Assessing the controllability of Arctic sea ice extent by sulfate aerosol geoengineering

L S Jackson,P M Forster,J A Crook,D Leedal,A Jarvis,A Ridgwell,N Vaughan

doi:10.1002/2014gl062240

Abstract

AbstractIn an assessment of how Arctic sea ice cover could be remediated in a warming world, we simulated the injection of SO2 into the Arctic stratosphere making annual adjustments to injection rates. We treated one climate model realization as a surrogate “real world” with imperfect “observations” and no rerunning or reference to control simulations. SO2 injection rates were proposed using a novel model predictive control regime which incorporated a second simpler climate model to forecast “optimal” decision pathways. Commencing the simulation in 2018, Arctic sea ice cover was remediated by 2043 and maintained until solar geoengineering was terminated. We found quantifying climate side effects problematic because internal climate variability hampered detection of regional climate changes beyond the Arctic. Nevertheless, through decision maker learning and the accumulation of at least 10 years time series data exploited through an annual review cycle, uncertainties in observations and forcings were successfully managed.

Highlights

September Arctic sea ice (ASI) area decreased by 12.2 ± 1.3% per decade during 1979–2013 [Fetterer et al, 2002]
This has important far-field consequences because ASI area is strongly coupled with surface air temperature (SAT) in the Northern Hemisphere high latitudes [Kumar et al, 2010] and anthropogenic warming is amplified by surface albedo feedback [Winton, 2006]
Performance of the model predictive control (MPC) SO2 injections was reviewed by comparing real world” (RW) annual minimum ASI area against target and by attempting to identify climate side effects associated with solar radiation management (SRM)

Summary

Introduction

September Arctic sea ice (ASI) area decreased by 12.2 ± 1.3% per decade during 1979–2013 [Fetterer et al, 2002]. Climate models project that this decline in ASI area will continue during the 21st century, with ice free conditions in September likely to occur between 2032 and 2046 [Snape and Forster, 2014]. Previous climate model studies have shown that the ASI area could be restored by SRM [Jones et al, 2010; Tilmes et al, 2014] While these studies have focused on the possible climate impacts, they have not addressed how the management of SRM deployment might work in practice. They typically assume fixed rates of geoengineering injection for a fixed period of time [e.g., Kravitz et al, 2011]. Persistent climate model errors like the failure to simulate the Northern Hemisphere dynamical response to volcanic eruptions [Driscoll et al, 2012] would be captured by an MPC learning process

HadGEM2-CCS as a Surrogate “Real World”

Model Predictive Control and Sequential Decision Making

Simulation Design

The “Real World” Simulation

Comparison of Surrogate “Real World” and Control Simulations

Discussion and Conclusions