The Rationale for Accelerating Regionally Focused Climate Intervention Research
Abstract
Ten years ago, Paul Crutzen asked whether the time had come to consider undertaking research into intentionally intervening in the climate system so that it might be considered a policy option comparable to reducing emissions for limiting human-induced climate change. Crutzen’s article pointed out how little progress had been made in reducing emissions and suggested that resurrecting decades-old ideas for imitating volcanic eruptions as a possible intervention might be needed. Today, model-based simulations, optimistically assuming that nations fulfill their commitments for future emissions reductions, project global average temperature to increase to 3–4°C above its preindustrial level by 2100, a level that Crutzen envisioned as likely to meriting active intervention. While research has begun to explore the means for intervening globally, such interventions raise challenging issues of governance, unintended consequences, intergenerational equity, and more. Initially, focusing research on potential tropospheric and surface-based approaches to altering energy flows as a means for moderating adverseregionalimpacts might well pose less difficult governance challenges and more regionally constrained evaluations of intended outcomes and unintended consequences. Because natural processes would tend to dissipate most types of tropospheric interventions, adjustments, and even termination, would be possible over periods of weeks to months. In addition to serving their particular purpose, regional interventions would also provide an opportunity for learning more about Earth system behavior and the potential effectiveness and risks of global-scale interventions, if such interventions might eventually be needed to counter-balance especially severe global warming.
1. Progress Over the Past Decade in Slowing the Growth of Warming Influences
Ten years ago, as a result of the slow international progress at that time in reducing emissions of carbon dioxide (CO2), Crutzen [2006] called for research to explore the possibility that enhancement of the Earth’s albedo could serve as a useful complement to reducing greenhouse gas emissions as an approach to slow and eventually limit or reverse global warming. Since that time, annual emissions of CO2 and other greenhouse gases in nations with relatively high per capita emissions have started to decline, now roughly balancing the increases in emissions from nations with relatively low per capita emissions that are rising due to population growth and the need for more energy to raise the standard-of-living in these nations [Timmer, 2015; Le Quéré et al., 2016]. As a result, despite overall good intentions, global annual emissions of CO2 are now ∼20% higher than in 2006 and the atmospheric CO2 concentration has increased from ∼380 ppmv to over 400 ppmv. There are also increasing indications that natural carbon feedbacks (including release of soil carbon in low latitudes and oxidation of thawing permafrost and methane clathrates in high latitudes) are being stimulated [Stocker et al., 2013], amplifying the ongoing warming influences of anthropogenic greenhouse gas emissions. Given the long persistence time of the atmospheric CO2increase, the commitment to future warming will thus continue to increase at least as long as global emissions are above a very small fraction of their current level [Clark et al., 2016].
That climate change is occurring in response to the increasing CO2 and greenhouse gas concentrations has also become more evident since 2006. The continuing increase in global average temperature, thinning, and retreat of snow cover and Arctic sea ice, increased rate of mass loss from the Greenland and Antarctic ice sheets, increased ocean heat content, rising sea level, and more all indicate ongoing climate change [Blunden and Arndt, 2016]. As the statistical distribution of the weather is being altered, departures from the traditional statistical norm are reaching three to four standard deviations and more [e.g., Hansen et al., 2012]. As a result, regions of the world are experiencing, for example, more intense precipitation and flooding, more intense and frequent periods of drought and consequent wildfire, greater and faster retreat of sea ice and snow cover, greater uptake of heat by the oceans, faster ice sheet loss, rising rates of sea level rise, and other conditions unprecedented in historical times. With the ongoing increase in global population, the increasing risks of severe impacts on grain and food production and on human health, the increasing occurrence of coastal inundation and storm damage, and the likelihood of severe environmental and societal disruption and dislocation are going up and already starting to impact particular regions [Intergovernmental Panel on Climate Change (IPCC), 2014].
That the 2015 Paris Agreement [UN Framework Convention on Climate Change (UNFCCC), 2015] was so widely agreed to reflects increasing international recognition that climate change has already started to have serious impacts, that substantial further impacts lie ahead, and that efforts to reduce CO2 and other greenhouse gas emissions must be urgently undertaken. Indeed, the Paris Agreement committed the signatories to phasing out all emissions of fossil-fuel-generated CO2 by no later than the latter part of the 21st century. With fossil fuels presently providing ∼80% of the world’s energy and with the demand for energy for transportation growing in developing nations, however, the near complete transformation of the global energy system seems unlikely to occur rapidly enough to avoid very disruptive impacts [IPCC, 2014] and to “prevent dangerous anthropogenic interference with the climate system,” as called for in 1992’s Framework Convention on Climate Change [UN Framework Convention on Climate Change (UNFCCC), 1992]. Indeed, analyses based on current national commitments project a warming of roughly 3–3.5°C by 2100 and indicate that considerably greater reductions in emissions, including achieving negative emissions of CO2 over many decades, will be needed to bring long-term warming below the proposed ceiling of 1.5–2°C above preindustrial temperatures [Jones et al., 2016].
For sea level rise, the situation is even more threatening. Analyses of paleoclimatic records from the most recent deglaciation and from much more distant periods that were 4–6°C warmer than present suggest that the sensitivity of sea level to changes in global average temperature must be roughly10–20 m per degree Celsius. While the response time for the ice loss to occur is not well understood, it is not implausible that the rate of future sea level rise could reach a few meters per century and that such a high rate could, on average, persist for many centuries, even if global CO2 emissions are pushed to zero before 2100 [Hansen et al., 2016].
2. The Prospects for Additional Steps to Limit Changes in Atmospheric Composition
Impressive initiatives being undertaken by many cities around the world to reduce their fossil-fuel footprints and the rapidly declining costs of non-CO2 emitting energy technologies suggest the potential for a strong ratcheting up of the national commitments for emissions reductions under the Paris Agreement [Rogelj et al., 2016]. The societal and environmental co-benefits of healthy forests and grazing lands are also increasing the prospects for reducing CO2 emissions resulting from land clearing and forest mismanagement [World Bank, 2015]. There is also emerging research indicating that reforestation, afforestation, and even burial of biochar and similar measures could lead to a useful increase in terrestrial, or perhaps even ocean, carbon storage, which would also help to limit ocean acidification [Houghton et al., 2015; National Research Council (NRC), 2015a].
The UNEP/WMO Integrated Assessment of Black Carbon and Tropospheric Ozone [United Nations Environment Programme (UNEP)/World Meteorological Organization (WMO), 2011; Shindell et al., 2012] made clear that near-term reductions in positive radiative forcing could most rapidly be achieved by reducing the atmospheric loadings of short-lived species (particularly methane, black carbon, and tropospheric ozone). The Assessment estimated that, using existing technologies, feasible emission reductions could cut the projected global warming from the present to 2050 by ∼0.5°C, so roughly halve their estimate of the projected warming over that period. And negotiations under the Montreal Protocol to reduce hydrofluorocarbon emissions have led to an agreement that is projected to reduce global warming in 2100 by as much as ∼0.5°C when both direct (i.e., reduced hydrofluorocarbon emissions) and indirect factors (e.g., opportunities and incentives for improved efficiency) are considered [Climate and Clean Air Coalition, 2016].
Emissions from coal-fired power plants are becoming a particular target for reductions because their generation of energy leads to relatively high emissions of CO2 per unit of delivered electricity. However, because the typical lifetime of sulfate aerosols in the atmosphere is roughly a week or two, closing coal-fired power plants that are not already scrubbing out SO2 will actually augment the net warming influence of human activities for several decades until offset by the long-term benefits of the associated reduction in CO2 emissions [Wigley, 1991]. As a result, while clearly beneficial in the long-term, the closing of coal-fired power plants may need to be paired with reductions in emissions of methane and black carbon so that the net change in radiative forcing from all policy actions tracks downward as the coal-fired plants are closed.
Thus, while the decade since Crutzen’s paper has seen the initiation of substantial local, regional, national, and international efforts, the predicament that concerned him remains at least as serious as it was then.
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