Vatnajökull Mass Loss Under Solar Geoengineering Due to the North Atlantic Meridional Overturning Circulation

Abstract

Icelandic glaciers are rather sensitive to climate warming (e.g., Guðmundsson et al., 2011;Schmidt et al., 2018Schmidt et al., , 2020. This is because of their extreme maritime setting giving them high snowfall, and ice temperatures that are always close to the melting point, leading to both large accumulation rates in their upper reaches and rapid melt in their ablation zones. Iceland sits at the confluence of the warm Irminger and cold East Iceland ocean currents (e.g., Ólafsson et al., 2007) leading to a temperate maritime climate (Vilhjálmsson, 2002). Iceland is situated close to the overturning regions of the Atlantic Meridional Overturning Circulation (AMOC) that brings significant heat to the North Atlantic region. Atmospheric greenhouse gas concentration increases tend to reduce AMOC, albeit with heavily model-dependent results (Caesar Abstract The objective of solar geoengineering by stratospheric aerosol injection (SAI) is to lower global temperatures, but it may also have adverse side effects. Iceland is situated close to the overturning regions of the Atlantic Meridional Overturning Circulation (AMOC) that warms the North Atlantic area. Hence, this may be one region where reduced irradiance by SAI may not be successful in reducing impacts from greenhouse gas warming. We examine this proposition by estimating how the Icelandic Vatnajökull ice cap (VIC) surface mass balance (SMB) and surface runoff changes in response to greenhouse gas and solar geoengineering scenarios over the period 1982-2089. We use the surface energy and mass balance model SEMIC driven by Earth System Model output under the GeoMIP G4, and CMIP RCP4.5 and RCP8.5 greenhouse gas scenarios. Geoengineering significantly reduces VIC near-surface air temperature by 0.4°C, downward longwave radiation by 2.4 Wm −2 and increases snowfall by 4.9 mm yr −1 relative to RCP4.5. During the SAI period 2020-2069, modeled annual mean SMB under G4, RCP4.5 and RCP8.5 are −0.34 ± 0.18 m yr −1 , −0.56 ± 0.06 m yr −1 and −0.66 ± 0.04 m yr −1 , respectively; surface runoff reduction under G4 is 6 ± 6% and 7 ± 6% (95% confidence interval uncertainties) compared with that under RCP4.5 and RCP8.5, which is far smaller than the 20 ± 2% and 32 ± 2% reductions for the Greenland ice sheet. The differences may be attributed to the reinvigoration of AMOC under G4 relative to RCP4.5 which brings more heat to Iceland than Greenland, leading to around half the cooling and longwave radiation reductions than for Greenland. Plain Language SummaryThe unique location of Iceland, near the overturning regions of the Atlantic thermohaline overturning circulation, makes its response to climate change rather atypical. The overturning circulation supplies a lot of heat to Iceland, but this has been slowing, and is projected to reduce further as greenhouse warming continues over the century. Solar geoengineering has the potential to lower temperatures by artificially increasing planetary albedo and is predicted to reverse this decline in ocean circulation heat transport, but als...

Reduced Ice Loss From Greenland Under Stratospheric Aerosol Injection

“…The decline in relative humidity was also critical—since water vapor is an important greenhouse gas—and solar geoengineered climates are inherently drier than pure GHG forced climates (Bala et al., 2008). Change in air‐sea heat exchange in the North Atlantic is also a contributing factor to differences under G4 and GHG forcing (Yue et al., 2021). These surface heat flux changes also vary with the advection of heat to the north by the Atlantic Meridional Overturning Circulation (AMOC) that GHG forcing reduces.…”