Nothing illustrates more strikingly the atmosphere’s tremendous reactivity than the seasonal change seen in the carbon dioxide concentration. The range of change is a full 3 percent every year! This discovery, made by Charles David Keeling while attempting to measure the precise level of carbon dioxide in the atmosphere, was a complete surprise.
Dr. Keeling was asked to determine precisely how much carbon dioxide is in the atmosphere. The idea was to make similar measurements some years later, to see whether carbon dioxide might be rising from human emissions. Everyone naturally assumed that a precise measurement would settle the matter for some time, because the atmosphere is well mixed and there was no reason to think that concentrations would change perceptibly from one year to the next and especially not within the same year.
Fortunately for carbon cyclescience, Dr. Keeling tended to be extremely skeptical toward all assumptions, reasonable or not. So, having already gone to the trouble of building an instrument for high-precision measurement, he decided to make measurements at least through a full year. After re-rechecking the instruments to exclude any malfunction, it was clear that the carbon dioxide changes seasonally over quite a large range. In addition, continuing the measurements showed that the values drift upward from one year to the next. After these discoveries, the science of the carbon cycle had changed forever. Since then, the "Keeling curve" has become the symbol of the ever-changing chemistry of the atmosphere and the associated warming of the planet.
The next problem was to figure out why the carbon dioxide varied the way it did during the year (i.e. the little squiggles). Was it the ocean with its large reservoir, warming and cooling? Or was it processes on land, having to do with plant growth? There are ways to distinguish between the two possibilities, and the answer is actually land plants. Since most of the land is in the northern hemisphere, the fluctuations are greatest here. (If the ocean were to blame, we should see a larger effect in the southern hemisphere.) Every spring, when trees leaf out and grasslands and farmlands green, the carbon dioxide in the air decreases, reflecting the demand from photosynthesis. Conversely, in fall, when leaves and wilted plants are returned to the soil and decay, the carbon dioxide rises again. Gaia “breathes” on an annual cycle, and we can measure how deeply.
Measurements of CO2 at Mauna Loa, Hawaii clearly show the annual “breathing” of Northern Hemisphere land plants. (From: Dave Keeling and Tim Whorf at Scripps Institution of Oceanography)
With the atmospheric reservoir so small that one can see the annual change from trading carbon with the biosphere, we should perhaps expect an equally vigorous exchange with the ocean. In fact, such an exchange exists and it results in a rather short residence time of the carbon in the atmosphere, about 10 years or somewhat less. The residence time is determined from the proportion of radioactive carbon in the atmosphere compared with the proportion of radiocarbon in the ocean. Recall that radiocarbon is only made in the atmosphere (from bombardment of nitrogen atoms with cosmic rays). It survives a limited time (half of it decays back to nitrogen in 6000 years). So, if the ocean is to hold as much radiocarbon as is observed, it cannot take a long time to get there. Other methods of determining the residence time all confirm this exchange rate, originally based on the radiocarbon studies of Dr. Hans Suess at UCSD.