Point of View

Global warming and the limits of theory

December 13, 2009 

— President Barack Obama will lay out his commitment to reducing greenhouse gas emissions at the United Nations climate conference in Copenhagen. Whatever political and economic road map we follow in response to climate change must draw from a solid scientific basis and a clear grasp of the uncertainties. The high stakes involved compel us all to understand what guidance we can expect from climate science, now more than ever.

The theoretical foundation for how the Earth's climate changes is straightforward, based on physical principles. Application of those principles, variations in Earth's orbit around the sun key among them, explains Earth's primary long-term climate signal - the alternating warm and cold periods within the current ice age.

Positive feedbacks in the climate system amplify these temperature changes. The most important involve the oceans and the ice-covered regions of Earth. The theory clearly predicts that if CO {-2} is injected into the climate system, as from human activities, the Earth will warm as a result. The warming should be greatest in the polar regions where the ice feedback is most active. That is in fact what is observed.

There is a very large body of published work spanning several decades over many disciplines of science that supports this theory of climate change. The recent "Climategate" e-mails notwithstanding, there are thousands of scientists working on the problem of climate change, not just two or three, whose combined efforts ensure that the scientific method won't let us down.

Why then are computer model predictions of future climate uncertain? Why is the crystal ball so clouded when we need it most?

The best we can expect from models are general climate trends. Climate models are inherently prone to error when we ask them to forecast future conditions at a specific place and time. To see why, let's think about the predictability of some simple natural systems that follow well-established theories.

A double pendulum (a pendulum that has another free-moving pendulum attached to its end) swings to the tune of Newton's laws of motion. But since the motion of the first pendulum influences the motion of the second one, its behavior becomes unpredictable after a short time. Does that invalidate the theory? Not at all - it just shows the limitation in the predictive capacity of a chaotic system.

The theory of evolution is one of the most successful scientific theories we have. Yet we can't use it to predict the precise form that a species will attain with time. The theory can tell us general functional features that will be favored, but not the details of what the future animals and plants will look like. This in no way invalidates the theory - again, it just highlights its predictive limits.

And so it is with climate change theory. Given these prognostic constraints, can we act on a climate model's vision of the future?

Here's an analogy. No one can predict exactly when and where the next big quake will hit California, but theory and data tell us to expect one every couple hundred years. Do we ignore the risk because of the limitations in the predictive capability of a very robust theory? Most would say no.

How should we respond to the climate change problem when the theory is sound, but its predictive capacity is intrinsically limited? We focus on what is known and on the risks.

Melting glaciers and warmer oceans lead to rising sea levels, up to 3 feet this century based on the trend in observed changes. Several feet of sea-level rise, combined with diminishing freshwater resources from disappearing glacial ice, will present tens of millions of people with intractable choices. Inevitably, those societies most at risk have limited means to mitigate their effects.

One way or another, we know enough to act.

Action won't play out in isolation. Obama will have much more than climate change on his mind when he addresses the Copenhagen climate conference this week. He's also thinking about dependence on unfriendly countries for oil. He wants to position his country at the forefront of the inevitable rise of the alternate energy economy. He is aware of the cost and sacrifice that carbon emissions reduction will entail.

Open and honest discussion from our leaders is the key to making the right choices.

We all have important roles to play in moving forward. Scientists must communicate the science clearly and honestly. Policymakers and voters must make educated choices. Scientific literacy is our birthright. Its cultivation is our society's greatest charge. We can and must rise to the challenge. Nothing less than our future lies in the balance.

Thomas Rickenbach is an assistant professor of atmospheric science in the Department of Geography at East Carolina University.

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