Influence of climate change on UV exposure

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Impact of greenhouse gases on the global ozone layer

Emissions of ozone-depleting substances as a result of human activities have led to a reduction in stratospheric ozone. At mid-latitudes in the northern hemisphere – and therefore also in Germany – there has been an approximately 3 % depletion of the stratospheric ozone layer. This has led sunburn-causing UV irradiance to increase by approximately 7 % in winter and spring and approximately 4 % in summer and autumn.

Although studies show that the ban on ozone-depleting substances (halogenated substances such as chlorofluorocarbons, CFCs) has halted the further depletion of the ozone layer at the global level and that the ozone layer is recovering again in the upper stratosphere, a significant increase in total ozone can be observed at mid-latitudes (60 degrees south to 60 degrees north). Initial analyses suggest that, although ozone levels in the upper stratosphere are recovering at mid-latitudes, continuing ozone depletion in the lower stratosphere since 1998 has simultaneously counteracted the recovery of total ozone at these latitudes. The reasons for this have not yet been fully clarified, but halogenated greenhouse gases (containing fluorine, chlorine, bromine or iodine), which can break down ozone, have been mooted as possible causes.

Low-ozone events (LOEs)

Low-ozone events are “passing” low-ozone air masses that cover a small area and lead to sudden, unexpectedly high UV irradiances and therefore to a greater risk of UV-related diseases – and especially UV-related cancers of the skin and eye. In late March/early April 2020, for example, a low-ozone event of this kind caused the UV index to rise from UVI 3 to UVI 6 – that is, to a UV radiation intensity that usually doesn’t occur until 4–6 weeks later in the year.

Low-ozone events in Germany are the result either of winter ozone depletion (a hole in the ozone layer) over the North Pole (the Arctic) (low-ozone events in March/April) or of equalisation of the concentration within the global ozone layer between the equator and the North Pole (low-ozone events in May/June). Low-ozone events in late March/early April are particularly relevant to health. In the last two decades, there has been an increase in the frequency of these Arctic ozone holes in winter and therefore the probability of low-ozone events in late March/early April. The latest analyses of the MOSAiC Arctic expedition by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) identify a clear relationship with anthropogenic greenhouse gas emissions – that is, with climate change. On the one hand, ozone is chemically depleted by halogenated greenhouse gases; on the other, greenhouse gases cause an extreme reduction in stratospheric temperature over the North Pole, further driving chemical ozone depletion. The colder the temperatures, the greater the depletion of ozone. Researchers predict that ozone depletion over the Arctic could intensify even further by the end of the century if global greenhouse gas emissions are not reduced quickly and consistently.

These low-ozone events in late March/early April are relevant to health because the occurrence of high UV irradiances that are normal in early summer is not expected at this time of year, and people therefore do not consider the necessary sun-protection measures. This, coupled with the fact that it can still be quite cold at this time of year – particularly in Northern Germany – and that people are keen to feel the warmth of the sun, leads to an increased risk of excessive UV exposure of the skin and eyes. As UV radiation is not perceptible to humans, it is not possible for people to assess how strong the UV radiation is themselves. It is therefore important to keep an eye on the UV index from late March/early April onwards.

Less cloud cover – more sunny days – more UV radiation

Anomaly in sunshine duration in Germany, 1951–2021 Source: German Meteorological Service (DWD)

As illustrated by satellite data and climate models, climate change is leading to changes in the known cloud cover scenarios for countries or parts of continents.

The reduction in cloud cover observed in Germany over recent decades has manifested in an increase in the number of sunny days and therefore of sunshine hours over recent years. Records from the German Meteorological Service (DWD) clearly show a linear increase in the number of sunshine hours, for the year as a whole, by 132 hours from 1951 to 2021.

More hours of sunshine mean more time during which UV radiation can reach the earth’s surface unimpeded. Analyses of data from the Germany-wide UV measuring network reveal that this is accompanied by an increase in total annual UV irradiance: in sunny and hot years, such as 2003 and 2018, this total is significantly higher than the 20-year average.

More heat – more UV exposure?

Temperature anomaly in Germany 1951 - 2021 Source: German Meteorological Service (DWD

Another factor that should be taken into account when it comes to climate change and its effects is the way in which people expose themselves to UV radiation – that is, people’s behaviour. This essentially depends on the prevailing weather conditions and the temperature, or rather the perceived temperature – parameters on which climate change has a profound impact.

Records from the German Meteorological Service (DWD) for recent decades show an increase not only in average sunshine hours but also in temperature and the number of “summer days” in Germany: from 1951 to 2021, the average temperature increased by 1.6 °C and the number of summer days (with a maximum air temperature greater than or equal to 25 °C) rose by 22.6 days.

Anomaly in number of summer days in Germany, 1951–2021 Source: German Meteorological Service (DWD)

Scientific studies support the theory that people spend more time outdoors due to changing weather conditions and higher temperatures as a result of climate change, thereby increasing their individual net UV exposure. Forecasts suggest that time spent outdoors in warmer weather is likely to increase most in areas at middle and high latitudes. For these areas, studies show that the frequency and duration of activities outdoors increase at higher temperatures – ranging from comfortable temperatures to slightly excessive heat. In addition, an individual’s attitude to warmth or heat plays a significant role. Studies show that people may adapt their behaviour to changing climatic conditions and that they might then also perceive intense heat as pleasant, for example, and spend time outdoors rather than looking for cool rooms, thereby increasing their own exposure to UV radiation. On the other hand, intense heat could lead people to spend more time in air-conditioned rooms, leading to a reduction in individual UV exposure.

At present, there is a lack of informative quantitative analyses of people’s net UV exposure as a result of their behaviour. Further research will be needed in this area. However, the fact is that people’s behaviour and the adjustment of behaviour to changing climatic conditions is an important factor when it comes to estimating UV radiation exposure as an impact of climate change and therefore the risk of UV-related diseases.