Although a growing number of countries are taking steps to reduce their carbon emissions, we're still nowhere close to where we need to be if we want to limit future temperature changes to 2 degrees Celsius. If the coming temperature changes become too disruptive, our future selves may wish that our present selves hadn't released so much carbon.

But they will have options other than looking back with regret. It's possible with existing technologies to pull carbon from the air or to limit the sunlight reaching Earth. These forms of geoengineering are the subject of a new report by the National Academies of Science, funded by everyone from the NOAA and NASA to the US intelligence community. The report concludes that carbon removal from the atmosphere is technically viable, but it's currently too expensive to see widespread use. Altering the amount of sunlight reaching the Earth, however, appears fraught with risks, both practical and political.

The report's authors make one thing clear from the very start: it would be much, much easier to simply limit our carbon emissions now. "There is no substitute for dramatic reductions in the emissions of CO2 and other greenhouse gases," they write, "to mitigate the negative consequences of climate change, and concurrently to reduce ocean acidification." The report's first recommendation is that we focus on mitigation and adaption efforts, as "these approaches do not present poorly defined and poorly quantified risks and are at a greater state of technological readiness."

But because dramatic reductions haven't happened yet, we should be prepared for a future in which we resort to other means to control the environmental and economic consequences. The report breaks those other means into two categories: carbon dioxide reduction, meaning removing it from the atmosphere, and albedo modification, which means reflecting more sunlight back into space.

Carbon dioxide removal is pretty well understood. One aspect involves promoting the Earth's natural removal systems, such as preserving ecosystems that sequester carbon. While effective, these approaches are limited in their capacity to remove carbon on the scale that we're emitting it. More radical solutions to ecosystem engineering, like fertilizing microbial growth in the oceans, don't look so great. "Previous studies nearly all agree that deploying ocean iron fertilization at climatically relevant levels poses risks that outweigh potential benefits," the study says.

The alternative approach to carbon dioxide removal is carbon capture and storage. The basic technology is well understood, but all the test projects so far have had limited capacity and very high costs. In fact, the report's authors find that the costs of deploying the current version of this technology broadly is likely to meet or exceed the costs of simply replacing fossil fuels with non-emitting energy sources. And although the technology for capturing and storing CO 2 is worked out, we know much less about how well the storage will work on longer time scales or with the volumes of gas that must be stored in order to have a significant impact on the climate.

Thus, the report recommends that we do the research needed to identify and quantify the environmental risks and bring the costs of the process down.

If carbon removal is expensive but relatively low-risk, albedo modification is its evil twin: cheap but with tremendous risks. Typically, this approach involves doing something that volcanoes do with regularity: inject lots of aerosol particles into the stratosphere, where they will scatter some incoming sunlight back into space. Doing so is relatively simple and cheap; estimates cited in the report indicate that it would be an order of magnitude less expensive than decarbonizing the global economy.

But the report helpfully points out that the past natural versions of this approach haven't always been benign; large eruptions have sometimes caused crop failures and famine. The approach also changes precipitation patterns and amounts and depletes the ozone layer. The effects of volcanoes also dissipate within a few years, while intentional albedo modification would have to go on indefinitely—were it to stop, we'd face sudden and potentially radical warming within a few years. Finally, it does nothing to solve that other carbon problem: ocean acidification, where dissolved CO 2 reduces the pH of the seas, with potentially severe impacts on the life present there.

All told, the report concludes that "there is significant potential for unanticipated, unmanageable, and regrettable consequences in multiple human dimensions from albedo modification at climate altering scales." It urges against deploying it at this time and recommends caution about doing any research on it. Any tests have the potential to bring these consequences to any nations near the test site—bad on their own, and made worse by the fact that we have nothing in the way of an international political structure to manage the ensuing problems.

What the authors recommend is a two-fold approach to albedo modification research. Currently, aerosols like those that would be used in such a process are a major source of uncertainty in our understanding of the climate. Further research in this area could help us project future climate changes and at the same time give us a better understanding of what we could accomplish through albedo modification. But the authors argue that it's time to start working out international research and governance frameworks that would allow us to pursue more aggressive research in this area should we decide it's needed.

In the end, the report clearly comes down in favor of research into carbon removal technology. "Overall, there is much to be gained and very low risk in pursuing multiple parts of a portfolio of [carbon removal] strategies that demonstrate practical solutions over the short term and develop more cost-effective, regional-scale and larger solutions for the long term," it concludes. "In contrast, even the best albedo modification strategies are currently limited by unfamiliar and unquantifiable risks and governance issues rather than direct costs."

But beyond the research programs, it's clear that neither of these approaches is ready for deployment, and it's not clear that either of them can ever be made ready, a fact driven home by the cancellation of what would have been the US' largest carbon capture experiment. That's in sharp contrast with non-emitting power sources, where technology is already mature and costs are in many cases already competitive with those of fossil fuels.

The report is available as a free download.

Listing image by NASA