Have there been any controversial or halted geoengineering field trials in the last two years and why were they stopped?

1. Harvard’s SCoPEx: a high-profile project that was shut down

Harvard’s Stratospheric Controlled Perturbation Experiment (SCoPEx), long proposed as a tiny, tightly controlled test of aerosol behavior in the stratosphere, was officially ceased in March 2024 after repeated delays, the departure of key researchers, and intense public opposition from environmental and Indigenous groups; Harvard said the SCoPEx platform will be repurposed for other stratospheric research ^{[1] [2] [3]}. Reporting in MIT Technology Review and Scientific American traces the closure to years of controversy and an advisory process that failed to clear the path for the planned balloon releases ^{[1] [3]}.

2. Local opposition and governance failures drove several cancellations

Multiple cancelled or halted trials share a pattern: organizers failed to win community buy-in or satisfy advisory bodies, provoking local officials and Indigenous groups to intervene. The University of Washington’s small Alameda sea-salt/particle test was halted by Alameda officials on health and consultation grounds, and commentators and analysts tied that decision to inadequate local engagement ^{[4] [5]}. Critics argued small tests could normalize a pathway to deployment; proponents argued small trials are needed to understand risks ^{[5] [8]}.

3. Practical and logistical reasons also delayed fieldwork

Not all stoppages were purely political. Woods Hole Oceanographic Institution’s first U.S. ocean alkalinity enhancement field trial (LOC‑NESS) was postponed into 2025 for shipping and ocean-condition reasons, even as it faced pushback from fishers and environmentalists worried about ecological impacts ^{[6] [7]}. These operational constraints underscore that field trials depend on narrow environmental windows and complex permitting and logistics ^[6].

4. Unauthorized private tests intensified scrutiny

Controversy escalated when private actors moved ahead with small, unauthorized releases: the start‑up Make Sunsets launched weather balloons with small amounts of sulfur dioxide into the stratosphere, prompting Mexico to ban experiments and governments (including the EPA) to review authorities to halt such activity ^{[3] [9]}. Those incidents hardened resistance to institutional trials and raised questions about governance, enforcement and the risks of rogue actors ^{[3] [9]}.

5. Scientific defenders vs. governance and ethical critics

Proponents — including some funders and researchers — argue small, well‑governed field tests are a “critical missing part” of climate science and are necessary to reduce uncertainties ^{[8] [10]}. Opponents warn that even small trials could normalize a path to deployment, distract from emissions cuts, and pose unpredictable environmental harms; networks of NGOs and Indigenous groups have demanded moratoria and wider governance frameworks ^{[11] [12] [3]}.

6. International and legal frictions complicate any restart

The controversy has spilled into diplomatic and legal arenas. The Convention on Biological Diversity has been invoked as creating a de facto moratorium on some outdoor SRM experiments, and critics say national funding programs (for example the UK’s ARIA programme) risk flouting precautionary norms and prompting international pushback ^{[12] [13]}. Reporting shows funders are proceeding despite these diplomatic and governance tensions ^{[5] [13]}.

7. What the halted trials reveal about future research

The recent pattern — halted university tests, cancelled local trials, unauthorized private launches, and high-profile funder commitments — shows that technical feasibility is no longer the only barrier: public trust, local consent, clear legal permits, and international governance now determine whether small field tests proceed ^{[1] [5] [12]}. Available sources do not mention any definitive resolution to a global governance framework that would clearly authorize or block such experiments.

Limitations and competing viewpoints: reporting from MIT Technology Review, Scientific American, The New York Times and specialist outlets document the cancellations and their causes; pro‑research voices (ARIA leaders, some scientists) stress scientific necessity for controlled trials ^{[8] [10]}, while critics (NGOs, Indigenous groups, some scientists) stress precaution, ethical concerns and political risks ^{[11] [12] [3]}. My summary relies solely on the provided reporting and does not assert events or motives beyond what those sources state.