On the one hand, greenhouse gases aren’t piling up in the atmosphere as fast as they used to. On the other hand, it can’t last.
Something surprising has happened: the accumulation of greenhouse gases in Earth’s atmosphere, the gases that trap heat and warm the planet, has slowed down. Carbon dioxide, the most important greenhouse gas, has in recent years been increasing at around half its previous rate. Methane, less abundant than carbon dioxide but 20 times as effective (molecule for molecule) at absorbing heat, may be on the verge of leveling off. The same is true of chlorofluorocarbons, which are better known as ozone destroyers but are the most efficient heat absorbers of all. Why aren’t climate mavens shouting joyfully from the rooftops? Because they don’t believe the good news will last. The recent changes, they say, are mostly natural fluctuations--the eruption of Mount Pinatubo is one possible cause--or unintentional side effects of human actions. Thus the welcome trend is unlikely to persist.
Chlorofluorocarbons are the one exception. Spurred by the dramatic growth of the ozone hole over the Antarctic in the mid-eighties, 128 countries have ratified an agreement, known as the Montreal Protocol, to phase out the production of the two main CFCs, CFC-11 and CFC-12, by 1996. The protocol has been remarkably effective. The National Oceanic and Atmospheric Administration maintains a network of monitoring stations around the world, and James Elkins and his colleagues at NOAA have been using it to measure CFCs in the atmosphere since 1977. In the first 11 years of their study, they found that the concentration of these compounds climbed steadily higher. But as production of CFCs fell off, their growth rate in the atmosphere quickly dropped. CFC-12 is still growing, but slowly; CFC-11 has now almost completely leveled off. We were predicting that levels would start to fall in 2000, but it looks now like we’re actually a couple years ahead of schedule, says Elkins.
As gratifying as the CFC slowing may be, it’s not terribly surprising. CFCs are synthetic chemicals and thus comparatively easy to control: you just stop making them. Their concentration in the atmosphere is measured in parts per trillion. The other greenhouse gases are far more abundant. They flow into the atmosphere from many different sources, both natural and artificial, and are removed from the atmosphere in just as many ways. All these factors affect the levels of the gases in the air at any moment, and many are barely understood.
Carbon dioxide in particular is confusing atmospheric scientists these days. The leading CO2-trend watcher is Charles Keeling, of the Scripps Institution of Oceanography in La Jolla, California, who started amassing a record of carbon dioxide on Mauna Loa in Hawaii in 1958. From that time to 1992 he found that the concentration of CO2 in the atmosphere increased from 315 parts per million to 356 parts per million. And the rate of increase itself increased, from about .7 parts per million per year in the 1960s to 1.4 parts per million per year in the 1980s. On the whole this acceleration was steady--as steady as the expansion of human industry and pollution--but every few years it was punctuated by sudden bursts of even greater CO2 input.
The bursts coincided with a phenomenon known as El Niño, a periodic shift in the direction of winds and ocean currents in the equatorial Pacific that has global effects on weather. Among those effects are droughts in parts of the tropics, such as Indonesia. Keeling and others believe that when vegetation in the drought-stricken areas dies and decays, it pumps about a billion metric tons of CO2 into the atmosphere. Each time El Niño disappears and the rains return, plants grow back, capturing more carbon dioxide than usual and slowing the gas’s accumulation in the atmosphere.
The latest El Niño began in 1990 and started to bring with it the usual CO2 acceleration. But then something baffling happened: although El Niño was still under way, the CO2 growth rate abruptly dropped from 1.4 parts per million per year back down to .5 in 1992. It was the most dramatic slowdown Keeling has ever seen. NOAA’s global network revealed that the slowing trend was more pronounced in the Northern Hemisphere than in the Southern.
Humans can take no credit for the slump. We’ve been steadily injecting about 7 billion metric tons of carbon dioxide annually into the atmosphere during the past few years, mostly through the burning of fossil fuels. The only hope researchers now have of explaining the CO2 slowdown is a climatological coincidence: the change came just after Mount Pinatubo erupted in June 1991. The volcano has already been fingered as a possible cause of the acceleration of ozone destruction and the cool temperatures that have since gripped the planet. Researchers wonder if yet another atmospheric change can be pinned on it.
There are at least two ideas about how the volcano might have affected CO2 levels. The rock Pinatubo hurled into the air contained a small percentage of iron, and iron is known to act as a fertilizer for plankton in some parts of the ocean. As the metal settled onto the sea surface, it may have caused these microscopic plants to grow rapidly, drawing CO2 out of the atmosphere. Unfortunately there is no evidence that this actually happened.
Alternatively, Pinatubo’s cooling shade of sulfuric acid droplets may have slowed the growth of soil bacteria on land. Since the bacteria eat dead plant matter and return carbon dioxide to the atmosphere, slowing them down would keep more CO2 out of the atmosphere. The fact that the CO2 slump was greater in the Northern Hemisphere fits this picture: there is more land and thus there are more soil bacteria in the north. That the little critters have actually been eating at a slower rate, however, has not been shown.
The reasons for the methane slump are almost as uncertain. Methane comes from a dizzying range of sources. It is produced by bacteria that live in oxygen-free environments such as wetlands, rice paddies, landfills, and the guts of cows. It is also released when vegetation burns, and it is the principal component of natural gas. Since the beginning of the industrial revolution its concentration in the atmosphere has more than doubled, from less than 800 parts per billion 200 years ago to about 1,700 parts per billion in 1992.
Four years ago two researchers made a surprising announcement: the increase in methane has been sagging for at least a decade. Like the NOAA researchers, Aslam Khalil and Rheinhold Rasmussen of the Oregon Graduate Institute have been measuring atmospheric levels of gases at stations around the globe since 1978. In 1981, they found, methane concentrations rose by 20 parts per billion. In 1990 the increase was only 11 parts per billion. These trends, Khalil says, probably reflect changes in agricultural practices that produce methane. For a variety of reasons, the total number of acres of rice being harvested hasn’t increased much in a decade, he points out. And while the world cattle population has doubled since the turn of the century, it hasn’t grown at all in the past eight years.
Since Khalil and Rasmussen reported their findings, however, the methane growth rate has really nose-dived. NOAA researchers now report that methane concentrations increased by only 4.7 parts per billion in 1992. In the Northern Hemisphere the levels increased by only 1.8 parts per billion. And while the climatologists are still putting together the numbers for 1993, they’ve found no evidence so far of a general reversal of this downward trend; at some stations the methane concentration didn’t increase at all last year.
Ed Dlugokencky, the atmospheric chemist in charge of NOAA’s methane measurements, believes that something other than cows and rice paddies must be behind this collapse. The changes were mostly happening in the industrialized latitudes, he notes, which suggests they might be attributed to changes in fossil fuel production. Dlugokencky points specifically to a pipeline network built by the former Soviet Union in the 1970s to carry natural gas from Siberia to cities in western Russia. The shoddily constructed pipes, he says, may have leaked untold amounts of methane--until 1989, when an explosion prompted the Russians to enlist Western corporations to help fix the leaks. Dlugokencky thinks those repairs alone could have substantially curtailed the methane growth rate. Since he has no hard numbers to back up this hypothesis, though (NOAA has no sampling sites in Russia), he’s not yet ruling out other causes.
Pinatubo is one alternative; the cool temperatures it produced may have stunted the growth of methane-producing bacteria in wetlands as well as that of CO2-producing bacteria in soil. But there is also a weirder possibility: human beings may be slowing the growth of methane by destroying the ozone layer. With less ozone in the stratosphere, more ultraviolet radiation has penetrated into the lower atmosphere. The UV rays destroy ozone there just as they do in the stratosphere, releasing excited oxygen atoms. Those atoms, in turn, react with water to form hydroxyl (OH)- -a highly reactive molecule that seeks out and destroys many atmospheric compounds, including methane. As ozone is depleted, some researchers suggest, more UV rays penetrate the stratosphere, producing more OH that destroys more methane.
Everything is connected, in other words, which is one reason climate researchers are in a bit of a muddle these days when it comes to understanding the recent changes in the atmosphere. Yet most of them agree on one thing: at best the changes will only slow down global warming. Pinatubo’s shade is lifting, and with it any possible effects on carbon dioxide and methane. CFCs will continue to decline, but as the ozone layer slowly heals itself, any downward push it may be exerting on methane will fade as well. Methane may level off anyway, or its concentration may start increasing again at an accelerating rate--no one knows for sure.
But the situation with carbon dioxide, by far the most significant greenhouse gas, is fundamentally different. Its concentration in the atmosphere will continue to rise. Indeed, its rate of increase seems to be rebounding upward again after the mysterious plunge of the last few years. As long as we keep putting 7 billion tons of CO2 into the atmosphere each year, a habit we show no signs of abandoning, the long-term forecast for planet Earth will remain reasonably clear. The outlook is for a warmer planet in the next century.