“This paper updates our knowledge of what controls the global distribution of tropospheric ozone and methane, and how sensitive these gases are to human influences.”

It sets a benchmark for current global model studies of tropospheric ozone, and it is a synthesis of results from most of the groups around the world currently working on this topic. It is part of a set of papers that present various model results to do with both the evaluation and intercomparison of global tropospheric chemistry models. The overall study is the largest of its kind to date. The paper also makes quantitative forecasts out to the year 2030 of the likely range of impacts on air quality and climate, including inter-model and emission scenario uncertainties, so has direct relevance to policymakers.

The paper synthesizes results from several state-of-the-art models, and evaluates new ozone precursor emission scenarios. These scenarios have changed quite dramatically in the last few years, as it has been realized that air quality legislation is having (and will have) a major effect that was previously not included in emission scenarios. Some of the results are significantly different from earlier work (e.g., the global tropospheric ozone budget), thus challenging some previously held views.

In this podcast, David Stevenson, Senior Lecturer in Atmospheric Modelling at The University of Edinburgh, discusses the global distribution of tropospheric ozone and methane, and how sensitive these gases are to human influences. Podcast formats: mp3 | wma

Methane and tropospheric ozone are the second and third largest greenhouse gas contributors to radiative forcing of climate change to date. In the near future, they are likely to be of similar importance. Ground level ozone is also a major air pollutant which damages human health, crops, and natural ecosystems. This paper updates our knowledge of what controls the global distribution of tropospheric ozone and methane, and how sensitive these gases are to human influences.

By evaluating a wide range of future emissions scenarios with multiple models, the paper illustrates that global society has a clear choice to make: if serious efforts are made to reduce emissions using currently available technologies, ozone levels will fall and methane will stabilize, with clear benefits for both air quality and climate; alternatively, if current legislation is not followed and emissions are allowed to rise steeply, ozone and methane will grow, worsening air quality and adding significantly to the radiative forcing of climate change.

The research highlighted key areas of uncertainty in the models, such as the stratospheric source of tropospheric ozone, and the influence of tropical convection. I am working on these areas, and also trying to better understand the potential feedbacks that climate change will have on tropospheric chemistry.

As outlined above, the evaluation of different future emission scenarios offers starkly different futures, which presents society and politicians with clear choices.