The US Environmental Protection Agency and the World Health Organisation regulate ozone concentrations; near the ground, ozone is a strong pollutant causing skin irritation and respiratory problems.
In the relatively pristine environment of Hawaii, science has observed increases in autumn ozone since 1973 – only recently have researchers found that this is largely due to climate variability.
The measurements of atmospheric ozone near the surface in Hawaii, taking place since 1973, comprise the longest record of ozone measurements in the USA.
As well as the length of the observational record, the location is sensitive to the interchange of air between the northern and southern hemispheres, making the record a particularly valuable one. Several decades ago, ozone amounts peaked in spring at about 55 parts per billion by volume (ppbv) and reached a minimum in the autumn at about 35 ppbv, dominated by the natural cycle.
However, since 1973, ozone has increased on average by about 10 ppbv in the autumn, but remained unchanged in spring. By contrast, in many remote sites, surface ozone has also increased. Until recently, researchers did not understand the autumnal increase in ozone.
Climate Model Simulations
Recent results, published on 26 January 2014 in Nature Geoscience, have simulated the increase in Hawaiian ozone. The study was a joint research effort by Princeton University and NOAA by Lin and coworkers. It was based on further simulations of the Geophysical Fluid Dynamics Laboratory (GFDL) climate model, which had previously been used for the 2013 IPCC climate assessment.
The GFDL model is one of the most advanced and comprehensive in the world, featuring relatively high spatial resolution of about 200 km (125 miles) as well as a detailed representation of atmospheric chemistry. Whereas many climate models have a similar resolution, and hence are capable of reasonably realistic representations of processes down to the 200 km size, such as individual weather systems, very few climate models represent atmospheric chemistry in a significant way. Consequently, the model is well suited to representing pollution episodes which give rise to increased ozone amounts.
Climate Model Results
In the figure, the September to October observations of ozone are given for the period 1975 to the present relative to the value in 1975. The mean observed increase is about 3.5 ppbv per decade. In comparison, two simulations of the climate model give increases of 2.3 ppbv per decade with fixed emissions, and 4.5 ppbv per decade when emissions increase as observed. The better simulation occurs when both climate change is included, and emissions increases are specified.
Moreover, the year to year variability in the observations, especially since 1990, is reproduced in the simulations thus providing confidence in the results. The region of enhanced ozone occurring specifically in the South Pacific is of particular importance to the Hawaiian Islands. Analysis showed that, in the model, ozone-rich air from Asia was transported by the winds to the South Pacific region.
In contrast, in spring, circulation changes masked the increase in pollutants from Asia, and ozone did not increase in Hawaii. Ozone has a lifetime of about 3 weeks in the lower atmosphere so the results show that changes which occur in the polluted Asian environment can influence the remote parts of the USA.
Implications For The Future
The Lin et al. study shows that local pollution amounts can be influenced by changes in large scale circulation patterns caused by climate variability and change. This is quite a complicated issue: The pollution from Asia is a problem because primarily of the change in winds, but with increased pollution adding to the effect. Temperature increase due to climate change (“Global warming”) is unlikely to have contributed directly to the increase of ozone in Hawaii. It is too early at this stage to determine whether the changes observed will be present for the long term. Nonetheless, as we have seen with the run of bad winter weather over the US and European continents in early 2014, global changes to the atmospheric circulation mean that the US in particular is not immune from the negative effects of climate variability and change.