Can Geoengineering save the Greenland ice sheet? Dan Lunt (d.j.lunt@bristol.ac.uk), Andy Ridgwell, Paul Valdes http://www.bridge.bris.ac.uk Dan Lunt, Copenhagen, 2009

Eventual near-total melting at CO2 >= 550ppmv [Alley et al, Science, 2005] Eventual near-total melting at CO2 >= 550ppmv Dan Lunt, Copenhagen, 2009

[Keith, Nature, 2001] Dan Lunt, Copenhagen, 2009

What did we do….? The General Circulation Model (GCM) HadCM3(L) - “State of the art” (for > ~100 year simulations) Dan Lunt, Copenhagen, 2009

(1) Pre-industrial (PI) (2) High CO2 (4* PI), 3 simulations: (1) Pre-industrial (PI) (2) High CO2 (4* PI), (3) High CO2 (4* PI), reduced solar constant 4*CO2 4.2% reduction in solar constant = 57 W/m2 Geoengineered Pre-industrial Dan Lunt, Copenhagen, 2009

Annual mean surface temperature change Geoengineered - pre-industrial Lunt et al, GRL, 2008 Annual mean surface temperature change Geoengineered - pre-industrial Dan Lunt, Copenhagen, 2009

Precipitation: ENSO: THC: Dan Lunt, Copenhagen, 2009

“Sunshade World” Precip “CO2 World” Precip “Sunshade World” Temp “CO2 World” Temp “Sunshade World” Precip “CO2 World” Precip Dan Lunt, Copenhagen, 2009

What about Greenland?? “Sunshade World” Temp “CO2 World” Temp “Sunshade World” Precip “CO2 World” Precip What about Greenland?? Dan Lunt, Copenhagen, 2009

The ice-sheet model GLIMMER Ice-sheet model – GLIMMER. Based on Payne (1999). Predicts evolution of ice-sheet geometry (extent, elevation). Mass balance scheme (surface of ice-sheet) Ice-sheet dynamics Driven offline by temperature and precipitation from the GCM. Temperature downscaled using high-resolution orography and a constant lapse rate. [Letreguilly et al, 1991] GLIMMER [Stone et al, in prep] Dan Lunt, Copenhagen, 2009

4*CO2, Lunt et al, Phil, Trans. Roy. Soc. A, 2009 control 4*CO2, Lunt et al, Phil, Trans. Roy. Soc. A, 2009 4*CO2, Ridley et al, Journal of Climate, 2005 Dan Lunt, Copenhagen, 2009

4*CO2, Lunt et al, Phil, Trans. Roy. Soc. A, 2009 4*CO2, geoengineered control 4*CO2, Lunt et al, Phil, Trans. Roy. Soc. A, 2009 4*CO2, geoengineered Dan Lunt, Copenhagen, 2009

Conclusions and Questions ‘Sunshade’ geoengineering will lead to warmer poles, cooler tropics. Effects amplified by snow and seaice albedo feedbacks. Decreased intensity of the hydrological cycle. Reduced ENSO variability, increased Atlantic MOC. BUT….changes are small relative to the unmitigated case….. and it could stop ‘dangerous’ sea level rise…… HOWEVER……. Dan Lunt, Copenhagen, 2009

Risks of catastrophic failure? Ocean acidification? Risks of catastrophic failure? Ice sheet model......? Lack of albedo feedbacks Lack of ‘fast’ processes Lack of ‘second order’ physics What about Antarctica? Dan Lunt, Copenhagen, 2009