Polar Ozone Depletion and Subtropical Precipitation

Research conducted by S M Kang andcolleagues this year has further demonstrated the varied climatic influence polar ozone depletion can have; not just in Antarctica but extending to the tropics.

Kang and colleagues compared the results of four contrasting models to the observed trend in global precipitation between 1979-2000 (years in which ozone depletion has occurred). Observational data showed that in austral summer (December to February) precipitation was increasing between the latitudes 15S and 35S. 

The models were constructed using the Canadian Middle Atmosphere Model (CMAM); one was coupled with ocean-atmosphere interactions and the other uncoupled – therefore using prescribed data on sea surface temperature and other parameters. Another two models were run using the NCAR Community Atmosphere Model, again one was coupled to atmosphere-ocean processes and the other was uncoupled. This fourth model also only had the inputs of ozone depletion southwards of 40S latitude in order to clarify whether polar ozone was in fact the main driver of the results.

All four models spatially agreed with the observed trend that subtropical precipitation was increasing (Figures 1 and 2). This agreement across varying models added to the robustness of the study’s findings and interpretations, which was vital due to the model dependent nature of precipitation results. Kang’s results supported that of previous studies in suggesting a shift in the southern hemisphere’s extra tropical westerly jet stream was responsible for changes in precipitation. This shift of the jet stream (Figure 3), suggested to be caused by polar ozone depletion, alters the storm track that brings increased precipitation to areas such as eastern Australia.

The most important conclusions from this study demonstrate that the hydrological cycle of the tropics can be dramatically influenced by interactions at high latitudes. From an anthropogenic perspective, changes in weather patterns that have been caused by ozone depletion again adds to the ever increasing list of feedbacks that human activity can cause.

Are you having a laugh?!

According to research carried out by A. R. Ravishankara and colleagues at NOAA (National Oceanic & Atmospheric Administration) Nitrous Oxide N2O (commonly known as laughing gas) is the "single most important ozone-depleting emission".

This statement is surprising considering N2O is not restricted by the Montreal Protocol and is therefore freely emitted at an estimated rate of 10.5million metric tons per year according to the IPCC.

N2O is produced by human activity in large scales as a by product in the use of fertiliser in agriculture and fossil fuel combustion. N2O is known to catalytically destroy ozone via a similar process to that of chlorine examined in a previous blog. Interestingly chlorine and nitrous oxide have a dampening effect each other's ozone depleting potential (ODP) by interfering with the chemical processes involved. This point is key to the understanding of the results provided by the study.

ODP allows for the comparison of the amount of stratospheric ozone depletion caused by the emission of a unit mass at the earth's surface to that caused by the release of CFC-11. For example, according to the World Health Organisation CFC-12 has an ODP of  1.03, therefore slightly more damaging to ozone than CFC-11

A 2D model used to calculate ODP showed that the (pre CFCs) 1959 ODP of N2O was at 0.026, compared to a year 2000 level of 0.017. This demonstrates a marked decrease due to the influx of CFCs into the stratosphere between the above years. By restricting CFCs through the Montreal Protocol the amount of chlorine in the stratosphere gradually reduces which in turn allows for an enhancement in N2O ozone depletion and it is therefore suggested that the ODP of N2O will increase to similar levels of 1959 (0.026), which is a 50% increase in ODP.

To compound this effect is that N2O is a potent greenhouse gas, in fact it has a global warming potential 300 times stronger than that of CO2. Although it is produced is smaller quantities than CO2 this two pronged effect on climate that N2O exhibits is hard to ignore. The graphs to the left demonstrate, using data from various studies and the models referenced by A. R.Ravishankara, that N2O (red) has a projected potential to reach almost 30% ODP of the total CFCs exhibited in the 1980s. The graph below that is Global Warming Potential (GWP) for 100 years, and this also shows the potential increase that unrestricted N2O emissions can have upon the earth's heat budget.



So what does the future hold for N2O?

Will N2O have the last laugh...?

Listen to this podcast, featuring an interview with A.R. Ravishankara to find out more.

Geoengineering and Ozone

The idea of geoengineering solutions to future climate change risks really appeals to me. Images of futuristic ships spraying aerosols into the stratosphere, reflective balloons ascending and man made clouds forming all sound like science fiction coming to fruition. However, with developing scientific knowledge and engineering capabilities, the need for research into testing and validating geoengineering methods along with their uncertainties and side effects are hugely important.

Simone Tilmes et al 2009 investigated the impact of geoengineered aerosols using a coupled chemistry climate model that predicted atmospheric changes from 2020 to 2050. Based on the injection of liquid sulphate aerosols (similar to that produced by volcanic eruptions) Tilmes et al assumed a constant stratospheric distribution. Of particular interest to me was their results in relation to projected ozone depletion.

The conclusion of the research was that the introduction of sulphate aerosols led to the increase of heterogeneous reactions in the stratosphere. In turn this caused the enhancement of the ozone depleting chlorine cycle over Antarctica. It was predicted that compared to a baseline scenario, the geo-engineering method resulted in delaying the recovery of the Antarctic ozone hole by 20-30 years.

Furthermore, models suggested Arctic ozone depletion rates would be larger than currently observed between 2040-2050 due to a modelled increase in the strength of the polar vortex (briefly explained in my blog The basics of ozone depletion). In addition to this, a predicted one to two fold increase in ozone depletion in the northern hemisphere could be expected.

Although the study did find an overall cooling of the troposphere with this geoengineering method it is clear from the issues arising with ozone depletion that problems may outweigh the benefits. When an increase of UV rays and delaying of the recovery of the ozone hole is considered there is certainly much for policy makers to ponder when addressing the issue of geo-engineering climate. In relation to the “Anthropocene” as a whole, this is certainly a stark example of problems we will encounter through man's role as “Stewards of the Earth System” described by Will Steffen et al.

ozone depletion and sun damage for whales

The blog this week focuses on the damage to skin caused by increased levels of UV rays, but not on humans. A study last year by Laura Martinez-Levasseur and colleagues found acute sun damage on the skin of several species of whales in the Gulf of California.

Using high resolution photography along with biopsies and statistical analysis, the research found that the amount of blisters observed on whales rose significantly between 2007 and 2009. Interestingly the characteristics of each species of whale determined the amount of sun damage. Fin whales exhibited the lowest amount of skin damage and the photoprotective role their darker pigmentation of skin provides was suggested to be the determining factor. However both blue and sperm whales showed similar levels of skin damage despite different pigmentation of skin. This was attributed to sea surface behaviour which is the amount of time spent at the surface during sunlight hours. Sperm whales have a darker skin colour; however they tend to spend approximately 7-10 minutes breathing at the surface compared to blue whales that spend just 2 minutes, thus increasing sun exposure. 

The researchers conclude that the results suggest the “thinning of the ozone layer poses a significant and rising threat to the health of our oceans whales”.

This study was the first to measure the effects of increased UV rays on mammals in the wild. It is clearly important that more research is carried out on other species with particular interest to species migrating at high latitudes where ozone depletion is more pronounced. This biological feedback loop from a human induced phenomenon of ozone depletion once again demonstrates the consequences involved. We may have the scientific awareness to protect ourselves with sun cream (barring the occasional accident) however the majority of animals can not naturally adapt at the pace required with rapidly increasing UV rays.

Ozone depletion and Antarctic Sea Ice extent

Source: Turner et al 2009

This blog is a departure from the processes described last week into the modelling and measuring of indirect impacts that ozone depletion can have on the environment.

A study by Turner et al 2009, brought to my attention last week in the polar ecosystems lecture, suggests a link between stratospheric ozone depletion and sea ice extent (SIE) in regions of the Antarctic. Using satellite data it was found that there has been a statistically significant increase of annual mean SIE by 0.97% per decade since the 1970s. Two key areas of interest emerged in which SIE appeared to be declining (-6.63%/decade) over the Amundsen Sea and increasing over the Ross Sea (as shown on figure two).

In order to differentiate the anthropogenic forcing from the natural variability the researchers created 2 different models. One used pre industrial ozone levels and the other used ozone levels recorded from the year 2000.

Figure 3d from Turner et al 2009

When the two models were compared spatially over a map of the Antarctic (figure 3d) it clearly showed a “trough” of low pressure over the Amundsen Sea, to the west of the Ross Sea. This area of low pressure creates a cyclone which enhances the strength of winds flowing from the Ross Ice Shelf towards the coast and in turn maintains lower temperatures.  These combine to produce improved conditions for sea ice production, thus SIE is found to increase in the Ross Sea.

The authors of the research raise important issues on the projections of SIE in Antarctica. With particular reference to the estimated ‘recovery’ of the ozone hole by the mid twenty first century along with ever increasing green house gases. It is proposed that the observed increase in SIE over the previous decades will therefore be reversed.

It is interesting to think that this feedback loop from ozone depletion can actually have such a profound effect on SIE in Antarctica and I look forward to investigating further contrasting examples of feedbacks associated with ozone depletion.