Faster carbon dioxide emissions will overwhelm capacity of land and ocean to absorb carbon

One in a new generation of computer climate models that include the effects of Earth’s carbon cycle indicates there are limits to the planet’s ability to absorb increased emissions of carbon dioxide.


"Plants are happy growing at a certain rate, and though they can accelerate to a certain extent with more CO2, the rate is limited by metabolic reactions in the plant, by water and nutrient availability, et cetera."

In addition, increasing temperatures and drought frequencies lower plant uptake of CO2 as plants breathe in less to conserve water. A second study she and colleagues published last week in PNAS report evidence for this temperature and drought effect. Since 1982, a greening of the Northern Hemisphere has occurred each spring and summer (except for 1992 and 1993, after Mt. Pinatubo erupted) as the climate has steadily warmed. As a result, there is a small but steady decline in atmospheric CO2 each growing season due to increasing photosynthesis at temperate latitudes in the northern hemisphere. When Fung and a team of her former and current post-doctoral fellows took a detailed look at this phenomenon, however, they discovered that since 1994, enhanced uptake of CO2 as photosynthesis revved up in the warm wet springs was offset by decreasing CO2 uptake during summers, which became increasingly hot and dry - an unsuspected browning in the past 10 years.

"This negative effect of hot, dry summers completely wiped out the benefits of warm, wet springs," Fung said. "A warming climate does not necessarily lead to higher CO2 growing-season uptake, even in temperate areas such as North America."

In the climate modeling study published this week in PNAS, she and colleagues found that similar water stress could slow the uptake of CO2 by terrestrial vegetation, and at some point, the rate of fossil fuel CO2 emissions will outstrip the ability of the vegetation to keep up, leading to a rise in atmospheric CO2, increased greenhouse temperatures and increased frequency of droughts. An amplifying loop leads to ever higher temperatures, more droughts and higher CO2 levels.

The oceans exhibit a similar trend, Fung said, though less pronounced. There, mixing by turbulence in the ocean is essential for moving CO2 down into the deep ocean, away from the top 100 meters of the ocean, where carbon absorption from the atmosphere takes place. With increased temperatures, the ocean stratifies more, mixing becomes harder, and CO2 accumulates in the surface ocean instead of in the deep ocean. This accumulation creates a back pressure, lowering CO2 absorption.