Once the forests and oceans become saturated, as scientists predict, the impact of greenhouse gas emissions is expected to double – but no-one yet knows when this time will come.
Researchers in the US studied global carbon dioxide (CO2) emissions over the past 50 years and compared them with rising levels of the gas in the atmosphere.
They found that while CO2 emissions had quadrupled in the last five decades, natural carbon “sinks” that capture the greenhouse gas doubled their uptake. This had lessened the impact of man-made CO2 emissions on the Earth’s climate.
“What we are seeing is that the Earth continues to do the heavy lifting by taking up huge amounts of carbon dioxide, even while humans have done very little to reduce carbon emissions,” lead researcher Dr Ashley Ballantyne, from the University of Colorado at Boulder, said.
“How long this will continue, we don’t know.”
A total of 33.6 billion tons of CO2 were emitted globally in 2010, climbing to 34.8 billion tons in 2011, according to the International Energy Agency.
Much of this carbon is absorbed by the oceans and soil, or captured by green plants.
Plants absorb around 66 billion tons of carbon from the atmosphere each year to generate energy and renew themselves.
Oceans absorb about a third of man-made CO2 emissions, roughly 22 million tons a day.
The researchers’ findings have been published in the journal Nature.
Co-author Caroline Alden, also from the University of Colorado, warned that the Earth could not continue gulping down carbon forever.
“It’s not a question of whether or not natural sinks will slow their uptake of carbon, but when,” she said. “We’re already seeing climate change happen despite the fact that only half of fossil fuel emissions stay in the atmosphere while the other half is drawn down by the land biosphere and oceans.
“If natural sinks saturate as models predict, the impact of human emissions on atmospheric CO2 will double.”
Recent studies have suggested that carbon sinks are declining in some regions, including areas of the southern hemisphere and parts of the oceans.
The new research showed that global CO2 uptake by natural sinks doubled overall between 1960 and 2010.
But increased variations year-to-year suggest that the mechanism may be growing unstable.
Dr Ballantyne added: “When carbon sinks become carbon sources, it will be a very critical time for Earth.”