GeoMIP Simulations: Testbed

The GeoMIP Testbed was introduced by Kravitz et al. (2015) as a way for other communities to participate in GeoMIP. The idea is that individuals can propose potential GeoMIP experiments to be performed by a small number of models. If these experiments turn out to be fruitful, they can then be considered for adoption by GeoMIP as official core experiments.

All current testbed experiments will be listed here as they are added:


G6sulfur_limits

Experiment G6sulfur is designed to reduce radiative forcing in a high emissions scenario to that of a moderate emissions scenario via simulation of stratospheric sulfate aerosol injection. This experiment would be useful in assessing the effectiveness of geoengineering as part of a portfolio of responses to climate change. However, this experiment only addresses one potential scenario, i.e., using geoengineering to achieve the forcing from a “medium” scenario. Increasing amounts of stratospheric SO2 injection would cause particles to coagulate and fall out more rapidly. Therefore, the relationship between the amount of injection and the resulting radiative forcing is projected to be sublinear. This problem prompts a natural question: how would the injection amount and the results from that injection differ if geoengineering were used to achieve a larger radiative forcing? This question is the first step in assessing any potential practical limits to stratospheric aerosol injection.

A natural first step in addressing this problem would in- volve a similar setup to that of G6sulfur. Against a background of the ScenarioMIP Tier 1 high forcing scenario, sulfate aerosol precursors would be injected into the stratosphere in sufficient amounts to reduce anthropogenic radiative forcing from the levels in the high forcing scenario to levels in the low forcing scenario.


GeoSulfur5, GeoSulfur20, GeoSulfur50
A different way of quantifying the effects of stratospheric aerosol geoengineering is to perform a series of experiments in which the hypothetical rate of injection of stratospheric sulfate aerosols is constrained. Such a simulation would be well suited to ascertain the range of model responses to a fixed amount of SO2 injection, highlighting model diversity. Against a background of the ScenarioMIP Tier 1 high forcing scenario, the modeling groups will inject 10, 20, or 50 Tg SO2 per year into the lower stratosphere, in a similar setup to experiment G4.

GeoLandAlbedo
Experiment G1ocean-albedo has simulated the effects of marine cloud brightening by increasing ocean albedo by a constant multiplication factor. However, GeoMIP has not yet explored land-based approaches towards solar radiation management. Such approaches could readily be implemented on the regional scale, as human activities already control the albedo of a significant fraction of the land surface. We therefore propose an alternative experiment in which the land surface albedo is increased, against a background of the CMIP5 abrupt4xCO2 experiment.

Under experiment GeoLandAlbedo, the land surface albedo would be increased by a uniform amount of 0.1 across all urban and agricultural areas. Such an increment represents a reasonable estimate of the maximum large-scale albedo increase that could be achieved in practice. The aim of experiment GeoLandAlbedo would not be to achieve a global energy balance but rather to determine the extent to which land surface albedo changes could offset the effects of increasing greenhouse gases on a regional basis.