May 31, 2007 -- Scientific conferences on the subject of climate change sometimes turn into intellectual free-for-alls, as experts who look at the same data reach opposite conclusions.
But believe it or not, that's an improvement over the situation just a decade ago, when climate scientists debated fiercely over whether current trends toward warmer temperatures were human-driven or just natural fluctuations in global climate.
There is little debate on that subject now. Few scientists today doubt that global climate change is at least partly anthropogenic, meaning we did it.
But uncertainty reigns supreme on even some of the most fundamental questions. Richard Houghton, deputy director and senior scientist at the Woods Hole Research Center, is an expert on Earth's carbon cycle, the process by which the planet and its atmosphere absorb and release carbon in various forms. That includes carbon dioxide, the primary cause of the greenhouse effect.
How Bad, How Soon?
Houghton is the author of a comprehensive report detailing the uncertainties over what humans have already done to the planet and what the future holds. The report, published in the Annual Review of Earth and Planetary Science, surveys scores of major studies and is laced with expressions like "agreement is poor" and "the results are mixed."
That's of great concern to him, as well as to many others, because if we don't really understand exactly what is happening, and what to expect in the future, we're not likely to succeed in mitigating the impact and managing such critical elements as the carbon cycle. Houghton ends his report with this basic question:
"Will we still be able to manage the carbon cycle by the time we have the answers?"
Forests and Oceans
The disagreement among experts includes some of the most basic issues. Some argue, for example, that increases in atmospheric carbon dioxide will cause forests and plants to grow more robustly, because carbon is the basis for organic material and warmth induces more growth. But other experts argue that the evidence shows there's only so much the forests can do, and the greenhouse effect will continue unabated.
Houghton says that forests are "particularly important as a carbon reservoir because trees hold much more carbon per unit area than other types of vegetation." But he suggests that at best trees and plants will only buy a little time because eventually they will die and rot and return carbon to the atmosphere. And as trees age, they are less efficient at sequestering carbon.
Many factors affect plant growth, and the resulting "terrestrial sinks," such as forest fires and drought, so it is "exceedingly difficult if not impossible" to determine their true impact, he writes.
But what about the oceans? The seas already contain nearly 50 times more carbon than the atmosphere, according to Houghton, and are the primary carbon sinks on the planet. Most carbon that falls on the oceans is taken up by near surface creatures and transported to deep waters that are rich with carbon. But will the seas continue to sequester the carbon released by burning fossil fuels? Houghton cites evidence that "the efficiency of oceanic uptake may also have declined."
Where to Put Carbon
Some experts argue that a warmer planet will make the oceans less capable of absorbing carbon, and thus accelerate climate change.
There are several proposals for developing artificial sinks to sequester carbon and lock it up until the problem goes away. Enrich the oceans with iron, for example, and they should be able to take up more carbon. But Houghton isn't convinced.
"The technical and economic aspects of an operational sequestration program require considerable research," he writes.
In other words, don't bank on it.
While many of the most important questions remain unanswered, progress has been made in some areas. In the early years, scientists didn't even include clouds in their computer models, because nobody knew how to do that. But perhaps the greatest advance has been a clear and candid recognition throughout the scientific community, and in the populace as well, that this is a real problem, anthropogenic in nature, and it isn't going to go away.
And in fact, climate change could accelerate dramatically as it begins to feed upon itself, as some experts have predicted.
"The natural processes on land and in the oceans that have removed carbon dioxide from the atmosphere for the last century may be starting to weaken," Houghton says.
If so, we've turned a major corner, creating a planet that can no longer heal itself.