The world may finally acknowledge that global warming is a major environmental hazard. But new research shows that reducing the main greenhouse gas behind it may be even more difficult than previously believed. The reason: the world's oceans and forests, which scientists were counting on to help hold off catastrophic rises in carbon dioxide, are already so full of CO2 that they are losing their ability to absorb this climate change culprit.

"For every ton of CO2 emitted [into] the atmosphere, the natural sinks are removing less carbon than before," says biologist Josep "Pep" Canadell, executive director of the Global Carbon Project—an Australia–based research consortium devoted to analyzing the pollution behind global warming. "This trend will continue into the future."

Specifically, oceans and plant growth absorbed only around 540 kilograms per metric ton (1,190 pounds per short ton) of the CO2 produced in 2006, compared with 600 kilograms per metric ton (1,322 pounds per short ton) in 2000. Coupled with an emissions growth rate of 3.3 percent—triple the growth rate of the 1990s—the atmospheric burden is now rising by nearly two parts per million of CO2 a year, the fastest growth rate since 1850, the international team of researchers reports in Proceedings of the National Academy of Sciences USA.

"We have yet to make real progress in turning the world toward decreasing CO2 emissions," says the study's co-author Chris Field, director of the Carnegie Institution of Washington's Department of Global Ecology in Stanford, Calif. "A greater buildup of CO2 means more warming."

Atmospheric concentrations of the most ubiquitous greenhouse gas reached 381 parts-per-million in 2006 after emissions of CO2 from burning fossil fuels rose to 8.4 billion metric tons (1.85 x 1013 pounds) per year, according to figures from the United Nations, British Petroleum and the U.S. Geological Survey.

All told, human activity released 9.9 billion metric tons (2.18 x 1013 pounds) of carbon in 2006, up from just 8.4 billion metric tons (1.85 x 1013 pounds) in 2000. At the same time, poleward shifts of westerly winds in the Southern Ocean reduced the region's ability to suck up CO2 as have mid-latitude droughts, which slowed the growth rate of forests and plants that capture carbon.

New maritime measurements over the past decade also show that the North Atlantic's ability to absorb CO2 has been cut in half, according to researchers from the University of East Anglia who were not affiliated with the study by Canadell and his colleagues. "Until now, we thought that the decline in the efficiency of natural sinks was going to happen during the 21st century and more strongly during [its] second half," Canadell says. "If we didn't [include in the assumptions] that this was going to happen [so soon], have we underestimated the decline in the efficiency into the future?"

In addition, this research shows that CO2 emissions over the past decade were higher than those considered in the most dire scenarios for future climate change, which means that even more drastic actions will be needed to stem global warming. "The longer we wait to reduce emissions," Canadell says, "the harder the cuts that will be required to stabilize atmospheric CO2 emissions."