Growing confidence in understanding the climate impact of aviation non-CO2 effects, says leading scientist

GreenAir Online

Friday 31 January 2014

It is readily accepted that in addition to CO2 there are other climate forcing effects caused by aircraft at high altitudes but scientists have been less clear as to the extent of the impact. However, according to Keith Shine, Regius Professor of Meteorology and Climate Science at the UK's University of Reading, there is now a growing confidence in the understanding of other contributory climate factors from aviation.

"The climate effects from the non-CO2 effects are a major contributor to the total climate impact of aviation - there is no doubt about this," Shine told a recent seminar on environmental risks to the aviation industry hosted by the University.

"However, uncertainty over the science has caused some within the aviation community to shrug their shoulders about the issue. But over recent years there has been some really significant understanding of the effects of NOx emissions, water vapour emissions and contrail forcing. So we are getting somewhere, although there remains uncertainty over the role of aerosols and sulphur emissions from aviation."

Aviation CO2 radiative forcing - a simple measurement of the climate effect - at 30 mW m-2 represents around 1.6% of the total CO2 forcing from all human activities. When non-CO2 effects (NCE) are included, aviation contributes 1.3 to 14% of the total radiative forcing, the wide variance indicative of the uncertainty level.

One of the reasons for this discrepancy, said Shine, is the disagreements between the various aviation inventories on the vertical distribution - the height - at which emissions take place. For example, the effects from water vapour and NOx have shown significant sensitivity according to the choice of inventory. "If we are to come up with estimates on the impact of aviation on the climate, we need to know to know where the global fleet is flying," he said. "This has quite an impact on the quantification of NCE."

Some effects, such as the increase in ozone (a greenhouse gas) as a result of NOx emissions, have a positive radiative forcing, i.e. a warming effect. This forcing tends to be short-lived - around a couple of weeks - and restricted to the hemisphere where the NOx is emitted. But this in turn leads to a decrease of methane, another powerful greenhouse gas, in the atmosphere and a longer-lived cooling effect, or negative forcing. Shine said the net likely effect was a positive forcing on an overall global mean. Although there is still some uncertainty, multi-model comparisons have led to significantly improved understanding. "We have made good progress in this area," he reported.

Whereas, CO2 emissions have a global impact wherever they take place, emissions of NOx are both latitude and altitude dependent. Flying higher leads to a larger warming net effect from NOx emissions than at a lower altitude, where the effect is likely to be a cooling.

Shine said recent research had confirmed that NOx emissions at lower latitudes, where much of the growth in aviation was taking place, tended to have a much larger positive forcing than NOx emissions at higher latitudes. "This regional and height dependency is now getting quite well mapped."

Aircraft contrails are another potential source of radiative forcing that have been subjected to considerable research. Some contrails are short-lived and have no impact, whereas others are more persistent and can evolve into clouds indistinguishable from natural cirrus, formation depending strongly on atmospheric conditions. Subtle changes in aircraft altitude or location greatly influence contrail formation.

Contrails can both reflect sunlight, providing a cooling effect, and trap infrared energy that leads to a warming effect. This makes calculating the net effect more difficult but, said Shine, an expanding database of contrail properties and occurrence was leading scientists to have a better handle on the issue.

"There has been a significant convergence over recent years in the estimates as a result of satellite observations and modelling, and we now have a much improved confidence that the actual value lies within stated ranges, despite the ranges still being quite large."

As well as being important in contrail formations, water vapour emissions are in their own right an important greenhouse gas. Earlier research had estimated a 3 mW m-2 forcing but with a very high range of uncertainty from 0.4 to 20 mW m-2. At the higher measure, this would have put water vapour impacts on a similar level to CO2 impacts from aviation emissions, said Shine.

The upper limit, he said, had now been revised down significantly as a result of further research at Reading that was supported by more recent work, and forcing was now estimated at 0.9 on a range of 0.3 to 1.4 mW m-2. "This has been a very useful clarification of a potentially large forcing impact from aviation."

One of the most challenging areas in climate science is measuring the impact of aerosols on clouds, revealed Shine. In aviation terms, these can be carbon aerosols emitted by aircraft or they could be sulphur gases emitted by the engines and condensed into aerosols. This is an area of huge uncertainty, he said.

Two recent studies in the United States (Gettelman & Chen, April 2013) and Germany (Righi, Hendricks & Sausen, May 2013) had found the effect of the aerosols was not on clouds at cruise levels but that the sulphate was affecting lower-level water liquid clouds, thereby making them more reflective of energy back to space and  indicating possibly large negative forcings. However, the value range was rather wide, he said, and this area would be the subject of much research activity over the coming years.

Weighing the impact of non-CO2 impacts from aviation remains a contentious issue, accepts Shine, and is largely dependent on a particular viewpoint. Using a Radiative Forcing Index, a measurement of total forcing compared to CO2 forcing that was introduced by the UN's Intergovernmental Panel on Climate Change (IPCC), the present-day value for aviation is around 2.8, meaning that aviation is responsible for a total climate impact 2.8 times greater than that from CO2 emissions alone. However, points out Shine, this neglects the fact that CO2 forcing is persistent - lasting for decades or centuries - whereas most of the NCE forcings are short-lived in comparison.

"You could come up with a CO2 multiplier but that depends on the choice of metric and the time period over which the effects are being looked at," he said. "If you are measuring the effects over, say, 20 years you will come up with one view but if it's over 100 years you will come up with another. This is a policy decision, not a science decision."

One metric is the Global Warming Potential (GWP) used by the UNFCCC to compare the effect of different emissions. Another is a metric developed at Reading called the Global Temperature-change Potential (GTP) that Shine suggests is more applicable for a target-based climate policy, such as keeping temperature change below 2C since pre-industrial times.

The table below shows the two metrics over two different time horizons. If there is a shorter perspective, say 20 years, then the effect of non-CO2 emissions is around four times higher than CO2 alone when measured using GWP but comes down sharply over 100 years. Using the GTP metric, the ratio is much closer to one. The ratio is even less if the estimates are included from the recent aerosol studies.

"There is uncertainty in this area but that is as much as to do with the policy we are trying to pursue as from a science point of view," said Shine.

Reading University is taking part in a European project led by Germany's DLR called REACT4C, which also includes Eurocontrol, Airbus and the UK Met Office. The project is looking at whether 'climate friendly' aircraft routings are possible to reduce NCE impacts and comparing the trade-offs with the most economic route in terms of fuel use and time, with a particular emphasis on transatlantic crossings.

According to Reading's Dr Emma Irvine, the pattern of upper-level winds over the North Atlantic, in particular the location and strength of the jet stream, influence both the optimal flight route and the climate impact of emissions from trans-Atlantic flights at cruise altitude. If the jet stream, which is strongest in winter, is in the right location then eastbound flights between the USA and Europe can be routed to take advantage of these strong tailwinds, reducing flight time, fuel burn and associated emissions. Conversely, aircraft travelling westbound will route to avoid the jet stream. However, under climate change, the mean location, wind speed and variability of the jet stream may change. Although the science is uncertain at present due to variances in climate models, said Irvine, it is possible the jet stream may move further north.

As part of a new project funded by the UK's Natural Environment Research Council (NERC), Reading is analysing upper-level winds in selected climate model simulations from the latest IPCC assessment. It is using optimum routing software to diagnose minimum-time routes based on the present-day and future climate in order to see how changes in upper-level winds will affect the optimal route location and flight times.

As well as contrail cirrus formation, Irvine and her colleagues are also beginning to study the potential of aircraft fuel freeze at cruise levels at higher latitudes. She said the impact of climate change was to cause a warming effect at surface level but a cooling effect in the stratosphere.

A strengthening of the jet stream in terms of wind speed would have the advantage of shorter flight times on east-bound transatlantic flights and therefore lower fuel burn and reduced emissions. However, said Reading's Dr Paul Williams, there is evidence to suggest that changes to the jet stream as a result of climate change could result in an increase in clear air turbulence suffered by aircraft on transatlantic crossings. Williams said around 50% of aircraft accidents were as a result of encounters with turbulence, and also led to injuries to hundreds of passengers each year. The annual cost to the industry was in the region of $100 million, he estimated.

Climate model simulations undertaken at Reading had shown clear air turbulence changes significantly when the atmospheric CO2 is doubled, said Williams, and within the North Atlantic flight corridor in winter, most measures show a 10-40% increase in the average strength of turbulence. They also show a 40-170% increase in the likelihood of encountering turbulence strong enough to dislodge unsecured objects.

"We conclude that climate change will lead to bumpier transatlantic flights by the middle of this century," he told the seminar and speculated that journey times, fuel burn and emissions could increase as a result of longer routes being taken to avoid turbulence.

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