Understanding complacency about climate change

Professor Sterman and colleague find flawed reasoning at the root of complacency

Overflowing bathtubJohn Sterman likens climate change to an overflowing bathtub. Even if the drain is open, when the inflow of water exceeds the outflow, the water level will rise over time — and eventually the tub will overflow.

Why is the general public not more concerned about the potential consequences of climate change? On many risk assessment issues, such as the risk of a plane crash, the public is far more fearful than scientists, observed John Sterman, the Jay W. Forrester Professor of Management at the MIT Sloan School of Management. But he noted, on the issue of climate, the situation is just the opposite.

“The science is unequivocal now, and the scientists are telling us that it's urgent that we reduce emissions,” he said. “That debate is basically done.” However, Sterman added, the public at large doesn't understand that.

What's behind this puzzling complacency? Sterman's research suggests some clues. In some important research conducted with Linda Booth Sweeney of the Harvard Graduate School of Education, Sterman, who is director of the Systems Dynamics Group at MIT Sloan, has found that even highly educated people have a poor understanding of some of the dynamics that affect climate change.

And if people don't have good mental models for understanding climate change, they may come to faulty conclusions about policy. For example, if people erroneously think that climate change is easily reversible, they may support waiting to see what the effects of climate change will be before taking substantial actions to reduce emissions of greenhouse gases like carbon dioxide.

More on systems with long time delays >>

The bathtub metaphor

To help people better understand climate change, Sterman uses the metaphor of a bathtub filling with water. Suppose you are adding water to a bathtub that has a drain that is open, and you keep adding water to the tub at a rate twice as fast as the water flows out through the drain. Even though water is constantly flowing out through the drain, the inflow of water into the tub exceeds the outflow, and the water level in the tub will rise over time. Eventually, the tub will overflow.

A similar dynamic, according to Sterman, is found in climate change. Carbon dioxide is removed from the atmosphere by natural processes — much the way the bathtub has a drain. The problem is that human activity now adds carbon dioxide to the atmosphere at roughly twice the natural removal rate — much as the spigot in the bathtub example adds water faster than the water flows out through the drain.

As a result, to halt greenhouse gas-induced climate change, it's not enough to stabilize carbon dioxide emissions at their current level or cut them slightly; to start to reduce the carbon dioxide concentration in the atmosphere, current carbon dioxide emissions would have to be cut by more than half, so that they would fall below the natural rate of carbon dioxide removal.

More on climate change dynamics >>

To test the hypothesis that people have poor mental models for understanding climate change, Sterman and Booth Sweeney conducted a study in which highly educated university students were given a short, nontechnical summary of information about climate change — including information about the fact that carbon dioxide emissions are currently twice the rate of natural carbon dioxide removal.

The study participants were then asked to draw a simple graph of what would have to happen to carbon dioxide emissions and removal rates in order for carbon dioxide concentrations in the atmosphere to stabilize or fall some over time.

The study participants had much stronger technical backgrounds than the average person: 98 percent of them were graduate students, and 60 percent had training in science, math or engineering. Most of the rest were trained in social sciences, particularly economics. But even among a group of graduate students with strong technical backgrounds, the researchers found, most people answered incorrectly.

For example, a substantial majority — 63 percent — thought that concentrations of carbon dioxide in the atmosphere could be stabilized even if emissions levels remained above the natural carbon dioxide removal rate. They apparently did not realize that the system would still be out of balance, with carbon dioxide continuing to be added at a faster rate than it is being removed.

More on mental models >>

Flawed reasoning not unique to climate change

The kind of flawed reasoning that the study revealed is not specific to climate change; in other, related research, Sterman and Booth Sweeney found that people generally don't have good mental models for understanding systems that involve inflows, outflows, and accumulation — whether those systems are bathtubs, business inventories or the concentrations of greenhouse gases in the earth's atmosphere.

Unfortunately, on the question of climate change, the stakes are extremely high. “To make a difference in the climate change challenge, we have to change the way everybody thinks about their personal energy choices,” Sterman observed.

“The good news,” says Sterman, “is that you don't need to know any math to understand that the level of water in a tub rises as long as you pour water in faster than it drains out. Once people understand that we're pouring greenhouse gases into the atmosphere far faster than they are removed, they understand why we can't wait and see — why we have to reduce our emissions today to protect the world we will pass on to our children, and their children.”

Want to test your own understanding of climate change dynamics? Try the System Dynamics Group's online Greenhouse Gas Emissions Simulator. You can also read a paper describing this research; the paper was published in the journal Climatic Change.


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Work starts at home: For a large institution like MIT, the battle against global warming begins with efforts to reduce its own carbon imprint. As part of MIT Sloan's S-Lab, co-taught by Professor John Sterman, a team of students last year consulted with MIT officials on how to accomplish just that. Listen to the podcast >>