At its July 2014 session, CAeM, which was partly held in conjunction with the Meteorology Divisional Meeting of the International Civil Aviation Organization (ICAO), decided to establish an expert team to tackle some of the challenges that the aviation industry is facing related to atmospheric science and climate. The ICAO Global Air Navigation Plan and Aviation System Block Upgrades provided a 15 -year forward vision of the global air traffic management system aimed at helping the industry cope with the pressing challenges of growth and related environmental effects. The meteorological and climatologic research communities can support this vision by providing their best possible estimates of potential climate change impacts. This information would enable aviation stakeholders to make informed decisions. While ICAO addresses relevant mitigation measures to reduce emissions by the sector, WMO will support the long-term adaptation strategies of aviation stakeholders.
Herbert Puempel, the Chairperson of the CAeM Expert Team on Science, Aviation and Climate, has been the WMO representative on the ICAO Committee on Aviation Environmental Protection (CAEP) since 2000. His explanations of the potential impact of climate change on aviation operations have been instrumental in raising aviation stakeholders’ interest in the related risks for the air transport sector. In this interview, Mr Puempel gives us a perspective of what flying through changed atmospheric conditions could look like in the near future.
Have challenges related to climate change been identified for the aviation industry?
The question of climate change impacts on aviation was addressed in the light of the Fourth (2007) and Fifth (2014) Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC). The aim was to identify impacts on aviation as a significant part of the transport sector. But there was a need to go beyond the interpretation of the “general” results of the two Assessment Reports, and look for specific scientific and user issues to be addressed in dedicated studies. Such studies are being carried out by several authors and we can now distinguish the impacts that will be caused by large-scale phenomena as well as small- to micro-scale effects.
What are the consequences of the large-scale phenomena related to overall temperature rise?
The scientific case for the effects of higher surface temperatures on aviation has been established and rests on a firm understanding of the physical processes involved in driving up these temperatures. The expected higher temperature maxima, coupled in some regions to higher values of specific humidity could have severe consequences on take-off performance at airports at high-altitudes or with short runways, limiting payload or fuel uptake.
These effects will require more detailed analyses for different regions, but will be a major concern for elevated airports in subtropical regions. The established method of scheduling long-haul departures for the cooler evening and night hours in some regions (the Middle East, and Central and Southern American high-altitude airports) will be further affected by reduced overnight cooling where high cloud cover, partially caused by long-lived contrails, is often present. In these cases, the warming effect of contrail-related cirrus clouds, which reduce radiative cooling at night, may have to be considered as an additional problem. This would reduce the already limited hours of operation even further in some regions.
For large-scale phenomena, what are the risks to air transport resulting from sea-level rise?
Linked to the higher temperatures, the consequential rise in sea level (through increased melting of ice caps and glaciers and thermal expansion of the oceans) is fairly well understood and documented. In regions with strong monsoons, tropical storms, sea-level rise and storm surges linked to more intense extratropical cyclones will threaten the viability of airports at coastal locations unless protective measures are taken. These effects are likely to be exacerbated in those regions by very intense precipitation linked to the storms. The intense precipitation can lead to flooding where rainfall runoff hits storm tides head-on, for example, the extreme floods that occurred in Myanmar during Tropical Storm Nargis. Effective planning of new airports in such regions requires hydrological, climatologic and technical expertise.
Is the aviation sector sensitive to global climate events such as El Niño and what adaptation measures could be put in place?
An in-depth analysis of the El Niño Southern Oscillation (ENSO) from the latest generation of climate models appears to support evidence from paleo-climatologic studies that point to an increase in the severity of El Niño. This trend may be visible in the 2015/2016 El Niño episode. Such high-amplitude El Niño effects will affect many regions of the world by exacerbating extreme droughts and heat waves. All these extreme situations will have strong negative impacts on all forms of transport, including aviation.
However, an understanding of the role of seasonal, inter-annual and decadal variations, such as ENSO, the North Atlantic Oscillation and other recurring phenomena, will require significantly more research efforts. Given the overwhelming amount of data resulting from climate model predictions, the initial approach to understanding future climate states was the analysis of a new quasi-equilibrium state predicted for the end of the 21st century, when climate would have settled at a warmer level in line with the increased CO2. This new equilibrium state was described in latitudinal and regional means of temperature and precipitation over extended periods of time to isolate, sometimes, conflicting signals. However, many climate model predictions exhibit noticeable biases in some regions and parameters, for example, in the Equatorial Pacific Ocean temperatures, when compared to the current climate.
Adaptation measures need to address future mean state, and local and regional extremes likely to occur over the next decade(s). Such extremes may already be exhibiting typical conditions that we only expected to become regular features by the end of the century.
To provide robust scientific advice to stakeholders, the scientific community will need to address typical scenarios and try to describe impacts linked to those scenarios. As an example, we may consider the emerging evidence of a sequence of high-amplitude, low wave atmospheric flow regimes in none El Niño years. For example, over the East Atlantic and Europe, these regimes have led to paradoxical occurrence of intense snowfall and low winter temperatures over the east coast of North America and large areas of Europe. This evidence was contrasted by a significant northward displacement of the westerly jets with very mild temperatures during the extreme El Niño years, which are probably closer to what the earlier, average-based predictions gave (high rainfall and strong winds over northern latitudes, and drought in the Mediterranean region). The preponderance of extended periods of quasi-stationary, large-amplitude planetary waves may persist, even in such years.
What about the potential impacts of smaller scale local phenomena, which affect flight safety?
Scientific research into future impacts of climate change on aviation encounters a problem in that many high-impact weather phenomena are linked to space and time scales far below those resolved by current forecast models. This problem is even more pronounced when using much coarser climate models, so that intelligent ways of downscaling, statistical post-processing and advanced methods of conceptual models would be needed to derive at least statistically reliable results for small- to micro-scale phenomena. This relates to high-impact weather phenomena such as convection and related effects ranging from low-level wind shear to hail and lightning strikes, clear-air turbulence (CAT) and mountain-wave turbulence, as well as turbulence near thunderstorm tops, icing and low-level wind shear, and low visibility and ceiling.
Improving our physical understanding of the generation of small-scale rotational movements in the atmosphere that play a part in reducing vertical wind shear – experienced as turbulence of varying intensities by crew and passengers – can help. For example, although CAT occurs on a micro-scale, our physical understanding tells us that the wind shear that generates it is driven by much larger scales. It is, therefore, potentially resolvable by the current generation of weather and climate models. More basic scientific research is needed to improve our understanding of these small-scale effects. This will require better atmospheric observations and operational data from aircraft (for example, for turbulence).1
Another area of research is in the changing behaviour of atmospheric jet streams as a response to climate change. The mid-latitude jet stream in each hemisphere is created and sustained by the temperature difference between the cold polar regions and the warm tropics. Climate models, satellite observations and physical theory all suggest that this temperature difference is changing in a complicated manner. It is decreasing at ground level because of polar warming, but it is increasing at flight cruising levels because of lower stratospheric cooling. One possibility is that changes in the prevailing jet stream wind patterns may modify optimal flight routes, journey times and fuel consumption. Another possibility is that increased shear within the jet streams at cruising levels may reduce the stability of the atmosphere and increase the likelihood of CAT breaking out.
Is there any research linking other weather aviation hazards, such as icing or sand/duststorms, in the context of climate change?
Airframe icing is traditionally seen as a problem for general aviation and, more specifically, for commuter aviation where there is limited engine power and rudimentary anti-icing devices. It nevertheless needs to be better understood in order to predict future scenarios. The presence of large supercooled droplets at a temperature range between −4 and −14 °C depends on a number of conditions. These include the availability of large amounts of water vapour, typically meso-scale bands of intense updrafts and a limited concentration of suitable aerosols acting as condensation nuclei, favouring the formation of large supercooled droplets.
The general warming trend and increase of moisture in some latitude bands, with a more active dynamic of the flow, all point to an increased chance of occurrences of conditions favourable to icing. They also lead to an upward extension of the upper limit of icing layers due to the higher temperatures.
High-altitude icing is caused by ingestion of a high density of icicles at very low temperatures (below −50 °C) in the vicinity of convective cloud tops with ice contents in excess of 5 g/m3 of air. It is likely to increase with more intense cumulonimbus clouds and a rise of the tropopause due to the higher temperature and moisture of tropical air masses. The most energy-efficient modern (lean-burn) aviation engines appear to be more susceptible to these events than older, robust but thirstier turbines.
The likely increase in the occurrence and intensity of sand- and duststorms, caused by longer drought periods and potentially stronger winds in subtropical latitudes, will require a thorough analysis of the impacts on safety and regularity of flights. There is emerging evidence that the drive to higher engine efficiency (not least in order to reduce specific fuel consumption!) has pushed the operating temperatures in the combustion chambers of the most modern engines towards temperatures in excess of 1 600 °C. At these temperatures, the silicates contained in typical sand- and duststorms when sucked into the engine would melt and, thus, in a way similar to the volcanic ash, affect the performance and maintenance requirements.
Should the air transport sector consider climate change related risk management?
Aviation is exposed to weather phenomena not only on the ground, but also at levels up to the higher troposphere and lower stratosphere. It probably has the strongest tradition of prioritizing safety in the transport industry and is thus a prime candidate for developing sound and balanced risk management.
Aviation is probably the only reliable means of disaster response and relief in cases of large-scale disasters. For example, it will be unrealistic to maintain or repair hundreds or thousands of kilometres of roads or rail connections across areas affected by flooding, landslides, fires or storms in order to bring relief and aid to those affected. Adaptation measures and risk management, therefore, need to pay particular attention to the hardening of aviation infrastructure to ensure a robust and sustainable relief mechanism.
How does CAeM and the aviation community plan to turn insights into actionable guidance?
International organizations such as ICAO or the European Aviation Safety Agency need to develop guidance material and best practice models to support risk management. These should involve all stakeholders, from operators, pilots, airport managers and manufacturers to governments and national safety regulators. A multidisciplinary effort by scientists and operational and safety experts could contribute to the drafting of such guidance material together with operational and safety experts. It will be important for the guidance material to be regularly revised and updated to reflect evolving and changing climate statistics.
The wider community, including stakeholders such as ICAO-CAEP, Eurocontrol, the airports council and aircraft and equipment manufacturers, has been in close contact over recent months and years. There is a growing consensus that a multidisciplinary workshop leading to guidance for adaptation should be held in the near future.
1 Research on the CAT regime changes with conclusive results has been conducted by Paul Williams and Manoj Joshi (www.met.rdg.ac.uk/~williams/publications/nclimate1866.pdf)