Science Briefs

Global Warming, Playing Dice, and Berenstain Bears

In the summer of 1988, I testified to the U.S. Senate that the world was getting warmer and that the dominant cause was probably human-made greenhouse gases.

The Senate, and the public, wanted to know the cause of parched conditions in the Midwest, where the Mississippi had practically dried up. I said that our numerical climate model indicated a tendency for more frequent and severe droughts as the world became warmer, but a specific drought was a matter of chance, dependent on fluctuating meteorological patterns.

Although that testimony increased public awareness of global warming, it was soon evident that I had communicated poorly. On a Jeopardy quiz show the "answer" was that I had said the Midwest drought was caused by the greenhouse effect.

People have a predilection for deterministic explanations of climate fluctuations. Even Albert Einstein abhorred the notion of chance in nature, saying, "God does not play dice". But the science of quantum mechanics, with Einstein a major contributor, proved that uncertainty plays a big role in physics and in the world.

Cartoon of bears playing with dice

One result is chaos in weather and climate. Temperature and precipitation patterns fluctuate in ways unpredictable beyond a few weeks at most. Yet climate, the average weather, can be changed in a deterministic way by a "forcing", such as an increase of atmospheric gases.

I tried to explain forcings and chaos with colored dice. One die represented normal climate for 1951-1980, with equal chances for warm, average and cool seasons. The other die was "loaded" due to forcing by greenhouse gases, such that the chance of an unusually warm season increased from 33 to about 60 percent, as calculated by our climate model for the late 1990s.

When Sen. Albert Gore asked me to testify before the Senate again, in 1989, I wanted to explain the greenhouse effect better. I held up a one-watt Christmas tree bulb, saying that the human greenhouse effect is heating the Earth by an amount equal to two of these bulbs over every square yard of the Earth's surface. In 100 years, this heating could double or quadruple, depending on how fast we put greenhouse gases into the air.

This added heating intensifies dry conditions, when and where it is dry. However, over oceans and wet land, added heating increases evaporation, which eventually falls as rain. So my testimony was that global warming, paradoxically, increases both extremes of the hydrologic cycle. It causes more intense droughts and forest fires, but, at other places and times, it causes heavier rainfall, more intense storms fueled by latent heat of water vapor, and greater flooding.

Unfortunately, this discussion was lost in a tempest caused by alterations to my testimony inserted by the White House Office of Management and Budget. The brouhaha may have helped keep attention on the global warming topic, but it failed to illuminate the scientific issues and uncertainties. And the public global warming "debate" continues to contrast opposite intransigent positions, rather than exemplifying how science research really works.

I suggest to students that they view the debate in the media the way young Berenstain Bear viewed the botched bicycle lessons of Papa Bear: "this is what you should not do". A good scientist does not act like a lawyer defending the position of a client.

The fun in science is its objectivity. First exhilaration occurs when a young scientist compares alternative ideas or models with observations and discovers how something works. When the observations are of the Earth's climate, it is awesome to think that our models can capture and predict the effect of the sun, volcanoes and greenhouse gases. But awe is tempered by realization that the "laboratory" is home to billions of people and wildlife.

What have we learned about the greenhouse effect in 10 years? Bad news and good news. The bad news is that the world is warming, as predicted. The frequency of unusually warm seasons has increased to about 60 percent. Record warm temperatures occur more often than record cold. The year 1998 was, on global average, the warmest year in the history of instrumental data.

Remarkable climate extremes have occurred recently: the Chicago heat wave of 1995, a run of 29 days of 100°F temperatures in Dallas in 1998, floods in the Midwest in 1993 and 1997 and in the Southeast in 1999. The high natural variability of climate prevents unique association of these events with global warming. But a quantitative index of temperature and moisture changes reveals that climate extremes are increasing at most places in the sense predicted for global warming. And we can predict with reasonable confidence that the record annual and decadal temperatures for the 48 contiguous United States, set in the 1930s, will soon be broken.

The good news is that the growth rate of greenhouse gases has slowed. In the 1980s the rate was four more light bulbs per square yard in 100 years. Despite increased population and energy use, the rate has slowed to three more light bulbs per 100 years, rather than increasing to the five bulbs that were in the most popular climate forcing scenarios. Credit for the slowdown belongs in part to the public, legislatures, and businesses that phased out chlorofluorocarbons. Also methane and carbon dioxide growth rates slowed, for reasons that are not well understood and are perhaps only temporary.

What's to be done? First, we must avoid providing "lessons in what not to do". Immediate, economically-wrenching constraints on energy use have negligible effect on climate forcings. But the other extreme, denial of the greenhouse problem, is equally foolish. Climate change is real, and it is a complex problem.

Climate will change in the next few decades, regardless of our actions. But we can slow the planetary experiment as we develop better understanding. We need bi-partisan common sense strategies to encourage greenhouse benign technologies that continue the positive changes in our long term energy use trajectory. This is good for business and it will provide us with the option to eventually stabilize climate, thus maintaining a healthy planet for humans and bears.


Hansen, J.E., M. Sato, A. Lacis, R. Ruedy, I. Tegen, and E. Matthews 1998. Climate forcings in the industrial era. Proc. Natl. Acad. Sci. 95, 12753-12758.

Hansen, J. 1999. Climate change in the new millennium. Columbia University State-of-the-Planet Conference, November 15, 1999.