Geoengineering vs. volcanic activity for a cooler climate

[More crazy talk from Carnegie Institute’s Ken Caldiera… Anthony]

Washington, DC– Major volcanic eruptions spew ash particles into the atmosphere, which reflect some of the Sun’s radiation back into space and cool the planet. But could this effect be intentionally recreated to fight climate change? A new paper in Geophysical Research Letters investigates.

Solar geoengineering is a theoretical approach to curbing the effects of climate change by seeding the atmosphere with a regularly replenished layer of intentionally released aerosol particles. Proponents sometimes describe it as being like a “human-made” volcano.

“Nobody likes the idea of intentionally tinkering with our climate system at global scale,” said Carnegie’s Ken Caldeira. “Even if we hope these approaches won’t ever have to be used, it is really important that we understand them because someday they might be needed to help alleviate suffering.”

He, along with Carnegie’s Lei Duan (a former student from Zhejiang University), Long Cao of Zhejiang University, and Govindasamy Bala of the Indian Institute of Science, set out to compare the effects on the climate of a volcanic eruption and of solar geoengineering. They used sophisticated models to investigate the impact of a single volcano-like event, which releases particulates that linger in the atmosphere for just a few years, and of a long-term geoengineering deployment, which requires maintaining an aerosol layer in the atmosphere.

They found that regardless of how it got there, when the particulate material is injected into the atmosphere, there is a rapid decrease in surface temperature, with the land cooling faster than the ocean.

However, the volcanic eruption created a greater temperature difference between the land and sea than did the geoengineering simulation. This resulted in different precipitation patterns between the two scenarios. In both situations, precipitation decreases over land–meaning less available water for many people living there–but the decrease was more significant in the aftermath of a volcanic eruption than it was in the geoengineering case.

“When a volcano goes off, the land cools substantially quicker than the ocean. This disrupts rainfall patterns in ways that you wouldn’t expect to happen with a sustained deployment of a geoengineering system,” said lead author Duan.

Overall, the authors say that their results demonstrate that volcanic eruptions are imperfect analogs for geoengineering and that scientists should be cautious about extrapolating too much from them.

“While it’s important to evaluate geoengineering proposals from an informed position, the best way to reduce climate risk is to reduce emissions,” Caldeira concluded.

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Climate Response to Pulse Versus Sustained Stratospheric Aerosol Forcing

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL083701 (open access)

Abstract

Solar geoengineering has been suggested as a potential means to counteract anthropogenic warming. Major volcanic eruptions have been used as natural analogues to large‐scale deployments of stratospheric aerosol geoengineering, yet difference in climate responses to these forcings remains unclear. Using the National Center for Atmospheric Research Community Earth System Model, we compare climate responses to two highly idealized stratospheric aerosol forcings that have different durations: a short‐term pulse representative of volcanic eruptions and a long‐term sustained forcing representative of geoengineering. For the same amount of global mean cooling, decreases in land temperature, precipitation, and runoff in the pulse case are much larger than that in the sustained case. The spatial pattern changes differ substantially between these two cases. Thus, direct extrapolations from volcanic eruption observations provide limited insight into impacts of potential stratospheric aerosol geoengineering. However, simulations of volcanic eruptions can be useful to test process representations in models that are used to simulate geoengineering deployments.

Plain Language Summary

Major volcanic eruptions are considered as natural analogues for stratospheric sulfate aerosol geoengineering that aims to cool the climate by increasing the burden of stratospheric sulfate aerosols. Volcanic eruptions produce a layer of sulfate aerosols that stays in the stratosphere for a couple of years, whereas geoengineering efforts would need to sustain the aerosol layer persistently to counteract CO₂‐induced warming. Here we use a climate model to compare climate changes in response to a volcano‐like pulse aerosol forcing and a geoengineering‐like sustained aerosol forcing. When producing similar amount of global mean cooling, the pulse aerosol forcing results in a much larger reduction in land temperature and land minus ocean temperature when compared to that induced by a sustained aerosol forcing. Also, both land precipitation and runoff decrease more in response to the pulse aerosol forcing. Spatial patterns of temperature and the hydrological cycle change also differ substantially between these two types of forcings. These differences in the climate response between the pulse forcing and sustained forcing clearly show that caution should be taken when using climate consequences of volcanic eruptions to directly infer climate responses to stratospheric aerosol geoengineering.

Added, a graphic from Willis Eschenbach which illustrates the issue:

And the data: