Tag: climate

Paper on the importance of using a proper counterfactual for assessing climate interventions published in Communications Earth & Environment

Figure 1 from Lee et al. (2024). Comparison of how Arctic maximum (TXx) and minimum (TNn) temperatures change under three “radiative forcing geoengineering” (RFG) proposals — stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT) — as compared to a world with very high greenhouse gas emissions but no RFG (RCP8.5) or a world with less greenhouse gas emissions that has the same global mean temperature as that achieved by the RFGs (RCP4.5). Importantly, the RFGs are applied on a high greenhouse gas background (RCP8.5), meaning the RFG-RCP8.5 differences (first and third column) show the causal effect of the RFGs, which is to cool the Arctic. Comparing the RFGs with RCP4.5 (second and fourth columns) gives an indication of how efficiently or inefficiently the RFG cools the Arctic as compared to decarbonization; in this model, the RFGs generally cool the Arctic less efficiently. However, it would be a conceptual mistake to say that the RFGs warm the Arctic: Their causal effect is clearly cooling, albeit less efficiently than decarbonization. A paper only focusing on the RFG-RCP8.5 differences could therefore oversell the benefits of the RFGs while a paper focusing on the RFG-RCP4.5 differences could mischaracterize the (imperfect) benefit of RFG as a harm due to RFG. For a balanced assessment of climate interventions, it’s necessary to keep both perspectives in mind simultaneously.

Our new Matters Arising paper, “Informative risk analyses of radiative forcing geoengineering require proper counterfactuals”, has been published in Communications Earth & Environment in response to another article that we felt did not properly communicate their results. The root of the disagreement lies in what we consider to be the proper “counterfactual”, or situation that would exist in the absence of the object of study. In this case, Müller and colleagues looked at simulations of different solar climate interventions (radiative forcing geoengineering methods in their terminology) that were designed to cool global temperatures to close to what would occur in a moderate greenhouse gas emission scenario (RCP4.5) despite actually having much higher greenhouse gas emissions (RCP8.5). Because there is residual warming in the Arctic, the authors claimed that RFG increased the risk in the Arctic; we believe this claim is confused and misleading, as the RFGs actually reduced the risk in the Arctic compared to the high emission scenario on which they were applied, just not by as much as if emissions had been reduced as in RCP4.5. Figure 1 from our paper, reproduced above, shows how a paper focusing only on the RFG-RCP8.5 differences may overstate the benefits of RFGs (if there exists any tradeoff between RFGs and decarbonization) whereas one focusing on the RFG-RCP4.5 differences would mischaracterize the imperfect benefits of the RFGs as being a direct harm from implementing the RFGs.

To read more, the introduction of our paper is provided below and the open-access full text is available at: https://www.nature.com/articles/s43247-024-01881-y.

The study “Radiative forcing geoengineering under high CO2 levels leads to higher risk of Arctic wildfires and permafrost thaw than a targeted mitigation scenario” by Müller, et al. examines three scenarios of radiative forcing geoengineering as simulated by the Norwegian Earth System Model. The authors compare high-latitude boreal summer maximum temperatures and winter minimum temperatures in the geoengineering scenarios – stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning – to high-warming and moderate-warming scenarios without geoengineering. They conclude that all three geoengineering interventions, which use the high-warming scenario as the baseline, worsen the risk of wildfire and permafrost thaw relative to the moderate-warming scenario because they cool the Arctic somewhat less than the global mean in their experiments. We have significant concerns about how this paper’s results and conclusions are framed.

First and foremost, Müller et al. claim that geoengineering increases the risk of wildfires and permafrost thaw; instead, what the authors show is that geoengineering reduces these risks, but not as much as an equivalent scenario and emissions cuts. We note that the original title, “Radiative forcing geoengineering causes a higher risk of wildfires and permafrost thawing over the Arctic regions”, made this claim more explicit than the revision, which is an improvement. However, both framings of “risk” suffer from the fundamental defect of comparing geoengineering to an inappropriate baseline: the three geoengineering scenarios use RCP8.5 (a high-emissions, high-warming scenario) as the background, but the authors primarily compare the geoengineering scenarios to RCP4.5 (a moderate-warming scenario) instead of the more appropriate counterfactual of higher emissions without geoengineering. Secondly, the authors overgeneralize from a limited set of simulations even though it is now well known that regional impacts are highly dependent on the specific geoengineering strategy employed.

Lee et al. (2024), Communications Earth & Environment, doi:10.1038/s43247-024-01881-y

Pants on Fire!: Michael contributes to Politifact debunking of social media “geoengineered hurricane” conspiracies

Politifact Truth-O-Meter rating for the idea that the pictured clouds are related to artificial hurricanes: Pants on fire!

Michael spoke with Politifact the day before Hurricane Milton made landfall in Florida to help debunk social media conspiracy theories that pictures of unusual and visually striking cloud patterns are evidence that the government is “geoengineering” hurricanes. You can read some of the highlights below, or check out the full fact check at Politifact.

Michael Diamond, Florida State University assistant professor of meteorology, and Erik Nielsen, instructional assistant professor at Texas A&M University’s Department of Atmospheric Sciences, identified these clouds as asperitas — rare clouds that “resemble rippling ocean waves in the sky.” 

There is no established explanation why these clouds form, according to the U.K. Met Office, but “atmospheric conditions must be unstable.” They don’t produce rainfall but typically appear after thunderstorms, the office said.

“(Asperitas) clouds are associated with thunderstorms but don’t produce storms themselves,” Diamond said. “These clouds cannot be formed through geoengineering or cloud seeding, to our present knowledge.”

Diamond also pointed out that Hurricane Milton was still far from the coast when these claims were made.

This video doesn’t show geoengineering occurring as Hurricane Milton approaches Florida. We rate that Pants on Fire!

Factcheck by Loreben Tuquero, 10/8/2024

Paper on the rise and fall of Earth’s strong clear-sky hemispheric albedo symmetry published in Communications Earth & Environment

Figure. Observed and average climate model breakdown of the clear-sky asymmetry into atmospheric and surface components. Satellite observations are shown in shades of orange; historical simulations averaged near the present-day are shown in gray; and future simulations under low- and high-emissions scenarios averaged around the end of the century are shown in blue and red, respectively. Diamond markers over the future scenarios represent the present-day values for reference. Errors bars represent 95% confidence in the mean value. “Tot.” stands for the total value and “Atm.” and “Sfc.” the atmospheric and surface components, respectively.

A striking and mysterious feature of the Earth system is that the Northern and Southern Hemispheres reflect identical amounts of sunlight. This hemispheric albedo symmetry comprises two asymmetries: The Northern Hemisphere is more reflective in clear skies, whereas the Southern Hemisphere is cloudier. The clear-sky asymmetry has surface and atmospheric components, with the relatively-bright continents and aerosol (airborne particulate matter) both disproportionately located in the Northern Hemisphere.

In our new (open-access) paper, we show that the continent-based component of the clear-sky surface asymmetry is largely offset by greater reflection from the Antarctic surface than the Arctic surface, allowing the net clear-sky asymmetry to be dominated by atmospheric aerosol. Climate model simulations suggest that anthropogenic aerosol emissions since the pre-industrial era have driven a large increase in the clear-sky asymmetry that would reverse in future low-emission scenarios featuring rapid decarbonization and decreases in co-emitted aerosol. High-emission scenarios also show a decrease in asymmetry, but instead driven by declines in Northern Hemisphere ice and snow cover. Strong clear-sky hemispheric albedo asymmetry is therefore a transient, rather than fixed, feature of Earth’s climate. If all-sky symmetry is maintained despite changes in the clear-sky asymmetry, compensating cloud changes would have uncertain but important implications for Earth’s energy balance and hydrological cycle.

For more information, please check out the paper or my “Behind the Paper” blog post in the Nature Portfolio Earth and Environment Community.