Quantifying Contributions of Ozone Changes to Global and Arctic Warming During the Second Half of the Twentieth Century
Since the 1950s, tropospheric ozone has increased rapidly by about 50% of the total growth since pre-industrial times, and surface temperatures have risen much faster in the Arctic than over the globe as a whole. Faster high-latitude warming, known as Arctic amplification (AA), is a prominent feature of anthropogenic climate change with important implications for human and natural systems. The rapid warming of the Arctic is believed to have arisen from increases in atmospheric concentrations of WMGHG, while anthropogenic aerosols could have offset about 60% of the Arctic warming induced by WMGHG during 1913–2012. A recent study finds that ozone-depleting substances very likely played a role in determining the magnitude of AA seen in recent decades. This study quantifies the contribution of ozone changes to observed global warming during 1955–2005 using historical simulations from the Community Earth System Model (CESM) Large Ensemble (LE) Project. Another purpose of this study is to assess the relative importance of ozone changes in observed Arctic warming by comparing ensembles of CESM1 simulations with and without transient ozone, with other forcings identical in each simulation.
Results indicate that ozone changes during 1955–2005 have strongly enhanced the downwelling longwave flux and increased net shortwave flux at the surface, and thus significantly contributed about 0.15 °C of global mean surface warming, roughly 21%, 26% and 16% of the observed, all-forcing and WMGHG-driven trends, respectively. In the Arctic in the same period, corresponding ozone-driven warming was about 0.63 °C, roughly 48%, 40%, and 25% of the same three trends. During 1979–2005, these ozone changes have markedly added about 0.25 × 106 km2 to the decrease in the Arctic sea ice extent (SIE), or roughly 25%, 48%, and 40% of the same three trends. Considering that the ozone-driven radiative forcing of about 0.22 (0.06) W·m−2 in 1955–2005 (1979–2005) was about 12% (6%) of the corresponding WMGHG forcing, ozone changes had contributed disproportionately to global and Arctic warming and Arctic sea ice decline during the second half of the twentieth century. Tropospheric ozone has shown relatively steady growth since 2006 and might have significantly contributed to recent observed warming over the globe and the Arctic.
Using ensemble simulations of the CESM1 coupled model with realistic time-dependent ozone changes for 1955–2005, we analyze the responses of global and Arctic climate to ozone changes by subtracting the fixed ozone ensemble average from the all forcing ensemble average. The forced annual or seasonal warming trends in the historical all forcing simulations are consistent with the corresponding observed trends over the globe and the Arctic, indicating that the CESM1_LE is an appropriate model for our analysis of relative contributions of ozone changes to observed or total forced trends. The spatial and temporal SAT responses are roughly consistent with the variations in FLDS resulting from ozone changes, which indicates that large-scale SAT trends due to ozone forcing are dominated by tropospheric ozone trends, although stratospheric ozone depletion may cause cooling in limited areas, as reported in other studies. Compared to the negative FSNS trend driven by all-forcing or weak FSNS trends driven by WMGHG forcing, the ozone-induced global mean FSNS trend is positive and has a warming effect on GMST throughout the year. The forced warming trends averaged over the globe and Arctic are indeed in accord with observations, suggesting that ozone changes, primarily in the troposphere, have significantly contributed to global and Arctic warming since 1955. These conclusions are generally consistent with findings from previous studies. The analyzed trends also indicate that recent marked declines in Arctic sea ice attributed to tropospheric ozone increases have further contributed to the accelerated warming in the Arctic, especially over the marginal Barents Sea, Kara Sea, and Laptev Sea.