November 2005

HARVARD GAZETTE ARCHIVES

Current Issue: October 06, 2005

Schrag's talk paralleled the new exhibit, "Climate Change: Our Global Experiment," which was developed in conjunction with the Harvard Center for the Environment. The exhibit opened Oct. 1 and highlights the work of Schrag and other scientists on the topic.

"A key element of our mission is to present current science on issues of importance," said HMNH Executive Director Elizabeth Werby in introducing Schrag. "It [the exhibit] is incredibly timely and incredibly urgent."

Schrag drew on the lessons offered by Venus' hot atmosphere, Mars' cold one, and Earth's own past of fire and ice to illustrate how atmospheric carbon dioxide has affected planetary temperatures.

Carbon dioxide in the atmosphere acts much like the glass in a greenhouse, letting the sun's rays in and preventing their energy, in the form of heat, from radiating back out into space.

Venus, with surface temperatures of 460 degrees Celsius, is an example of a planet where the greenhouse effect has run out of control. Though Venus is closer to the sun than Earth is, its high temperature is largely the result of its atmosphere trapping and holding the sun's energy. The atmosphere is about 100 times denser than the Earth's and is made up almost entirely of carbon dioxide.

At the other extreme is Mars, with surface temperatures of minus 50 degrees Celsius. Though Mars' atmosphere is also largely made up of carbon dioxide, it is 100 times thinner than the Earth's, making it unable to retain enough heat to warm it up.

The Earth itself has gone through wide swings in temperature, also connected to the levels of carbon dioxide. During ice ages over the past 400,000 years, Schrag said, carbon dioxide levels fell as ice sheets grew and rose when they melted, staying between roughly 180 parts per million (ppm) and 280 ppm.

On at least two occasions in the distant past, the Earth's cooling has reached a tipping point and run out of control, Schrag said. Evidence shows that once glaciers have covered about half of the Earth's surface, their ability to reflect the sun's rays created a runaway cooling that froze the entire planet.

The "snowball Earth" condition lasted for millions of years until the planet was rescued by its volcanism. Despite the icy covering, volcanoes, driven by heat deep within the planet, would continue to erupt, spewing carbon dioxide into the atmosphere.

Once in the air, the carbon dioxide slowly built up in the atmosphere until it grew warm enough to melt the ice.

But it may be a 60-million-year-long global "heat wave" that provides the closest analogy for what's to come.

During this time, called the Eocene epoch, scientists believe carbon dioxide levels were about 500 ppm, approximating those expected 100 years from now.

The Eocene climate was very warm with pine forests growing in Antarctica, palm trees in Wyoming, and crocodiles swimming in the now icy seas off of Greenland.

Scientists are particularly interested in a dramatic warming period at the beginning of the Eocene, when global temperatures shot up 8 to 10 degrees Celsius in the geologically short period of 10,000 years. Some researchers believe the temperature spike resulted from a bubble of greenhouse gas - perhaps methane - bursting from the ocean floor or from the relatively sudden drying of what had been an ocean after India collided with the Asian continent, releasing greenhouse gases as organic material on the sea floor dried and decomposed.

Whatever the cause, Schrag said the event is similar to what's going on now, except that now we are increasing greenhouse gases more rapidly with the burning of fossil fuels.

"We're doing it more quickly by burning coal, oil, and gas, but it's really the same thing," Schrag said.

Scientists have seen the levels of carbon dioxide in the Earth's atmosphere rise over the past few decades, from a level of about 315 parts per million in 1955 to about 380 ppm today. Projections show a "best-case" scenario of carbon dioxide levels topping out at about 500 ppm - similar to levels in the Eocene - by 2100.

"We don't think carbon dioxide has been much higher than 300 parts per million for 400,000 years, and not much higher than that for 30 million years," Schrag said.

The worst-case scenario couples today's energy use patterns with rapid economic growth in large developing nations like India and China and would result in carbon dioxide levels as high as 1,000 ppm.

Schrag admits that scientists don't know everything about what's happening and can't predict clearly what the future holds. But to skeptics who point out the flaws in today's knowledge as a reason for inaction, Schrag said, understanding things perfectly doesn't necessarily mean things will get better.

"We don't understand everything about the Earth, that's true, but that's not necessarily going to help us," Schrag said.

The most worrying evidence to Schrag is the melting of tropical mountain glaciers, such as the one on top of Mount Kilimanjaro in Tanzania. Evidence has shown that these glaciers - which have been in place for thousands of years - are melting, some of them quite quickly.

Scientists believe that the global average temperature will increase between 1 and 6 degrees Celsius by century's end. That may not sound like a lot, Schrag said, but global temperature was just 4 to 5 degrees Celsius lower during the last glacial maximum, when the Boston area was buried in ice.

That means the coming temperature change is potentially dramatic. Sea levels could rise by one-and-a-half meters owing to expansion of warming seas. Should either the glacier covering Greenland or part of Antarctica collapse and slide into the sea, sea levels could rise far more, six to seven meters.

Schrag said that the greatest impact will likely be on the poor, who have fewer resources with which to adapt to changes, but that doesn't mean the wealthy will be unaffected.

"In general, people will be worst affected by climate change when they are too poor to adapt to climate change," Schrag said. "But rich people will be affected by climate change too, in ways that will cost a lot."

A problem in any effort to address the issue, Schrag said, is the inertia built into both the Earth's atmospheric systems and in the world's energy use habits. Atmospheric systems take a long time to change and will respond to efforts to lower carbon dioxide only slowly. In addition, any reforms in energy use are unlikely to immediately decommission existing power plants or to stop fast-developing nations like China - which is set to surpass the United States as the world's largest carbon dioxide emitter in the near future - from investing in economic growth.

Possible solutions include capturing the carbon dioxide emitted by these plants and burying it in the earth's crust, increasing efficiency of energy use, and using more renewable power. Schrag said all these will be needed.

"We don't have forever to make these choices," Schrag said. "We're performing an experiment on the planet that hasn't been done for millions or tens of millions of years."

Daniel Schrag, Climate Change, October 2005

November 7, 2005