Weakening of the extratropical storm tracks in idealized solar geoengineering scenarios
Andrew Lockley
[PDF] Weakening of the extratropical storm tracks in idealized solar geoengineering scenarios
Stephen Salter
Hi All
Do the brown bits in figure S1 mean polar warming would be caused by solar dimming in all six models?
Stephen
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Renaud de RICHTER
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Douglas MacMartin
No. Every model that has ever simulated solar dimming shows that the poles would cool relative to not dimming the sun. But, if you just turn the sun down then you “undercool” the poles. The plot is G1 relative to pre-industrial, not G1 relative to 4xCO2.
(These are the exact same model simulations that you’ve asked the exact same question about before.)
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Stephen Salter
Doug
If we are more
confident about numbers for temperatures now than what they
would be with four times CO2 or what they were in pre-industrial
perhaps it would be less confusing to use now as the baseline.
Below is what marine
cloud brightening would do with a 50% increase in the
concentration of condensation nuclei in sea areas with low
cloud. It looks good to me but might be even better with more
sophisticated spray patterns.
Stephen
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Douglas MacMartin
Hi Stephen,
The highly idealized simulations in that paper are the GeoMIP G1, so starting with preindustrial, doing an abrupt 4xCO2, while simultaneously turning down the sun, so the only comparisons that can be done with that are either the effect of increased CO2 relative to not increased CO2, or the simultaneous effect of increased CO2 + solar reduction relative to not-increased-CO2. The result would not be substantially different if “today” was used as a baseline instead of preindustrial. These simulations are useful for understanding physics, but not useful (at least directly) for policy relevance, especially since solar reduction is not the same as any actual geoengineering method other than space-based.
I don’t personally believe that we know enough about where/when MCB would be effective to make any useful policy-relevant predictions about what the effects would be; GCM simulations at this point are quite hypothetical as the ultimate effects on temperature and precipitation will depend on where it can be made to work. (Which I agree ought to be a research priority… there’s lots of things that ought to be research priorities if anyone ever decides to fund this research.)
doug
Renaud de RICHTER
How can the world combat the continued rise in global temperatures? How about shading the Earth from a portion of the sun's heat by injecting the stratosphere with reflective aerosols? After all, volcanoes do essentially the same thing, albeit in short, dramatic bursts: When a Vesuvius erupts, it blasts fine ash into the atmosphere, where the particles can linger as a kind of cloud cover, reflecting solar radiation back into space and temporarily cooling the planet.
Some researchers are exploring proposals to engineer similar effects, for example by launching reflective aerosols into the stratosphere—via planes, balloons, and even blimps—in order to block the sun's heat and counteract global warming. But such solar geoengineering schemes, as they are known, could have other long-lasting effects on the climate.
Now scientists at MIT have found that solar geoengineering would significantly change extratropical storm tracks—the zones in the middle and high latitudes where storms form year-round and are steered by the jet stream across the oceans and land. Extratropical storm tracks give rise to extratropical cyclones, and not their tropical cousins, hurricanes. The strength of extratropical storm tracks determines the severity and frequency of storms such as nor'easters in the United States.
The team considered an idealized scenario in which solar radiation was reflected enough to offset the warming that would occur if carbon dioxide were to quadruple in concentration. In a number of global climate models under this scenario, the strength of storm tracks in both the northern and southern hemispheres weakened significantly in response.
Weakened storm tracks would mean less powerful winter storms, but the team cautions that weaker storm tracks also lead to stagnant conditions, particularly in summer, and less wind to clear away air pollution. Changes in winds could also affect the circulation of ocean waters and, in turn, the stability of ice sheets.
"About half the world's population lives in the extratropical regions where storm tracks dominate weather," says Charles Gertler, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS). "Our results show that solar geoengineering will not simply reverse climate change. Instead, it has the potential itself to induce novel changes in climate."
Gertler and his colleagues have published their results this week in the journal Geophysical Research Letters. Co-authors include EAPS Professor Paul O'Gorman, along with Ben Kravitz of Indiana State University, John Moore of Beijing Normal University, Steven Phipps of the University of Tasmania, and Shingo Watanabe of the Japan Agency for Marine-Earth Science and Technology
A not-so-sunny picture
Scientists have previously modeled what Earth's climate might look like if solar geoengineering scenarios were to play out on a global scale, with mixed results. On the one hand, spraying aerosols into the stratosphere would reduce incoming solar heat and, to a degree, counteract the warming caused by carbon dioxide emissions. On the other hand, such cooling of the planet would not prevent other greenhouse gas-induced effects such as regional reductions in rainfall and ocean acidification.
There have also been signs that intentionally reducing solar radiation would shrink the temperature difference between the Earth's equator and poles or, in climate parlance, weaken the planet's meridional temperature gradient, cooling the equator while the poles continue to warm. This last consequence was especially intriguing to Gertler and O'Gorman.
"Storm tracks feed off of meridional temperature gradients, and storm tracks are interesting because they help us to understand weather extremes," Gertler says. "So we were interested in how geoengineering affects storm tracks."
The team looked at how extratropical storm tracks might change under a scenario of solar geoengineering known to climate scientists as experiment G1 of the Geoengineering Model Intercomparison Project (GeoMIP), a project that provides various geoengineering scenarios for scientists to run on climate models to assess their various climate effects.
The G1 experiment assumes an idealized scenario in which a solar geoengineering scheme blocks enough solar radiation to counterbalance the warming that would occur if carbon dioxide concentrations were to quadruple.
The researchers used results from various climate models run forward in time under the conditions of the G1 experiment. They also used results from a more sophisticated geoengineering scenario with doubling of carbon dioxide concentrations and aerosols injected into the stratosphere at more than one latitude. In each model they recorded the day-to-day change in air pressure at sea level pressure at various locations along the storm tracks. These changes reflect the passage of storms and measure a storm track's energy.
"If we look at the variance in sea level pressure, we have a sense of how often and how strongly cyclones pass over each area," Gertler explains. "We then average the variance across the whole extratropical region, to get an average value of storm track strength for the northern and southern hemispheres."
An imperfect counterbalance
Their results, across climate models, showed that solar geoengineering would weaken storm tracks in both Northern and Southern hemispheres. Depending on the scenario they considered, the storm track in the Northern Hemisphere would be 5 to 17% weaker than it is today.
"A weakened storm track, in both hemispheres, would mean weaker winter storms but also lead to more stagnant weather, which could affect heat waves," Gertler says. "Across all seasons, this could affect ventilation of air pollution. It also may contribute to a weakening of the hydrological cycle, with regional reductions in rainfall. These are not good changes, compared to a baseline climate that we are used to."
The researchers were curious to see how the same storm tracks would respond to just global warming alone, without the addition of social geoengineering, so they ran the climate models again under several warming-only scenarios. Surprisingly, they found that, in the northern hemisphere, global warming would also weaken storm tracks, by the same magnitude as with the addition of solar geoengineering. This suggests solar geoengineering, and efforts to cool the Earth by reducing incoming heat, would not do much to alter global warming's effects, at least on storm tracks—a puzzling outcome that the researchers are unsure how to explain.
In the Southern Hemisphere, there is a slightly different story. They found that global warming alone would strengthen storm tracks there, whereas the addition of solar geoengineering would prevent this strengthening, and even further, would weaken the storm tracks there.
"In the Southern Hemisphere, winds drive ocean circulation, which in turn could affect uptake of carbon dioxide, and the stability of the Antarctic ice sheet," O'Gorman adds. "So how storm tracks change over the Southern Hemisphere is quite important."
The team also observed that the weakening of storm tracks was strongly correlated with changes in temperature and humidity. Specifically, the climate models showed that in response to reduced incoming solar radiation, the equator cooled significantly as the poles continued to warm. This reduced temperature gradient appears to be sufficient to explain the weakening storm tracks—a result that the group is the first to demonstrate.
"This work highlights that solar geoengineering is not reversing climate change, but is substituting one unprecedented climate state for another," Gertler says. "Reflecting sunlight isn't a perfect counterbalance to the greenhouse effect."
Adds O'Gorman: "There are multiple reasons to avoid doing this, and instead to favor reducing emissions of CO2 and other greenhouse gases."
Weakening of the Extratropical Storm Tracks in Solar Geoengineering Scenarios
Abstract
Solar geoengineering that aims to offset global warming could nonetheless alter atmospheric temperature gradients and humidity and thus affect the extratropical storm tracks. Here, we first analyze climate model simulations from experiment G1 of the Geoengineering Model Intercomparison Project, in which a reduction in incoming solar radiation balances a quadrupling of CO2. The Northern Hemisphere extratropical storm track weakens by a comparable amount in G1 as it does for increased CO2 only. The Southern Hemisphere storm track also weakens in G1, in contrast to a strengthening and poleward shift for increased CO2. Using mean available potential energy, we show that the changes in zonal‐mean temperature and humidity are sufficient to explain the different responses of storm‐track intensity. We also demonstrate similar weakening in a more complex geoengineering scenario. Our results offer insight into how geoengineering affects storm tracks, highlighting the potential for geoengineering to induce novel climate changes.
Plain Language Summary
Solar geoengineering refers to reflecting incoming sunlight to counteract the greenhouse effect of increased carbon dioxide concentrations and is one proposed intervention to avoid the most dramatic risks of global warming. Climate under solar geoengineering would nonetheless be meaningfully different from a baseline climate without increased carbon dioxide. The extratropical storm tracks, regions with heightened incidence of extratropical cyclones, are important components of weather and climate outside of the tropics. In simulations with global climate models, we find that the storm track in the Northern Hemisphere is similarly weakened in a solar geoengineering scenario with little change in global mean temperature as in a global warming scenario. The storm track in the Southern Hemisphere also weakens in the geoengineering scenario in contrast to a strengthening with global warming. The weakening of the storm tracks in the geoengineering scenario is partly related to a weakening of the pole‐to‐equator temperature gradient in both hemispheres. This means that reflecting incoming sunlight may not prevent changes in the strength of extratropical cyclones in the Northern Hemisphere and may overcorrect in the Southern Hemisphere.
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