Climate engineering refers to large-scale, deliberate interventions in Earth’s climate system designed to counteract the effects of global warming. These proposals range from reflecting sunlight back into space to pulling carbon dioxide directly out of the atmosphere. None are currently deployed at scale, but research programs are expanding as global temperatures continue to rise and conventional emissions cuts fall behind targets.
The field divides into two broad categories: methods that remove greenhouse gases already in the atmosphere, and methods that reduce how much solar energy the planet absorbs. Both carry significant promise and significant risk, and understanding the difference between them is essential to following the growing public debate.
Carbon Dioxide Removal
Carbon dioxide removal (CDR) tackles the root cause of warming by pulling CO₂ out of the air and storing it somewhere durable. The simplest version is planting trees, which absorb carbon as they grow. But forests alone can’t absorb enough to offset current emissions, and they release their stored carbon when they burn or decompose. That limitation has driven interest in more engineered approaches.
Direct air capture uses industrial machines fitted with chemical filters to strip CO₂ from ambient air. The captured gas is then compressed and injected deep underground into geological formations, where it mineralizes over time. Several pilot plants exist today, the largest of which capture around 36,000 tons of CO₂ per year. That sounds like a lot, but global emissions exceed 37 billion tons annually, so the gap between current capacity and what’s needed is enormous. The main barriers are cost and energy. Running these machines requires significant electricity, and if that electricity comes from fossil fuels, the net benefit shrinks.
Another CDR approach is enhanced weathering. Certain minerals, particularly silicate rocks like basalt, naturally react with CO₂ and lock it into stable carbonate compounds. Crushing these rocks into fine particles and spreading them on farmland or coastlines accelerates a process that ordinarily takes thousands of years. Early field trials suggest this could sequester meaningful quantities of carbon while also improving soil health, though scaling it up would require a massive mining and distribution infrastructure.
Ocean-based CDR methods include fertilizing patches of ocean with iron or other nutrients to stimulate phytoplankton blooms that absorb carbon. When these organisms die and sink to the deep ocean floor, the carbon stays sequestered for centuries. However, experiments with ocean iron fertilization have produced mixed results, and ecologists worry about cascading effects on marine food webs.
Solar Radiation Management
Solar radiation management (SRM) doesn’t reduce greenhouse gas concentrations at all. Instead, it aims to cool the planet by reflecting a small percentage of incoming sunlight back into space. Think of it as turning down the thermostat without fixing the furnace.
The most discussed SRM proposal is stratospheric aerosol injection: using high-altitude aircraft to release tiny reflective particles, most commonly sulfur dioxide, into the stratosphere. The concept isn’t theoretical fantasy. When Mount Pinatubo erupted in the Philippines in 1991, it launched roughly 20 million tons of sulfur dioxide into the upper atmosphere. Global average temperatures dropped by about 0.5°C for the following year. SRM proposals essentially aim to replicate this effect on a continuous, controlled basis.
Marine cloud brightening is a more localized approach. Spraying fine sea salt particles into low-lying clouds over the ocean would make those clouds more reflective, bouncing more sunlight away from the surface below. Early modeling suggests this could help protect coral reefs and cool specific regions, but the effects would be temporary and geographically uneven.
A third idea involves increasing the reflectivity of surfaces on Earth itself, sometimes called albedo modification. Proposals include painting roofs white, growing more reflective crop varieties, or placing reflective covers over glaciers. These are low-tech and low-risk compared to stratospheric interventions, but their cooling effect is too small to make a dent in global temperature trends on their own.
Why SRM Worries Scientists and Policymakers
Solar radiation management could theoretically reduce global temperatures within months rather than decades, which makes it appealing as an emergency measure. But the risks are substantial and unlike anything in conventional climate policy.
The most alarming concern is called “termination shock.” If aerosol injection were deployed for years and then suddenly stopped, whether due to war, political collapse, or budget cuts, temperatures would snap back rapidly. Ecosystems and societies adapted to the artificially cooled climate would face a rate of warming far faster than what’s happening now. This creates a commitment problem: once you start, stopping becomes dangerous.
Regional side effects add another layer of concern. Climate models consistently show that stratospheric aerosols could shift rainfall patterns, potentially reducing monsoon precipitation in South and Southeast Asia while altering weather in sub-Saharan Africa. Cooling one part of the globe could mean drought in another. This raises profound questions about who decides to deploy SRM and who bears the consequences, questions that have no existing international legal framework to resolve.
SRM also does nothing to address ocean acidification. As long as CO₂ concentrations keep rising, the oceans keep absorbing it, lowering pH levels and threatening shellfish, coral, and the broader marine food chain. A cooler but increasingly acidic ocean is still an ocean in crisis.
How CDR and SRM Compare
- Speed: SRM could lower temperatures within a year. CDR works over decades to centuries.
- Root cause: CDR addresses the buildup of greenhouse gases directly. SRM masks the symptoms.
- Reversibility: Stopping CDR has no negative rebound effect. Stopping SRM can trigger rapid warming.
- Cost: SRM is estimated to be relatively cheap to deploy (a few billion dollars per year for stratospheric aerosols). CDR at climate-relevant scales would cost hundreds of billions annually.
- Side effects: CDR approaches carry localized environmental risks. SRM carries global, potentially unequal consequences.
Most climate scientists view the two not as competing options but as potentially complementary ones, with CDR as the long-term solution and SRM as a possible bridge if warming reaches crisis levels before emissions drop fast enough.
The Moral Hazard Debate
One of the most persistent arguments against climate engineering research is the idea that it creates a moral hazard. If governments believe a technological fix is available, they may invest less in reducing emissions, the one strategy everyone agrees is necessary. Critics point out that climate engineering could become a political excuse to delay the hard economic transitions away from fossil fuels.
Supporters counter that emissions reductions alone are already failing to keep pace with warming targets. The Paris Agreement’s goal of limiting warming to 1.5°C above preindustrial levels is widely considered out of reach without some form of carbon removal. In that context, researching climate engineering isn’t a distraction but a necessity. The question is how to pursue it without undermining the urgency of cutting emissions at the source.
Where Things Stand Today
No country has deployed climate engineering at a scale large enough to measurably affect global temperatures. What does exist is a patchwork of research programs. The United States, European Union, China, and several other nations fund modeling studies and small-scale field experiments. A handful of startups sell carbon removal credits from direct air capture, though total removal remains a tiny fraction of what’s emitted.
Governance remains the biggest gap. There is no international treaty or regulatory body specifically overseeing climate engineering research or deployment. The closest existing framework is the London Protocol, which regulates ocean dumping and has been interpreted to cover some forms of ocean-based carbon removal. Calls for formal governance are growing, particularly as private actors and individual nations gain the technical capacity to attempt unilateral interventions.
Climate engineering sits at an uncomfortable intersection of necessity, uncertainty, and politics. The science suggests some of these tools could genuinely help, but deploying them without robust international coordination could introduce risks as serious as the ones they’re meant to solve.