KRAVITZ ET AL.: ARCTIC SPRING GEOENGINEERING 1 Climate Model Simulations of Stratospheric Geoengineering in the Arctic Spring 2 3 4 Ben Kravitz, Alan Robock, and Allison Marquardt 5 6 7 Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey 8 9 10 11 12 13 14 15 16 17 18 19 Submitted to Journal of Geophysical Research - Atmospheres 20 21 September, 2010 22 23 24 25 26 27 28 29 30 31 Ben Kravitz, Department of Environmental Sciences, Rutgers University, 14 College Farm 32 Road, New Brunswick, NJ 08901, USA. ([email protected]) (Corresponding 33 Author) 34 35 Alan Robock, Department of Environmental Sciences, Rutgers University, 14 College Farm 36 Road, New Brunswick, NJ 08901, USA. ([email protected]) 37 38 Allison Marquardt, Department of Environmental Sciences, Rutgers University, 14 College Farm 39 Road, New Brunswick, NJ 08901, USA. ([email protected]) 40 41 42 - 1 - KRAVITZ ET AL.: ARCTIC SPRING GEOENGINEERING 43 Abstract 44 We use a general circulation model of Earth’s climate to conduct simulations of 45 stratospheric sulfate aerosol geoengineering in the Arctic spring to determine whether 46 geoengineering during the months of maximum insolation is as effective as geoengineering year- 47 round. As control cases, we simulate a global warming ensemble and an ensemble of global 48 warming combined with daily stratospheric injections of SO2 at high latitude, totaling 3 Tg SO2 49 per year. We compare these to two ensembles, each with global warming forcing and high 50 latitude stratospheric injections of 0.75 Tg SO2 per year: daily injections throughout April, May, 51 and June; and daily injections throughout April. These spring injection experiments show 52 smaller aerosol optical depth than the year-round injections, especially in the winter, during 53 which all of the sulfate aerosols from the spring injection experiments are removed each year. 54 They also show summer cooling over the Northern Hemisphere continents, as is seen in large 55 volcanic eruptions, although not as much as in the year-round injections. No significant 56 monsoonal precipitation perturbation is detected, in contrast to previous simulations with this 57 same model. Year-round injection results in an increase in Arctic sea ice from a control 58 scenario, and the spring injection experiments show reduced sea ice loss from the global 59 warming simulations. Further simulations are required, but these results suggest that while SO2 60 injections only in the spring are not as effective as year-round injections, a strategy of injections 61 in spring and summer combined would maximize the cooling of the aerosol cloud, requiring 62 slightly smaller total annual injections than a year-round strategy. 63 - 2 - KRAVITZ ET AL.: ARCTIC SPRING GEOENGINEERING 64 1. Introduction 65 Geoengineering with stratospheric sulfate aerosols has been proposed [e.g., Crutzen, 66 2006] as a cheap [e.g., Robock et al., 2009], effective [e.g., Rasch et al., 2008], and temporary 67 [e.g., Wigley, 2006] means of reducing global average surface air temperature to alleviate 68 negative climate impacts from increasing greenhouse gas concentrations. In an effort to tailor 69 geoengineering and reduce the degree to which humans directly interfere with and modify the 70 climate, some have suggested geoengineering only in the Arctic [Caldeira and Wood, 2008]. 71 They propose that this would have the effect of cooling the Northern Hemisphere continents and 72 potentially “saving” the Arctic sea ice, as is seen temporarily in the case of large volcanic 73 eruptions, but would not impact temperatures in the tropics or the Southern Hemisphere. Due to 74 the reduced area needing to be shaded by sulfate aerosols, geoengineering in the Arctic would in 75 theory require smaller injections of SO2 than geoengineering in the tropics. 76 Robock et al. [2008] performed simulations of both tropical and Arctic geoengineering. 77 They found features similar to those of large volcanic eruptions: summer cooling over the 78 Northern Hemisphere continents and weakening of the Indian/African summer monsoon, which 79 was more pronounced for the case of the Arctic injection. 80 These simulations by Robock et al. involved year-round injections of SO2. However, in 81 the Arctic, year-round injections would not be necessary, as there is no sunlight for the aerosols 82 to backscatter during the winter. This motivated our study to investigate Arctic geoengineering 83 that is tailored to backscatter solar radiation only during the summer, which is the period of 84 maximum insolation. 85 In addition to replicating the Arctic injection experiment from Robock et al. [2008], we 86 designed two additional experiments of geoengineering only in the Arctic spring. Assuming the - 3 - KRAVITZ ET AL.: ARCTIC SPRING GEOENGINEERING 87 same daily rate of injection as the year-round experiment, this would reduce the amount of SO2 88 that is injected into the stratosphere, lowering the cost and the degree to which humans directly 89 interfere with the climate system. However, we hypothesize that since the aerosols would be 90 present during summer, the radiative effects of the aerosols would be similar to those of a year- 91 round injection. We also wished to investigate whether the Asian/African monsoon system is 92 negatively impacted under these scenarios and whether they can prevent the loss of Arctic sea 93 ice. 94 2. Experiment 95 We conducted simulations with the coupled atmosphere-ocean general circulation model 96 ModelE, which was developed by the National Aeronautics and Space Administration Goddard 97 Institute for Space Studies [Schmidt et al., 2006]. We used the stratospheric version with 4° 98 latitude by 5° longitude horizontal resolution and 23 vertical levels up to 80 km. It is fully 99 coupled to a 4° latitude by 5° longitude dynamic ocean with 13 vertical levels [Russell et al., 100 1995]. 101 The aerosol module [Koch et al., 2006] accounts for SO2 conversion to sulfate aerosols, 102 as well as transport and removal of the aerosols. The chemical model calculates the sulfur cycle 103 in the stratosphere, where the conversion rate of SO2 to sulfate is based on the respective 104 concentrations of SO2 and the hydroxyl radical, the latter of which is prescribed [Oman et al., 105 2006a]. We specified the dry aerosol effective radius to be 0.25 µm, which is the value used for 106 simulation of past volcanic eruptions and geoengineering. The model hydrates the aerosols 107 based on ambient humidity values according to formulas prescribed by Tang [1996], resulting in 108 a distribution of hydrated aerosols with an effective radius of approximately 0.30-0.35 µm, 109 which is consistent with the findings of Stothers [1997]. Radiative forcing from the aerosols is - 4 - KRAVITZ ET AL.: ARCTIC SPRING GEOENGINEERING 110 fully interactive with the atmospheric circulation and in our paper is the conventional one as 111 defined by the Intergovernmental Panel on Climate Change [IPCC, 2001], also called “adjusted 112 forcing” (Fa) by Hansen et al. [2005]. For more details, we refer the reader to Kravitz et al. 113 [2010a], which used the same model version and setup. 114 This version of ModelE has been successfully used in the past to simulate both volcanic 115 eruptions and geoengineering. Simulations have been conducted for the eruptions of Laki in 116 1783-1784 [Oman et al., 2006a, 2006b], Katmai in 1912 [Oman et al., 2005], Pinatubo in 1991 117 [Robock et al., 2007], and the recent eruption of Kasatochi in 2008 [Kravitz et al., 2010a]. In all 118 of these cases, ModelE was shown to be reliable in recreating the climate impacts of the 119 eruption. Moreover, Robock et al. [2008] used this model to simulate geoengineering, and 120 results from this study agreed with similar experiments performed by the Hadley Centre [Jones 121 et al., 2010]. Therefore, we are confident in this model’s ability to simulate geoengineering to a 122 degree of accuracy that is scientifically useful. 123 We used the same version of ModelE that was used by Robock et al. [2008], using the 124 same specifications except for two tuning parameters and the atmospheric and oceanic initial 125 conditions. Robock et al. used atmospheric and oceanic conditions from the year 1999, whereas 126 we used those conditions from the year 2007. The current version of ModelE was tuned by 127 modifying two parameters that change planetary albedo, and hence, the net radiation of the 128 planet. This model was tuned because Robock et al. detected a significant temperature trend 129 during the period over which they conducted their simulations, due to insufficient time allowed 130 for model spin-up. These tuning parameters modify the critical humidities for ice cloud and 131 water cloud condensation. Specifically, Robock et al. used tuning parameters U00ice=0.590 and 132 U00wtrx=1.33. The version used for the simulations in our study used parameters U00ice=0.595 - 5 - KRAVITZ ET AL.: ARCTIC SPRING GEOENGINEERING 133 and U00wtrx=1.40. Before simulations were performed with these new parameters, the model 134 was spun-up for an additional 100 years. This resulted in a much smaller trend over the model 135 period 2007-2026, and, after conducting further simulation with a control run, the temperature 136 trend under these new tuning parameters is largely negligible. In Section 5, we discuss the 137 effects this tuning had on our results. 138 We began with a 6-member control ensemble of 20-year runs (2007-2026), during which 139 global greenhouse gas concentrations, as well as aerosol concentrations, remained fixed at 140 constant 2007 conditions. We then simulated a 6-member ensemble of 20-year runs covering the 141 same period, in which global greenhouse gas concentrations increased according to the IPCC’s 142 A1B scenario [IPCC, 2007].