Stratospheric geoengineering impacts on El Niño/Southern Oscillation

C J Gabriel,A Robock

doi:10.5194/acp-15-11949-2015

Abstract

Abstract. To examine the impact of proposed stratospheric geoengineering schemes on the amplitude and frequency of El Niño/Southern Oscillation (ENSO) variations we examine climate model simulations from the Geoengineering Model Intercomparison Project (GeoMIP) G1–G4 experiments. Here we compare tropical Pacific behavior under anthropogenic global warming (AGW) using several scenarios: an instantaneous quadrupling of the atmosphere's CO2 concentration, a 1 % annual increase in CO2 concentration, and the representative concentration pathway resulting in 4.5 W m−2 radiative forcing at the end of the 21st century, the Representative Concentration Pathway 4.5 scenario, with that under G1–G4 and under historical model simulations. Climate models under AGW project relatively uniform warming across the tropical Pacific over the next several decades. We find no statistically significant change in ENSO frequency or amplitude under stratospheric geoengineering as compared with those that would occur under ongoing AGW, although the relative brevity of the G1–G4 simulations may have limited detectability of such changes. We also find that the amplitude and frequency of ENSO events do not vary significantly under either AGW scenarios or G1–G4 from the variability found within historical simulations or observations going back to the mid-19th century. Finally, while warming of the Niño3.4 region in the tropical Pacific is fully offset in G1 and G2 during the 40-year simulations, the region continues to warm significantly in G3 and G4, which both start from a present-day climate.

Highlights

1.1 BackgroundThe warming of Earth in the Industrial Age is unequivocal, and it is extremely likely that the warming since 1950 is primarily the result of anthropogenic emission of heat trapping gases rather than natural climate variability (IPCC, 2013)
We begin with the simple question of whether or not, in a single Geoengineering Model Intercomparison Project (GeoMIP) participating model that simulates El Niño/Southern Oscillation (ENSO) well, a difference in ENSO amplitude or frequency is evident in a comparison between one experiment and its control
Might detectability of changes in ENSO be more evident from analyzing changes in non-sea surface temperatures (SSTs)-based ENSO indices? First, we considered the Southern Oscillation Index (SOI), which is a standardized index based on the atmospheric pressure difference between Darwin, Australia and Tahiti, because climate change does not produce SOI trends, except for a trivial increase as a result of increased water vapor concentration in a warmer world

Summary

Introduction

1.1 BackgroundThe warming of Earth in the Industrial Age is unequivocal, and it is extremely likely that the warming since 1950 is primarily the result of anthropogenic emission of heat trapping gases rather than natural climate variability (IPCC, 2013). Ice core records from the European Project for Ice Coring in Antarctica reveal that current concentrations of the heat trapping gases carbon dioxide and methane are higher than at any time during the past 650 000 years (Siegenthaler et al, 2005). The realization that weathering the impacts of this warming may be beyond human adaptive capacity has generated many proposed mitigation techniques, which focus on limiting emission or increasing storage of heat-trapping gases such as carbon dioxide. Implementation costs and economic, political and societal factors limit societies’ will and ability to impose mitigation measures. This has forced a recent consideration of geoengineering: intentional manipulation of global-scale physical processes (Crutzen, 2006). Continuous sulfate injections into the tropical stratosphere have the potential to create a long-lasting, well-mixed sulfate aerosol layer, which could reduce incoming shortwave radi-

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Conclusion