The climate cooling potential of different geoengineering options

Transcription

1 The climate cooling potential of different geoengineering options Tim Lenton & Naomi Vaughan (GEAR) initiative School of Environmental Sciences, University of East Anglia, Norwich, UK

2 Overview Climate context Method Cooling potential Side effects Conclusion

3 Prevention versus medicine Mitigation - Reducing greenhouse gas (especially CO 2 ) emissions Geoengineering large scale engineering of our environment in order to combat or counteract the effects of changes in atmospheric chemistry. National Academy of Sciences (1992) 1. Solar radiation management 2. Carbon dioxide removal

4 Future projections = High growth = Mid growth = Low growth IPCC (2007)

5 Potential climate tipping points Lenton et al. (2008) PNAS 105(6):

6 Some tipping points may be too close Lenton and Schellnhuber (2007) Nature Reports Climate Change

7 Geoengineering options Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9:

8 Geoengineering options Reflect more sunlight back to space Remove CO 2 from atmosphere and store it Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9:

9 How to quantify and compare them? Use radiative forcing (RF in W m -2 ) Linearly related to surface temperature change T s = RF Uncertain climate sensitivity parameter = K W -1 m 2 (best guess = 0.86) Useful reference points: Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9: Current anthropogenic RF ~ 1.6 W m -2 Doubling CO 2 gives RF = 3.71 W m -2

10 Solar radiation management Reflect more sunlight back to space Sunshades Stratospheric aerosols Increase surface albedo Increase cloud albedo Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9:

11 Quantification Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9: Radiative forcing at top of atmosphere Depends on change in planetary albedo Change in planetary albedo depends on: Albedo change of layer affected Fraction of Earth s area affected Prior absorption/reflection Changes in subsequent absorption/reflection Use global mean energy balance (land or ocean)

12 Reduce sunlight reaching the surface Sunshades Stratospheric aerosols Cloud albedo - CCN biological, mechanical Sunshades Stratospheric aerosols Increase cloud albedo Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9:

13 Sunshades in space At L1 point (Angel 2006) 4.7 million km 2 Multiple flyers ~0.3m 2 800,000 flyers/launch Just to counteract CO 2 rise of 2 ppm yr -1 requires: 35,700 km 2 yr -1 ~150,000 launches yr -1 Cartoon of reflectors in space RF = W m -2 In principle but not in practice

14 Stratospheric aerosol injection Depends on aerosol type, amount, effective radius Inject into lower stratosphere over tropics Mt S yr -1 to offset doubling CO 2 c.f. current ~50 Mt S yr -1 added to troposphere Stratospheric aerosols injection Mt Pinatubo eruption 1991 RF = W m -2 Feasible

15 Enhancing cloud albedo Cloud albedo depends on density of cloud droplets Enhance this by adding condensation nuclei Sea salt, dimethyl sulphide Sensitivity depends on: Hygroscopicity of aerosol Optical depth of cloud Changes in entrainment Mechanically enhance cloud albedo Stephen Salter RF = W m -2 Feasible but uneven

16 Increase reflection at the surface Cropland, Grassland Urban, human settlement Desert Increase surface albedo Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9:

17 Increase surface albedo Desert Reflective cover over Cropland, grassland Variegated species, light shrubs, leaf waxes Human settlement, urban areas Paved surfaces, roofs Urban heat island 50% global population Reflective roof surfaces Californian legislation Urban RF = W m -2

18 CO 2 removal from the atmosphere Remove CO 2 from atmosphere and store it Air capture Biochar Afforestation & reforestation Nutrient addition Carbonate addition Enhance downwelling Bio-energy capture Enhance upwelling Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9:

19 Quantification Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9: Radiative forcing a logarithmic function of CO 2 RF = 5.35 ln(co 2 /CO 2 ref ) Must specify reference concentration; CO 2 ref Any perturbation to atmospheric CO 2 decays away over time due to exchange with the ocean and land f(t) = e t/ e t/ e t/ e t/3.4 Removal flux of CO 2 is itself a function of time Must specify timescale of interest, chose 2050 and 2100

20 Removal of CO 2 - Land BioEnergy Capture & Storage Air capture - Artificial trees Biochar Afforestation Air capture Biochar Afforestation & reforestation Nutrient addition Carbonate addition Enhance downwelling Bio-energy capture Enhance upwelling Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9:

21 Air capture and storage of CO 2 Chemical air capture More costly Biomass energy with carbon capture & storage (BECS) Maximum estimate Biofuels replace oil in transport Biomass replaces coal for electricity production 11.8 PgC yr -1 in PgC stored by 2100 Direct air capture Klaus Lackner s artificial trees RF (2050) = W m -2 RF (2100) = -2.5 W m -2

22 Biochar CO 2 removal Pyrolysis converts up to 50% of carbon in biomass to charcoal Maximum estimate 0.56 PgC yr -1 at present Scaling up using biomass energy production 3.15 PgC yr -1 in 2100 Total biochar reservoir 148 PgC by 2100 Long term storage potential: 224 PgC global cropland 175 PgC temperate grassland Biochar biomass burnt in near zero oxygen (pyrolysis) RF (2050) = W m -2 RF (2100) = W m -2

23 Removal of CO 2 - Ocean Addition of Iron, Nitrate, Phosphorus Ocean pipes Add carbonate Increase downwelling Air capture Biochar Afforestation & reforestation Nutrient addition Carbonate addition Enhance downwelling Bio-energy capture Enhance upwelling Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9:

24 Ocean fertilisation Macronutrient addition Nitrate, Phosphate Micronutrient addition Iron fertilisation 12 Fe-addition patch experiments to date Assume removal of iron limitation globally Consider increase of sinking organic carbon flux below the depth of winter mixing Iron addition to Fe-limited surface waters to stimulate productivity Iron Fertilisation RF (2050) = W m -2 RF (2100) = W m -2

25 Radiative forcing potential (in 2100) All blue values are in W m -2 >-3.1 All green values are in ppm of CO 2 > desert grassland cropland settlements urban areas ~ ~ ppm +3.1 W m Lenton & Vaughan (2009) Atmospheric Chemistry and Physics 9:

26 Radiative forcing potential

27 Radiative forcing potential

28 Side effects of reflecting sunlight Weakens the water cycle Promotes drought in monsoon regions e.g. India Commitment to long-term maintenance Sudden stop of activity causes rapid warming Ocean acidification Does not address this impact (may increase it) Regional climate changes Residual differences in global climate

29 Side effects of CO 2 removal Land Possible conflicts with other land uses Other greenhouse gases and albedo effects Ocean Ecosystem impacts Difficult verification and monitoring

30 Conclusion Climate change is a problem of risk management Already a risk of dangerous climate change even with strong mitigation Need to weigh up the risks of using or not using geoengineering 2 types of geoengineering with very different side effects and risks Sunlight reflection options could be reserved for use in emergency Carbon dioxide removal could complement mitigation efforts It is the only way to return to the pre-industrial atmospheric CO 2 level