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Global temperatures have risen about .8 degrees Celsius since the mid-1800s. Studying the heat content in the oceans can tell us why.

Bruce Anderson

Why the Earth is Warming//

Study disproves two of three hypotheses, aims blame at man-made pollutants

Bruce Anderson didn’t set out to prove that the rise in global temperatures since the start of the Industrial Revolution is caused by human activity. And the five-year study that he and four colleagues published in the October 2012 Journal of Climate doesn’t quite draw that conclusion. But the research does suggest that man-made pollutants are to blame.

The study, which tested three hypotheses about causes of the warming trend, debunks alternative theories that have been floated in recent years. At the same time, says Anderson, a College of Arts & Sciences associate professor of earth and environment, the research strengthens the theory that humans are responsible for the phenomenon, in which carbon dioxide, methane, nitrous oxide, and the other gases we emit accumulate in the atmosphere, trapping heat that radiates outward from the Earth.

The consensus among scientists is that global temperatures have risen about .8 degrees Celsius since the mid-1800s. Anderson believes that focusing on the heat content in the oceans can tell us why. He notes that the oceans store and release nearly 100 times more heat than land surfaces and that nearly 95 percent of the heat added to the environment over the last 50 years has gone into the oceans.

For their study, Anderson and his colleagues—from Washington state, the United Kingdom, and Italy—looked at the heat content of the oceans from 1950 to 2000 and used complex computer models to test the three hypotheses.

First, Anderson calculated the expected increase in ocean heat resulting from the changes in levels of carbon dioxide and other chemicals. Then he compared those numbers with the observed, or measured, increases in ocean heat over the same period—data that are typically collected from the top 700 meters (about 2,300 feet) of oceans. The two sets of numbers matched.

“What we find,” says Anderson, who led the study, “is that the heating of the interior oceans is fully consistent with what we’d expect. That wasn’t entirely new. What was new was that we were able, using the same set of data, to also test alternative hypotheses that other people hadn’t looked at.”

One hypothesis suggests that global warming is the result of what’s called internal climate variability, or changes in the interactions between the oceans and the atmosphere. It argues that temperatures are rising because they are drawing heat from deep within the oceans.

That can certainly happen, Anderson says, pointing to El Niño, a warming of the waters in the equatorial Pacific that affects climate around the world.

He estimated how much energy would be needed to drive the increases in global temperatures, and as a result, how much that process would reduce the heat of the interior oceans. He compared those calculations to the measured heat content of the oceans over the five decades. The two sets of numbers did not match.

For the hypothesis to hold water, he says, there would have to be a significant drop in ocean heat in order to feed the increasing global temperatures. But scientists have actually seen a steadily increasing amount of heat over a 50-year period.

The third theory, says Anderson, was the trickiest one to test. “Every so often, particularly in nonscientific literature, there will be a slew of hypotheses that are put out there: increases in the energy output of the sun, changes in gamma radiation, galactic cosmic rays.”

Anderson assumed that there is some unknown source of heat, and proposed that the source added as much heat to the climate system as all of the greenhouse gases combined. How would the heat content of the oceans respond?

He calculated that response and compared it to the observed heat content of the oceans. As with the second hypothesis, the two sets of numbers failed to match.

“We came to the conclusion that, at most, 15 percent of the warming over the last half-century could have been the result of some unknown mechanism for heating the planet,” he says.

Anderson believes that the study rejects “in one fell swoop” the idea that there are mechanisms other than human activity that cause climate change.

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