked with the newer data.

“Generally, the longer the time series, the better chance we have of unscrambling what is going on and seeing if we have a decadal oscillation here. It will also become a good test for how well our models are agreeing,” says Wielicki. In the long run, the information may also help scientists understand more in general about how clouds behave. Scientists may be able to incorporate this new knowledge into their climate models to improve forecasts of future climate change.

References:

  1. Wielicki, B. A., T. Wong, R. P. Allan, A. Singo, J. T. Kiehl, B. J. Soden, C. T. Gordon, A. J. Miller, S.-K. Yang, D. A. Randall, F. Robertson, J. Susskind, and H. Jacobowitz, 2002: Evidence for large decadal variability in the tropical mean radiative energy budget, Science, 295, 841-844.
  2. Chen, J., B. E. Carlson, and A. D. Del Genio, 2002: Evidence for Strengthening of the Tropical General Circulation in the 1990s, Science, 295, 838-841.
  1. Wielicki, B. A., T. Wong, R. P. Allan, A. Singo, J. T. Kiehl, B. J. Soden, C. T. Gordon, A. J. Miller, S.-K. Yang, D. A. Randall, F. Robertson, J. Susskind, and H. Jacobowitz, 2002: Evidence for large decadal variability in the tropical mean radiative energy budget, Science, 295, 841-844.
  2. Chen, J., B. E. Carlson, and A. D. Del Genio, 2002: Evidence for Strengthening of the Tropical General Circulation in the 1990s, Science, 295, 838-841.