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Earth’s ecosystems still soaking up half of human carbon emissions

Coral reefs off Maui. The extra carbon sequestered by the oceans has put the world's coral reefs at risk due to acidification. Photo by: Rhett A. Butler.
Coral reefs off Maui. The extra carbon sequestered by the oceans has put the world’s coral reefs at risk due to acidification. Photo by: Rhett A. Butler.

Even as humans emit ever more carbon dioxide into the atmosphere, Earth’s ecosystems are still sequestering about half, according to new research in Nature. The study finds that the planet’s oceans, forests, and other vegetation have stepped into overdrive to deal with the influx of carbon emitted from burning fossil fuels, but notes that this doesn’t come without a price, including the acidification of the oceans.

“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,” co-author Caroline Alden with the University of Colorado, Boulder said in a press release. “If natural sinks saturate as models predict, the impact of human emissions on atmospheric CO2 will double.”

The research found that while global carbon emissions quadrupled in the past 50 years, Earth’s carbon sinks doubled the amount of carbon they sequestered, lessening, but not neutralizing the warming impact of the carbon emissions into the atmosphere.

“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,” explains lead author Ashley Ballantyne, also with the University of Colorado, Boulder. “How long this will continue, we don’t know.”

To date, global temperatures have risen around 0.8 degrees Celsius (1.44 degrees Fahrenheit) due to greenhouse gas emissions, but that temperature rise would have been greater if not for the planet’s uptick in carbon sequestration.

“It is important to understand that CO2 sinks are not really sinks in the sense that the extra carbon is still present in Earth’s vegetation, soils and the ocean,” explains co-author Pieter Tans with the National Oceanic and Atmospheric Administration (NOAA). “It hasn’t disappeared. What we really are seeing is a global carbon system that has been pushed out of equilibrium by the human burning of fossil fuels.”

Sequestration of carbon can take its toll: the world’s oceans are acidifying due to the extra influx of carbon. This chemical change imperils coral reefs, the most biodiverse ecosystem in the oceans, and marine molluscs, which depend on calcium carbonate to build their shells, a mineral that becomes scarcer in more acidic oceans. Ocean acidification has become one trend—in addition to overfishing and pollution—that has led marine biologists to warn that the oceans could suffer a mass extinction event.

In the end, the researchers expect that eventually carbon sinks will fill up, putting any new emissions into the atmosphere.

“As the oceans acidify, we know it becomes harder to stuff even more CO2 into the oceans,” Tans says.

The only way to tackle climate change is to rapidly and aggressively cut global carbon emissions, according to scientists. However such emissions continue to rise: last year, emissions hit a new record of 34 billion tons.

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