Introducing a Comprehensive Set of Stratospheric Aerosol Injection Strategies
Abstract. Stratospheric aerosol injection (SAI) comes with a wide range of possible design choices, such as the location and timing of the injection. Different injection strategies can yield different climate responses; therefore, making informed future decisions on SAI requires an understanding of the range of possible climate outcomes. Yet to date, there has been no systematic exploration of a comprehensive set of SAI strategies. This limits the ability to determine which effects are robust across different strategies and which depend on specific injection choices, or to determine if there are underlying trade-offs between different climate goals.
This study systematically explores how the choice of SAI strategy affects climate responses. Here, we introduce four hemispherically-symmetric injection strategies, all of which are designed to maintain the same global mean surface temperature: an annual injection at the equator (EQ), an annual injection of equal amounts of SO2 at 15° N and 15° S (15N+15S), an annual injection of equal amounts of SO2 at 30° N and 30° S (30N+30S), and a polar injection strategy that injects equal amounts of SO2 at 60° N and 60° S only during spring in each hemisphere (60N+60S). We compare these four hemispherically-symmetric SAI strategies with a more complex injection strategy that injects different quantities of SO2 at 30° N, 15° N, 15° S, and 30° S in order to maintain not only the global mean surface temperature but also its large scale horizontal gradients. We find that the choice of SAI strategy notably affects the spatial distribution of aerosol optical depths, injection efficiency, and various surface climate responses. Among other findings, we show that injecting in subtropics produces more global cooling per unit injection, with the EQ and the 60N+60S cases requiring, respectively, 59 % and 50 % more injection than the 30N+30S case to meet the same global mean temperature target. Injecting at higher latitudes results in larger equator-to-pole temperature gradients. While all five strategies restore September Arctic sea ice, the high-latitude injection one is more effective due to the SAI-induced cooling occurring preferentially at higher latitudes.
Yan Zhang et al.
Status: open (until 23 Apr 2023)
- RC1: 'Comment on egusphere-2023-117', Anonymous Referee #1, 11 Mar 2023 reply
Yan Zhang et al.
Data from: Introducing a Comprehensive Set of Stratospheric Aerosol Injection Strategies https://zenodo.org/record/7545452#.Y9QqLnbMK3A
Yan Zhang et al.
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The title of the paper is very misleading. The study done is certainly not comprehensive. And why “Introducing?”
The abstract is poorly written. It does not explain what global warming scenario is used. It does not mention what climate models are used. It jumps right into SAI while ignoring the fact that it does not exist, and is only a proposed scheme. It ignores the need to assess a wide range of potential benefits and risks before it is ever implemented. It does not say what is being injected. In fact the experiments are injecting gas and not aerosol.
The scientific questions being addressed by this paper are obscure. The paper says it wants to examine the response to certain sulfur dioxide emissions with respect to one global warming scenario using one climate model and specified injection altitudes and temperature reduction goal. It is by no means comprehensive. But why are they doing it? Is it a technical exercise for one specific modeling group? If so, this should be a technical report and not a journal article. I am not convinced it should be otherwise.
The paper is very long and detailed, going through many variables from the climate model simulations they did. I lost interest before I got halfway through. Why would other readers find this interesting? The paper does not pose interesting scientific questions that are then answered by specific experiments. I could not find any interesting new science in the paper. Many of the results are what would be expected.
On line 65 the paper says, “The understanding that comes from the analysis of the differences between these strategies lays the foundation for future work.” That is what a technical report should be doing, not a journal article which needs new science to justify publication.
In several places, “We note that” is in the text and should be deleted. Every sentence should be noted or it should not be in the paper.
There are 45 additional comments in the annotated manuscript. If the authors chose to reply to this review, a response of “we addressed all the comments” would not be sufficient. Each comment should be listed with a specific response.