Extratropical Climate Change in the Upper Troposphere and the Routing of Aircraft (EXTRA)

The upper troposphere in mid-latitudes is the region encompassing altitudes of around 8-12 km, which includes the jet streams, regions of very strong winds, which are closely related to strengths and paths of the mid-latitude depressions. Climate change is expected to change the nature of the upper troposphere at mid-latitudes - climate models indicate that over coming decades, it will warm, the relative humidity will increase and the strength and orientation of the jet stream might change, and the boundary between the troposphere and the overlying stratosphere (the tropopause) will increase in altitude. However, when different climate models are used to predict future climate change, there is a significant spread in the results they produce; the reasons for this spread are not fully understood.

Understanding climate change in the mid-latitude upper troposphere is of importance in its own right, but it has a wider economic significance. The cruise altititude of commercial aircraft is in the upper troposphere and flight times can be strongly affected by the wind conditions. Most obviously, the duration of flights between (as an example) London and New York are normally more than an hour faster when going eastbound, as the aircraft attempt to fly in the jet stream and receive an extra "push" - by contrast, westbound flights normally try to avoid the jet stream as this would impede progress. However, day-to-day variations in weather conditions in the north Atlantic mean that flight durations of both eastbound and westbound flights can vary by up to 100 minutes, depending largely on the strength and position of the jet stream. Since fuel use, and hence carbon dioxide emissions, are closely related to the flight duration, there are both economic and climate consequences for this variation. In our recent research we have shown that the weather in the upper troposphere in the North Atlantic can be split into characeristic patterns (5 in winter and 3 in summer) for which the aircraft routes are distinct. In addition we have shown that other climate effects of aircraft emissions (for example, contrails and ozone change resulting from emissions of oxides of nitrogen) very likely vary between these weather patterns.

Since aircraft routing is dependent on the weather situation in the upper troposphere, it is natural to ask whether climate change could impact on aircraft routing. There has been much research on the effect of aviation on climate change, but surprisingly little that asks the reverse question: what is the effect of climate change on aviation? Our proposal aims to answer this question, while at the same time improving understanding of upper tropospheric climate change. Since the aviation industry aims to put constraints on its carbon dioxide emissions, the effect of climate change on aviation routing could either assist or work against these aims.

We will consider how the routes of individual aircraft may be affected by the changes in the frequency of different weather patterns in the North Atlantic, predicted by a number of different climate models. We will exploit a recent, large and easily available set of simulations of possible future climate change from a range of world-leading climate models that have been produced for the fifth assessment report of the Intergovernmental Panel on Climate Change, which is currently being written. We will assess how well the climate models reproduce the present-day weather patterns in the North Atlantic and then look at how these patterns change for various possible future climates. We will then see how aircraft routing is affected by these weather patterns and compute the impact of this carbon dioxide emissions. We will also investigate the impact of both the climate change and re-routing on the other climate impacts of aviation.

We will extend this work to cover the North Pacific, which is expected to show a significant increase in air traffic over coming decades.

There are a very wide range of users associated with the aviation industry that will benefit directly from EXTRA. Possible changes in aircraft routing and associated changes in the climate impact of aviation will be of importance to airlines, air traffic control, aircraft manufacturers, airport operators and non-governmental organisations interested in the environmental impact of aircraft. There are industry-wide efforts (for example under the "Sustainable Aviation" initiative (www.sustainableaviation.co.uk/) to control the growth in aircraft emissions of carbon dioxide, and associated climate effects). Our work will add a new dimension to their considerations. Our work may also have implications for the management of air space, with consequent impacts on airlines, air traffic control, aircraft manufacturers and airport operators. Finally, there is an important economic aspect, if routing, and hence fuel use, is impacted by climate change.The airline industry is extremely competitive, often working on quite tight margins, and fuel use is one of their major operational costs.

With the likely imminent inclusion of international aviation CO2 emissions within the EU Emissions Trading System, our work will be of direct interest to national Government departments (particularly DECC and DfT), the European Union and to bodies that advise Government (such as the Committee on Climate Change (CCC)), as well as the aviation industry itself.

Other atmospheric science researchers outside of academia, either specifically engaged in aviation and climate research, or more generally in climate change research, such as the Met Office (and particularly the Hadley Centre) will find this work of importance, and indeed it will contribute to generic understanding of atmospheric processes beyond its specific aviation focus.