One of the seemingly ideal and direct solutions to climate change is to efficiently vacuum up greenhouse gases straight from the atmosphere. But a new study finds that such a proposal is very far-fetched and tremendously expensive.

The president's science adviser, John Holdren, and Energy Secretary Steven Chu have expressed support in the past for capturing and storing pollution from the air as a measure to mitigate global temperature increases. However, in a paper published earlier this month in the Proceedings of the National Academy of Sciences, researchers found that trying to scrub the air is much more expensive than keeping it from getting dirty in the first place.

For the scientists conducting the study, air capture was shown to be largely wishful thinking that distracts from more effective strategies for combating pollution and climate change. "We thought it was important to set the record straight because [air capture] has policy implications," said Howard Herzog, a senior research engineer at the Massachusetts Institute of Technology's Energy Initiative and one of the report's authors. He said that air capture is appealing because it allows people to get away with not changing anything about their energy use.

Air capture involves using filters, chemical reactions or special materials to collect greenhouse gases like carbon dioxide. Many of these technologies already exist for industrial use to keep carbon dioxide out of critical processes and to purify the air on spacecraft and submarines. The problem with using these tools to fight climate change is that pulling carbon dioxide out of the atmosphere is resource-intensive. "[Air capture] takes a lot of energy. The reason we have CO2 emissions is because we use a lot of energy. Controlling CO2 by burning a lot of energy doesn't make a lot of sense," Herzog said.

Another challenge for air capture is that the atmosphere blanketing the Earth's surface is very big, and carbon dioxide is a relatively small part of it. The scientists studied some of the existing air capture strategies on the market and calculated how efficient and how costly they would be in cleaning the air at large. "I suggested looking into what the efficiencies should be as a function of the dilution of the target materials," said Kurt Zenz House, president of C12 Energy, a carbon dioxide management firm, and one of the paper's co-authors.

Small needles, big haystack
He explained that the team examined how much energy it takes for carbon extraction systems to clean the air outdoors when the gas is spread out, unlike the richer concentrations in smokestacks for factories and coal power plants where carbon scrubbing systems are commonly advised. Collecting carbon dioxide from the atmosphere would require combing through 300 times as much air as you would need in a power plant. "It's harder to find a needle in large haystack than a small haystack," said House.

The researchers found that previous cost and efficiency estimates for air capture from entrepreneurs and scientists were far too optimistic. Extracting carbon dioxide from the air would likely cost more than $1,000 per ton, compared to $50 to $100 per ton from a system installed in a chimney. "We're not saying it's infeasible to take CO2 out of the air; we're asking if this is an economic way to mitigate climate change, and here we're very clear it's not," said Herzog.

House also noted that the energy needed to pull a given quantity of carbon dioxide from the air is greater than the energy you get from burning the coal that produced it. In other words, running an air capture system with coal power would produce more pollution than it cleans up. "If you power it with natural gas, you break even, which is pointless," said House.

The only way air capture would be effective in fighting climate change is if it were powered by renewable energy like solar or wind power, in which case, it is better to feed the energy back into the grid to displace fossil fuel generation, according to House. "For air capture to work, people would basically have to substantially improve on what we've achieved so far in commercial separation systems," he said.

However, there are other ways of capturing carbon. "Probably the most practical one is biological, like trees and vegetation. That's driven by solar energy. That's not inexpensive, but it's a lot more feasible [than air capture]," said Herzog, noting that using plants to control carbon emissions would require a significant amount of land.

Another approach is to stop carbon at the source, using existing technologies to capture carbon dioxide from industrial sites and either storing it underground or reusing it for manufacturing. There are already fossil fuel energy plants using these systems, but cost is still an issue. "It's in the same ballpark as large-scale renewables and nuclear power," said Herzog. "None of them are cheap compared to base systems."

More frequent wildfires and insect infestations—themselves influenced by climate change -- can help speed that transition, said Pieter Beck, an ecologist at the Woods Hole Research Center.

Beck's work, which combines satellite observations of vegetation growth with tree-ring data, shows that while the northern reaches of the boreal forest are thriving, the growth of trees south of Alaska's Brooks Range is slowing due to drought stress.

"We're seeing the ideal envelope for boreal forests move north," said Beck. "I don't have an answer at what time scale the tipping canoe and widespread changes in regime would occur, but I think we're at a stage now where the question is whether they could occur on the time scale of the 21st century."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500