Removal or Failure of Climate Engineering by Sulfate Aerosol Injection Poses Dangerously Abrupt Temperature Increases

Sulfate aerosol injection as a method of geoengineering and cooling the planet has been showing great promise and growing in popularity in regards to fixing the climate crisis. It has long been suggested that geoengineering could function independently and provide researchers with more time to improve and create methods of removing CO2 from the atmosphere, which is the real fix. Ross et al. (2009), however, claim that if we rely on geoengineering and do not implement it alongside the removal of CO2, there will be drastic temperature changes if the geoengineering strategy is removed or fails. If CO2 emissions do not decrease, the failure or removal of climate engineering methods would result in a large temperature spike, increasing global temperatures by a maximum of 4.5 °C, which would have catastrophic impacts on the planet’s ecosystems and possibly result in mass species extinctions.— Ellie Pickrell
Ross, A., Matthews, H., 2009. Climate engineering and the risk of rapid climate change. Environmental Resolution Letters 4, 045103.

 In this study, Ross et al. used a climate model to predict the effects of the implementation and subsequent removal of climate engineering by injection of sulfate aerosols with the A1B emissions scenario. The control group consisted of a business as usual emissions scenario. The second simulation consisted of a model exposed to climate engineering that started in the year 2020 and was removed in 2060. These two simulations were repeated 40 times each, varying with climate sensitivity of the model from 0.5 to 10 ° C. Climate sensitivity is the response of global mean surface air temperature to a doubling of atmospheric CO2 concentrations. An estimated climate sensitivity probability density function was used from another paper (Hegerl et al. 2006) to identify the likelihood of each set of model situations.
In the control group where climate engineering was not applied, temperatures increased consistently from 1990 to 2100, ranging from 0.6 to 5.1 °C for climate sensitivities ranging from 0.5 to 10 °C.  Atmospheric CO2 concentrations at the year 2100 ranged from 690 to 739 ppmv, with higher climate sensitivities containing the higher concentrations. In the climate engineered simulations, temperatures dropped to values very similar to the temperatures in 1990 between 2020 and 2059, with respect to the control scenario. As soon as the engineering was removed, however, temperatures increased rapidly, ranging from 0.15 to 4.5 °C between 2060 and 2100. The temperature change after the removal of the engineering was higher with higher values of climate sensitivity. The final CO2 concentrations in the geoengineering runs were similar to those in the control simulations (between 689 and 722 ppmv).
Next, Ross et al. looked at the annual rate of temperature change between 1990 and 2100 for each simulation. In the control scenario, the annual rate of temperature change increased until 2060, whens greenhouse gas emissions decline with the A1B emissions scenario. This resulted in a decreased rate of temperature change. In the climate engineering scenarios, the rate of temperature change was relatively small up until 2020, when geoeningeering was implemented and temperatures dropped. From 2020 to 2060 the rate of temperature change was insignificant, until temperatures abruptly increased after the removal of geoengineering. The maximum rate of warming ranged from 0.13 to 0.76 °C/year. These high rates of warming, however, only lasted for a few years and within a decade, the rates decreased to less than 0.1 °C/year. The maximum rate of sea level rise was also higher in the geoengineering simulations than in the controls.
Finally, Ross et al. looked at the probability density functions between 1990 and 2100, which measures the likelihood that these temperatures will change. For the control group, the most likely maximum annual temperate change was 0.031°/year. The geoengineering simulation showed a likely maximum rate of temperature change just under 0.5 °C/year, which occurred in 2060 at the high temperature spike. 

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