Abstract
The Montreal Protocol on Substances that Deplete the Ozone Layer has been hailed as the most successful environmental treaty ever (https://www.unenvironment.org/news-and-stories/story/montreal-protocol-triumph-treaty). Yet, although our main concern about ozone depletion is the subsequent increase in harmful solar UV radiation at the Earth’s surface, no studies to date have demonstrated its effectiveness in that regard. Here we use long-term UV Index (UVI) data derived from high-quality UV spectroradiometer measurements to demonstrate its success in curbing increases in UV radiation. Without this landmark agreement, UVI values would have increased at mid-latitude locations by approximately 20% between the early 1990s and today and would approximately quadruple at mid-latitudes by 2100. In contrast, an analysis of UVI data from multiple clean-air sites shows that maximum daily UVI values have remained essentially constant over the last ~20 years in all seasons, and may even have decreased slightly in the southern hemisphere, especially in Antarctica, where effects of ozone depletion were larger. Reconstructions of the UVI from total ozone data show evidence of increasing UVI levels in the 1980s, but unfortunately, there are no high-quality UV measurements available prior to the early 1990s to confirm these increases with direct observations.
Highlights
The network was established in 1991 to ensure the highest quality monitoring of stratospheric properties that may be influenced by mankind[9]
Close agreement is not expected at time scales of less than 5 years because there is a random component to the way modes of variability such as the quasi biennial oscillation (QBO) manifest themselves in the model runs
For the reference period 1978–1987, total ozone column amounts (TOCs) calculated with the NIWA-UKCA model for sites between 45°S and 45°N – which represents 71% of the globe –are on average 8 ± 4% (±1σ) higher than the measured TOCs
Summary
We begin by comparing projected ozone fields from the two World Avoided models (Scenarios (2) and (3)). There is a significant divergence between the measured values and World Expected values at higher latitudes: measured and calculated UVI ratios are more or less constant over time while the World Expected curves have a broad maximum during the first decade of the 21st century (last two panels of Fig. 4) This suggests that the impact of the large spring-time ozone depletions persists for too long into the following summer in the model. For these high-latitude sites, the “calculated” and “measured” data sets are in almost perfect agreement, in Antarctica, indicating that ozone variation is the largest contributor to UVI changes.
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