Alarmed, nations of the world now are drawing up new conservation policies to reduce fossil fuel burning, in hopes of reducing greenhouse gases in the atmosphere. But David Schimel of the National Center for Atmospheric Research, a co-author of the new study, says that before determining how much to reduce fossil fuel burning we should consider the effects of natural climate variations on the ability of plants to absorb CO2.
Schimel said satellite measurements of CO2, plant growth and temperature show that natural warming events such as El Ni No at first cause more CO2 to be released into the atmosphere, probably as the result of accelerated decay of dead plant matter in the soil. But later, within two years, there is an explosion of growth in forests and grasslands, which means plants suck more carbon dioxide out of the atmosphere.

"We think that there is a delayed response in vegetation and soil to the warming effects of such phenomena as El Ni No, and this leads to increased plant growth," said Schimel.
However, he said, it is not clear whether the warming by El Ni NO causes a net decrease in the build-up of CO2 over the long haul. "We don't really know that yet," said Schimel. What the study does show, however, is that the rise and fall of CO2 in the atmosphere is strongly influenced by natural changes in global temperature, said B. H. Braswell of the University of New Hampshire, another co-author of the study.
Braswell said that in years when the global weather is cooler than normal, there is a decrease in both the decay of dead plants and in new plant growth. This causes an effect that is the opposite of El Ni No warming: CO2 atmosphere levels first decline and later increase.