What scientific evidence supports or opposes solar geoengineering?

1. Major claims on the table: what proponents and opponents both say

Public summaries and reviews identify four central, competing claims about solar geoengineering that structure scientific debate. First, climate models and historical volcanic eruptions support the claim that reflecting sunlight can cool the planet quickly, reducing global mean temperature, with the IPCC and multiple model studies endorsing that outcome ^[1]. Second, cost estimates for research and potential deployment are relatively low compared with mitigation costs, which proponents cite to argue feasibility ^[1]. Third, critics argue SAI and other techniques fail to address atmospheric CO₂ and ocean acidification and could produce uneven regional benefits and harms, undermining justice and ecological integrity ^{[1] [4]}. Fourth, public-interest groups warn that research or deployment could cause mitigation deterrence or geopolitical conflict and therefore call for moratoria or strict governance ^{[3] [4]}.

2. Empirical and model-based support: how strong is the cooling claim?

Climate-model ensembles repeatedly show that injecting reflective aerosols into the stratosphere could offset a substantial fraction of anthropogenic warming on a global average and do so rapidly, a conclusion reflected in the IPCC 1.5 °C literature and governance reviews ^[1]. The 1991 Mount Pinatubo eruption provides a natural experiment: global temperatures fell by roughly 0.5–0.6 °C for about a year, demonstrating the physical mechanism at planetary scale ^[1]. Programmatic research hubs such as the Harvard Solar Geoengineering Research Program have coordinated experiments and modeling since the early 2020s and continue publishing work that refines deployment scenarios and quantified impacts; these institutional efforts aim to reduce scientific uncertainties and inform policy (p2_s1, ^[7], 2025-05-20; 2025-05-27).

3. Physical, ecological, and climate-system downsides that science flags

Model studies and literature reviews identify consistent but complex trade-offs: SAI would not recreate preindustrial climate patterns and is likely to generate regional temperature and precipitation anomalies, alter monsoons, and risk impacts on ecosystems and biodiversity. It could also delay stratospheric ozone recovery, increase acid-rain precursors, and disrupt ocean–atmosphere processes, while any cessation (‘termination shock’) would produce abrupt warming ^{[1] [4] [5]}. Importantly, models cannot yet resolve many ecosystem cascade effects, and some uncertainties only become knowable through real-world interventions, creating a scientific impasse where the act of reducing uncertainty requires risky experiments or deployment ^[4].

4. Political friction lines: governance, equity, and contested research agendas

Scientific evidence intersects urgently with governance: numerous analyses document social opposition, Indigenous concerns, and geopolitical risk that could magnify harms or spur conflict if uncoordinated deployment occurs ^[4]. Advocacy groups such as the Union of Concerned Scientists explicitly oppose deployment on the basis that the risks are unacceptably high and that geoengineering could reduce incentives to cut emissions, a position rooted in both scientific and ethical judgments ^[3]. Conversely, university programs and some funders promote expanded research aimed at reducing uncertainty and informing democratic debate; these actors often emphasize transparency and multidisciplinary inquiry, framing research as necessary to prepare governance options rather than to enable immediate deployment (p2_s1, ^[7], 2025-05-27).

5. What the most recent programmatic work says about research posture

Between 2022 and mid‑2025, scholarship and institutional programs have moved from theoretical modeling toward structured field experimentation plans and governance frameworks. Yale and Carnegie analyses and program statements outline that research must be coupled with legitimacy, public engagement, and strict oversight, noting that most scientific evidence remains incomplete and that field experiments should be small, transparent, and socially reviewed (p2_s3, 2022-10-24; ^[8], 2024-04-25; p1_s3). The most recent institutional reports from 2025 emphasize multidisciplinary approaches—natural sciences, social sciences, and humanities—to evaluate trade-offs, with explicit calls to prevent mitigation deterrence and avoid lock‑in to risky pathways (p2_s1, ^[7], 2025-05-20; 2025-05-27).

6. Bottom line: the scientific balance and what’s missing from public debate

The consolidated evidence shows a robust physical mechanism by which solar geoengineering could cool the planet quickly, supported by models and volcanic analogues, but it also shows systemic limitations—regional harms, ozone and ecosystem risks, and zero effect on CO₂ and acidification—that are significant and partly irreducible without risky testing ^{[1] [4]}. The debate now pivots less on whether cooling is physically possible and more on how to govern research, who decides acceptable risk, and how to ensure emissions reductions are not undermined by promises of technological fixes; these governance and equity questions are as central as the climate‑science uncertainties and will determine whether any future use is judged legitimate ^{[3] [6]}.