Despite efforts to stabilize CO2 concentrations, it is possible that the climate system could respond abruptly with catastrophic consequences.
Intentional intervention in the climate system to avoid or ameliorate such consequences has been proposed as one possible response should such a scenario arise.
In a one-week study, the authors of this report conducted a technical review and evaluation of proposed climate engineering concepts that might serve as a rapid palliative response to such climate emergency scenarios. Because of their potential to induce a prompt (<1 yr) global cooling, this study concentrated on Shortwave Climate Engineering (SWCE) methods for moderately reducing the amount of shortwave solar radiation absorbed by the Earth.
The study’s main objective was to outline a decade-long agenda of technical research that would maximally reduce the uncertainty surrounding the benefits and risks associated with SWCE. For rigor of technical analysis, the study focused the research agenda on one particular SWCE concept—stratospheric aerosol injection—and in doing so developed several conceptual frameworks and methods valuable for assessing any SWCE proposal.
Basic physical science considerations, exploratory climate modeling, and the impacts of volcanic aerosols on climate all suggest that SWCE could partially compensate for some effects—particularly net global warming—of increased atmospheric CO2. However, existing data also reveal important limits to the range of CO2 impacts that SWCE could ameliorate; for example, ongoing ocean acidification would not be affected, and some categories of climate emergency scenario might prove unresponsive to SWCE.
Moreover, significant uncertainty presently surrounds the spatial and temporal response of numerous climate and ecological parameters to SWCE, making the near-term deployment of large-scale SWCE extraordinarily risky.
The core questions that need to be addressed can also be clustered into three streams of research:
Engineering (intervention system development)
Climate Science (modeling and experimentation to understand and anticipate impacts of the intervention)
Climate Monitoring (detecting and assessing the actual impacts, both anticipated and unanticipated)
While a number of studies have suggested the engineering feasibility of specific SWCE proposals, the questions in the Climate Science and Climate Monitoring streams present far greater challenges due to the inherent complexity of temporal and spatial delays and feedbacks within the climate system. Components of any comprehensive research agenda for reducing these uncertainties can be divided into three progressive phases:
(I) Non-Invasive Laboratory and Computational Research
(II) Field Experiments
(III) Monitored Deployment
Each phase involves distinct and escalating risks (both technical and socio-political), while simultaneously providing data of greater value for reducing uncertainties. These frameworks are applied to structure the comprehensive research agenda outlined for stratospheric aerosol SWCE in Part 3 of this report.
For the Engineering stream, current understanding, questions and methods guiding the necessary research into aerosol material, stratospheric lofting and dispersion are all defined.
For the Climate Science and Climate Monitoring streams, emphasis is placed on identifying, predicting and monitoring the response of important climate parameters across four broad categories: Radiative, Geophysical, Geochemical and Ecological.
Finally, the components within each stream are identified as belonging to Phase I or II research, and the limits placed by the natural variability of the climate system on what can be learned from low-level Phase II field-testing are roughly assessed.
This report does not attempt to evaluate whether stratospheric aerosol (or any other) SWCE systems should be developed or deployed—or even whether any parts of the outlined research program should be pursued.
Such questions are the subject of an intense ongoing debate, involving socio-political and economic issues beyond the scope of this study.
This report aims to better inform that debate by elucidating the technical research agenda that would be necessary to reduce the uncertainty in potential SWCE interventions.