What inspired you to focus your research on CDR technologies and their role in climate policy?
I have been focused on the solutions to climate change since I was at the U.S. Department of Energy from 1993 to 1998 and acting assistant secretary of energy for energy efficiency and renewable energy in 1997. Over the last quarter century, the technologies that we invested in back then heavily—solar, wind, geothermal advance batteries, alternative fuel vehicles, and various energy efficiency technologies in buildings and industry—have emerged as the scalable solutions to the urgent need to reduce global greenhouse gas emissions.
But in the last decade as global emissions have soared to 50 billion tons (Gt) of CO2 equivalent, carbon dioxide removal strategies have generated great interest. The three most widely analyzed and modeled are direct air carbon capture and storage which pulls CO2 directly out of the air and stores it underground; planting trees; and bioenergy with carbon capture and storage, whereby growing biomass removes CO2 from the air and a CCS system on the bioenergy plant could permanently bury it.
Many people have even argued that because of these CDR strategies, we don’t have to actually reduce greenhouse gas emissions so rapidly. But I had always been concerned that DACCS and BECCS are not even commercial yet, so relying on them was risky. So, that is why I took a close look at CDR.
What are some of the primary barriers to the scalability of DACCS that you’ve identified?
The first barrier is simply how difficult it is to extract CO2 out of the air in large quantities. The overall efficiency of DACCS is very low (5% to 10%) and the price very high because CO2 in the air is so diluted—420 parts per million. For context, the entire Houston Astrodome contains only about 1 ton of CO2.
Per ton of CO2 captured and stored, current costs range from $600 to $1,000 or more. Yet as a document, many experts in and out of the industry believe that it will be difficult to bring costs substantially below $300 per ton.
Also, you must run the system on a huge amount of renewable energy to actually reduce net CO2 emissions. A 2020 review concluded ‘renewables-powered DAC would require all of the wind and solar energy generated in the U.S. in 2018 to capture just 1/10th of a Gt of CO2.’
But the opportunity cost of all this money and renewables is huge because you could have achieved far more CO2 emissions reductions for a far lower cost simply by using them to replace existing fossil fuel plants on the grid and to power electric vehicles to replace gasoline powered cars. So, studies make clear that until you have eliminated most fossil fuel use, which is unlikely to occur before 2050, DACCS is a costly distraction.
What does ‘net zero’ mean, and why has it become such a focal point in climate discussions?
The science says that to stabilize global temperatures at levels needed to avoid dangerous climate change, we need to reduce emissions from 50 GT/year to zero by 2050. But, in theory, with a scaled-up CDR effort, we might only have to make reductions of, say, 50 GT/year if we could remove 10 GT/year.
This is a dangerous myth because it could easily delay or deter real emissions reductions in the next decade or two. And one 2020 study argues that if in fact all that CDR does not pan out, then that could result in ‘an additional temperature rise of up to 1.4°C’ (2.5°F).
And a great many people have even argued that with enough CDR we could conceivably overshoot a temperature target by mid-century and then turn global emissions massively negative to quickly cool back down. But this is dangerous magical thinking since, in reality, we don’t have anywhere near that amount of scalable CDR.
Given the complexity of climate models, how do you simplify the concept of carbon capture for a broader audience without losing the essential details?
I think that people understand the essential idea that we have to dramatically reduce total CO2 emissions sharply by mid-century to avoid catastrophic climate impacts. The basic idea of CCS is that one way to reduce emissions is to put a system on an existing coal plant to capture the CO2 in the exhaust gas of the power plant and then bury the captured CO2 underground. So far, efforts to do this have failed to make this a practical, affordable, and scalable strategy. The U.S. government has created a large subsidy for CCS under the Inflation Reduction Act to see if that will fix things.
With BECCS, all you are doing is putting the CCS system on a bioenergy plant. DACCS is more complicated because you have to build a huge device to pull CO2 out of the air and then you use a CCS system to capture and bury it.
You mention that massive tree-planting is not a practical climate solution. What are the limitations of this strategy?
The basic problem is that it requires a huge amount of land to achieve even modest impacts.
As an August piece I wrote with Climate Interactive Executive Director Andrew Jones explained, their modeling ‘found that planting 1 trillion trees, under optimistic conditions, would remove only 6% of the needed CO2 reduction [to limit total warming to 1.5°C]. And that would require a wildly unrealistic amount of land, over 2 billion acres, which is to say over 2 billion football fields—greater than the total land area of the contiguous United States.’
If carbon capture isn’t the silver bullet, what alternatives do you believe should be our focus to reduce CO2 emissions?
The core strategy to reduce CO2 emissions is well understood. You need to rapidly decarbonize the electric grid by replacing coal and gas plants with solar and wind power–and other renewables as they become available. You also have to add storage technologies and strategies to deal with the fact that many renewables have variable emissions. You also need to continue the development of new carbon-free power sources that don’t need storage.
At the same time, you need to replace the technologies that rely on fossil fuels for transportation and heating and industrial processes with ones that rely on electricity. These include electric vehicles, electric heat pumps for heating, and the like.
This way by mid-century you end up with a zero-carbon electric grid and the vast majority of the economy running on electricity.
Looking beyond COP28, what gives you hope about our ability to address the climate crisis?
What gives me the most hope is the tremendous advances in the last two decades in bringing down the cost of various technologies that are essential to rapidly reducing emissions. These include solar and wind power, batteries, electric vehicles, and various forms of energy efficiency such as LED lights. The vast majority of the technologies needed to address the climate crisis are already commercial—and the rest of technologies are near commercial. What we lack now is only the political will to deploy these technologies fast enough to minimize the worst impacts of climate change.
