Climate Controls

If we treated global warming as a technical problem instead of a moral outrage, we could cool the world.

Gregory Benford from the November 1997 issue

(Page 2 of 5)

Some geoengineering systems appear possible to deploy now, and at reasonable cost. They could be turned on and off quickly if we got unintended effects. It would be relatively easy to run small-scale experiments to answer questions about how our current atmosphere behaves when one alters the kind of dust, or aerosols, in it. Nuanced knowledge is crucial; the biosphere is a highly nonlinear system, one that has experienced climatic lurches before (glaciation, droughts) and can go into unstable modes, too.

Indeed, some critics argue that this simple fact precludes our tinkering with the "only Earth we have." Earth's climate might be chaotically unstable, so that a state with only slightly different beginning conditions would evolve to end up markedly different: An engineered early frost this year might mean an ice age the next. But we also know that Earth suffers natural injections of dust and aerosols from volcanoes, driving weather changes. Experiments that affect the planet within this range of natural variability could be allowed with little to no risk.

The simplest way to remove carbon dioxide, the main greenhouse gas, is to grow plants--preferably trees, since they tie up more of the gas in cellulose, meaning it will not return to the air within a season or two. Plants build themselves out of air and water, taking only a tiny fraction of their mass from the soil. Forests, which cover about a third of the land, have shrunk by a third in the last 10,000 years (though they have grown over the last half-century in the United States, mostly due to market forces).

Like the ocean, land plants hold about three times as much carbon as the atmosphere. While oceans take many centuries to exchange this mass with the air, flora take only a few years. As tropical societies clear the rain forest, the temperate nations have actually been growing more trees, slightly offsetting this effect. In the United States, we have lost about a quarter of our forest cover since Columbus, and replanting occurs mostly in the South, where pine trees are a big cash crop for the paper industry. But globally we destroy a forested acre every second. Just staying even with this loss demands a considerable planting program.

Trees soak up carbon fastest when young. Planting fast-growing species will give a big early effect, but what happens when they mature? Eventually they either die and rot on the ground, returning nutrients to the soil, or we burn them. If this burning replaces oil or coal burning, fine and good. Even felling all the trees still leaves some carbon stored longer as roots and lumber. Buildings can hold lumber out of this cycle for a century or so.

About half the U.S. carbon dioxide emissions could be captured if we grew tree crops on economically marginal croplands and pasture. More forests could enhance biodiversity, wildlife, and water quality (forests are natural filters); make for better recreation; and give us more natural wood products. Even better, one can do the cheapest part first, with land nobody uses now. This would cost about $5 billion a year, and a feel-good campaign would sell easily, with merchants able to proclaim their eco-virtue ("Buy a car, plant a grove of trees").

This would work reasonably well in the short run. But trees take water, and one must be careful not to exhaust the soil, so this is a solution with a clear horizon of about 40 years. Soaking up the world's present carbon dioxide increase solely through trees would take up an Australia-sized land area--that is, a continent. Most such land is in private hands, so the job cannot be done by government fiat. Still, a regional effort could make a perceptible dent in overall carbon dioxide levels.

The Geritol Solution

The oceans comprise the other great sink of greenhouse gases; some researchers estimate that they absorb 40 percent of fossil- fuel emissions. In coastal waters rich in runoff, plankton can swarm densely, a million in a drop of water. They color the sea brown and green where deltas form from big rivers, or cities dump their sewage. Tiny yet hugely important, plankton govern how well the sea harvests the sun's bounty, and so are the foundation of the ocean's food chain. Far offshore, the sea returns to its plankton-starved blue.

The oceans are huge drivers in the environmental equations, because within them the plankton process vast stores of gases. Though cause and effect are not quite clear, we do know that in ice ages, carbon dioxide levels dropped 30 percent.

Could we do this today? Driving carbon dioxide down should lower temperatures, certainly. But how?

The answer may lie not in the tropics but in the polar oceans, where huge reserves of key ingredients for plant growth--nitrates and phosphates--drift unused. The problem is not weak sunlight or bitter cold, but lack of iron. Electrons move readily in its presence, playing a leading role in trapping sunlight.

A radical fix would be to seed these oceans with dissolved iron dust. This may have been the trigger that caused the big carbon dioxide drop in the ice ages: The continents dried, so more dust blew into the oceans, carrying iron and stimulating plankton to absorb carbon dioxide. Mother Nature can be subtle.

Such an idea crosses the momentous boundary between quasi-natural mitigation such as tree planting and self-evidently artificial means. Here is the nub of it, the conceptual chasm. With a boast that may cost his cause dearly, the inventor of the idea, John Martin of the Moss Landing Marine Laboratories in California, said, "Give me half a tanker full of iron, and I'll give you another ice age."

The captured carbon gets tied up in a "standing crop" of plankton. These tiny creatures dwell within a few meters of the surface. To truly bury the gas, they must somehow carry it into the vast bulk of the whole ocean. Some biologists believe that from the plankton the carbon dioxide should slowly dissolve into the lower waters, though we are uncertain of this. Perhaps the carbon dioxide eventually is deposited on the seabed. This last process no one has checked. Somehow, though, a good deal
of carbon does end up in the deep ocean sinks.