The high-altitude effects of non-CO2 greenhouse gases caused by aviation are still uncertain, say scientists on GreenAir Online

Mon 6 Oct 2008 – Scientists studying the effects of aircraft contrails and non-CO2 greenhouse gas emissions from aviation, such as nitrogen oxides (NOx), report that whilst the science is improving they are still unsure of the magnitude of aviation’s impact on global warming. There are still major uncertainties that require further research and they advise policy makers to steer clear of basing decisions on so-called ‘multipliers’.

Their findings were presented to aviation stakeholders at a seminar held at the Royal Society in London last week organized by Omega, a collaborative body of leading UK universities engaged in researching the impacts of aviation on the environment. The presentations, led by Prof. David Lee of Manchester Metropolitan University’s Centre for Air Transport and the Environment, brought together the work of leading climatologists and atmospheric scientists from Europe and the United States following a meeting in Oxford in July.

Prof. Keith Shine of Reading University reported there was “exciting” work being carried out on the effect of oxides of nitrogen (NOx) in the troposphere – the lower part of the atmosphere – that was awaiting peer review and validation. “NOx has some particularly slippery issues we have to grapple with,” he said. “It is a very reactive gas in the atmosphere and has different impacts.”

The major effects are that NOx leads to the formation of ozone, a greenhouse gas, in the troposphere but above an altitude of 20km – the stratosphere – it causes ozone depletion. In addition, NOx leads to the formation of a hydroxyl (OH) radical that acts as a detergent which cleans many of the pollutants out of the atmosphere and leads to the increased rate of destruction of the harmful greenhouse gas methane. “So it turns out that by emitting one greenhouse gas into the atmosphere we are also partly destroying another,” explained Prof. Shine. “To add complexity to the subject, methane is one of the important molecules that lead to ozone formation, so NOx is creating more ozone but we are also destroying methane, so leading to less ozone.”

He said the effects of NOx also depend not just on the altitude of the emissions but also the geographical location of the emissions. “With CO2, it doesn’t matter where the emissions take place – whether it be the tropics or the North Pole, the impact is the same, which isn’t the case for NOx emissions.” Emissions of nitrogen oxides at the equator appear to have a greater effect than those at northern latitudes.

Another confusing aspect, said Prof. Shine, is that most of the present day commercial aviation fleet flies in the northern hemisphere and because ozone is fairly short-lived, most of the warming ozone effects are therefore going on in the northern hemisphere. However, because methane is a much longer-lived gas (a decade or so) its effects are felt globally.

Some of the impacts of NOx are short-lived, a timescale of about a week, whereas other effects have a very long lifetime, summarized Prof. Shine, which leads to the difficulties in reaching accurate conclusions on radiative forcing and metrics issues.

There is the compensation effect between ozone increase causing a positive radiative forcing and the methane decrease causing a negative radiative forcing but there remain differences between the scientists on the level of this compensation.

“Is our advice any clearer than it was a decade ago when the IPCC report came out? Probably not,” said Prof. Shine. “But what we want to put over is that although there is underlying uncertainty we do now understand the issues considerably better.

“We know that NOx emissions do produce ozone and do destroy methane. Over the past ten years we have built up a much better understanding of timescales and the geographical and altitude dependency response to NOx emissions.

Some of the difficulties that need to be overcome include isolating the impact of aircraft NOx and knowing just how much NOx is produced by aircraft engine exhausts compared to other anthropogenic sources such as road traffic, but also from natural sources. “There is still significant debate about how much NOx is produced in the immense heat of a lightning stroke,” said Prof. Shine.

He said the spread of results amongst models remains high and there is a need to know why different models come up with different answers. Understanding of the compensation between the ozone increase and the methane decrease is also something scientists have been struggling with for some years.

“Long term monitoring of the state of the atmosphere is crucial,” stated Prof. Shine. “The observing systems are not in place to tell us with precision what is going on, certainly within the upper troposphere.”

With regard to the advice he would give policy makers and aviation stakeholders, he said it depends on the question being asked. “This is important because if we are talking about short-term climate change over, say, the next 10 or 20 years, there may be a different answer to the one if the worry is about climate change over a 50- or 100-year period. If it is the latter, in my view the only thing we should be concerned with is CO2 emissions.”

Prof. Piers Forster from the University of Leeds says there is also major uncertainty about the effects of aircraft-induced contrails and cirrus clouds on global warming.

What we do know, he said, is that contrails reflect solar radiation, causing a cooling effect, and trap thermal radiation from the earth, causing a warming effect. There is though almost always a net warming effect, he pointed out. Scientists also have a better understanding of the thermodynamics of how and where contrails are formed.

There is research underway on what is called ice supersaturation and it is now possible to predict regions of ice supersaturation, those regions where aircraft contrails are persistent, using routine satellite meteorological instruments.

However, it is difficult to measure contrails from satellites. Prof. Forster would like to see aircraft fitted with sensors that could take measurements on, for example, outside temperature and humidity, and perhaps even rear-facing cameras to detect when and whether contrails are formed.

Prof. Lee echoed Prof. Shine’s observation that in trying to get to the heart of measuring the impacts of aviation on climate change and for scientists to provide answers, it is necessary to first define the question: what is the policy consideration, what is the context, what is the purpose of the metric and, most critically, over what timescale are the impacts to be measured.

He warned of the perils of applying these metrics to making policy decisions. “Depending on the metric you use and the time horizon, it is possible to show different non-CO2 impacts.”

RFI is the widely quoted metric that was used in the IPCC 1999 aviation study and measures the total radiative forcing caused by aircraft – primarily through CO2 and NOx emissions and contrails – against the radiative forcing of CO2 alone. It is used by some NGOs and politicians who press for tougher emissions regulations to be applied to airlines. However, pointed out Prof. Lee, if it was applied to shipping emissions – which European legislators are considering including in the Emissions Trading Scheme along with aviation – a negative RFI would result.

Although radiative forcing is a useful metric for scientists as it quantifies impacts, he says, it is not ideal for policy or regulatory purposes because it measures what has already taken place, not what may happen in the future. RFI should not be used as an emissions multiplier, he stated, nor should it be used in carbon emissions calculators. “It was never intended for these purposes.”

With regard to CO2, Prof. Lee says research carried out since the IPCC 1999 report, which said aviation was responsible for 2% of global emissions (based on 1992 traffic statistics), has been updated to show that in 2000 the percentage peaked at 2.7% but fell back to 2.5% in 2005.