The ozone layer protects life on land from damage caused by too much UV radiation. By protecting the ozone layer, the Montreal Protocol helps to protect biodiversity and the terrestrial ecosystems that we all rely on for clean air and clean water and to pollinate our crops.
The Montreal Protocol helps to achieve SDG15 aims that include ‘ensure the conservation, restoration and sustainable use of terrestrial and inland freshwater ecosystems and their services’ (Target 15.1) and ‘to reduce the degradation of natural habitats, halt the loss of biodiversity’ (Target 15.5).
Just as excess UV radiation can damage human health, it is clear that it can have multiple effects on other organisms. The successful implementation of the Montreal Protocol has protected SDG15 Life on Land by preventing the very large increases in UV radiation that would have occurred without the effective protection provided by the stratospheric ozone layer.
Plants need sunlight for photosynthesis, so they cannot avoid exposure to UV radiation. They have evolved systems that reduce or repair UV damage, including producing pigments that act as sunscreens. Some UV responses can actually benefit plants by stimulating their resistance to attack by pests or diseases. As with human health, there is a balance between these positive and negative effects of UVB on plants. Uncontrolled ozone depletion would have shifted this balance very much towards the negative.
Different plants would have been more or less sensitive to damage from excess UV radiation. However, as an overall ball-park figure, the Protocol’s Environmental Effects Assessment Panel concluded that plant growth is reduced by around 6 per cent by an increase in UV radiation equivalent to a 10 per cent reduction in stratospheric ozone. A 10 per cent ozone depletion is small when compared to the 50 per cent or more that models indicate would have occurred by the end of this century without the Montreal Protocol. If the estimate of growth reduction due to limited ozone depletion can be extended to these much more severe ozone depletion, then plant growth worldwide might have been reduced by at least 25 per cent by the end of this century.
Plants are the base of all terrestrial food-webs, so such a large reduction in productivity would have had severe impacts on animal species and biodiversity in general. Forests and other terrestrial ecosystems also provide ecosystem services by absorbing air pollutants and regulating the flow of clean water into lakes and rivers. A large reduction in plant growth due to uncontrolled ozone depletion would not only compromise these ecosystem services, it would also lower the capacity of vegetation to absorb carbon dioxide from the atmosphere. This would reduce the absorption of CO2 produced by human activities, leading to higher concentrations in the atmosphere. In this way, by protecting SDG15 Life on Land, the Montreal Protocol also protects the climate (SDG13).
The Montreal Protocol has already made a large contribution to protecting the climate by phasing-out ozone-depleting substances, such as chlorofluorocarbons (CFCs), that are also very potent greenhouse gases. The reduction in ODS emissions achieved by the Protocol is already equivalent to around 135 billion tonnes of CO2. Looking to the future, modelling studies suggest that by controlling ODS emissions the Montreal Protocol will have prevented temperature increases of 4-6°C at the poles and over 2°C in the tropics by 2070. Building on that success, the Kigali Amendment to the Protocol is set to avoid another 0.4°C by controlling high global warming hydrofluorocarbons (HFCs).
The temperature increases that have been avoided by the Montreal Protocol are similar in magnitude to those expected if we fail to control CO2 emissions. As a result, the reports of the Intergovernmental Panel on Climate Change (IPCC) gives some insight to how the temperature increases avoided by the Protocol would have affected SDG15. For example, IPCC highlight a high risk of damage to terrestrial ecosystems worldwide from temperature increases by 2°C or more. They also conclude that temperature increases over 2°C would bring a very high risk of severe impacts in the Arctic region. The Arctic and its indigenous people is one of IPCC’s ‘unique and threatened systems’ that are at very high risk of severe impact. The recent 2019 IPPC report on the cryosphere highlight the risks of large reductions in permafrost, changes in the flow of water from the land into rivers, increase wildfires, and changes in the health and distribution of tundra and high-latitude forests.
While these IPPC assessments clearly focus on the unresolved challenge of warming due largely to CO2 emissions, they illustrate the likely magnitude of impacts that have been avoided by successful climate protection through the Montreal Protocol. These climate-related benefits of the Montreal Protocol in protecting life on land seem likely to be substantial, although most remain less well studied than benefits related to avoiding increased UV radiation. The one exception is the growing understanding of the ecological effects of on-going changes in climate in the Southern hemisphere that are linked to the Antarctic ozone hole. Stratospheric ozone depletion has contributed to changes in wind patterns and rainfall that extend well beyond the Antarctic. The nature of these changes varies between regions, resulting in contrasting responses in different ecosystems. Increased rainfall in some areas has increased productivity in some regions but elsewhere increased aridity has damaged vegetation and increased the risk of forest fires.