Ozone and the Montreal Protocol

Introduction

One of the major anthropogenic influences on the atmosphere results from the release of ozone depleting substances (ODSs). Rowland and Molina (1974) warned against human-produced chemicals playing an important role in stratospheric ozone depletion, leading to a thinning of the ozone layer, thereby increasing the incidents of skin cancer and eye cataracts, but also affecting plants, crops and the ocean ecosystem. Observations confirmed the global ozone depletion, and revealed that the maximum ozone depletion occurred in the springtime Antarctic stratosphere, a phenomenon commonly known as the “ozone hole”. As a response to ozone depletion, the Montreal Protocol was established in 1987, which prohibited emissions of certain ODS into the atmosphere. In their latest report on the ozone layer, the World Meteorological Organization (WMO) and United Nations Environmental Programme (UNEP) projected that the reduction of ODS will lead to an increase in ozone in the 21st century, reaching pre-1980 levels in the second half of the century, with detailed recovery times depending on latitude (WMO, 2014).

Results and Discussion

To assess the effectiveness of the Montreal Protocol and its Amendments (MPA), we carried out two 140 yr long transient simulations with CCM SOCOL v.2.0 (Schraner et al., 2008) spanning 1960–2100 and driven by the prescribed evolution of the Sea Surface Temperature (SST), Sea Ice (SI), Greenhouse Gases (GHG), Ozone Depleting Substances (ODS) and source of CO and NO_x.

In the reference simulation GHG (N₂O, CH₄ , and CO₂ ) are taken from the IPCC (2001) “A1B” scenario. Surface mixing ratios of ODS are based on the halogen scenario A1 from WMO (2007) and on adjusted HCFCs scenario of nearly a full phase out in 2030. In the stratosphere background, non-volcanic sulfate aerosol loading is assumed, while the tropospheric aerosols are not considered.

For the second scenario simulation (noMPA), which was designed to estimate the role of the Montreal Protocol, we applied ODS from the so called world avoided scenario proposed by Velders et al. (2007) where ODS is increasing by 3% per year due to the absence of the limitation introduced by the MPA. The scenario starts in 1987 and shows what would have happened without any further national regulations, international agreements, or public actions. It is a transient simulation similar to the reference simulation, but with halogen loading evolution taken from the world avoided scenario throughout the simulation, whereas GHGs and SSTs/SIs are the same as in the reference simulation.

More results and discussion can be found in Egorova et al. (2013).

Conclusions

We evaluate the usefulness of international agreements such as the MPA on the basis of numerical experiments carried out with the modern chemistry-climate model SOCOL. Analysis of the simulated data allows the following conclusions to be drawn:

• In the absence of the MPA by 2100, the mesosphere and stratosphere cool down by 40K and 20 K, respectively, as a consequence of dramatic ozone depletion, which by the end of the 21st century could exceed 80 %. For the total ozone the simulated benefits of the MPA reaches 50% in the tropical latitudes and 70–80% in high latitudes of both hemispheres. For the global annual ozone benefits of the MPA reach 50% by the 2050 and 80% by the 2100. The tropospheric ozone depletion by additional halogens and less ozone influx from the stratosphere reaches 60–80%.
• Pronounced ozone depletion leads to: i) a substantial alteration of the strength of polar vortices by up to 40 m.s^-1 and latitudinal location by up to 20, ii) an increase of the BDC intensity by up to 100% and shift of the NAM to a negative phase. In the absence of the MPA, we model substantial changes in surface temperatures over Europe and the Russian Federation, and Poles as well as some changes in precipitation.

The predicted changes of the circulation and climate should be taken with caution because of the absence of an interactive ocean in our model. These issues therefore require further investigation with more sophisticated models. When the Montreal Protocol limitations are not implemented, UV radiation undergoes a dramatic increase in the 21st century, with 5-fold increases in populated areas, corresponding to UV Index values in excess of 50 in the summer months.

In contrast, UV levels tend to decrease in the 21st century under the Montreal Protocol scenario, by 5% to 10% at middle latitudes with respect to the “pre-ozone hole” period. This decrease is partly due to an increase in total column ozone in excess of the ozone levels found in the 1960s, but also due to an increase in overall cloud cover, as estimated by CCM SOCOL. We find that the expected ozone increases and changes in clouds have quantitatively similar influences on future surface UV radiation levels. Therefore, cloud processes and their radiative impacts need to be further studied in CCM model validation studies to better constrain this important parameter. All our results confirm the important role of the Montreal Protocol in protecting the ozone layer and the Earth climate.

References