Does the concentration of CO2 gas in the atmosphere cause warming of the earth's atmosphere? Or does it lead to less rainfall when it warms up? Or does the warming of the earth's atmosphere lead to an increase in rainfall on the earth's surface?
Equilibrium Climate Sensitivity (ECS) is the global mean change in surface temperature for a doubling of CO2 from the pre-industrial (PI) value. ECS is one of the key metrics used in assessing future global warming, and therefore plays a very important role in climate change related policy-making. One important question in this regard is how ECS changes in a warmer world. Several studies found that ECS increases at higher CO2 concentrations (e.g., Bloch-Johnson et al., 2021; Colman & McAvaney, 2009; Gregory et al., 2015; Meraner et al., 2013). And, more recently, Mitevski et al. (2021) found a non-linear and non-monotonic dependence of ECS on CO2 concentrations. In addition to the surface temperature response, the precipitation response is another critical aspect of climate change. To evaluate precipitation changes, the key metric used is Hydrological Sensitivity (HS). HS is defined as the difference in global mean precipitation per one degree of global mean temperature change from the PI control state. Previous studies have explored the response of the hydrological cycle to global warming by examining HS in terms of the global energy budget, and have described the mechanisms affecting it (e.g., Allen & Ingram, 2002; Held & Soden, 2006; Jeevanjee & Romps, 2018; O'Gorman et al., 2011). The fact that HS is energetically constrained means that the precipitation response can be separated into fast and slow components. The fast response depends only on the CO2 concentrations in the atmosphere, before the surface temperature has time to warm, and results in a decrease in precipitation. The slow response, in contrast, is associated with surface warming, and results in an increase in precipitation (Andrews et al., 2010).