Even if all emissions of greenhouse gases and aerosol precursors ended today, more warming would occur in coming decades than the 0.8°C that has occurred to date, greatly intensifying climate change and its associated impacts. Already, sea level is rising, sea ice and mountain glaciers are retreating, the ranges of plant and animal species are shifting poleward and upward, and the Greenland and Antarctic ice sheets are both losing mass. Each round of the assessments of the Intergovernmental Panel on Climate Change has found that change is occurring more rapidly than previously projected.
With coal, petroleum, and natural gas providing over 80% of the world’s energy, greenhouse gas emissions cannot be stopped today, or next year, or even in a few decades. While international negotiations have the potential to limit emissions and the ultimate changes in climate to be experienced by future generations, much greater changes in climate than have occurred to date are inevitable, and their impacts will continue to intensify. The changes are so rapid and large that it is becoming much more likely that critical thresholds will be exceeded, triggering potentially unstoppable Earth system responses. Most serious would be initiation of large releases of methane from thawing permafrost or sea level rise of over a meter per century as deterioration of the major ice sheets accelerates. The resources required to deal with the consequences if this occurs would be enormous.
While limiting emissions through mitigation is essential and can help slow climate change, the problem is that just reducing emissions will only very slowly limit climate change over the next few decades. And while adaptation is also essential, there are many consequences of climate change for which no practical adaptive response exists—societies will have to suffer, migrate, or find new ways to provide the lost services. With international negotiations moving slowly, the situation continues to become more and more problematic.
The urgency of the situation has resurrected consideration about whether it might be appropriate to not just limit what we are doing to the climate, but, in addition, to attempt to deliberately manipulate the climate and/or atmospheric composition in a way that would counteract the changes being induced by the energy-related emissions of carbon dioxide and other greenhouse gases. This category of effort has been dubbed geoengineering, although it might better be termed georestoring of the climate, because the intent would be to moderate climate change and its impacts.
For example, nations might choose to try to emulate the cooling influence of major volcanic eruptions by, on a continuing basis, augmenting the amount of particulate matter in the stratosphere. Injecting sulfate aerosols globally or at the right latitudes could be done in a way that would reflect an amount of solar radiation equal to the increased trapping of energy by the greenhouse gases. Although this could limit global warming, thus reducing impacts like the loss of sea ice and slow the increase in sea level, there would be several side effects, including whitening of the sky, perhaps altering the course of storm systems, and reducing the efficiency of many types of solar energy systems. While this might seem an acceptable tradeoff today, it is worth noting that future generations would have to continue these injections for centuries, or the warming that was being offset would reappear relatively rapidly. The report, Beyond Mitigation: Potential Options for Counter-Balancing the Climatic and Environmental Consequences of the Rising Concentrations of Greenhouse Gases, just published provides an overview of the range of proposals that have been made over the past few decades.
In addition to limiting global change through mitigation, a conceivable complementary approach could be to augment the international approach could be augmented with specially designed efforts to limit the intensity of specific, particularly severe impacts. In my view, four possible actions deserve intense analysis because the potential losses appear to be far larger than the likely cost of implementation:
- Limiting the solar radiation that reaches the Arctic and Antarctic in order to restore conditions needed by the region’s species and to limit sea level rise from the melting of the Greenland and West Antarctic Ice Sheets.
- Enhancing uptake of carbon by the ocean, storing it in the deep ocean in order to moderate ocean acidification and limit damage to the marine food web. Alternatively, ocean acidification might be limited in specific areas such as the Great Barrier Reef by adding a buffering compound to ocean waters.
- Limiting the warming of the ocean in the regions that contribute to intensification of tropical cyclones (i.e., hurricanes, typhoons, etc.).
- Actively managing the global emissions of sulfur dioxide in order to maintain, or even enhance, the global cooling influence of tropospheric aerosols.
Each of these actions, and there may be others worthy of consideration, would focus on intervening to moderate a specific impact. There are viable technological approaches for each of these activities, and they would be readily reversible if unexpected, adverse consequences arose. What is needed now is an aggressive research and development effort that determines the optimum approach, carries out small scale tests, investigates and compares unintended side effects with the impacts of greenhouse gases that are alleviated, and puts forth a near-term plan for active consideration at an appropriate regional or global forum.
None of these actions would be a substitute for aggressive global mitigation of emissions or alleviate all of the adverse consequences. However, they could more evenly spread the burden of global warming and potentially slow the onset of at least some of the irreversible consequences. In this way, geoengineering could buy a small amount of time for global mitigation to be negotiated and take hold. Undertaking research on these impact interventions seems important and timely.