How efficient is an approach of geoengineering to mitigate the global warming? I.I. Mokhov, A.V. Eliseev, and A.V. Chernokulsky A.M. Obukhov Institute of Atmospheric Physics RAS ENVIROMIS-2008
Motivation - 1 - Globally, climate has warmed in the 20th century by 0.6 K (0.4-0.8 K). - Such warming on a century timescale was not observed for any previous epoch and most likely to be attributed to human activities [IPCC, 2007]. This warming is expected to proceed for the whole 21st century and beyond. adopted from [IPCC, 2007]
Motivation - 2 It was suggested [Budyko, 1974] to mitigate global warming by injection of sulphur in the stratosphere. Recently, this approach is considered as a form of geoengineering [Schneider 1996; Schneider 2001; Izrael, 2005; Crutzen, 2006; Wigley, 2006]. Natural examples: cooling after volcanic eruptions; less direct: cooling due tropospheric sulphates Benefits: low cost due to large residence time of aerosols in stratosphere (~2-3 yr) Possible disadvantages: strong decrease of precipitation [Trenberth and Dai, 2007] possible enhancement of the stratospheric ozone depletion [Tilmes et al., 2008] Emissions required to compensate the atmopsheric CO 2 doubling: [Izrael, 2005]: 0.6 TgS/ yr [Crutzen, 2006]: 1-2 TgS/ yr [Wigley, 2006]: 5 TgS/ yr
IAP RAS CM Resolution : 4.5 o *6 o , L8 - atmosphere, L4 - ocean, L1 - land; ∆ t = 5 days Atmosphere : 3D quasigeostrophic large- scale dynamics. Synoptic- scale dynamics is parametrised based on their representation as Gaussian ensembles. In any atmospheric layer, temperature depends linearly on height. Fully interactive hydrological cycle. Partly interactive methane cycle. Ocean : Prognostic equation for sea surface temperature. Geostrophic large- scale dynamics. Universal vertical profiles in any oceanic layer. Oceanic salinity is prescribed. Interactive, globally averaged oceanic carbon cycle. Sea ice : Diagnostic, based on the local SST Vegetation : Spatial distribution of ecozones is prescribed. Fully interactive globally averaged terrestrial carbon cycle. Interactive CH 4 emissions from natural wetlands. Turnaround time: ~ 17 sec per model year (Intel Zeon)
Top-of-the atmosphere stratospheric aerosol radiative forcing F strat = - a strat τ strat , a strat = 22 W/ m 2 [Hansen et al, 2005], optical depth τ strat = k ext,strat M strat M strat is stratospheric aerosol mass per unit area, extinction coefficient k ext,strat = 7.6 m 2 / g (derived from the Mt. Pinatubo A.D. 1991 eruption observations)
Annual mean surface air temperature [K] response to volcanic forcing [Amman et al., 2003] (1891-2000) o b n u A.D. 1992 o t a g h n c n (aftermath for i i P u h C g . t Mt.Pinatubo eruption) A M l E IAP RAS CM ∆ T g , K obs., ENSO removed [Wigley, 2000]
Ensemble numerical experiments with a climate mitigation via stratospheric aerosol loading - duration: 1860-2100 - historical+SRES A1B anthropogenic CO 2 and CH 4 emissions - historical+SRES A1B atmospheric concentrations of N 2 O (BernCC) and tropospheric sulphates (MOZART 2.0) + mitigation via controlled sulphur emissions in the stratosphere with values of governing parameters varying between different ensemble members The total number of ensemble members: 2331 Cumulative length: 564 102 yr
Parameters of st ratospheric aerosols Global burden: d M strat,g / d t = E - M strat,g / τ res { Emissions: 0, before A.D. 2012 E = E 0 , from A.D. 2015 to t 0 0, after t 0 Local burden: Earth ) * Y( φ ) M strat = ( M strat,g / S Depending on the ensemble member Y E 0 = from 0.6 to 4 TgS/ yr t 0 = A.D. 2100 or A.D. 2075 k ext, strat = 5-20 m 2 / g residence time τ res = 1-4 yr latitudinal profile Y( φ ) is varied between uniform, triangular, and trapezoidal functions of x = sin φ with varying either x 0 or x 1 (see Figure). x NP x 0 x 1 EQ -x 1 SP
Change in global surface air temperature ensemble members with τ res =2 yr, k ext,strat =7.6 m 2 /g, whole ensemble and uniform Y( φ ) ∆ T g , K ∆ T g , K no mitigation E 0 = 3 TgS/ yr E 0 = 0.6 TgS/ yr E 0 = 4 TgS/ yr E 0 = 1 TgS/ yr obs. (CRU UEA) E 0 = 2 TgS/ yr
Mitigat ion eff iciency of diff erent latitudina l profiles for stratospheric aerosol Y(sin φ ) τ strat,* = E 0 τ res k ext,strat / S Earth ∆ T mitigation,g - ∆ T anthrop,g in year 2100, K E 0 - emissions of stratospheric aerosols τ res - residence time k ext,strat - extinction coefficient S Earth - area of the Earth's surface uniform triangular with φ 0 =70 o N trapezoidal with φ 1 =50 o N triangular with φ 0 =30 o N τ strat,* trapezoidal with φ 1 =30 o N
Spat ial pattern of mit igation eff iciency for 2050-2060: -( ∆ T m itigation - ∆ T anthrop )/ ( ∆ T mitigation,g - ∆ T anthrop,g ) T m itigation - ensemble members with climate mitigation T anthrop - ensemble member without mitigation uniform Y(sin φ ) triangular Y(sin φ ) with φ 0 =70 o N
Global precipitation change ensemble members with τ res =2 yr, k ext,strat =7.6 m 2 /g, whole ensemble and uniform Y( φ ) ∆ P ∆ P g , % g , % E 0 = 2 TgS/ yr no mitigation E 0 = 3 TgS/ yr E 0 = 0.6 TgS/ yr E 0 = 4 TgS/ yr E 0 = 1 TgS/ yr
Pattern of relative precipitat ion response to mitigation for 2050-2060: 100*( ∆ P m itigation - ∆ P anthrop )/ P 0 P mitigation - ensemble members with climate mitigation P anthrop - ensemble member without mitigation P 0 - present-day annual precipitation uniform Y(sin φ ) triangular Y(sin φ ) with φ 0 =70 o N
Change in global surface air temperature in experiments with a mitigation emission stop in 2075 ∆ T g , K dT g /dt, K/yr no mitigation E 0 = 2 TgS/ yr obs. (CRU UEA)
SAT change rate [K/ decade] for 2076-2085 after the mit igation stop in 2075 (E 0 = 2 TgS/ yr) without mitigation triangular Y(sin φ ) with φ 0 =70 o N, triangular Y(sin φ ) with φ 0 =70 o N, τ res =4, k ext,strat =20 m 2 / g τ res =2.5, k ext,strat =7.6 m 2 / g
Globally averaged energy-balance model C dT g / dt = Q [ 1 - α (T g ) ] - ( A + B T g ) η + F strat,g , C - heat capacity per unit area, T g - globally averaged surface air temperature, t - time, Q - insolation, α - planetary albedo, A and B - constants, correction factor for anthropogenic greenhouse effect η = 1- c 0 log (q C / q C,0 ), c 0 = 2.3*10 -2 , q C (q C,0 ) is the current (initial) atmospheric CO 2 concentration. Equilibrium climate sensitivity to CO 2 doubling in the atmosphere [Mokhov, 1981]: ∆ T 2CO2 = ( c 0 I 0 log 2 ) / ( 1 - Q k α + B) nd k a = d α / dT g , subscript '0' where I 0 = A + B T g,0 a indicates the present-day state
EBM forcings specification i) q C = q C,0 exp ( t / t p ) t p - prescribed time scale F strat,g = - a strat τ strat,g , a strat = 22 W/ m 2, , ii) τ strat,g = k ext,strat M strat,g and M strat,g = τ life E 0 = const (stationarity approximation for M strat,g ). Governing parameters are varied between the different ensemble members: E 0 = 0.6-5 TgS/ yr τ res = 1-4 yr k ext,strat = 5-20 m 2 / g ∆ T 2CO2 = 1.5-4.5 K t p = 50-250 yr -1
Global temperature change in years 0-100 obtained with an energy-balance model mitigation with τ res =2 yr, k ext,strat =7.6 m 2 / g no mitigation E 0 =1 TgS/ yr t p , centuries t p , centuries E 0 =4 TgS/ yr ∆ T 2CO2 , K t p , centuries IAP RAS CM ∆ T , K
Emissions required to compensate the greenhouse-gases-induced warming (energy-balance model) E, TgS/yr t p =100 yr (~SRES A2), 12 τ res =2 yr 11 t p =100 yr (~SRES A2), 10 τ res =3 9 t p =136 yr (~SRES A1B), 8 τ res =2 7 t p =136 yr (~SRES A1B), 6 τ res =3 5 t p =230 yr (~SRES B1), τ res =2 4 3 t p =230 yr (~SRES B1), τ res =3 2 1 0 0 10 20 30 40 50 60 70 80 90 100 year
Conclusions - 1 - For large annual emissions, large residence time of sulphates in the stratosphere, and large extinction coefficient it is possible to mitigate both global and regional warming to a large extent. However, if the ranges for above parameters are narrowed to presumably more realistic widths, the residual warming is > 1.8 K in the 21st century. Globally, the most efficient latitudinal distribution of geoengineering aerosols is that with high loading in the extratropics. At regional scale, other latitudinal distributions may be preferable. - However, stratospheric aerosol climate mitigation leads to less humidification of arid regions in comparison to non-mitigated anthropogenically induced warming. A caveat in this result is due to prescribed atmospheric relative humidity in the IAP RAS CM.
Conclusions - 2 - Due to the fast removal of the mitigation effect if the corresponding emissions are stopped climate trajectory returns to the non-mitigated one within a few decades. This results in a necessity to continue mitigation very long in future, perhaps for several centuries in order to make it efficient. - The results obtained with the IAP RAS CM are further supported and interpreted by making use of an energy-balance climate model. It is shown that very high stratospheric sulphate emissions (up to 12 TgS/yr) are needed to compensate global warming expected in the 21st century.
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