Pumping aerosols into the stratosphere may buy us more time, but it's no substitute for cutting carbon emissions—and we still don't know enough to do it responsibly.

Many economists have long held that carbon pricing—either through a carbon tax or cap-and-trade—is the most cost-effective way to decarbonize energy systems, along with subsidies for basic research and development. Meanwhile, green innovation and industrial policies aimed at fostering low-carbon energy technologies have proliferated widely. Most of these predate direct carbon pricing. Low-carbon leaders such as California and the European Union (EU) have followed a distinct policy sequence that helps overcome some of the political challenges facing low-carbon policy by building economic interest groups in support of decarbonization and reducing the cost of technologies required for emissions reductions. However, while politically effective, this policy pathway faces significant challenges to environmental and cost effectiveness.

Solar geoengineering is no substitute for cutting emissions, but could nevertheless help reduce the atmospheric carbon burden. In the extreme, if solar geoengineering were used to hold radiative forcing constant under RCP8.5, the carbon burden may be reduced by ∼100 GTC, equivalent to 12–26 percent of twenty-first-century emissions at a cost of under US$0.5 per tCO2.

"...[E]ven if the impacts of a Trump slump in U.S. domestic climate action are manageable, the impact on global policy may not be. The concern here is not immediate support for new coal plants or the immediate risk of conflict; it is the way in which a weakening of international institutions might initiate a negative feedback loop played out over many years and decades."

"Of course, solar geoengineering alone cannot stabilize the climate. The world must cut carbon dioxide emissions. But solar geoengineering is unique because it can help address already committed climate change—future climate change locked in due to past emissions. Cutting carbon emissions alone will not have immediate effects, given the length of time that carbon already in the atmosphere stays there."

Estimates of damages from climate change are dependent on estimates of global-average-temperature increase, which in turn depend on how marginal increases in greenhouse-gas concentrations affect temperature. The "likely" range of temperature increase from a doubling of concentrations has stalled for 35 years at 1.5–4.5° C—making estimates of damages difficult and unreliable.

"China has initiated a limited research program on albedo modification. The U.S. has not. Given that albedo modification is the kind of technology that necessitates an open, transparent, and international research effort — precisely the kind of effort in which the U.S. excels — this is a serious failing."

Uncertainty about "climate sensitivity"—the impact on temperature of increased concentrations of greenhouse gases—grew from the IPCC's 4th to 5th Assessment Reports. The authors conclude that this "bad news" outweighs the "good news": a lower value for the bottom end of the range for temperature rise.