Detecting Anthropogenic Influence in Observed Upper Tropospheric Ozone Trends

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, in contrast to changes in lower tropospheric ozone which is more directly influenced by near-surface anthropogenic precursor emissions. The extent to which this increase is due to external forcings (either human or natural) is unknown. Attribution of UT O₃ changes is complicated by large internal climate variability, including the strong influence from exchange with the stratosphere. By applying techniques developed for detecting anthropogenic climate change to UT O₃, we show that the anthropogenic signal (“fingerprint”) in the increasing UT O₃ trend stands out from the background noise of internal variability. The time-invariant fingerprint of UT O₃ changes derived from a 16-member initial-condition ensemble with a chemistry-climate model (CESM2-WACCM6) is largest between 30S and 40N, especially in the latitudinal zone near 30N. In contrast, the noise in UT O₃ is largely associated with ENSO. We find that even a relatively short satellite record can identify the UT O₃ fingerprint pattern with high statistical confidence within only 13 years of the monitoring UT O₃ changes (starting from 2005). Whereas the UT O₃ fingerprint is spatially more extensive, the lower tropospheric (LT) O₃ fingerprint varies significantly over time and space in response to large-scale changes in anthropogenic precursor emissions. Locations with strong anthropogenic change signals in UT O₃ relative to internal variability are identified around the tropical Indian Ocean. In the LT, the highest signal-to-noise ratios are located near 40N in Asia and Europe. Our analysis reveals a significant human effect on Earth’s atmospheric chemistry in the UT and provides a scientific pathway for identifying fingerprints of anthropogenic climate change on observed tropospheric ozone trends. It also reveals atmospheric regions where anthropogenic impacts may be detected earlier by observing systems.