The ozone layer shows first signs of recovery, but …

By Michaela I. Hegglin

Just over two weeks ago, the United Nations (UN) held a press conference in New York to announce the release of the Assessment for Decision Makers (ADM), a summary document of the WMO/UNEP Scientific Assessment of Ozone Depletion 2014. The report is the work of a UN panel of 300 scientists from around the world (including four scientists from our Meteorology department) and represents the latest comprehensive update on the state of the Earth’s ozone layer, which protects the Earth from the Sun’s harmful ultraviolet radiation.

Figure 1: The ozone layer is on its way to recovery. The past evolution of ozone observations is well understood and can be modeled by complex chemistry-climate models (CCMVal-2 simulations in black with uncertainty in grey). Pink curve shows the potential of ozone-depleting substances to destroy ozone in the stratosphere (EESC; pink). Other colors show ozone evolution for different representative pathway scenarios (RCPs). (Source: ADM, WMO/UNEP ozone assessment, 2014).

The encouraging finding of this year’s Assessment is that the ozone layer is showing first signs of recovery. A reduction in ozone depletion is expected given the decline in stratospheric chlorine abundances by 10-15% since peak values in the late 1990s/early 2000s. However, the detection of ozone recovery has been anything but trivial, since the signal has to be disentangled from natural variability, enhanced ozone depletion after the Mt Pinatubo volcanic eruption in 1991, increases in tropospheric ozone, and the impact of climate change, all of which affect total column ozone in addition to ozone-depleting substances. While the observed increase in total column ozone seen in Figure 1 is consistent with model predictions, the authors of the Assessment were not ready to attribute it with high confidence to the decline in ozone-depleting substances (although a subsequent study by University of Reading authors has now done so*). Therefore the Assessment states that there are ‘indications of recovery’ and not yet recovery itself.

Astonishingly, there are skeptics who deny ozone depletion has ever happened. Reactions such as ‘Oh, so the ozone hole was just yet another scare-story of the environmentalists, such as acid rain and climate change?!’ were to be heard days after the ADM-release. Well, we definitely know better. If the world would not have reacted quickly to the threat from ozone-depleting substances, we would be in serious trouble now. At the time of the Montreal Protocol, ozone-depleting substances were set to grow at a rate that by today would have caused at least twice the ozone depletion that we have experienced (see Figure 2). Stopping and reversing the growth of the atmospheric concentrations of ozone-depleting substances has thereby helped avert an estimated 2 million skin cancer cases per year by the year 2030.

Figure 2: The world avoided. Without the Montreal Protocol, the global ozone layer would have experienced serious depletion around the globe. The change from red in 1989 to green-yellowish colours in 2028 indicate a thinning of the ozone layer by around 20%  (credit: Paul Newman, NASA Goddard, USA)

The bad news… Although the Montreal Protocol has averted the worst outcome of ozone depletion, the ozone layer is never expected to return to a pristine state. The culprit is climate change as discussed in the ADM.  Models predict that the ozone layer will be strongly affected by greenhouse gases due to both physical and chemical mechanisms. Nitrous oxide (N2O) and methane (CH4) both affect ozone chemically. Carbon dioxide (CO2) cools the stratosphere and also leads to increases in stratospheric ozone due to a slowing down of chemical loss reaction rates, ironically ‘helping’ the ozone layer to heal. Finally, the combined greenhouse effect of these gases increases the strength of the stratospheric overturning circulation. The strengthened circulation leads to a decrease in total column ozone in the tropics, and to an increase in the extratropics, the extent of which is dependent on the greenhouse-gas scenario the world will follow into the future (see Figure 1). However, too little UV radiation (as a consequence of a thicker ozone layer) can have adverse health impacts too, especially at higher latitudes where it is well known to lead to Vitamin-D deficiency and ailments such as rickets.  But because the ozone decrease expected from climate change will be in the tropics, it may this time affect people who are least educated about the ozone layer, and who have the least means to protect themselves.

The work is not done yet, moreover, since the substitute gases currently used by industry to replace the ozone-depleting substances are themselves strong greenhouse gases and if left unabated may contribute 10% or more to the climate forcing from CO2 by the year 2050. The problem is identified and industry seems to be at least prepared to investigate solutions. The latter was a key aspect throughout the process of the Montreal Protocol: bringing all stakeholders to the table — scientists, industrialists, economists, legal experts and policymakers — to find a compromise on how to deal with this global environmental issue. It took almost 30 years before the policy action has borne fruit. But full recovery of the global ozone layer is not expected before the middle of the century, and even later in the Antarctic –  a reminder of how long environment damage can accompany us. Even more so, the Montreal Protocol should give us hope that humankind is able to tackle climate change, and its mechanisms may well serve as an example of how to do it (see also commentary in the Guardian

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