To combat climate change, engineers and scientists have proposed and started to develop methods that would pull carbon dioxide out of Earth’s atmosphere.
But results from two climate modeling studies published recently suggest that without significant cuts to CO2 emissions from power plants and other sources, these extraction technologies probably won’t do much to mitigate the gas’s effects on Earth’s climate or oceans.
To keep global warming at reasonable levels—less than a 2 ºC increase in global average temperature by the end of the century, according to modeling studies—climate scientists think CO2 levels in the atmosphere can’t rise much past 420 ppm. Earlier this year, CO2 levels passed 400 ppm.
One tool to help reach this goal could be so-called negative emission strategies to remove CO2: capturing the gas from power plant emissions or establishing new forests to pull down more CO2, for example.
But how much help could these carbon-removal methods provide? To find out, Thomas Gasser of the Climate & Environmental Sciences Laboratory, in Gif-sur-Yvette, France, and colleagues compared different combinations of emissions cuts and amounts of carbon removal over the next century. For each level of emissions being cut, they determined the amount of carbon extraction needed to keep CO2 levels on track to stay below 420 ppm by 2100.
The lowest estimate of required removal was about 1.8 billion metric tons of CO2 per year (Nat. Comm. 2015, DOI: 10.1038/ncomms8958). Currently planned carbon capture and storage (CCS) projects worldwide have a combined capacity of only40 million metric tons per year, according to the Global CCS Institute, an international organization that promotes the technology. Other carbon removal efforts, which are still being developed, aren’t counted in this estimate.
Besides warming the planet, increasing CO2 levels in the atmosphere affect the oceans. As more of the gas dissolves, it acidifies Earth’s oceans, which can harm many organisms at the base of marine food webs.
Sabine Mathesius of Potsdam Institute for Climate Impact Research, in Germany, and colleagues wanted to determine how much carbon removal could mitigate these effects on the oceans. From their models, they estimated that if current emissions trends continue and carbon removal technologies extract 18.3 billion metric tons of CO2 per year, average ocean pH would improve just 0.1 units by 2500, compared to conditions without carbon removal. Even using an extreme extraction rate of 91.7 billion metric tons of CO2 per year in their models, ocean pH didn’t return to preindustrial levels (Nat. Clim. Change 2015, DOI: 10.1038/nclimate2729).
Both Gasser and Mathesius point out that their findings don’t suggest that carbon removal technology won’t be a useful tool for addressing rising CO2 levels. “It just won’t be strong enough to counteract a business-as-usual [emissions] scenario,” Mathesius says. Instead, both say, carbon extraction could be combined with cuts in fossil fuel use to lower net CO2 emissions to safer levels.
Some climate scientists hope that international talks in Paris this December will lead to pledges from nations to make such cuts (see page 46).
For both studies, “The simple message is that starting to attack the [emissions] problem sooner rather than later is important,” says Richard Matear, a climate researcher at the Commonwealth Scientific & Industrial Research Organization, in Australia, who wrote an accompanying commentary of the ocean study in Nature Climate Change. “Reducing emissions now is a good idea, rather than thinking we’re going to solve the problem through technology in 20 or 50 years.”