Abstract
Abstract. The stratospheric ozone layer shields surface life from harmful ultraviolet radiation. Following the Montreal Protocol ban on long-lived ozone-depleting substances (ODSs), rapid depletion of total column ozone (TCO) ceased in the late 1990s, and ozone above 32 km is now clearly recovering. However, there is still no confirmation of TCO recovery, and evidence has emerged that ongoing quasi-global (60∘ S–60∘ N) lower stratospheric ozone decreases may be responsible, dominated by low latitudes (30∘ S–30∘ N). Chemistry–climate models (CCMs) used to project future changes predict that lower stratospheric ozone will decrease in the tropics by 2100 but not at mid-latitudes (30–60∘). Here, we show that CCMs display an ozone decline similar to that observed in the tropics over 1998–2016, likely driven by an increase in tropical upwelling. On the other hand, mid-latitude lower stratospheric ozone is observed to decrease, while CCMs that specify real-world historical meteorological fields instead show an increase up to present day. However, these cannot be used to simulate future changes; we demonstrate here that free-running CCMs used for projections also show increases. Despite opposing lower stratospheric ozone changes, which should induce opposite temperature trends, CCMs and observed temperature trends agree; we demonstrate that opposing model–observation stratospheric water vapour (SWV) trends, and their associated radiative effects, explain why temperature changes agree in spite of opposing ozone trends. We provide new evidence that the observed mid-latitude trends can be explained by enhanced mixing between the tropics and extratropics. We further show that the temperature trends are consistent with the observed mid-latitude ozone decrease. Together, our results suggest that large-scale circulation changes expected in the future from increased greenhouse gases (GHGs) may now already be underway but that most CCMs do not simulate mid-latitude ozone layer changes well. However, it is important to emphasise that the periods considered here are short, and internal variability that is both intrinsic to each CCM and different to observed historical variability is not well-characterised and can influence trend estimates. Nevertheless, the reason CCMs do not exhibit the observed changes needs to be identified to allow models to be improved in order to build confidence in future projections of the ozone layer.
Highlights
In the latter half of the 20th century, emissions of halogencontaining ozone-depleting substances (ODSs) led to a decline of the ozone layer at all latitudes across the globe (WMO, 2014)
The 60◦ S– 60◦ N ozone layer is observed to have likely continued to thin due to lower stratospheric ozone decreases (Fig. 1a) that counteract upper stratospheric recovery (Ball et al, 2018), which are not reproduced by the multi-model mean (MMM) (Fig. 1b)
The same qualitative similarities in total column ozone (TCO) and lower stratospheric ozone trends is seen for the MMM, but with acceleration 5 times larger compared to the observations (Fig. 1d)
Summary
In the latter half of the 20th century, emissions of halogencontaining ozone-depleting substances (ODSs) led to a decline of the ozone layer at all latitudes across the globe (WMO, 2014). It was revealed that global total column ozone levels had stopped falling by around 1998– 2000 thanks to the MPA (WMO, 2007), and research has turned to identifying ozone recovery related to ODS declines (Chipperfield et al, 2017). There are clear indications that the MPA has worked in reducing atmospheric ODSs, that further significant and serious depletion of the ozone layer has been avoided (Egorova et al, 2013; Chipperfield et al, 2015), and that some regions exhibit MPA-dependent recovery
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