UV higher than normal?

Maybe a bit. But it’s the summer that matters ...

As I’ve noted in earlier posts, the amount of UV radiation reaching Earth’s surface tends to be low in spring because there’s more ozone overhead in that season.

That’s still true this spring, but it’s just been reported that UV levels have been slightly higher than in past years because the springtime peak in ozone is not as pronounced as usual. You can see the ozone anomaly in the graph below, which I’ve copied from NIWA’s very useful Ozone/UV web pages.

The graph - updated since the press release - shows that in the three months or so since the middle of July (more than 100 days), ozone amounts measured with our Dobson spectrometer (red points) have exceeded the seasonal average (blue line) only twice. The rest of the time they’ve been below the mean. On several days - especially in mid-August - ozone values this year were well below the previous absolute minimum since satellite data became available over 43 years ago. The anomaly is also visible, though not quite as obvious, in the lower-accuracy ozone measurements from the UV spectrometer (black points). The spring maximum at Lauder is normally about 360 DU, but this year it’s closer to 330 DU. As a result UV values at Lauder this spring should have been about 10% greater than normal, all other things being equal. But in the case of UV, all other factors aren’t usually equal. Cloud differences - for example - come into play.

Reasons for the persistently low ozone values are not clear, but may be related to the eruption of the Hunga-Tonga-Hunga-Ha'apai volcano in Tonga on 15 January 2022. The eruption injected unprecedented amounts of water vapour directly into the stratosphere, which can lead to chemical reactions that quickly destroy ozone.

I first heard the story on the Radio New Zealand news of 21 November, which included quotes from my NIWA scientist colleague, Richard Turner. No problems there. But I was disappointed to hear TVNZ’s take on the story in the TV1 News. In their efforts to convert news to infotainment, they obtained a couple of sound bites from a spokesperson for the Cancer Society. While they might have sounded OK, they were unfortunately both wrong, and perpetuated longstanding myths.

It’s not true to say New Zealand has “some of the world’s lowest ozone values” (particularly at this time of year). They are indeed lower than at northern mid-latitudes, but about half the globe has lower ozone than New Zealand, and the lowest ozone values over New Zealand are three times as large as in the Antarctic spring.

It’s also not true to say the “New Zealand has some of the highest UV anywhere in the world”. Granted, it’s much higher than at northern mid-latitudes where the sensitive skin types of the European population evolved. But, as we noted eight years ago, unpolluted locations in about half the world’s area (essentially everywhere between latitudes 30 N and 30 S, and high altitudes outside that range) experience UV levels higher than in New Zealand. The highest levels anywhere in the world are nearly double those where I live. It’s a pity the TV News programmers didn’t consult people who understand these things.

How much did these ozone changes really affect our UV? The plot below was obtained from UV spectrometer data at Lauder. It shows all UVI measurements derived from spectra taken within an hour of solar noon each day of the year. The grey points are from all years from 2006 to 2021, a period when ozone values have been quite stable (after a decline late last century). The red data points are for the current year.

The tight groupings near the top of the envelope correspond to clear-sky conditions, and the lower groupings correspond to conditions where clouds block direct sunlight. The graph shows that the effects of those low ozone values really wasn’t a big deal, especially in the context of the large seasonal changes and cloud effects.

The red data points from this year have pretty much the same distribution as the black points for other years. As expected from the ozone differences, they have been slightly higher in recent months than in previous years at the same season, with the red clear-sky points being closer to the upper edge of the grey clear-sky cluster in recent months.

But it’s worth noting that the skin types of many Kiwis evolved for the much lower UV conditions present in the British Isles. At Lauder, near the southern tip of New Zealand, UV levels are already approaching the summer maxima that occur in the UK. And by summer, they will be twice as high and far exceed UK levels. You’ll see in the plot that the maximum for Lauder is UVI =13. It’s a bit higher in the north of the country, reaching perhaps 14 at times. But, although that’s much higher than in Europe, it’s nowhere near the global maxima of around 25 that occur at high altitudes in the tropics.

But, if these lower levels of ozone persist through the summer months, there will be an important story to be told. Our summer UV is already far too strong for many Kiwi skin types.

Should we forgive TVNZ and the Cancer Society for promulgating those false claims? I don’t think so.

Of course it’s understandable for TV1. Their ‘news’ is - after all - really just a vehicle to sell advertising. For that purpose the entertainment part may even trump truth, especially if it implicitly contains the catch-all “we’re all going to die” message.

The Cancer Society’s motives are much more noble. They’re just trying to prolong our lives by alerting us to the dangers of UV in the upcoming summer.

But both really should stick to their knitting and leave the atmospheric science comments to specialists. Inaccuracies like those reported are anathema to reputable scientists. Association with such falsehoods can cause credibility earned over years to evaporate in a flash.