What proposals for deliberate large-scale geoengineering have been published, and how do they differ from operational cloud seeding?
Executive summary
Deliberate large-scale geoengineering proposals fall into two broad camps—solar radiation management (SRM) methods that try to reflect sunlight and rapidly cool the planet, and carbon dioxide removal (CDR) methods that aim to extract and store CO2; both have been described in numerous academic reviews and policy reports [1] [2] [3]. Operational cloud seeding, by contrast, is a long-standing, regional weather-modification practice focused on altering precipitation or local cloud properties and is already deployed by scores of governments, making it qualitatively different in purpose, scale, and governance from the hypothetical planetary interventions that dominate geoengineering literature [4].
1. What the literature classifies as “large‑scale geoengineering”
Scholars and policy reviews define geoengineering as deliberate, large‑scale interventions in the Earth’s climate system and categorize proposals principally as CDR (remove CO2) or SRM (change the planet’s radiative balance) [5] [1]. CDR examples discussed in the literature include afforestation and reforestation, bioenergy with carbon capture and storage (BECCS), ocean carbon removal routes, enhanced weathering, and engineered approaches like direct air capture; these are presented as efforts to reduce atmospheric CO2 concentrations over decadal to century timescales [5] [6] [1]. SRM proposals span techniques intended to increase planetary reflectivity—most prominently stratospheric aerosol injection, marine cloud brightening, and space‑based reflectors or sunshields—as well as surface albedo modification and proposals for regional interventions such as glacier stabilization [7] [8] [9] [10].
2. How the leading SRM proposals would work and why they worry scientists
Stratospheric aerosol injection (SAI) would mimic volcanic cooling by dispersing sulfate or other reflective particles into the upper atmosphere using proposed delivery systems like aircraft, rockets, or high‑altitude pipes, producing a planet‑spanning radiative effect if scaled up [7] [3]. Marine cloud brightening (MCB) aims to increase the reflectivity of low marine clouds by adding cloud condensation nuclei so clouds scatter more sunlight, while space‑based sunshields would physically intercept sunlight before it reaches Earth—each has substantial uncertainties about efficacy, regional climate side‑effects, and governance [7] [8] [2]. Reviews caution that SRM does not remove greenhouse gases, can produce uneven regional impacts (e.g., altered precipitation), and raises profound international‑decision and liability questions [3] [11].
3. Carbon dioxide removal: promises, scales, and trade‑offs
CDR proposals aim to address the root cause—elevated CO2—through biological, chemical, and engineered sinks: afforestation and soil carbon enhancement increase natural sinks; BECCS couples biomass energy with sequestration but raises land‑use and food security concerns; direct air capture and enhanced weathering are engineered options with high energy and cost considerations [5] [6]. Nature Communications and review articles emphasize that CDR methods vary widely in timelines, permanence, monitoring needs, and social impacts, and none yet scale reliably to the gigaton‑per‑year removals envisioned in some scenarios [6] [1].
4. Why operational cloud seeding is different in scope and intent
Cloud seeding and other weather‑modification activities are regional, operational, and generally targeted at precipitation enhancement or localized cloud albedo changes rather than altering the global radiative balance; the World Meteorological Organization notes that roughly 50 countries and several U.S. states deploy such weather‑modification activities today [4]. Unlike hypothetical SRM schemes designed to cool the planet at planetary scale, operational cloud seeding is governed as weather management, addresses local objectives (e.g., drought relief), and is not framed in the literature as a substitute for emissions reduction or global climate intervention [4].
5. Governance, risk framing, and the political line between “research” and “deployment”
Reviews and policy pieces repeatedly stress governance gaps and the political problem that unilateral or poorly coordinated deployment of SRM could induce cross‑border harms; many experts call for precautionary research and robust global governance before any large‑scale implementation, distinguishing staged CDR pilots and low‑risk restoration from risky SRM field experiments [11] [3] [6]. Public engagement studies also show broad unfamiliarity and divided views, with SRM notably prompting ethical and geopolitical objections that are less salient for many CDR or small‑scale adaptation projects [5] [8].