Environmental effects of stratospheric ozone depletion, UV radiation, and interactions with climate change: UNEP Environmental Effects Assessment Panel, Update 2021

Abstract

The Environmental Effects Assessment Panel of the Montreal Protocol under the United Nations Environment Programme evaluates effects on the environment and human health that arise from changes in the stratospheric ozone layer and concomitant variations in ultraviolet (UV) radiation at the Earth's surface. The current update is based on scientific advances that have accumulated since our last assessment (Photochem and Photobiol Sci 20(1):1-67, 2021). We also discuss how climate change affects stratospheric ozone depletion and ultraviolet radiation, and how stratospheric ozone depletion affects climate change. The resulting interlinking effects of stratospheric ozone depletion, UV radiation, and climate change are assessed in terms of air quality, carbon sinks, ecosystems, human health, and natural and synthetic materials. We further highlight potential impacts on the biosphere from extreme climate events that are occurring with increasing frequency as a consequence of climate change. These and other interactive effects are examined with respect to the benefits that the Montreal Protocol and its Amendments are providing to life on Earth by controlling the production of various substances that contribute to both stratospheric ozone depletion and climate change.

Fig. 1

Daily maximum UV index (UVI) measured at the South Pole (a) and Arrival Heights (b) in 2019 (blue line) and 2020 (red line), compared with the average (white line) and the range (grey shading) of daily maximum observations of the years indicated in the legends. The UVI was calculated from spectra measured by SUV-100 spectroradiometers. Up to 2009, the instruments were part of the NSF UV monitoring network [13] and they are now a node in the NOAA Antarctic UV Monitoring Network (https://www.esrl.noaa.gov/gmd/grad/antuv/). Consistent data processing methods were applied for all years [14, 15]. In 2020, the UVI was typically above the long-term average at both sites due to the sustained and deep stratospheric ozone hole in that year. Conversely, the UVI in 2019 was close to the lower limit of historical observations because warming of the Antarctic stratosphere produced one of the smallest ozone holes on record in that year [1, 8]

Fig. 2

Change (%) in global tropospheric OH concentrations from 1980 to 2010, estimated with different Earth System Models (ESMs). The net change in OH (rightmost column) had contributions from increased emissions of nitrogen oxides and other precursors of tropospheric ozone (∆NO_x); increased emissions of methane (∆CH₄); accumulation of ozone-depleting substances now regulated under the Montreal Protocol (∆ODSs); emissions of particulate matter and its precursors (∆PM); and other undifferentiated changes attributed to underlying climate change as well as interactions among these separate factors (∆Other). The ODSs have contributed to the net increase in OH concentrations by depleting stratospheric ozone, thus allowing more UV radiation to penetrate into the troposphere and increase the rates of photochemical reactions that generate OH. Modified from Stevenson et al. [36]

Fig. 3

Pathways by which extreme climate events driven by changes in stratospheric ozone and climate can modify exposures and responses of terrestrial organisms and ecosystems to solar UV radiation (solid lines). Dotted line shows modulating effects of climate change factors on response to UV radiation, while dashed line indicates feedback effects of the biosphere on the climate system. Increases in exposure to UV radiation are shown as plus signs (+), and decreases as negative signs (−)

Fig. 4

Depth-integrated methane photoproduction rate (top 150 m) calculated with a photochemical model based on remote sensing reflectance data (Figure by Rachele Ossola, adapted from Li et al. [138])

Fig. 5

a Schematic illustration of the three-layer structure of the world ocean. The upper mixed layer is stirred by a range of turbulent processes driven by the wind. Warming, and glacier and sea ice melting have increased the density contrast (that is, stratification) between surface and deep waters that meet at the pycnocline (a layer of density transition between the upper and lower layers). Mixing incorporates water at the top of the pycnocline into the surface layer, deepening it, but also making the pycnocline sharper. b Global trends in ocean mixed layer depth during summer show localised regions of deepening at high southern latitudes (blue shading) and shallowing mostly at lower latitudes (orange shading). White indicates areas of zero or non-significant trends (modified from [154])