Signal-To-Noise Calculations of Emergence and De-Emergence of Stratospheric Ozone Depletion

Abstract

The year when total column ozone (TCO) returns to 1980 levels is commonly used to quantify recovery from disturbance caused by ozone-depleting substances. This date is reasonable but somewhat arbitrary. Here we borrow an indicator from climate change research, the signal-to-noise (S/N) metric to investigate how TCO might return to statistically similar levels to those of the pre-ozone hole era (1960–1970). We calculate S/N for ozone projections from the Chemistry-Climate Model Initiative. In most regions, a return to 1980 levels doesn't represent TCO recovery to pre-disturbance conditions because it does not account for internal variability or reflect when statistically significant TCO losses occurred. Future projections show that, in most regions, TCO “de-emerges” (i.e., S/N becomes less than one standard deviation from the pre-disturbance mean) before it returns to its 1980 value. We conclude that S/N is an appropriate, perhaps complementary metric for determining when TCO returns to a pre-disturbance state.