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LOTUS: evaluation of long-term ozone trends for the 2018 WMO/UNEP Ozone Assessment

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The Montreal Protocol, an international treaty signed in 1987, bans the production of chemical substances responsible for the dramatic depletion of the ozone layer. By 2006, it was clear that the thinning of this protective UV-shield had halted in the mid-1990s. However, it took until 2018 to conclude with confidence that stratospheric ozone concentrations are slowly rising again. The confirmation of this long-standing prediction is of great scientific and societal importance, and it was made possible by researchers at BIRA-IASB who initiated and coordinated the international community-wide activity LOTUS, under the auspices of the World Climate Research Programme.
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Scientific Assessments of Ozone Depletion

Ozone in the stratosphere shields humans and the biosphere against solar ultraviolet radiation that increases the risks of skin cancer, cataract and suppression of the immune system. The unpleasant discoveries in the early 1980s of a hole in the ozone layer over Antarctica every springtime, and of its rapid thinning over the rest of the globe, urged policy makers to sign the Vienna Convention in 1985 and the Montreal Protocol in 1987, thereby banning the production and emission of ozone depleting chlorofluorocarbons.

In addition, the research community was assigned to asses -every four years- the state of:

  • the ozone layer
  • the substances related to its depletion
  • climate change effects on the ozone layer

Those four-yearly assessments are coordinated by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP).

Halt of the depletion process

Subsequent measurements by various ground-based and satellite instruments showed that the depletion process slowed down and that ozone levels in the stratosphere stabilized around 1997. This crucial observation confirmed that the ban of chlorine- and bromine-containing gases had helped protect the ozone layer.

The next question then was whether ozone concentrations would be rising again towards their initial level as a result of the declining levels of harmful gases.

Ozone concentrations are rising again in the upper stratosphere

Finding an answer to this question proved to be very challenging because of:

  • the naturally occurring variability in the atmosphere
  • the uncertainties associated with the measurements and with the trend estimation methods

The influence of the latter two factors was reconsidered in the last two years by the international, community-wide research activity LOTUS (Long-term Ozone Trends and Uncertainties in the Stratosphere), carried out under the auspices of the World Climate Research Programme (WCRP) and its Stratosphere-troposphere Processes And their Role in Climate (SPARC) project.

Researchers at BIRA-IASB initiated, coordinated and contributed to LOTUS and, together with their international colleagues, they recently concluded that ozone levels have increased, by 1.5% between 2000 and 2016 in the upper stratosphere at middle Northern latitudes. This long-awaited positive result confirms that the recovery process of the ozone layer has indeed started and that the measures taken by the Montreal Protocol have been effective.

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Figure 2 caption (legend)
Regions in the atmosphere with increasing (red) or decreasing (blue) ozone concentrations between 2000 and 2016. Stipples indicate results of lower confidence. © LOTUS
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