Climate Dynamics

, Volume 40, Issue 1–2, pp 295–316 | Cite as

Asymmetries in tropical rainfall and circulation patterns in idealised CO2 removal experiments

  • Robin Chadwick
  • Peili Wu
  • Peter Good
  • Timothy Andrews
Article

Abstract

Atmospheric CO2 removal is currently receiving serious consideration as a supplement or even alternative to emissions reduction. However the possible consequences of such a strategy for the climate system, and particularly for regional changes to the hydrological cycle, are not well understood. Two idealised general circulation model experiments are described, where CO2 concentrations are steadily increased, then decreased along the same path. Global mean precipitation continues to increase for several decades after CO2 begins to decrease. The mean tropical circulation shows associated changes due to the constraint on the global circulation imposed by precipitation and water vapour. The patterns of precipitation and circulation change also exhibit asymmetries with regard to changes in both CO2 and global mean temperature, but while the lag in global precipitation can be ascribed to different levels of CO2 at the same temperature state, the regional changes cannot. Instead, ocean memory and heat transfer are important here. In particular the equatorial East Pacific continues to warm relative to the West Pacific during CO2 ramp-down, producing an anomalously large equatorial Pacific sea surface temperature gradient and associated rainfall anomalies. The mechanism is likely to be a lag in response to atmospheric forcing between mixed-layer water in the east Pacific and the sub-thermocline water below, due to transport through the ocean circulation. The implication of this study is that a CO2 pathway of increasing then decreasing atmospheric CO2 concentrations may lead us to climate states during CO2 decrease that have not been experienced during the increase.

Keywords

CO2 removal Rainfall Tropical circulation Geoengineering 

Notes

Acknowledgments

The authors were supported by the Joint Department of Energy and Climate Change (DECC) and Department for Environment, Food and Rural Affairs (Defra) Met Office Hadley Centre Climate Programme, DECC/Defra (GA01101). We would like to thank Marie Boucher-Boucher for providing the HadGEM2-ES ramp-up/ramp-down data. Thanks also to Mark Ringer, Gill Martin and Graeme Stephens for useful discussions, and to two anonymous reviewers for helpful comments.

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Copyright information

© Crown Copyright 2012

Authors and Affiliations

  • Robin Chadwick
    • 1
  • Peili Wu
    • 1
  • Peter Good
    • 1
  • Timothy Andrews
    • 1
  1. 1.Met Office Hadley CentreExeterUK

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