While it is clear that the dominant forcing of the observed increase in global average temperatures over the last hundred years has been the increase in greenhouse gases due to human activity, it is less clear what is the relative contribution of other factors, such as the increase in atmospheric aerosols or of unforced natural climate variability. A number of studies have shown that a number of simple indices of climate variability and climate change, in addition to the global mean temperature, can be helpful in evaluating the performance of climate models in simulating large-scale variations of surface air temperature, as well as the  responses to different forcings. These simple indices include the temperature contrast between the Northern and Southern Hemispheres and the temperature contrast between the land and oceans in each hemisphere.

Hence, the average temperature over the oceans in the Southern Hemisphere, and its contrast with the Northern Hemisphere oceans, and with the average temperature over land in the Southern Hemisphere, is a key metric for assessing the role of different forcings in human-caused climate change. We have used the recent CMIP6 simulations of the historical period to assess the high and low climate sensitivity models using these metrics, particularly the temperature variations over the Southern Hemisphere.