Budgeting Carbon Dioxide

Lately I've been razzed about my bias in favor of things northern. The comments arose because I asserted in a recent column that weather in the north during autumn 1492 helped make possible Columbus' encountering America. That, my critics thought, was pushing things a bit far.

Hah! It's possible to push much farther. For example: according to some authorities, the north may be saving the world from roasting. If they're right, this region may be responsible for holding down the supply of an important culprit in the greenhouse effect.

The authorities are Tare Takahashi, Pieter P. Tans, and Inez Fung, all experts on different aspects of carbon dioxide's interactions with the rest of the world. They base their idea on a study of how much carbon dioxide enters the atmosphere from what sources, and how much leaves the atmosphere into what sinks. (Anyone who puts off doing the dishes should like the idea of a "sink" as a final resting place, which is just about how scientists use the term.) If asked, they'd probably say they study the global carbon budget.

Like that of the U.S. government, the global carbon budget really hasn't balanced for years. Scientists can measure the amount of carbon dioxide in the atmosphere accurately to better than one part per million (which, a technically inclined bartender once noted, is like detecting a drop of vermouth in 16 gallons of gin). And they can estimate fairly accurately how many tons of carbon are emitted every year through industrial processes, fossil fuel burning, and deforestation.

With that much knowledge in hand, scientists should be able to calculate how much carbon dioxide will enter the air and how much will be there in a year or two. But nature and calculations don't agree. The observed rate of increase in atmospheric carbon dioxide is about half what it should be. The air should contain perhaps 25 parts per million more carbon dioxide than it does. Somewhere, somehow, the world is sopping up extra carbon dioxide. In effect, it's coming closer to balancing the carbon budget by making deposits in a hidden account.

Almost as soon as the carbon budget deficit was recognized, the ocean became the most popular candidate as the atmosphere's equivalent of a numbered Swiss account. The biological and physical reasons for that view are still strong---the ocean can certainly hold a great deal of dissolved carbon dioxide, and carbon in particles (such as fish scales or dead plankton) can literally sink to the bottom, to be out of the cycle for decades at least---but Takahashi, Tans, and Fung are not convinced. They believe that recent, more precise and more numerous measurements of atmospheric components, combined with newer, more complex computer models of global atmospheric circulation point away from the oceans as the most important sink for human-generated excess carbon dioxide. Given the timing and location of atmospheric carbon dioxide peaks and valleys, with less in northern summer and more in winter, they think the major sink for this important greenhouse gas is the northern forest.

To be fair, they take "northern forest" as just about any tree north of the Panama Canal, not just the great boreal forests of Alaska, Canada, Scandinavia, and Siberia. Their calculations need the regrowing forests of New England, too. Yet, judging by the numbers Takahashi's team offers in their publication on the idea, even the scruffy black spruces of interior Alaska might accomplish the required carbon-sinking. According to the group's calculations, each square yard of forest would need to catch and hold an additional one or two pounds of carbon over a period of 25 years.

Whatever the successful sink may be, it's gathering only half the greenhouse-building excess carbon dioxide. So even if our trees are responsible, they're not really saving us. They're only buying us time.