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and the University Corporation for Atmospheric Research

OCTOBER 1995 — VOLUME 3, NO. 1

Copyright | Note to Teachers


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Clouds Impact Climate Change Scenarios

The role of clouds is one of the wild cards in predicting global climate change, but whether clouds will enhance or moderate global warming is still not understood. Because cloud formation and behavior is complex and global in scope, scientists find it difficult to create accurate computer models to simulate clouds' effect on climate change. Researchers at the National Center for Atmospheric Research (NCAR) are striving to refine computer climate models to improve estimates about global warming.

Clouds are often viewed as a passive part of our weather, the result of atmospheric behavior. However, clouds play an active role in maintaining the earth's average temperature. Some of the incoming radiation (energy from the sun) is reflected by clouds, keeping that energy from reaching the earth. Clouds also trap some of the heat rising from the earth's surface, creating a greenhouse effect that makes earth a habitable planet.

Although these facts are clear, the big picture is more complex. Some clouds -- such as low, thick stratocumulus -- are best at reflecting solar input. Others -- in particular the high, thin cirrus varieties -- hold in more heat than they reflect from the sun. The problem, then, is figuring out how much of each cloud type is present around the globe at any time.


Clouds affect the global climate by absorbing and reflecting
energy from the sun and trapping heat being reflected by the earth.
Recent studies have found that clouds absorb four times more energy than
was previously thought. The new value is changing the predictions of global
climate models and forecasts for global warming.
Photo by NCAR

Modern satellites can not only take frequent pictures of cloud cover over the earth, they can directly sense incoming and outgoing radiation. In 1989, scientists determined that the average total cloud cover worldwide reflected more energy to space than it trapped, ultimately causing a cooling effect on the planet. This cooling could potentially counteract an enhanced greenhouse effect, and negate the impact of increasing levels of carbon dioxide and other greenhouse gases in the atmosphere.

Does this mean that more clouds will cool the planet even more? That depends on the type of clouds that increase. If the current cloud cover increased, but with exactly the same mix of cloud types, then the greater cooling could offset at least some of the warming due to increased greenhouse gases. If the proportion of low clouds went up, the increase in cooling would be even larger. Conversely, if cirrus clouds increased, but stratus clouds did not, the balance would tip in the other direction. Global cloud cover would provide less cooling than it does now. Clearly, scientists need to know what kinds of clouds would increase or decrease with the greenhouse effect. A warmer earth might cause changes in the ocean or atmosphere that change the cloud types present, which would then "feed back" into the warming itself, making it either stronger or weaker. Also, changes in the location of the various cloud types could in turn change the existing pattern of global temperatures.

Climate modelers use computer models to examine future scenarios, but the accuracy of the predictions they generate is limited by the computing capacity available to run them. Scientists cannot track conditions at every point on the globe, so they must use a grid of points which might be separated by several hundred kilometers. Similarly, only 10 or 12 vertical slices at different heights through the atmosphere are examined.

The physics of cloud formation and dissipation must be simplified in a global climate model. Different clouds have different numbers of ice crystals or water droplets at different sizes; in a model these must be held fixed for various lengths of time, even though in the atmosphere they are constantly evolving. Because of these problems, even the most sophisticated climate models do not agree on the impact of cloud changes on global temperature. But that may be changing with new research findings.

In 1994 NCAR climate modeler Jeffrey Kiehl and other researchers from various institutions reported that clouds absorb about four times as much solar energy as had been thought previously. The accuracy of the new absorption value is supported by the fact that each study reached the new value by a different method. However, current theory cannot explain the new value.

Cirrus clouds

Cirrus clouds, thin, wispy, and feathery, are composed
entirely of ice crystals. They usually form at 6,000 meters (20,000 feet)
and above, and where the temperature is always far below freezing. Cirrus
clouds trap more heat in the earth's atmosphere than they reflect from
the sun, so a change in the percentage of this cloud type
could affect global change scenarios.
Photo by NCAR

Kiehl says that before researchers can say for certain what effect this new knowledge will have on global warming scenarios, scientists will first have to learn why and how clouds absorb this additional energy. By using the new value in a climate model, Kiehl arrived at preliminary results that predict a warmer, moister climate than that shown in previous models. In a simulation of present climate, the new figures generated a model that is much closer to the observed atmosphere, indicating that the new cloud absorption values may improve the accuracy of climate models.

Despite this improved modeling capacity, Kiehl says researchers are frustrated by their inability to explain the finding. Before this phenomenon can be fully integrated into climate models, scientists must understand how it works. As researchers examine how clouds absorb more energy, they must re-examine how energy is transferred around the globe from the equator to the poles. Previously accepted theories may have to change to correlate with current research. In any case, there is much for a scientist to study.

In all of this research, one thing is clear: clouds play an important role in the present and future climate of earth.

Jeff Kiehl: Beyond the Scientific Method

Jeff Kiehl is a scientist who doesn't believe in the scientific method. "If science proceeded the way the textbooks say it does, every scientific problem would have been solved long ago," he quips. Instead, he points to creativity and intuition as critical elements in solving the complex puzzles the natural world poses.

Jeff describes his own thought process as a surmounting of successive walls. The first wall presents itself when initially contemplating a problem. Jeff does background reading and data gathering, immersing himself in the area he is interested in. This, the conventional "textbook" science, is the necessary first step. It is followed by a sense of total saturation or exhaustion, the first of Jeff's walls. The answer will only come once he has given up, stopped thinking about the problem altogether. But even when he is over this first barrier, he faces a second and more formidable one. In science, the answer to a question merely suggests further, often broader and more difficult, issues. A new finding may point to a new process or underlying principle. This is the second, and most important, creative wall.

To breach both kinds of barriers to understanding, Jeff first needs to reach the point of despair and to give up consciously on the problem at hand. The solution comes to him later, in the shower, or taking a walk to get away from his desk. He describes it as an "intuitive and random" sensation. For example, one day he was lying on the couch immobilized by a migraine headache. Out of nowhere he had the explanation to an anomalous result of earlier research that had defied him for three years. The subsequent mathematical proofs have so far validated that insight. He had been discussing the problem for a few weeks but had not had any conscious new insights into it before his sudden brainstorm.

He claims to have breached this second conceptual battlement only three or four times in nearly 20 years of research. (The rest of the time, he says, has been filled with more routine work.) "Some scientists stop at the first wall," Jeff says. "A few really brilliant ones seem to have streams of major creative insights all the time."

"You have to be like a fighting dog that sinks his teeth in and doesn't let go. You have to have faith that the solution will come. The process is painful. But the exhilaration of breaking through the second wall is why I do what I do. It's the irrational and intuitive part that is the fun of science."

Lucy Warner


Observe How Clouds Affect Temperature

Scientists use satellite data and computer models to determine the possible effects of clouds on climate change, but you can use your eyes, a thermometer, and some paper on which to record your observations to conduct some scientific research of your own. A cloud chart will help you identify different types of clouds.

1. Use a thermometer to record temperatures on sunny and cloudy days over several weeks or the entire school year. Note the type of cloud present and the extent of cloud coverage. Take nighttime readings on clear, partially cloudy, and cloudy nights as well.

2. Make a chart showing how you gathered your data. Include time of day, season of the year, type of cloud, extent of coverage, and temperatures.

3. Consult a local source for climate information in your area. Usually a chamber of commerce provides information about average daily temperature, the number of sunny days, and other climate information. Two other sources are your local National Weather Service office or the book, Places Rated Almanac by R. Boyer (ISBN 0136-7700-61). Compare your information to the average temperatures for the area. How close are the numbers?

4. Analyze your data to determine the impact of cloud cover on temperatures. Did you notice a difference between types of clouds and the effect on temperature? Does your research from the ground concur with what scientists hypothesize? Why do you think measurements from satellites are important?


Science Now is jointly published by the Walter Orr Roberts Institute at the University Corporation for Atmospheric Research and SIRS Publishing, Inc. (Social Issues Resources Series.) Science Now is published three times during the school year and is distributed to SIRS subscribers. Comments and questions should be directed to Joyce Gellhorn via Internet at jgellhorn@sprynet.com. You can also contact your SIRS representative or write to:

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Caroline Hanson

Scientific Editor:
Pat Kennedy

Bob Henson, UCAR Communications
Jeffrey Kiehl, Climate and Global Dynamics Division

UCAR is a consortium of over 60 universities in the U.S. and Canada with doctoral programs in atmospheric and related sciences. UCAR manages and operates the National Center for Atmospheric Research under the sponsorship of the National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. Anyone who undertakes any of the activities described herein shall do so at their own risk; UCAR and SIRS Publishing, Inc. assume no liability, whatsoever, for any injury or harm, which may result therefrom.


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