No one disputes that the level of the greenhouse gas carbon dioxide in the atmosphere is rising. But many people don't know that rise is subject to annual cycles. A new study indicates that those once-predictable annual cycles may be getting less predictable, with much wider variation between the seasons -- a possible indicator that the Earth's climate is destabilizing.
In spring, plants remove CO2 from the atmosphere as they grow, and then they release it again in fall and winter. The result is an annual increase and decrease in CO2 in the atmosphere, shown as the tightly packed waves on the graph above. The curve on that graph is called a "Keeling Curve," after the late atmospheric chemist Charles David Keeling, who founded the Mauna Loa Observatory station where the increase in atmospheric CO2 was first noted.
A study conducted by Scripps Institution of Oceanography at UC San Diego, published today in the journal Science, compared seasonal CO2 variations in atmospheric samples gathered above the Arctic and North Atlantic Oceans northward of the 45th parallel in the late 1950s and early 1960s, and then compared them to more recent samples.
The result? There's a significantly wider swing between annual lows and highs now, as much as 50 percent wider than back during the Eisenhower administration.
Researchers from the National Oceanic and Atmospheric Administration (NOAA) and the Cooperative Institute for Research in the Environmental Sciences (CIRES) also took part in the study.
The reason for the increase in those seasonal CO2 swings isn't quite clear, but it's very likely that plants in the Northern hemisphere are responding to increased CO2 levels by taking up much more of the gas during the growing season. Plants, as you may remember from your high school biology classes, use the energy in sunlight in a process called photosynthesis to chemically combine CO2 and water to create sugars and starches, with oxygen as the waste byproduct. (Which is why we can breathe. Thank you, plants.) But when they're not photosynthesizing, plants take those sugars and starches and burn them as cellular fuel to keep themselves alive in the metabolic process called "respiration." When they do so, they give off CO2 the same way we do, and that CO2 goes right back into the atmosphere.
What's more, vegetation often dies back during the winter in colder climates. Trees and shrubs drop their leaves, and herbaceous plants die back to the ground. That means each winter starts with a whole lot of dead and dying vegetative matter that almost immediately starts being broken down by decomposer organisms like insects and other animals, fungi, and bacteria. That decomposition can release most of the carbon stored in the dead plant matter into the atmosphere as CO2.
So it would seem that at least in the colder parts of the Northern Hemisphere, plants are eating up a bit more CO2 during the growing season, then breathing more of it back out during the cold season, than they were in 1960. Part of the reason may be that trees can grow a bit farther north than before as the planet warms.
"The atmospheric CO2 observations are important because they show the combined effect of ecological changes over large regions," said the study's lead author Heather Graven, a postdoctoral researcher in the Scripps CO2 Program. (That program is led by geochemist Ralph Keeling, son of Mauna Loa founder Charles David.) "This reinforces ground-based studies that show substantial changes are occurring as a result of rising CO2 concentrations, warming temperatures, and changing land management," added Graven, "including the expansion of forests in some regions and the poleward migration of ecosystems."
Though plant growth changes as described above surely account for some of the increase in seasonal CO2 differences, say the researchers, the shift is far more than can be accounted for by increased seasonal plant productivity or poleward forest shift. Other possibilities include plants growing larger leaves in a high-CO2 environment, changes in timing of photosynthesis and respiration, or changes in the species makeup of ecosystems.
One thing's for sure: whatever changes we're making by hacking the atmosphere, they're going to be complicated.