Features: Atmospheric oxygen--an endangered resource?

B R E A T H I N G  E A S Y 
Et tu, O2?

With forest resources--"the lungs of the Earth"-- under attack in many regions, some have raised concerns about the planet's oxygen supply. A leading geochemist assesses these claims and finds that we can probably breathe easy

By WALLACE S. BROECKER

AN OFT-HEARD WARNING with regard to our planet's future is that by cutting back tropical forests we put our supply of oxygen gas at risk. Many good reasons exist for placing deforestation near the top of our list of environmental sins, but fortunately the fate of the Earth's O2 supply does not hang in the balance. Simply put, our atmosphere is endowed with such an enormous reserve of this gas that even if we were to burn all our fossil fuel reserves, all our trees, and all the organic matter stored in soils, we would use up only a few percent of the available O2. No matter how foolishly we treat our environmental heritage, we simply don't have the capacity to put more than a small dent in our O2 supply. Furthermore, the Earth's forests do not play a dominant role in maintaining O2 reserves, because they consume just as much of this gas as they produce. In the tropics, ants, termites, bacteria, and fungi eat nearly the entire photosynthetic O2 product. Only a tiny fraction of the organic matter they produce accumulates in swamps and soils or is carried down the rivers for burial on the sea floor.
While no danger exists that our O2 reserve will be depleted, nevertheless the O2 content of our atmosphere is slowly declining--so slowly that a sufficiently accurate technique to measure this change wasn't developed until the late 1980s. Ralph Keeling, its developer, showed that between 1989 and 1994 the O2 content of the atmosphere decreased at an average annual rate of 2 parts per million. Considering that the atmosphere contains 210,000 parts per million, one can see why this measurement proved so difficult.
This drop was not unexpected, for the combustion of fossil fuels destroys O2. For each 100 atoms of fossil-fuel carbon burned, about 140 molecules of O2 are consumed. The surprise came when Keeling's measurements showed that the rate of decline of O2 was only about two-thirds of that attributable to fossil-fuel combustion during this period. Only one explanation can be given for this observation: Losses of biomass through deforestation must have been outweighed by a fattening of biomass elsewhere, termed global "greening" by geochemists. Although the details as to just how and where remain obscure, the buildup of extra CO2 in our atmosphere and of extra fixed nitrogen in our soils probably allows plants to grow a bit faster than before, leading to a greater storage of carbon in tree wood and soil humus. For each atom of extra carbon stored in this way, roughly one molecule of extra oxygen accumulates in the atmosphere.
At first glance, this finding appeared to be good news to those worried about the climatic effects of the ongoing buildup of anthropogenic CO2 in the atmosphere, for it suggested that during this five-year period an amount of carbon equal to one-third of that burned for energy production had taken up residence in the biosphere. As another third was taken up by the ocean, this meant that between 1989 and 1994 only one-third of the CO2 we produced by burning fossil fuels accumulated in the atmosphere. However, this enormous biospheric storage is likely an anomaly reflecting an unusual climate, perhaps related to persistent El Niño conditions or emissions by the volcano Pinatubo. A burst of plant growth during this period allowed carbon storage to exceed respiratory losses temporarily, but once climate conditions return to normal the products of this burst will be eaten up, releasing this carbon stored in organic matter back into the atmosphere as CO2 gas. Thus, we can't use Keeling's observation as evidence that the biosphere will serve as a major sink for the CO2 we generate. But through Keeling's O2 measurements we now have a reliable means to monitor the ongoing changes in global biomass. Eventually his record will allow us to diagnose the response of the Earth's biomass to changing climate and nutrient availability.
While my research deals primarily with the Earth's geochemical cycles, four years ago I became interested in what was going on in Biosphere 2 (as I have described elsewhere), then operated as a matter-sealed habitat. As widely heralded in the press, the inhabitants of Bio2 ran short of O2. Sixteen months into Bio2's two-year mission, tank trucks carrying liquid O2 had to be brought in to replace the 10 or so tons of O2 that had disappeared from Bio2's atmosphere. At the time of this injection, the eight Biospherians were breathing air containing only 14 percent O2 (ours contains 21 percent)--as if they were living at 17,000 feet elevation.
One difference is that while the Earth's plants produce through photosynthesis an amount of O2 equal to that of the atmosphere in about 2,000 years, those in Bio2 complete this task in only one year. Hence, the Earth's oxygen supply is far better buffered against change than Bio2's. Another difference is that the Earth's ecosystems have over the eons achieved a balance between photosynthetic production and respiratory consumption of O2; the tendency for the atmospheric O2 reserve to change is quite small. By contrast, the designers of Bio2 so generously stocked its soils with humus that the hungry bacteria were able to outcompete its fledgling plants. Each day roughly 30 percent more O2 was lost to respiration than was gained from photosynthesis. Taken together with its low storage capacity, this imbalance caused the O2 content of Bio2's air to drop at an alarming rate. Torn between the desires to maintain a matter-sealed environment and to protect the health of the Biospherians, the managers had to yield to the health needs and pipe in replacement oxygen.
The question naturally arises as to whether the Earth has ever experienced an oxygen emergency. Unfortunately, no one has come up with a reliable paleo-O2 proxy. A decade ago, the claim that bubbles trapped in amber preserved 60 million-year-old air generated excitement in the geochemical community, but this claim quickly faded with the discovery that the gas in these bubbles exchanges with that in the surroundings once each thousand years or so. In the absence of a valid proxy, our knowledge of possible fluctuations in the Earth's O2 reserves is based on inferences made from isotope ratio measurements conducted on the element carbon contained in ancient limestones (calcium carbonate) and on the element sulfur contained in ancient evaporites (calcium sulfate). While not telling us even the sense of the O2 content changes, let alone their magnitude, this approach allows a strong argument that variations in O2 have surely occurred.
The basis for these inferences is that the O2 in our atmosphere polices the flow of oxidized and reduced matter from the ocean atmosphere system to sediments. If for some reason the combined oxidation state of the carbon and sulfur leaving this system does not match that of the material supplied by erosion and volcanism, then atmospheric O2 takes up the slack. If for example too much carbon is being buried as organic residues, the O2 coproduced during photosynthesis accumulates in the atmosphere. This buildup would eventually allow O2 to invade those anaerobic nooks and crannies where the extra organic matter is being buried and thus restore a balance between the oxidation state of the output and input material.
Many reasons might be given for swings in the atmosphere's O2 content. One in particular stands out. As life evolved, the makeup of the food web responsible for the degradation of the organic matter manufactured by plants has changed. A startling example of such a change is found at the geologic boundary separating the Paleozoic and Mesozoic periods. At this point in planetary history, life experienced a terrible catastrophe. Ninety percent of species living in the ocean and 70 percent of those on land suddenly died out, never to reappear. This loss must have totally disrupted the finely tuned food web that had developed over the 300-million-year-long Paleozoic period. Suddenly most of the key players were gone. Replacements surely stepped in on short notice, but their scheme for dividing up the food supply must surely have been different and at least initially less efficient. Indeed, this is what the isotopes of carbon and sulfur tell us. Before the extinction event, the reduced material accumulating in sediments was dominated by carbon; afterward, iron sulfide shared a far greater part of the pie. It would be surprising if a major police action by the atmosphere's O2 was not required in order to re-establish a smooth flow of electrons through the ocean-atmosphere system.

Perhaps the most amazing thing about our planet is that we have any O2 at all. The cloud of gas and dust from which our solar system formed was dominated by hydrogen gas. As hydrogen atoms eagerly donate electrons to any element capable of latching onto them, our Earth was constructed from highly reduced (electron-rich) material. Except at its very surface, it remains so. Were our atmosphere, ocean, and soils stirred back into the Earth's interior, our O2 would be totally annihilated. Just how O2 came into being remains a mystery. The most likely explanation is that water molecules that wandered to the outer edges of the atmosphere were knocked apart by ultraviolet rays from the Sun. The light hydrogen atoms were able to evaporate to space, while the much heavier oxygen atoms were bound to Earth by gravity and mated with the reduced sulfur and carbon exposed at the Earth's surface. Only when this conversion had been completed could O2 begin to accumulate in our atmosphere. Records kept in sediments tell us this task took at least 2.5 billion years (more than half of geologic time). The evolution of multicellular organisms, and hence of our ancestors, awaited this transition from an O2-free to an O2-bearing atmosphere. Fortunately, this buildup was large enough that the Earth, unlike Bio2, became endowed with an adequate supply of this precious gas. The size of this inventory has surely varied, but once established it has never dipped low enough to threaten the existence of those who depend on it, nor will it in the future.

Note: See also "The Biosphere and Me," Prof. Broecker's account of his involvement with Biosphere 2 in the Geological Society of America's magazine GSA Today (which requires the Adobe Acrobat document reader; download and save, renaming with the extension ".pdf").
WALLACE S. BROECKER, Ph.D., is Newberry Professor of Geology at Columbia's Lamont-Doherty Earth Observatory and chief scientist at Biosphere 2. He received both the 1996 Blue Planet Prize and the National Medal of Science for achievements in global environmental research.
PHOTO CREDITS: Howard Roberts (montage); Bill Millard (Bio2).