Science Briefs

Oh Where Oh Where Does the CO2 Go?

Carbon dioxide (CO2) is the principal greenhouse gas generated by anthropogenic activities. An accumulation of CO2 in Earth's atmosphere due to these activities over the last 120 years is widely believed to be responsible for the observed global warming of around 0.5°C during this period. A better understanding of both the contemporary and past ways in which carbon is cycled through the Earth system will enable us to improve our predictions of future CO2 abundances and hence of future global climate greenhouse gas change.

Schematic illustration of biogeochemical cycles; arrows denote major pathways of the biogeochemical cycling of trace gases.

In the past 200 years, atmospheric CO2 levels have increased by a quarter, going from 0.028% around 1700 to 0.035% in 1995. This increase has been caused by growing human populations using and burning increasing amounts of coal, oil and gas. Also, deforestation puts additional CO2 in the atmosphere, by burning of trees and by disturbing vegetation and soil dynamics, which permits the detritus of forest clearing to decompose and to oxidize the soil carbon.

However, when these sources of CO2 are tallied up, they are more than double the observed increase of CO2 in the atmosphere. Some of the anthropogenic CO2 is absorbed by the ocean, and the rest by the vegetation and soils. But how much? where? and by what mechanisms? Places where the CO2 winds up are termed "sinks". It is important to know how the anthropogenic CO2 sink is partitioned between land and sea because the lifetime of a molecule of carbon in vegetation is 10-100 times shorter than in the ocean. Thus, the CO2 that has gone into the land's biosphere may not stay there for long.

Scientists at the Goddard Institute for Space Studies (GISS) are attacking the problem from several approaches. Atmospheric transport models are being used to infer, from the way in which CO2 (and its isotopes) in the atmosphere varies geographically, where the CO2 sources and sinks may be located. Satellite observations of the land's surface are analyzed in order to understand how photosynthesis reacts to year-to-year changes in climate and whether the biosphere is changing its biomass on the long-term. GISS scientists are also collaborating with oceanographers to figure out where the oceans are absorbing and outgassing CO2. By analyzing all components of the atmosphere-land-ocean evidence together, we aim to constrain our interpretation of the carbon cycle.

Carbon dioxide from gas bubbles trapped in Greenland and Antarctic ice sheets shows variations in concentration that correspond to the glacial and interglacial cycles. Twenty thousand years ago, during the last glacial maximum, the atmospheric CO2 content was only 0.020%. By the beginning of the Holocene, ten thousand years ago, the atmospheric CO2 concentration had increased to 0.027%, very close to the pre-industrial value of 0.028%. Understanding how the carbon cycle dynamics have changed in past climates will also provide significant clues to how the carbon cycle may respond to future climate change.