Factors influencing (future) stratospheric ozone levels

The status of the ozone layer (and therefore ground level UVR) is influenced by emissions of ODS (regulated by the Montreal Protocol), but also by emissions of methane (CH₄) and nitrous oxide (N₂O).

The international mechanism for protecting the ozone layer is the “Montreal Protocol on Substances That Deplete the Ozone Layer”, which came into force in 1989, and its subsequent amendments (UNEP 2006). Over the next few decades, the benefits of these measures will become apparent as compliance has been high. Overall, the Montreal Protocol is seen as the most successful international environmental agreement to date. The production of chlorofluorocarbons (CFCs), halons and methylbromide, the most harmful ozone depleting substances, are now phased out worldwide. The concentrations of these gases in the atmosphere is anticipated to reduce over the next decades. As a result, ozone levels are expected to increase in the coming decades, although the evolution will also depend on the changing climate system (IPCC 2005). However, it will probably take several decades until atmospheric ODS concentrations will fall below pre-1980 levels.

CH₄ acts as a precursor of the radicals that determine ozone chemistry. In addition, CH₄ is the primary mechanisms for the conversion of reactive Cl to the unreactive HCl, affecting the efficiency of chlorine-driven ozone loss (IPCC 2005). The oxidation of CH₄ increases water vapour and, subsequently, ozone losses in the HOx catalytic cycle in the upper stratosphere and lower mesosphere. In the troposphere and lower stratosphere, ozone is increased because oxidation of CH4 catalyzed by NOx produces ozone.

N₂O acts as a precursor of the radicals that determine ozone chemistry (IPCC 2005). In the mid to upper stratosphere, N₂O induces reduction of ozone. Increases in N₂O lead to increases in the NO_x catalytic loss cycle for ozone in the mid to upper stratosphere, because N₂O decomposes to form NO_x in the stratosphere (N₂O induced reduction of ozone).

Future scenarios (2030, 2050): IPCC SRES-A2 and SRES-B1

The scenarios used in this study are based on the SRES- scenarios by the IPCC (2000) (Box 1). The SRES scenarios are widely used for climate change assessments. They quantify the emissions of greenhouse gases (including CH₄ and N₂O) and ODS over the coming century, and their key drivers (population projections, economic growth, energy choices). The time horizon for the SRES-scenario is up to 2100. The WP3.7-UVR assessment primarily focuses on baseline (2000), 2030 and 2050. Besides developments in stratospheric ozone, the ageing of the population is expected to have an effect on future skin cancer in the population as well (UVR-related skin cancer incidence and mortality is higher at older ages).

Of the four SRES-scenarios, WP3.7-UVR explored SRES-A2 and SRES-B1. For our assessment, several scenario characteristics are important  (Table 1):

UVR exposure under SRES-A2 and SRES-B1.

Hence, the WP3.7UVR-exposure scenarios are based on the IPCC emission scenarios (resulting from the SRES-storylines) and associated emission modelling and resulting atmospheric concentrations. For more information we refer to the IPCC data and publications:

• http://www.grida.no/publications/other/ipcc%5Fsr/?src=/climate/ipcc/emission/index.htm
• http://sres.ciesin.org/final_data.html
• http://ipcc.ch/ipccreports/tar/

Based on these (future) atmospheric concentrations of ODS, CH₄ and N₂O, The University of Oslo has calculated total ozone columns for the SRES-A2 and SRES-B1 scenarios (2000, 2030, 2050): the resulting ozone fields for SRES-A2 and SRES-B1 were estimated (personal communication with B. Rognerrud). Within WP3.7-UVR, these have been translated into ambient UVR levels for Rome, London and Helsinki. The results show that UVR exposures are higher (and ozone recovery slower) in SRES-B1 compared to SRES-A2. The larger increase in ozone column in SRES-A2 can probably be explained by the higher abundance of CH₄in this scenario.