Going Negative

By Timothy Beatley

Last year saw a peak in global carbon emissions, raising new concerns about how realistic it will be to reach the reduction targets set by the Paris Accord and our ability to limit the global temperature increase to 1.5 degrees Celsius. In response, much discussion has recently focused on so-called negative emissions as a partial remedy.

NE refers to the various techniques and technologies that remove carbon from the atmosphere. For many, NE represents our only hope to forestall the worst impacts of climate change. But how feasible are these techniques and technologies, and what will the role of cities be?

Climeworks founders Christoph Gebald and Jan Wurzbacher at their direct air capture plant in Hinwil, Switzerland. Watch a video to learn more about how the technology can mitigate carbon dioxide production. Photo by Julia Dunlop/©Climeworks.

Can NE save us?

I recently spoke to Steve Pacala, a Princeton University professor and codirector of the Carbon Mitigation Initiative there, who chaired the National Academy of Sciences committee looking into negative emissions. The committee analyzed a range of NE technologies, he says, and the findings in their November report Negative Emissions Technologies and Reliable Sequestration: A Research Agenda, are at once hopeful — there are new and emerging technologies — but also discouraging, as many of the key ones are still in development and probably a decade or more away from full-scale commercial application.

NE should not be considered a panacea or solution so much as a part of the mitigation portfolio, Pacala says. It is especially important in offsetting emissions that will be very difficult to mitigate or reduce. For example, there are currently few practical alternatives to mitigate the carbon emissions from the chemical fuels burned by airliners (though use of biofuels and even electric-powered airline flight may be possible in the longer run).

Some techniques reviewed by the committee, like aforestation and reforestation (tree planting, essentially), have been around a long time, but Pacala feels they will be limited by land scarcity (as more land will be needed for growing food and biodiversity conservation, and for a growing global population). Here, there is certainly an important role for cities.

Cities can and should sequester more carbon through tree planting (perhaps repurposing urban land from roadways and parking to urban forests), as well as protecting the carbon already stored in older trees. And we need more trees in cities, of course, to address the growing problem of urban heat, to enhance urban quality of life, and to provide habitat.

But Pacala is skeptical that this will take us very far toward reaching the needed 10 gigatons of carbon reduction.

Climeworks offers a number of technologies that remove carbon dioxide from the air, including a process that transforms it into rock for underground storage. Graphic courtesy Climeworks.

Keeping carbon in mind

Pacala believes one of the most promising new technologies is direct air capture. This involves pumping air through a solid sorbent or liquid solvent, causing chemical bonding, and then, through heating, the collection and permanent storage of the carbon. A number of small start-up companies are working on this technology, and several have commercially viable products to sell. One Swiss company, Climeworks, manufactures DAC modules (using a solid sorbent) and has several functioning plants. Another in Zurich extracts the carbon for use as a fertilizer in an adjoining greenhouse, using energy produced from burning municipal waste. And a new agreement with Coca-Cola will see extracted carbon used in their drinks and sparkling water.

A pilot plant near Reykjavik, Iceland, is closest to demonstrating what is possible when it comes to locking up carbon. Here, geothermal energy is used to extract carbon, which is then mixed with water and pumped underground, permanently mineralizing (essentially fixing the carbon to basalt rock).

The Reykjavik plant captures and permanently stores the carbon there underground — "insetting," they call it, instead of offsetting. Louise Charles of Climeworks tells me the technology has been coming down in price with each new plant and gaining traction mostly in the voluntary carbon market. The Reykjavik plant has been operating successfully for about a year, and has plans to significantly scale up.

We will see more on the DAC front, Pacala says, and improvements in energy efficiency and lower energy costs. The NAS report considers other technologies and approaches that will also take us some of the way: coastal blue carbon (protecting and growing coastal wetlands and mangroves), bioenergy with carbon capture; and carbon mineralization (exposed weathering of rock). Each could play a role. Coastal cities have a special opportunity to protect their ecological edges and the blue carbon stored there, as well as to restore and create new living shorelines — an adaptation trend we could see as more cities seek shorelines that better respond to dynamic and variable physical conditions (tides, sea-level rise), and a new generation of floodable parks. Equally true, planners could begin to emphasize the carbon sequestration benefits of grass-fed, rotational farming systems that lead to soil formation, rather than depletion.

Planners must begin to think about the many potential opportunities for NE in cities. As DAC evolves and matures, we should imagine buildings and homes designed to include them. (Can we rethink and redefine conventional HVAC systems to include a carbon capture and storage function?) Pacala also wonders whether cities should rethink their approach to waste management — perhaps there is a need for two kinds of landfills, with one designed for slow decomposition and the entombing of carbon. And to the extent that cities can divert trees into "long-lived wood products," some carbon could be sequestered in this way. The emerging trend of using wood in construction of larger high-rise structures is yet another way cities could lock some carbon away.

But will it be enough? It's hard to say, and it must not detract from the work of reducing the production of emissions in the first place. Cities must continue to work quickly on climate mitigation (in all the ways emissions can be reduced, from bicycles to renewable energy), and we must work creatively on the global scene to help reduce deforestation. But NE will also need to be in the planner's toolkit if we have any hope of reaching our global targets.

Timothy Beatley is the Teresa Heinz Professor of Sustainable Communities at the University of Virginia, where he directs the Biophilic Cities Project.