There is significant interannual variability in the atmospheric concentrati
on of carbon dioxide (CO2) even when the effect of anthropogenic sources ha
s been accounted for. This variability is well correlated with the El Nino-
Southern Oscillation (ENSO) cycle. This behavior of the natural carbon cycl
e provides a valuable mechanism for validating carbon cycle models. The mod
el in turn is a valuable tool for examining the processes involved in the r
elationship between ENSO and the carbon cycle.
A GCM coupled climate-carbon cycle model is used to study the mechanisms in
volved. The model simulates the observed temperature, precipitation, and CO
2 response of the climate to the ENSO cycle. Climatic changes over land dur
ing El Nino events lead to decreased gross primary productivity and increas
ed plant and soil respiration, and hence the terrestrial biosphere becomes
a source of CO2 to the atmosphere. Conversely, during El Nino events, the o
cean becomes a sink of CO2 because of reduction of equatorial Pacific outga
ssing as a result of decreased upwelling of carbon-rich deep water. During
La Nina events the opposite occurs; the land becomes a sink and the ocean a
source of CO2.
The magnitude of the model's response is such that the terrestrial biospher
e releases about 1.8 GtC yr(-1) for an El Nino with a Nino-3 index of magni
tude 1 degreesC, and the oceans take up about 0.5 GtC yr(-1).(1 GtC = 10(15
) g of carbon). The net global response is thus an increase in atmospheric
CO2 of about 0.6 ppmv yr(-1). This is in close agreement with the sensitivi
ty of the observed CO2 record to ENSO events.