Since we’ve made only moderate progress in reducing greenhouse gas emissions, climate science has turned to seriously exploring options that typically fell into the “far-fetched” category. That includes something called “solar radiation management” — increasing the reflectivity of the atmosphere to essentially obscure the planet. That could provide a little man-made cooling to temporarily offset a man-made warming.
Eliminating greenhouse gas emissions is the only long-term solution, but managing solar radiation can give us time to get our act together.
Such a global intervention certainly makes many uncomfortable, and we have known for a long time that it would have some side effects. For example, while global warming generally causes an increase in global precipitation, cooling the planet in this way would cause an even greater fall in precipitation, more than negating it. And while most ideas revolve around injecting tiny aerosol particles into the upper atmosphere, the spatial pattern of cooling may depend on where those injections are made.
In a new study led by John Fasullo of the US National Center for Atmospheric Research, researchers performed a model simulation of a specific design of this technique intended to minimize those side effects. But instead they saw a surprising new ones 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 reduce its ever-increasing greenhouse gas emissions. (While this is probably an unlikely extreme scenario, the side effects do become more apparent.) The injection pattern ensures that the planet is cooled quite evenly, and the Earth’s temperature is kept constant throughout the 21st century. It also successfully minimizes precipitation drop across the continents.
But these changes come at the cost of driving precipitation changes across the oceans, and that’s where it gets interesting.
With varying changes in evaporation and precipitation over different areas, some areas of the ocean become somewhat saltier at the surface, including the North Atlantic. Water in the ocean circulates in a global pattern similar to a conveyor belt, with surface water mixing toward the bottom as it becomes cooler and saltier, and thus denser. Water normally mixes in the North Atlantic, so the salt water in the model works to speed up this Atlantic conveyor belt.
This is unexpected, but is kind of a mirror image of simulations of strong future warming, which can sometimes have a large to slow down of the Atlantic Conveyor Belt which has significant climatic effects.
Less ice, more heat
The effects of an accelerated conveyor found in this simulation include greater warming of the deep ocean as more warm surface water moves downward. But on the surface, more warm water is also being sent north through Greenland to the Arctic Ocean, where it would threaten sea ice.
So even if the Earth’s temperature remains constant, the water around Greenland (and Antarctica too) is just as warm as it would be in the full warming scenario until at least 2050. This would cause glacial ice to melt and sea levels to rise despite the lack of water. global warming .
Since the circulation of the Atlantic Ocean 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, but you want to know what you’re getting into and intelligently minimize the side effects.
The conclusion the researchers want to emphasize is that any plan to inject sunlight-reflecting particles into the atmosphere must be carefully simulated so that we can find these little surprises ahead of time, rather than blundering into it. Sometimes you can reasonably estimate the consequences of a change in the Earth’s climate system, but it’s too complex to calculate all the little interactions in your head. That is what a climate model does. And sometimes the little interaction you didn’t predict can be a big deal.
Natural Geosciences2018. DOI: 10.1038/s41561-018-0249-7 (About DOIs).