You'd hardly credit it. There are umpteen articles in which Anthony Watts and his crew denied the ozone hole (it's existence and that human activity caused it - yep! Probably both at the same time.). Now he's now posted an article where it's real and blamed for global warming (archived here, latest here). Some new bloke called Steven Capozzola has written an article in which he says: "The evidence is compelling, and the subject deserves further scrutiny."
The evidence for what exactly? Well, that's not so easy to figure out. I'll see if I can step past the waffly bits to the nuts and bolts of his hypothesis. It goes something like this:
- A reduction in stratospheric ozone cools the stratosphere. That's correct. Ozone absorbs UV radiation from the sun and heats the surrounding air. There's a bit of a domino effect. The colder stratosphere means more polar stratospheric clouds form, which destroy more ozone than dry air does. As ozone levels rise again, this effect should reverse.
- Could the stabilization of ozone levels in the stratosphere help to explain the subsequent ‘pause?' in the temperature rise at the surface’: Steven posed this as a question. There is no evidence he sought an answer. The answer is "no". More on that below.
Some points about ozone in the stratosphere
I've already written some articles on ozone science, so I'll not repeat everything already written. Here are some basic facts about stratospheric ozone.
- The troposphere contains about 90% of all the air in the atmosphere. The rest is in the upper layers, with most of the rest in the stratosphere.
- Ozone is only about 10 parts per million by volume of the stratospheric atmosphere (which itself has much less air than the troposphere). By contrast, CO2 is around 400 parts per million in the troposphere.
- If all the ozone in the stratosphere were compressed at the surface of the planet, it would only be around 3mm thick, about the thickness of two five cent pieces or thinner (depending on your currency). (If all the CO2 in the troposphere were compressed at the surface of the planet, it would probably be around 400 times thicker.)
- The ozone layer in the stratosphere protects the surface by absorbing around 95% of UV radiation, preventing harmful UV radiation from reaching the surface.
- Ozone is created and destroyed by chemical reaction in the stratosphere. If not for human emissions of ozone destroying substances, the amount destroyed would balance the amount created.
- We inadvertently emitted long-lived ozone depleting substances, causing the ozone layer to thin, which increased UV radiation at the surface particularly at the poles. Ozone depleting substances have been progressively banned under a world-wide agreement called the Montreal Protocol, which was agreed in September 1987.
- Many ozone-depleting substances, and some of the replacements for ozone-depleting substances are potent greenhouse gases. Ozone itself is a greenhouse gas as well as being an absorber of short wave radiation (UV). In the troposphere these greenhouse gases add to global warming. In the stratosphere, they add to stratospheric cooling.
Here's a video showing what is expected to happen to stratospheric ozone as a result of the Montreal Protocol, with what would have happened if we'd continued to dump chlorofluorocarbons into the air.
What would have happened to the ozone layer if chlorofluorocarbons (CFCs) had not been regulated in the 1986 Montreal Protocol ? This NASA Goddard model runs from the year 1974 to 2064 and compares projected concentrations of ozone based on current emissions (left sphere), with the scenario we avoided if these emissions hadn't been reduced (right).
Here is another short video with Dr. Geir Braathen, answering some questions about the ozone hole. It's from a few years ago, back in 2009:
If you are interested in ozone, you can't go past the latest ozone report: Scientific Assessment of Ozone Depletion: 2014. It's to ozone science what the IPCC reports are to climate science.
Here are some more key facts I've learnt from Chapter Four of that report, on Stratospheric Ozone Changes and Climate. Direct quotes from the report are in italics.
Most of the observed drop in temperature of the lower stratosphere is because of the destruction of the ozone layer: Stratospheric ozone changes are the dominant driver of observed globally averaged long-term temperature changes in the lower stratosphere. Between 1979 and 1995 global mean lower stratospheric temperature decreased by about 1 K but has since remained approximately constant.
- When volcanoes erupt, the aerosols getting up to the stratosphere cause the lower stratosphere to warm (temporarily), and
- Greenhouse gases have had only a minor effect on the temperature of the lower stratosphere.
In the mid and upper stratosphere, greenhouse gases and ozone changes made comparable contributions to changes in temperature, though the magnitude of change is not certain. That's based on modeling. Observed mid- and upper-stratospheric temperatures decreased from 1979 to 2005, but the magnitude of the cooling is uncertain. A newly reprocessed data set of satellite measurements exhibits substantially different cooling trends compared to the existing data set.
Over Antarctica, ozone depletion has caused changes in the atmospheric circulation in the troposphere, with associated impacts at the surface and in the ocean. Among other things: Stratospheric ozone depletion has likely contributed to the observed expansion of the Southern Hemisphere Hadley Cell in austral summer.
In other words, weather is shifting poleward (south) in the southern hemisphere. Climate models simulate a poleward shift of the Southern Hemisphere midlatitude maximum in precipitation and a moistening of the subtropics in response to stratospheric ozone depletion in austral summer. There is some evidence of a consistent pattern of trends in observations.
In the northern hemisphere, there's not seen to be any link between climate (in the troposphere) and stratospheric ozone changes. It's all happening down south. No robust link between stratospheric ozone changes and Northern Hemisphere tropospheric climate has been found, consistent with the conclusions of the previous Ozone Assessment.
Water vapour in the stratosphere plays a very important role. It is important for Earth’s radiative balance (influencing stratospheric and tropospheric temperatures) and for stratospheric chemistry. Most of it comes from the tropopause in the tropics (thunderstorms) and from the oxidation of methane (CH4).
- Water vapor is the main source of reactive hydrogen oxide molecules (HOx) that destroy ozone, and
- changes in water vapor alter stratospheric temperatures (and hence rates of chemical reactions) and the formation of polar stratospheric clouds.
There's a diagram in Chapter 1 of the report that looks relevant, although it was included in the report to show how ozone-depleting substances (rather than water), get into the stratosphere. That is, mostly through convection in tropical regions:
How much did Steven get right?
I've also put together some facts relating to Steven's argument (summarised above):
Do greenhouse gas increases lead to stratospheric cooling? Short answer: yes, but...there is a lot of confusion about greenhouse warming and the temperature in the stratosphere. In the lower stratosphere, most of the fall in temperature observed in recent decades has been through ozone depletion. In the mid and upper stratosphere, the cooling is about equally from greenhouse gases and ozone depletion. It's the lower stratosphere that has the biggest impact on the troposphere - that is, on climate. For an explanation of why greenhouse gases cool the stratosphere, there is a good article at realclimate.org. It's to do with spectral absorption of CO2, and how it emits radiation in the much thinner stratosphere, with most just going up to space. This is different in the much denser troposphere, where long wave absorption by CO2, and other greenhouse gases like water, can warm the surface and each layer above and below it in the air.
Ozone destruction also causes stratospheric cooling: Ozone absorbs solar UV radiation and that heats up the (admittedly very thin) air in the stratosphere. There's a fair bit of air up there, but considerably less than in the troposphere, and its much less dense. Most greenhouse gases such as CO2 and water only absorb long wave radiation, not the short wave that comes in from the sun. (Ozone itself absorbs both long wave and short wave radiation.)
A colder stratosphere leads to more ozone depletion. One of the main reasons the ozone hole has been much larger over Antarctica is because the stratosphere there is colder in winter and spring. The importance of polar stratospheric clouds, and the chemical reactions that take place, are discussed in Section 3.3 of the 2014 ozone scientific assessment. There's too much detail to go into in this blog article, but the paragraph below, while not complete, will give you some idea of what happens. It's from an old article by Drew Shindle of NASA:
Ozone holes are caused by chemical reactions that take place primarily on the surface of polar stratospheric clouds, ice particles, or liquid droplets, which form at high altitudes in the extreme cold of the polar regions. The number of particles that form, and therefore the amount of chemical ozone destruction, is extremely sensitive to small changes in stratospheric temperature. Hence, even small amounts of stratospheric cooling can greatly increase ozone depletion.
Ozone depletion in the stratosphere is a negative forcing, not a positive forcing. Steven Steven Capozzola asked whether the slowdown in the rise in surface temperature could have been caused by a leveling off of the destruction of ozone in the stratosphere. I wrote up top that the short answer is no. This is why:
stratospheric ozone depletion since 1979 likely contributes a net negative radiative forcing of around −0.05 W m-2 ± 0.1 W m-2.
There's an article by M. Baker (University of Washington) on the website of the Union of Concerned Scientists, which discusses global warming and ozone depletion. In part it states:
Stratospheric ozone absorbs energy from the ultraviolet part of the solar spectrum, heating the lower stratosphere. This part of the spectrum accounts for less than one percent of the total solar energy reaching our atmosphere.  Stratospheric ozone is important because it prevent dangerous ultraviolet rays from harming plants and animals on Earth's surface, but reductions in the amount of radiation absorbed does not have a measurable impact on temperatures below.
In other words, the amount of ozone by itself doesn't make any real difference to global warming at the surface or in the troposphere. But it's not quite than simple (is it ever?). The amount of ozone in the stratosphere, particularly over Antarctica, does affect climate through winds, atmospheric circulation, oceans and quite possibly the amount of CO2 that's removed from the atmosphere in the Southern Ocean (a critical carbon sink). The 2014 scientific assessment has a section setting out projections, which include all sorts of interactions:
...involve a complex interaction between projected changes in GHGs, ODSs, and the climate system. While a declining stratospheric halogen loading is expected to lead to an increase in stratospheric ozone, GHG changes will also influence ozone concentrations through direct chemical effects, climate-induced changes in chemistry, and changes in the stratospheric circulation.Without the Montreal Protocol, the effects of depleted ozone would have been severe. Not just in regard to UV transmission to the surface, but in regard to the impact on climate. The 2014 ozone report discussed simulations of what might have happened if we'd done nothing:
Such simulations show significant stratospheric cooling in the next 20–50 years, with a maximum cooling in the Antarctic lower stratosphere in response to the increased ODS concentrations (e.g., Wu et al., 2013). This cooling would maintain the polar vortex year-round by the mid-21st century (Newman et al., 2009), resulting in no final warming in the Southern Hemisphere stratosphere.
Global warming and ozone depleting substances
Because of the Montreal Protocol, many ozone depleting substances that are also greenhouse gases have been banned. (While some have been replaced by other greenhouse gases, which is a problem.) However, they linger long. Overall, because ozone depleting substances don't break down readily, in the troposphere they add to global warming, and in the stratosphere they add to stratospheric cooling (and ozone depletion). The 2014 Ozone scientific assessment reports a recent study:
Garcia et al. (2012) found surface warming of over 2 K in response to enhanced ODSs in the tropics, 6 K in the Arctic, and about 4 K in Antarctic from 2000 to 2070 (Figure 4-20). This is of comparable magnitude to GHG warming under the RCP4.5 scenario (Garcia et al., 2012), indicating that global warming over next few decades could have been doubled in the absence of the Montreal Protocol.
Below is a figure from the report, which shows what we have probably avoided (b) in regard to increasing temperature, as a result of the Montreal Protocol:
There is a lot of information about ozone that I have yet to discover. And it's quite possible that I've not got everything right in this article - so feel free to point out any errors or omissions.
More that Steven Capozzola got wrong
At one point, Steven wrote about stratosphereric cooling and how it's tapered off a bit in recent years, as the ozone hole seems to be starting to stabilise (and may be even starting to recover). He wrote:
Unfortunately, at the IPCC, one hand seems not to know what the other is doing. And so it’s questionable whether the IPCC has considered the ozone variable when citing stratospheric cooling as an invalidator of the solar activity thesis.I'd say he hasn't read any IPCC reports. The latest one reflects what was in the 2014 ozone report - that in the lower stratosphere, ozone depletion has been having the biggest effect on temperature recently. In time though, it will be mostly CO2 all the way through from top to bottom.
From the WUWT comments
Stephen Wilde seems to be entertaining the wrong notion that "it's the sun". Deniers want to blame the sun for everything, don't they. (He doesn't explain why the sun would have suddenly done something different at precisely the time we started putting ozone depleting substances into the air.)
August 14, 2015 at 1:25 pm
If it was the sun that caused the decline in stratospheric ozone all along then there are potentially serious consequences for some.
Many WUWT deniers are sticking to the original WUWT script, and denying ozone science. For example, Leonard Lane wrote:
August 14, 2015 at 10:33 pm
Agree, I never did give much credence to CFCs and the “Ozone holes.”
RWturner has a fit of Dunning-Krugeritis, and wrongly thinks that the World Meteorological Organisation people don't know there are oceans on this planet. I suspect RWT doesn't know that by the "upper atmosphere" the WMO is referring to layers above the troposphere:
August 14, 2015 at 1:30 pm
the World Meteorological Organization (WMO) explains the prevailing sentiment against solar variability: “…if warming had been caused by a more active sun, scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere and a warming at the surface and in the lower parts of the atmosphere.”
Quick, someone inform the WMO about the oceans, their heat capacity, and their effect of global climate.
ulriclyons thinks that less sun means a hotter planet. Talk about denier contrarians!
August 15, 2015 at 10:35 pm (extract)
Wrong. Surface temperature initially increases with *decreasing* solar activty, by warming of the AMO and the associated drying of continental interiors, which is what happened from 1995. High solar activity levels do the reverse, particularly by stronger solar wind states as we saw in the mid 1970’s.
Rob is too easily impressed by WUWT "science":
August 17, 2015 at 12:28 am
This is potentially huge. Definitely one of the more intriguing topics of late.
References and further reading
Scientific Assessment of Ozone Depletion: 2014 - latest report from the World Meteorological Organisation
A Saturated Gassy Argument - an article by Spencer Weart, in collaboration with Raymond T. Pierrehumbert, which includes an explanation of upper atmospheric cooling as CO2 increases (see also the comment by Ray Pierrehumbert)
Is There a Connection between the Ozone Hole and Global Warming? - article by M. Baker at the Union of Concerned Scientists
Vertical Human Fingerprint Found in Stratospheric Cooling, Tropospheric Warming - article by Zeke Hausfather at Yale Climate Connections
From the HotWhopper archives
- The ozone hole and the scientific illiterati @wattsupwiththat - December 2013 (with some chemistry)
- WUWT is taking a break to indulge in ozone hole denial - March 2014
- Denier weirdness: WUWT denies human-caused ozone depletion - again! - March 2014
- Denier weirdness: Paranoid conspiracy blog WUWT branches out into ozone hole denial - September 2014
- Anthony Watts and his ozone hole deniers are out in force (again) - May 2015