Geoengineering a cooler Earth: The benefits and hazards of a radical response to the climate crisis

We are quite good at setting targets for reducing emissions of warming greenhouse gases into the atmosphere, but unfortunately not so good at achieving these targets.

It now seems clear that our efforts to reduce greenhouse gas emissions are not moving fast enough to ward off the increasingly destructive effects of storms, floods and heatwaves that are exacerbated by climate change.

Geoengineering techniques designed to cool the planet, for example by adding alkaline chemicals to the ocean, spraying salty mixtures into clouds and reflective particles into the stratosphere, are now entering preliminary small-scale trials to assess their safety and effectiveness. These trials are nicely summarised by Eric Niller in the Wall Street Journal during February.

The brightened clouds will reflect more sunlight away from Earth, cooling the waters around the Great Barrier Reef where ocean warming is causing large coral die-offs

A volcano called Mount Pinatubo in the Philippines erupted in 1991, spewing ash and volcanic gases such as sulphur dioxide and carbon dioxide into the upper atmosphere. Volcanic carbon dioxide release in contemporary eruptions has never caused measurable warming of the lower atmosphere but volcanic sulphur dioxide can cause global cooling. The sulphur dioxide is converted to sulphuric acid which condenses in the stratosphere into fine sulphate aerosols that increase reflection of radiation from the sun back into space, cooling the earth’s lower atmosphere.

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The Pinatubo eruption, probably the largest aerosol disturbance of the stratosphere in the 20th century, lowered Earth’s surface temperature, resulting in a global temperature decrease of about 0.5 degrees over the ensuing year. In an attempt to mimic this cooling effect, an Israeli start-up company called Stardust Solutions plans to disperse clouds of tiny reflective particles 60,000ft (18,300m) high in the atmosphere.

In another project called Marine Clouds Brightening, part of a $65 million (€60 million) programme led by Southern Cross University, a ship off the northwestern Australia coast is spraying trillions of nano-sized ocean salt crystals into the air. These crystals should mix with and brighten low-altitude clouds that form over the ocean. The brightened clouds will reflect more sunlight away from Earth, cooling the waters around the Great Barrier Reef where ocean warming is causing large coral die-offs. The UN Intergovernmental Panel on Climate Change says 70-90 per cent of coral reefs will die as global heating rises to 1.5 degrees.

And, in a $10 million project scheduled to begin next August, researchers at Woods Hole Oceanographic Institute plan to pour 6,000 gallons of sodium hydroxide solution into the ocean 10 miles south of Martha’s Vineyard. Sodium hydroxide is chemically alkaline and will dissolve in the ocean surface water and will react with CO2 in the air forming sodium bicarbonate and, it is hoped, trapping 600 metric tonnes of atmospheric CO2 as sodium bicarbonate in the ocean where it will remain undisturbed for millenniums — see further details in an article by Warren Cornwall in Science, last December.

Enthusiasts for this approach argue that absorbing CO2 from the atmosphere on a grand scale and storing it in the oceans could absorb as much atmospheric greenhouse gas as the Amazon rainforest, helping keep global warming under 2 degrees.

There are dangers in pursuing large-scale geoengineering approaches. One danger is that any significant tinkering with nature carries a significant risk that things will go wrong

These three projects are only initial field trials. Full global deployment of any such initiatives would require large-scale international co-operation and agreement and would call for the investment of trillions of dollars. Any such developments are many years away.

There are dangers in pursuing large-scale geoengineering approaches. One danger is that any significant tinkering with nature carries a significant risk that things will go wrong. For example, large-scale injection of reflective particles into the stratosphere might deplete Earth’s ozone layer and disrupt rainfall patterns. Also, uncertainties remain about how much carbon could be captured by artificially enhancing the alkalinity of oceans and what environmental side effects might kick in if humans start to modify the oceans on a global scale.

Possibly an even bigger risk posed by geoengineering approaches to cooling our planet is that such initiatives would deflect us from the intensive pursuit of the goal of achieving net zero emissions of warming gases to the atmosphere. We know how to achieve this goal, although it will be a difficult task, and the great majority of climate scientists assure us this is the way to go.

• William Reville is an emeritus professor of biochemistry at UCC