A Draft in the Greenhouse
Alaskans may be the least worried people on earth about global warming. On a chilly August afternoon or a frigid January night anywhere north of Dixon Entrance, a few degrees more warmth doesn't sound like a threat. It sounds like a promise.
Nevertheless, a shifted climate would affect us, and not always pleasantly. If sea levels rise significantly, for example, Homer Spit could become Homer Sandbar; the Yukon-Kuskokwim Delta might soon become Y-K Bay. So we too should keep a wary eye on those things that seem to be turning up the global thermostat.
Or not turning it up, as the case may be. And---suddenly---that may be the case with methane, one of the chief culprits in changing Earth's atmosphere into a heat-trapping greenhouse. According to a recent issue of the journal Science, the decades-long steady and spectacular increase in the amount of atmospheric methane has stopped.
In fact, it stopped in 1991. The rate of increase had been slowing during the 1980s, but measurements taken at 28 sites around the world, and along two shipping routes, showed that by 1992 the rate had plummeted in the Southern Hemisphere and reached zero in the Northern Hemisphere. Nobody is quite sure why.
Methane is released into the atmosphere by a variety of sources. Decomposition processes in wetlands, such as Alaska's muskeg and soggy tundra, are one source; such warming as we've seen so far has apparently increased the amount of methane output from the Arctic, so the decrease should not be because northern bogs are holding their breath, so to speak.
Other natural methane sources include the digestive processes of cattle and of termites, but the world seemed to hold no fewer termites nor less flatulent bovines in 1991 than 1981. Among the human-caused sources of excess methane is the decomposition associated with landfills, but again, our species is no less trashy than it's ever been. We're a bit more responsible with recycling and composting, but not yet enough to make the difference.
And the difference is significant. For the rate of increase to hit zero, as it did by 1992, meant that some 10 million tons of methane were no longer being released every year. This fortunate no-increase situation is not the same as a decline, please note. It means merely that, for now, the processes by which methane is added to and taken from the atmosphere have achieved a new balance. The actual amount of methane in the air is more than twice what it was 200 years ago. At present levels, it accounts for about a quarter of the warming effect that carbon dioxide does.
Of the explanations for the leveling-off of methane that atmospheric scientists find plausible, one amounts to the optimists' view and the other belongs to---well, they'd call themselves realists. The more hopeful explanation is that the world's leakiest natural gas pipelines have been fixed. It may be hard to imagine that anyone would tolerate leaks of that magnitude, but the pipelines of the huge Siberian gas fields and attendant distribution systems were infamously leaky. After a disastrous gas explosion in 1989, the Russian authorities directed attention and resources to fixing the leaks. By 1992, the optimists say, the Russians might have patched enough pipeline to zero out the increase in atmospheric methane.
The less optimistic note that the rate began to decline well before the Siberian pipes stopped leaking methane, if indeed they have stopped. They think something more complicated is going on, and point to a pair of other oddities in the concentrations of atmospheric gases. In 1991, the chronic rise in carbon dioxide concentrations abruptly slowed, and the amount of oxygen suddenly jumped. Biological effects? Mount Pinatubo's eruption? The interaction of all of the above? It's an exciting, if frustrating, time to work in atmospheric science.
