Synopsis: A clear signal of human influence on upper tropospheric ozone trends is identifiable with high statistical confidence in a 17-year satellite dataset.

Abstract: Tropospheric ozone (O₃) is a strong greenhouse gas, particularly in the upper troposphere (UT). Limited observations point to a continuous increase in UT O₃ in recent decades, but the attribution of UT O₃ changes is complicated by large internal climate variability.

We show that the anthropogenic signal (“fingerprint”) in the patterns of UT O₃ increases is distinguishable from the background noise of internal variability. The time-invariant fingerprint of human-caused UT O₃ changes is derived from a 16-member initial-condition ensemble performed with a chemistry-climate model (CESM2-WACCM6). The fingerprint is largest between 30°S and 40°N, especially near 30°N. In contrast, the noise pattern in UT O₃ is mainly associated with the El Niño–Southern Oscillation (ENSO). The UT O₃ fingerprint pattern can be discerned with high confidence within only 13 years of the 2005 start of the OMI/MLS satellite record. Unlike the UT O₃ fingerprint, the lower tropospheric (LT) O₃ fingerprint varies significantly over time and space in response to large-scale changes in anthropogenic precursor emissions, with the highest signal-to-noise ratios near 40°N in Asia and Europe.

Our analysis reveals a significant human effect on Earth’s atmospheric chemistry in the UT and indicates promise for identifying fingerprints of specific sources of ozone precursors.