Evaluating climate geoengineering proposals in the context of the Paris Agreement temperature goals

Abstract

Current mitigation efforts and existing future commitments are inadequate to accomplish the Paris Agreement temperature goals. In light of this, research and debate are intensifying on the possibilities of additionally employing proposed climate geoengineering technologies, either through atmospheric carbon dioxide removal or farther-reaching interventions altering the Earth’s radiative energy budget. Although research indicates that several techniques may eventually have the physical potential to contribute to limiting climate change, all are in early stages of development, involve substantial uncertainties and risks, and raise ethical and governance dilemmas. Based on present knowledge, climate geoengineering techniques cannot be relied on to significantly contribute to meeting the Paris Agreement temperature goals.

Inequal Responses of Drylands to Radiative Forcing Geoengineering Methods

“…In general, adopting three RFG methodologies alleviates the expansion of drylands under the RCP8.5 scenario because the decrease in global mean temperature due to RFGs generally reduces the climate changes according to the increase in atmospheric concentration of greenhouse gasses (Irvine et al, 2019;Lawrence et al, 2018). In general, adopting three RFG methodologies alleviates the expansion of drylands under the RCP8.5 scenario because the decrease in global mean temperature due to RFGs generally reduces the climate changes according to the increase in atmospheric concentration of greenhouse gasses (Irvine et al, 2019;Lawrence et al, 2018).…”

“…When CCT is implemented, thinner cirrus clouds increase incoming solar radiation and strengthens radiative cooling in the upper troposphere. Under SAI, in contrast, the decreased incoming solar radiation and surface cooling combined with heating of the upper troposphere from the stratospheric aerosol layer generally reduces the vertical temperature gradient (increase in static stability) and hence vertical motion in the troposphere (Ferraro & Griffiths, 2016;Lawrence et al, 2018). Also, the warming in lower troposphere and cooling in upper troposphere promote convections and enhance the global hydrological cycle (Kristjánsson et al, 2015).…”

“…This study primarily determines the responses of drylands to three individual RFGs, which shows similar potential to cool the Earth. In general, adopting three RFG methodologies alleviates the expansion of drylands under the RCP8.5 scenario because the decrease in global mean temperature due to RFGs generally reduces the climate changes according to the increase in atmospheric concentration of greenhouse gasses (Irvine et al, 2019;Lawrence et al, 2018). For example, in MED and SAF, both decrease in P and increase in PET under RCP8.5 are mitigated by the three RFGs, resulting in alleviation of drylands expansion ( Figure S8).…”

“…These changes enhance convective activities over the western Pacific and eastern Indian Ocean, which alters the Pacific Walker circulation similarly to that under a La Ninã event (Niemeier et al, 2013;Stjern et al, 2018). Under SAI, in contrast, the decreased incoming solar radiation and surface cooling combined with heating of the upper troposphere from the stratospheric aerosol layer generally reduces the vertical temperature gradient (increase in static stability) and hence vertical motion in the troposphere (Ferraro & Griffiths, 2016;Lawrence et al, 2018). These dominant features in the large-scale circulation changes can explain the regional variability in dryland responses.…”

“…The idea of altering the Earth's radiative budget through engineered approaches (i.e., radiative forcing geoengineering [RFG]) has been proposed to mitigate anthropogenic greenhouse gas-induced warming (e.g., Lawrence et al, 2018;Tilmes et al, 2013). Three RFG methodologies based on aerosols have been widely discussed: cirrus cloud thinning (CCT), marine sky brightening (MSB), and stratospheric aerosol injection (SAI) .…”