To what extent can cirrus cloud seeding counteract global warming?

Abstract

The idea of modifying cirrus clouds to directly counteract greenhouse gas warming has gained momentum in recent years, despite disputes over its physical feasibility. Previous studies that analyzed modifications of cirrus clouds by seeding of ice nucleating particles showed large uncertainties in both cloud and surface climate responses, ranging from no effect or even a small warming to a globally averaged cooling of about 2.5°C. We use two general circulation models that showed very different responses in previous studies, ECHAM6-HAM and CESM-CAM5, to determine which radiative and climatic responses to cirrus cloud seeding in a 1.5×CO 2 world are common and which are not. Seeding reduces the net cirrus radiative effect for −1.8 W m −2 in CESM compared with only −0.8 W m −2 in ECHAM. Accordingly, the surface temperature decrease is larger in CESM, counteracting about 70% of the global mean temperature increase due to CO 2 and only 30% in ECHAM. While seeding impacts on mean precipitation were addressed in past studies, we are the first to analyze extreme precipitation responses to cirrus seeding. Seeding decreases the frequency of the most extreme precipitation globally. However, the extreme precipitation events occur more frequently in the Sahel and Central America, following the mean precipitation increase due to a northward shift of the Intertropical Convergence Zone. In addition, we use a quadratic climate damage metric to evaluate the amount of CO 2 -induced damage cirrus seeding can counteract. Seeding decreases the damage by about 50% in ECHAM, and by 85% in CESM over the 21 selected land regions. Climate damage due to CO 2 increase is significantly reduced as a result of seeding in all of the considered land regions.

Is Turning Down the Sun a Good Proxy for Stratospheric Sulfate Geoengineering?

“…Even without considering geoengineering, there are uncertainties in the projected local changes under climate change, although with improvements in climate models, these uncertainties are decreasing (Christensen et al., 2007; Matte et al., 2019). For solar geoengineering, our assessment of local changes does however depend on more factors than for climate change: aside from the uncertainty in specific physical processes (Kravitz & MacMartin, 2020), these factors include (i) the desired level of cooling (P. Irvine et al., 2019; MacMartin et al., 2019; Tilmes et al., 2020); (ii) the specific technique simulated (i.e., the method chosen to reduce surface temperatures, Gasparini et al., 2020; Niemeier et al., 2013), and (iii) within the same technique, the specific strategy deployed (Kravitz et al., 2019; Visioni, MacMartin, Kravitz, Richter, et al., 2020). There is thus a compound of different kinds of uncertainties (those listed, and those we do not know we do not know about) that result in challenges in clearly determining—and communicating—what effects geoengineering would have locally.…”

The response of terrestrial ecosystem carbon cycling under different aerosol-based radiation management geoengineering

“…We conducted three different aerosol-based geoengineering experiments using the fully coupled NorESM1-ME, with which we investigated the impacts of idealized scenarios of aerosol-based geoengineering under the high-CO 2 RCP8.5 and the target temperature scenario RCP4.5. NorESM1-ME is based on the Community Earth System Model (CESM; Gent et al, 2011). Some of the key differences in NorESM1-ME from CESM are (1) a more sophisticated tropospheric chemistry-aerosol-cloud scheme (Kirkevag et al, 2013), (2) a different ocean circulation model based on the Miami Isopycnic Coordinate Ocean Model (MICOM) with extensive modifications (Bentsen et al, 2013), and (3) the ocean biogeochemical model, which originated from the Hamburg Oceanic Carbon Cycle (HAMOCC) model (Tjiputra et al, 2013).…”

Potential limitations of using a modal aerosol approach for sulfate geoengineering applications in climate models

“…Lastly, this observation would also be crucial in studies that aim to combine sulfate injections with the artificial seeding of upper tropospheric ice clouds with solid nuclei, to increase the size of the ice crystal and make them sediment faster (cloud seeding, Gasparini et al (2020)). Cao et al (2017), for instance, proposed a combination of the two methods to stabilize global temperatures and precipitation: such simulations, performed with CESM1(WACCM) or any model with a similar microphysical approach would not give meaningful results.…”