Attributing changes of global stratospheric temperatures to natural and anthropogenic forcings

Observations show that the cooling of global stratospheric temperature from 1979 through 2014 took place in two major steps coincident with the 1982 El Chichón and 1991 Mount Pinatubo volcanic eruptions. In order to attribute the features of the global stratospheric temperature time series to the main forcing agents, we performed a set of simulations using the NASA Goddard Earth Observing System Chemistry-Climate Model. In the lower stratosphere, the model experiments demonstrate that the increase in ozone-depleting substances is the main cause of the observed cooling trend. In the middle and upper stratosphere, greenhouse gases are the dominant forcing factor. The model simulations show that even though sporadic volcanic events and the solar cycle can have a distinct signature in the time dependence of the temperature trend, these forcing mechanisms do not play a statistically significant role in the long-term change. The characteristic step-like behavior in the time series can be attributed to the effects of the 11-year solar cycle, except for the post-1995 flattening of the lower stratospheric temperatures, where the decrease in ozone depleting-substances due to the Montreal Protocol slowed ozone depletion and therefore also the cooling of the lower stratosphere.