Shading the planet has not come with side effects of rain

This sounds like a radical course of action: injecting substances that are very present in the Earth's atmosphere to reflect a little sun and fight against global warming. Again, injecting a quantity of greenhouse gases into the atmosphere and warming the planet has also been drastic.

The key to a sensible reflection on this "solar geoengineering" is to avoid extremes and to consider the most plausible scenarios of use. That means we can ignore things like using solar geoengineering to undo all warming while emitting as much CO₂ as we see fit, it's just not plausible.

There are a number of reasons to remove it from the table. The fact that the atmospheric injections have a cooling effect is only temporary: stopping immediately reveals all the strength of the warming that you counterbalance. There is also the fact that this program is only counteracting the warming: ocean acidification will continue rapidly. And for another example, the inadequacy of physics between cooling based on solar geo-engineering and warming greenhouse effect means that precipitation can decrease even if the temperature remains the same.

A new study by a team led by Peter Irvine of Harvard tackles this problem of precipitation. The Irvine team sought to determine whether a more plausible geoengineering system could minimize negative impacts on the hydrological cycle.

Avoid the extremes

The scenario used consisted of doubling the concentration of CO₂ in the atmosphere, which should cause a lot of warming. But instead, we react by doing enough solar geoengineering to halve the resulting warming for a hundred years. There are ways to customize the location of your injections so that the tiny particles of reflective aerosols in the stratosphere mix uniformly across the globe, which would be crucial.

This means that the authors could simply reduce incoming sunlight in climate models by an equivalent amount, which allowed them to avoid a potential complication: the 13 different models that they used could simulate the chemical and physical behavior of atmospheric aerosol particles in a slightly different way. undesirable variable in the results.

In each model, simulations were performed with and without geo-solar engineering to see its effect on events such as extreme rainfall events, the overall equilibrium of precipitation with respect to evaporation, or even the hurricane resistance. Overall, the simulated scenario has a direct effect on temperatures. By offsetting half of the global warming, each site experiences less warming. This also significantly reduces the increase in the amount of steroids that warming gives to hurricanes, thus reducing their future strengths.

Although global warming usually results in a dry-dry-dry-dry pattern, twice as much CO₂ simulation increases the overall average three percent precipitation. With solar geoengineering, this number has dropped to 0.5%. It's not an overall reduction, it's a smaller increase.

Small local changes

However, with regard to precipitation, the local changes observed in these models are important: if one region sees a benefit to geoengineering and another to another damage, everyone in both regions will want to know it. But in these models, the situation is not very different at the local level. Worldwide, only 0.4% of ice-free land experiences a statistically significant decline in precipitation through solar geoengineering. About 25 to 45% of the land area (depending on the aspect of rainfall you are checking) is experiencing a slight increase.

It may very well be that, on closer inspection, half of our future warming is not the optimal scenario, but it suggests that we could do a lot of solar geo-engineering without significant side effects on precipitation.

"Our results are not […] support common claims that [solar geoengineering] inevitably lead to significant damage for some regions, "the researchers write," nor the claims that [solar geoengineering’s] the advantages and disadvantages always have a very unequal distribution. "

Many climate scientists are fiercely opposed – or at least deeply skeptical – to the concept of solar geoengineering. As a result, search like that always results in certain animated discussion. But the proponents of research argue that we can not debate something we do not understand, and a constructive debate should focus on plausible scenarios rather than the most extreme scenario you can imagine.

Everyone agrees that the only solution is to eliminate greenhouse gas emissions, but what if we do not do it fast enough and it remains an additional option? Studies like this one clarify what a hypothetical rescue plan might entail.

Nature Climate change, 2019. DOI: 10.1038 / s41558-019-0398-8 (About the DOIs).