Since we've only made moderate progress in cutting greenhouse gas emissions, climate science has turned to seriously investigating options that have typically been in the “far-fetched” category. That includes something called “solar radiation management”—increasing the reflectivity of the atmosphere to, in essence, shade the planet. That could provide a bit of human-caused cooling to temporarily offset some human-caused warming.

Eliminating greenhouse gas emissions is the only long-term solution, but solar radiation management could buy us time to get our act together.

Such a planet-wide intervention certainly makes many uneasy, and we’ve long known that it would come with some side-effects. For example, while greenhouse warming generally causes an increase in global precipitation, cooling the planet in this way would cause an even stronger precipitation decrease that more than cancels that out. And while most ideas revolve around injecting tiny aerosol particles into the upper atmosphere, the spatial pattern of cooling can depend on where those injections are done.

In a new study led by John Fasullo of the US National Center for Atmospheric Research, researchers ran a model simulation of one specific design of this technique meant to minimize those side-effects. But instead, they saw a surprising new side-effect.

Change the oceans

In the simulation, reflective aerosols are injected at 15 and 30 degrees latitude in both the Northern and Southern Hemispheres, and the world makes no effort to cut its ever-increasing greenhouse gas emissions. (While this is probably an implausibly extreme scenario, it does make side-effects pop out more clearly.) The injection pattern does a good job of cooling the planet pretty evenly, and global temperature is kept constant throughout the 21st century. It also successfully minimizes the precipitation decrease over the continents.

But these changes come at the cost of boosting the precipitation changes over the oceans, which is where things get interesting.

With varying changes in evaporation and precipitation over different areas, it just so happens that some regions of the ocean get a little saltier at the surface—including the North Atlantic. Water in the ocean circulates in a global conveyor-belt-like pattern where surface waters mix down to the bottom when they get cooler and saltier, and therefore more dense. Water normally mixes down in the North Atlantic, so the saltier water in the model acts to accelerate this Atlantic conveyor belt.

That’s unexpected but is sort of a mirror image to simulations of strong future warming, which sometimes produce a major slowdown of the Atlantic conveyor belt that has considerable climatic impacts.

Less ice, more heat

The effects of an accelerated conveyor belt found in this simulation include greater warming of the deep ocean because more warm surface water moves downward. But up at the surface, there is also more warm water sent north past Greenland and into the Arctic Ocean, where it would threaten sea ice.

So even though global temperature stays constant, the water around Greenland (and Antarctica as well) is just as warm as in the unmitigated-warming scenario through at least 2050. This would keep glacial ice melting and sea levels rising despite the lack of global warming.

Because Atlantic Ocean circulation can affect everything from Atlantic hurricanes to the Indian monsoon, sea-level rise is probably not the only impact in this simulation. That doesn’t necessarily mean this scenario would be worse than allowing continued warming to occur, but you want to know what you’re getting into and intelligently minimize the side-effects.

The conclusion the researchers want to highlight is that any plan to inject sunlight-reflecting particles into the atmosphere will have to be simulated carefully so we can find these little surprises in advance, rather than blundering into them. Sometimes you can make a pretty educated guess about the consequences of a change to Earth’s climate system, but it’s too complex to compute all the little interactions in your head. That’s what a climate model does. And sometimes the little interaction you didn’t predict can be a big deal.

Nature Geoscience, 2018. DOI: 10.1038/s41561-018-0249-7 (About DOIs).