From a distance, Earth’s climate is a simple matter of equilibrium. Eric Barron made this point with a single vivid image, of Earth from space. There was the mottled orb in high relief: sunlight and shadow, bright reflective cloud-cover and deep absorbing ocean, bone-pale desert and dark tropical forest. Balanced precisely between incoming solar warmth and cold, receiving dark. Yin and yang. It’s when you zoom in a little that things start to get complicated.

"The first thing you notice," Barron said, "is that the energy received at the Earth’s surface is not evenly distributed. The poles get a lot less than the equator. The poles are actually giving up net energy, while the equator is running a surplus."

The resulting temperature gradient drives the great wind belts, and moves the seas. These giant heat transporters, atmospheric and ocean circulation, are the primary shapers of global climate. But many, many other factors play their parts. Plants, for example, are sponges for heat. Snow and ice cover tend to perpetuate cold—not by their coldness, but by efficiently turning sunlight away. Even the texture of the soil is important, helping to define its moisture content, and so how much moisture the air can draw by evaporation. "You can begin to see the level of complexity we’re talking about," said Barron, director of Penn State’s Earth System Science Center.

Barron, a professor of geosciences, gave the fifth of this year’s Penn State Lectures on the Frontiers of Science. When considering "How Climate Change Works," he said, "It’s a good thing we have that underlying rule: Energy coming in must equal energy going out."

Do the Math
There are, it follows, only three basic ways to alter the climatic equation: Change the amount of energy that reaches us from the Sun; change the percent of energy reflected back into space; or change the composition of Earth’s atmosphere, so that more (or less) energy is trapped by that life-sustaining insulated blanket we know as the greenhouse effect. (Another heat source, Earth’s molten core, has only miniscule climatic impact, Barron said.)

Our climate has always been changing. The sun’s power waxes and wanes along cycles of orbit, and we get seasons. On much different time scales, the continents have shifted their positions; there has been more or less ice cover, more or less forest. Volcanic eruptions, spewing their dust, have played havoc—for brief periods—with the composition of the atmosphere.

These "disturbances" are known to climatologists as forcing factors. Alone, Barron said, their effects are relatively minor. Yet, "The fossil record shows us periods when there were alligators off the coast of Greenland. There was a time when over 400 species of plants flourished on the north slope of Alaska. There have been times when palm trees grew in Chicago, and others when all of Pennsylvania was covered in ice." For this kind of change to occur, Barron said, there have to be positive feedbacks: systematic responses that loop back around, spurring more change, and still more, until the original effect is many times amplified.

Feedbacks
Water vapor, for example, is a greenhouse gas. It acts as an insulator, absorbing heat radiated up from the planet’s surface and redirecting some of it back toward Earth. A small drop in temperature, however, causes water vapor to condense—and then it falls as rain or snow. That leaves less insulation in the atmosphere, which reduces the greenhouse effect, which in turn causes surface temperature to drop even more—and on and on, until some other limiting factor intervenes. Conversely, if air temperature rises even slightly, more water vapor will be evaporated from the oceans. A moister atmosphere means tighter insulation, and in turn more warming. In both cases, the continued action of the loop ends up being far more important than the original disturbance.

Some of the important feedbacks that affect climate, Barron said, have only recently been observed. "As a student, for example, I was taught that the distribution and character of vegetation were passive responses to climate. Now we know that’s not true. Life on Earth is part of the feedback system."

Over the last century and a half, in fact, life on Earth—of the human variety—has become an increasingly potent factor in climate change. Gas samples retrieved from bubbles in prehistoric ice cores and measurements taken at Hawaii’s Mauna Loa volcano document a thirty-percent increase, since the beginning of the Industrial Revolution, in the atmospheric concentration of CO2. "There is no question," Barron said, "that this increase is due to the burning of fossil fuels—and, to a smaller extent, to deforestation."

There is some question about just what effect this massive jump in CO2, a green-house gas, has had on global climate. From the available data, Barron said, it appears that Earth is getting warmer. But taking surface temperature readings on a global scale is a tricky business. "There are plenty of ways for errors to creep into those measurements. Still, after factoring out as many errors as possible," it appears that the Earth’s temperature has crept up 0.5*C, (about 0.9*F) during this century.

Model Planet
An even bigger debate looms over the effects of rising CO2 levels on future climate. Here’s where Barron and other atmospheric scientists have to turn to computer simulations of the atmosphere and the oceans. These General Circulation Models, or GCMs, can’t settle the question of what sort of warming is going on right now, Barron explained. "What they can do is tell us what should happen, given what we know, in response to a wide variety of changes."

Because modeling is extremely computer-intensive, climate processes and feedbacks, in even the best GCMs, must be highly simplified. Important details are left out. "Cloud response, for example, is huge," Barron said, but clouds are a relatively small-scale physical process. "At our present level of resolution, anything smaller than the state of Pennsylvania is too small to get right."

What current models have produced is a range of possibilities, and taken together, these results provide a consensus: At current rates of increase of atmospheric CO2, global surface temperature will increase from 0.5 to 2.0*C over the next 50 years.

If CO2 doubles, as it will during the 21st century unless fossilfuel burning is seriously curbed, that warming will be between 1.5*C and 4.5*C. These may not sound like big numbers, but they are. At the high end of possibility, a change of four degrees C would match—although in the opposite direction—the global temperature difference between 1999 and the last Ice Age.

"So what do you do? What do you do when your best science says look out, the change will be enormous, but there’s all this room for error?"

Warm Response
One thing you do is work on improving your models. "Here at Penn State we’re developing a high-resolution regional-scale model," Barron said. During the next decade, such models will increasingly be deployed in combination with GCMs to fine-tune predictions and lower the possibility for error. In the meantime, however, "we have to talk in terms of probabilities."

It is very probable, he continued, that over the next 50 years, at a global level, surface temperature will increase, precipitation levels will increase, sea ice will shrink toward the poles, and sea levels will rise. "No one really questions these things. What we can’t predict with confidence is how much, and what the local-scale effects will be."

Does that mean there’s nothing to worry about?

"Here’s where it gets personal," Barron said. "Let’s say, as some models predict, the range for beech trees shifts a couple hundred miles north. There’s a range of possible responses. You might say, ÔThat’s unacceptable. I can’t live without beech trees.’ Or you might say, ÔHumans can adapt.’

"The potential human-level impacts of climate change are myriad—and they’re not just aesthetic. Climate is intimately connected to human health, for one. Milder winters would mean more deer in Pennsylvania. More deer mean more deer ticks. More ticks mean more Lyme disease.

"Or you could look at something like denguehemorrhagic fever. It’s delivered by a mosquito, which can’t live in cold winter areas. That means right now it’s limited to the southern hemisphere—Africa and South America, mostly. But if the Earth gets warmer, will dengue fever come to, say, Tennessee?

"In the end," Barron concluded, "how you respond to this information becomes a very personal decision. You have to decide how vulnerable you are to climate change. You have to weigh the probabilities. You have to decide how much risk you are willing to tolerate."

Eric J. Barron, Ph.D., is professor of geosciences and director of the Earth System Science Center in the College of Earth and Mineral Sciences, 248 Deike Building, University Park, PA 16802; 814-865-1619; eric@essc.psu.edu. He is a member of the National Research Council’s Board on Climate Change and chair of its Climate Research Committee, and a member of the Science Executive Committee for NASA’s Earth Orbiting Satellite.