Separating ozone from greenhouse gas signals in stratospheric temperature trends

Ozone depletion over the last 3 decades has led to a decrease in stratospheric temperature through decrease of shortwave heating. Greenhouse gases such as carbon dioxide and methane have likewise caused a decrease in stratospheric temperature through longwave radiation to space. Over the next 2-3 decades, ozone should increase in response to the provisions of the Montreal Protocol. Increased ozone and increased carbon dioxide lead to opposite effects on stratospheric temperature. Thus the projection of how stratospheric temperature will change over the next few decades depends on an accurate evaluation of the relative sensitivity of the temperature to ozone change and carbon dioxide change. We have previously evaluated those sensitivities in our chemistry-climate model by time-series analysis of the 140-year simulation. We now extend the analysis to ask how long the data set has to be to achieve a statistically-significant separation of the ozone and greenhouse terms in the temperature time series. We find that in our simulation, the terms can be significantly separated for the upper stratosphere using output from 1979 through 2010. In the lower mid-latitude stratosphere we require output through about 2020 to achieve a significant separation of the terms. We will discuss the implication of these findings for determining the causes of trends in temperature measurements in the stratosphere.