For the past 39 years Charles Keeling and his colleagues at the Scripps Institution of Oceanography in La Jolla, California, have been scrupulously keeping track of carbon dioxide in the atmosphere. No one had done that before, so when we want to know CO2 levels from before 1958, we have to resort to imperfect records such as ice cores from Antarctica. They suggest that in 1750, before the industrial age, CO2 made up 280 parts per million of the air. When Keeling began his work, fossil fuel emissions and other sources had boosted the level to 315, and today it is 360. Global temperatures have also been rising, and most climatologists think the culprit is a greenhouse effect created in large part by the carbon dioxide.
This past summer, however, Keeling’s careful record revealed a surprising new wrinkle on global warming: it appears to be changing the course of the growing seasons of plants, making spring come a little earlier than it used to.
The annual growth of plants in the Northern Hemisphere has a marked effect on atmospheric CO2. Springtime photosynthesis uses up CO2, converting it into wood, stems, and leaves. In the fall and winter, as the plants shut down or die and are decomposed, a pulse of carbon returns to the air. The graph Keeling has been recording all these years therefore shows a rising sawtooth pattern.
But when Keeling and his colleagues factored out the rising caused by the burning of fossil fuels and analyzed the annual sawteeth in detail, they got a surprising result. The midpoint of each tooth, when plant growth is at a maximum and atmospheric CO2 is falling fastest, now comes about seven days earlier in the year than it did in the mid-1970s. Keeling thinks the best explanation for this change is that the growing season is starting earlier. The shift we measured was later on, when the plants are removing carbon dioxide at a more rapid rate, because that is easier to spot in the data. But how did they get to that stage? The easiest way to explain that is they put their leaves on earlier.
Although the link to global warming is hard to prove, Keeling’s earlier springtime jibes well with a report from James Hansen of nasa’s Goddard Institute for Space Studies in New York City that winter and spring temperatures in the high northern latitudes have increased by 4 degrees over the last 30 years. Because the temperatures are warmer, it is greening up faster in the spring, and the plants are getting a head start on their growth, Keeling says.
That head start seems to be having an effect on how much carbon plants are pulling out of the atmosphere. Keeling’s team found that the difference between the summertime lows and wintertime highs of CO2 readings has increased by up to 40 percent. In a place like Canada there is a very short growing season, Keeling says. If you add seven days, you are adding a substantial amount of time. In a place like Canada, an earlier spring might be welcome. In other parts of the world, though, the effects of global warming are not likely to be as pretty.