Paul Brent Goddard

and 5 more

Owing to increasing greenhouse gas emissions, the West Antarctic Ice Sheet as well as a few subglacial basins in East Antarctica are vulnerable to rapid ice loss in the upcoming decades and centuries, respectively. This study examines the effectiveness of using Stratospheric Aerosol Injection (SAI) that minimizes global mean temperature (GMT) change to slow projected 21st century Antarctic ice loss. We use eleven different SAI cases which vary by the latitudinal location(s) and the amount(s) of the injection(s) to examine the climatic response near Antarctica in each case as compared to the reference climate at the turn of the last century. We demonstrate that injecting at a single latitude in the northern hemisphere or at the Equator increases Antarctic shelf ocean temperatures pertinent to ice shelf basal melt, while injecting only in the southern hemisphere minimizes this temperature change. We use these results to analyze the results of more complex multi-latitude injection strategies that maintain GMT at or below 1.5°C above the pre-industrial. All these cases will slow Antarctic ice loss relative to the mid-to-late 21st century SSP2-4.5 emissions pathway. Yet, to avoid a GMT threshold estimated by previous studies pertaining to rapid West Antarctic ice loss (~1.5°C above the pre-industrial), our study suggests SAI would need to cool below this threshold and predominately inject at low southern hemisphere latitudes. These results highlight the complexity of factors impacting the Antarctic response to SAI and the critical role of the injection strategy in preventing future ice loss.

Daniele Visioni

and 4 more

The specifics of the simulated injection choices in the case of Stratospheric Aerosol Injections (SAI) are part of the crucial context necessary for meaningfully discussing the impacts that a deployment of SAI would have on the planet. One of the main choices is the desired amount of cooling that the injections are aiming to achieve. Previous SAI simulations have usually either simulated a fixed amount of injection, resulting in a fixed amount of warming being offset, or have specified one target temperature, so that the amount of cooling is only dependent on the underlying trajectory of greenhouse gases. Here, we use three sets of SAI simulations achieving different amounts of global mean surface cooling while following a middle-of-the-road greenhouse gas emission trajectory: one SAI scenario maintains temperatures at 1.5ºC above preindustrial levels (PI), and two other scenarios which achieve additional cooling to 1.0ºC and 0.5ºC above PI. We demonstrate that various surface impacts scale proportionally with respect to the amount of cooling, such as global mean precipitation changes, changes to the Atlantic Meridional Overturning Circulation (AMOC) and to the Walker Cell. We also highlight the importance of the choice of the baseline period when comparing the SAI responses to one another and to the greenhouse gas emission pathway. This analysis leads to policy-relevant discussions around the concept of a reference period altogether, and to what constitutes a relevant, or significant, change produced by SAI.
Stratospheric Aerosol Injection (SAI) is a proposed method of climate intervention aiming to reduce the impacts of human-induced global warming by reflecting a portion of incoming solar radiation. Many studies have demonstrated that SAI would successfully reduce global-mean surface air temperatures, however the vast array of potential scenarios and strategies for deployment result in a diverse range of climate impacts. Here we compare two SAI strategies - a quasi- equatorial injection and a multi-latitude off-equatorial injection - simulated with the UK Earth System Model (UKESM1), both aiming to reduce the global-mean surface temperature from that of a high-end emissions scenario to that of a moderate emissions scenario. Both strategies effectively reduce global mean surface air temperatures by around 3°C by the end of the century; however, there are significant differences in the resulting regional temperature and precipitation patterns. We compare changes in the surface and stratospheric climate under each strategy to determine how the climate response depends on the injection location. In agreement with previous studies, an equatorial injection results in a tropospheric overcooling in the tropics and a residual warming in the polar regions, with substantial changes to stratospheric temperatures, water vapour and circulation. However, we demonstrate that by utilising a feedback controller in an off-equatorial injection strategy, regional surface temperature and precipitation changes relative to the target can be minimised. We conclude that moving the injection away from the equator minimises unfavourable changes to the climate, calling for a new series of inter-model SAI comparisons using an off-equatorial strategy.
Simulating whole atmosphere dynamics, chemistry, and physics is computationally expensive. It can require high vertical resolution throughout the middle and upper atmosphere, as well as a comprehensive chemistry and aerosol scheme coupled to radiation physics. An unintentional outcome of the development of one of the most sophisticated and hence computationally expensive model configurations is that it often excludes a broad community of users with limited computational resources. Here, we analyze two configurations of the Community Earth System Model Version 2, Whole Atmosphere Community Climate Model Version 6 (CESM2(WACCM6)) with simplified “middle atmosphere” chemistry at nominal 1 and 2 degree horizontal resolutions. Using observations, a reanalysis, and direct model comparisons, we find that these configurations generally reproduce the climate, variability, and climate sensitivity of the 1 degree nominal horizontal resolution configuration with comprehensive chemistry. While the background stratospheric aerosol optical depth is elevated in the middle atmosphere configurations as compared to the comprehensive chemistry configuration, it is comparable between all configurations during volcanic eruptions. For any purposes other than those needing an accurate representation of tropospheric organic chemistry and secondary organic aerosols, these simplified chemistry configurations deliver reliable simulations of the whole atmosphere that require 35% to 86% fewer computational resources at nominal 1 and 2 degree horizontal resolution, respectively.

Yan Zhang

and 3 more

Stratospheric aerosol injection (SAI) can provide global cooling by adding aerosols to the lower stratosphere, and thus is considered as a possible supplement to emission reduction. Previous studies have shown that injecting aerosols at different latitude(s) and season(s) can lead to differences in regional surface climate, and there are at least three independent degrees of freedom (DOF) that can be used to simultaneously manage three different climate goals. To understand the fundamental limits of how well SAI might compensate for anthropogenic climate change, we need to know the possible surface climate responses to SAI by evaluating the SAI design space. This research work quantifies the number of meaningfully-independent DOFs of the SAI design space. This number of meaningfully-independent DOF depends on both the climate metrics that we care about and the amount of cooling. From the available simulation data of different SAI strategies, we observe that between surface air temperature and precipitation, surface air temperature dominates the change of surface climate. The number of injection choices that produce detectably different surface temperature is more than the number of injection choices that produce detectably different precipitation. At low levels of cooling, only a small set of injection choices yield detectably different surface climate responses. As more cooling is needed, more injection choices produce detectably different surface climate. For a cooling level of 1-2C, we find that there are likely between 6 and 12 DOFs. This reveals new opportunities for exploring alternate SAI designs with different distributions of climate impacts and evaluating the underlying trade-offs between different climate goals.

Daniele Visioni

and 2 more

Deliberately blocking out a small portion of the incoming solar radiation would cool the climate. One such approach would be injecting SO$_2$ into the stratosphere, which would produce sulfate aerosols that would remain in the atmosphere for 1–3 years, reflecting part of the incoming shortwave radiation. This would not affect the climate the same way as increased greenhouse gas (GHG) concentrations, leading to residual differences when a GHG increase is offset by stratospheric sulfate geoengineering. Many climate model simulations of geoengineering have used a uniform reduction of the incoming solar radiation as a proxy for stratospheric aerosols, both because many models are not designed to adequately capture relevant stratospheric aerosol processes, and because a solar reduction has often been assumed to capture the most important differences between how stratospheric aerosols and GHG would affect the climate. Here we show that dimming the sun does not produce the same surface climate effects as simulating aerosols in the stratosphere. By more closely matching the spatial pattern of solar reduction to that of the aerosols, some improvements in this idealized representation are possible, with further improvements if the stratospheric heating produced by the aerosols is included. This is relevant both for our understanding of the physical mechanisms driving the changes observed in stratospheric sulfate geoengineering simulations, and in terms of the relevance of impact assessments that use a uniform solar dimming.