Strøtanker om bæredygtighed

Luftfarten er stigende over alt i verden, og klimapåvirkningen fra luftfarten udgør en stadig stigende del af verdens samlede udledninger. Dette til trods for, at der arbejdes på teknologiske forbedringer over en bred kam, lettere fly, mindre luftmodstand, mere effektive motorer, iblanding af biobrændstof og omlægning af uhensigtsmæssige flyruter (kortere afstande), for ikke at tale om en gennemgang af de mange tilhørende aktiviteter på landjorden. Selv i de lande, hvor man har fået vendt udviklingen, så de samlede klimapåvirkning er aftagende, stiger udledningerne fra luftfarten.

Når det gælder klimapåvirkningen fra luftfarten, så er der ikke kun tale om selve CO₂-påvirkningen. Man taler tværtimod om en RFI-faktor, som er et mål for, hvor mange gange større den samlede klimapåvirkning er i forhold til selve CO₂-udledningen.

Når et tungt legeme som et fly med jetmotorer drives gennem atmosfæren med stor fart, sker der en række processer, som øger drivhuseffekten. I RFI-faktoren indregnes derfor en lang række faktorer ud over CO₂-udledningerne, som har indflydelse på luftfartens samlede klimapåvirkning, størrelser som ozon, partikler, gasser, vanddamp og skydannelser i flyenes kølvand (som ikke er det samme som motorernes udstødning).

Nogle af disse faktorer hører blandt de mindst kendte i klimaprocesserne – eller rettere man kender overordnet deres virkning, men stadig med en væsentlig usikkerhedsmargin. Man ser derfor i figur 6 en markant usikkerhed på de enkelte komponenter, men samtidig, at de tilsammen betyder markant mere for den samlede klimapåvirkning end blot den primært udledte CO₂-mængde.

Som det fremgår af nedenstående tekst fra Carbon Planet, ligger RFI i dag omkring 3,0, men med en række teknologiske nybrud, som mindsker de afledte klimapåvirkninger, vil forventes det at man i 2050 kan sænke den til omkring 2,5. Endda regner man i dag med en RFI på 1,9, fordi man endnu ikke har tilstrækkelig viden til præcist at kunne fastsætte drivhuseffekten for flyenes skydannelse.

I de seneste år har man set forsøg med at bruge en flybenzin iblandet biobrændstof – og i enkelte tilfælde flyveture på rent biobrændstof. For mange ville det være en forjættende tanke at kunne flyve klimavenligt. Men som EU også har måttet sande, så er klimagevinsten ved 1. generations biobrændstoffer i bedste fald minimal, samtidig med at den er med til at øge presset på fødevarepriser og fældning af regnskovsområder. Og selv hvis vi lykkedes at finde en form for 2. eller 3. generations biobrændstof, som vi kunne lave i store mængder uden at gribe forstyrrende ind i klodens madforsyning (måske enzym/algefarme i Sahara), så er den dårlige nyhed, at den sekundære klimapåvirkning, som RFI-faktoren medregner, stadig vil være der. Så selv den dag vi flyver på rent biobrændstof, vil flyvningen have en voldsom klimapåvirkning.

Meget få har hørt om RFI-faktoren, og man se ofte, at den udelades, når der tales om luftfartens klimapåvirkning. Jeg har derfor tilladt mig at indkopiere nedenstående tekst fra Carbon Planet,¹ som giver en god præsentation af RFI – Radiative Forcing Index.

Disse skystriber efter flyene er relativt kortlivede, men i 1992 anslog man, at 0,1% af Jordens overflade i gennemsnit var dækket af disse contrails, skyer dannet af flyene, og at dette areal i 2050 ville være steget til 0,5%.² Disse skyer tilbageholde jordens varmestråling og bidrager hermed til den globale opvarmning.

Yderligere er RFI markant højere om natten end om dagen, og om vinteren end om sommeren, ligesom RFI er markant stigende med flyvehøjden. Man kunne derfor alene ved at flyve primært om dagen og i lavere flyvehøjde end den typiske langdstance-flyvehøjde reducere den samlede klimabelastning ved flyvning markant (mod til gengæld at få større brændstofforbrug). Men som situationen er i dag, er der lang vej til, at der tages hensyn til den slags. Og som det fremgår af gårsdagens blogindlæg, EU indstiller CO₂-flyafgift midlertidigt for at fremme global aftale, har EU haft en stor modstand fra vedrenssamfundet overfor sig forsøg på at lægge en klimaafgift på den del af luftfarten.

Se også tidligere blog-indlæg: CO₂-regulering af europæisk luftfart fra 2012, Airbus udvikler fly med 50% CO₂-reduktion, Flyver på alger, Miljøbevidste er ikke nødvendigvis miljøvenlige og Erhvervslivet vil flyve mindre.

Se samtlige blog-indlæg i kategorien luftfart.

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Radiative Forcing Index

Uddrag fra GHG Emissions resulting form Aircraft Travelling, Carbon Planet 2009 (pdf).

However, an important consideration when dealing with aircraft emissions is that the effect of aviation is more than just that from CO₂ alone. The impact of aviation on the atmosphere needs to also include the effects of NO_x compounds, ozone, methane, water, contrails and particles which are all emitted from aircraft exhausts at the same time as CO₂. As these compounds are released directly into the atmosphere, their potential to effect the anthropogenic radiative forcing that naturally takes place in this region, is much greater than that for fossil fuel production, due to the longer residence times required for the latter. This effect is taken into account by the development of the radiative forcing index (RFI) which compares the total radiative forcing effect caused by aviation to that caused by CO₂ alone [7]:

In 1992, the RFI for aircraft was estimated at approximately 2.7, with an uncertainty of at least ±1.5 [7]. 3-D inventories for present and projected future aviation operations have been produced with permission of NASA’s Atmospheric Effects of Aviation Project (AEAP), the European Civil Aviation Conference’s ANCAT and EC Emissions Inventory Database Group (EIDG), and DLR (German Aerospace Centre) and used to predict changes in RFI up to the year 2050. RFI was deduced as 3.0 by 2015 but then drops to 2.6 for alternative technological scenarios by 2050 [7]. These studies revealed that the index can range from 2.2 to 3.4 within the year 2050 for the various subsonic aviation and technical options considered.

The UK Royal Commission of Environmental Pollution and the Commission of Integrated Transport [8] have both acknowledged that while there is continued debate and scientific review as to the exact value of the RFI, the present scientific evidence clearly indicates that RFI is an important factor towards the assessment of total GHG emissions from flying.

By definition, radiative forcing ranks the instantaneous effect of accumulated emissions up to a given point in time. Radiative forcing for aviation, represents the radiative forcing at a given time due to all prior and current aviation activity (accumulated CO₂ emissions, plus present day, short-lived impacts like contrails). Since different climate effects have different time scales, radiative forcing estimates can produce a misleading comparison of the relative contribution from short lived and long-lived effects.

A recent review by Forster et al. (2006) [11] suggested that the current RFI fails to account for resident time scales of emissions, and that this exaggerates the impact of non-CO₂ climate effects of aviation. Figure 5 displays predicted radiative forcing of CO₂ and non-CO₂ compounds from aircraft-induced emissions using a simplified carbon cycle model.

The scenario assumes constant current emissions as a basis (taking year 2000 as the reference point) and as a consequence, non-CO₂ RF’s are set to be in equilibrium with their environment, i.e. they do not change (the constant line as shown in Figure 5). Non-CO₂ residence times in the atmosphere are much lower than CO₂. However, this is subject to high uncertainties and variances across individual components as shown in Figure 6 [12].

What results is a continual rise in CO₂ radiating forcing due to its long residence time, and hence a decrease in the radiative forcing index (RFI) over time.

The integrated RFI over a 100 year lifetime results in a value of ~1.8, a lower value than what is currently accepted by the IPCC. Unfortunately, the science behind non-CO₂ climate effects requires further assessment in order to accurately include them in the current RFI models.

Sausen et al. (2005) [12] provide estimates of the various contributions to the radiative forcing (RF) from aviation, presented mainly based on results from the TRADEOFF project [13], that updates those of the Intergovernmental Panel on Climate Change (IPCC, 1999) [17,14]. The new estimate of the total RF from aviation for 2000 is approximately the same as that of the IPCC’s estimate for 1992. This is mainly a consequence of the strongly reduced RF from contrails, which compensates the increase due to increased traffic from 1992 to 2000.

The RF from other aviation-induced cirrus clouds might be as large as the present estimate of the total RF (without cirrus). However, the present day knowledge on aircraft-induced cirrus clouds is poorly understood to provide a reliable estimate K.
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Apart from triggering linear contrails, aviation has the potential to change cirrus clouds in the following ways:

✴ Contrails do not always evaporate after short intervals (minutes). If the background atmosphere is sufficiently supersaturated with respect to the ice phase, they can grow to larger cirrus clouds, or contrail cirrus, which cannot be distinguished from natural cirrus clouds if their history is unknown. Mannstein and Schumann (2005) [15] estimated that the cover by contrail cirrus in central Europe is approximately 10 times larger than the cover by linear contrails.

✴ Aircraft directly emit particles (e.g., soot) and also precursors of volatile particles (e.g., sulphur oxides). These aerosols are eventually transformed into cloud condensation nuclei, which may trigger the formation of cirrus clouds much later than the original emission, if the background atmosphere has changed to a state allowing cloud formation (supersaturation). An observational proof of this effect is still lacking although theory allows this process.

✴ Aircraft-induced aerosols can additionally modify the micro-physical properties of cirrus clouds, change cloud particle sizes and forms, and the number of cloud particles. The result of such modification may include a change in the precipitation rate, in cloud lifetime and in cloud radiative properties. A quantification of this effect is still the subject of research.

Zerefos et al. (2003) [16], after removing the influences of natural phenomena found an increase in cirrus cover in high air traffic areas over Europe of +1.3% per decade, contrasting with a decrease of -0.2% per decade in adjacent low air traffic areas. Similar positive trends in cirrus cloudiness were found in other regions with high aircraft density.

Stordal et al. (2005) [17] extrapolated their result in time to cover a longer period of aircraft operations and on the global scale, assuming the radiative efficiency of cirrus to equal that of contrails. This yielded a “mean” RF of 30 mWm-2, associated with a large uncertainty range of 10 to 80 mWm-2.

Both the studies of Zerefos et al. (2003) and Stordal et al. (2005) are based primarily on correlation analyses. These studies can provide statistical evidence of an association between aviation and observed changes in cloudiness, but do not prove causality.

Coincidentally, the heaviest air traffic is found in regions where the subtropical and the subpolar jets are often found, i.e., the cloudiness in these regions is potentially sensitive to decadal natural climate variability and to anthropogenic climate change. Therefore, there are reservations in considering the RF above as a best estimate; and it is, rather, an order of magnitude estimate of the RF from aircraft-induced cirrus changes.

The assumption that the radiative efficiency of cirrus equals that of contrails is highly uncertain. Nevertheless, based on current knowledge the best estimate of aviation-induced cirrus RF is somewhere between zero and an estimated upper bound. Due to this lack of knowledge it is not yet possible to add the RF for aviation-induced cirrus changes to the total aviation RF. Evidently, if the actual value was close to the “mean” value of Stordal et al. (2005), the total RF would increase substantially, and consequently aviation’s share to the total anthropogenic RF.

In summary, the recent TRADEOFF study has updated the RFI figure and a value of 1.9 is now the best-quantified estimate of radiative forcing index of aviation emissions excluding the probable but unproven effects of cirrus clouds [13].

Hence, aviation CO_2e emissions in this calculator use the current consensus estimate by DEFRA and TRADEOFF for the radiative forcing index of 1.9.

Fra GHG Emissions resulting form Aircraft Travelling, Carbon Planet 2009 pp. 7-9 (pdf).¹

Contrails and Cirrus Clouds from Aviation, Stockholm Environmental Institute 2011.²

Geographic Phenomenon: Contrails, The Backyard Geographer 02.08.2012.

Boucher in Nature Climate Change: Contrails can evolve into cirrus clouds causing more climate warming today than all the carbon dioxide emitted by aircrafts, Francis’ world 28.04.2011.

Impacts of Aviation on the Climate, manchester metropolitan University 2012.

Mehri Hashemi Devin & Ali Akbar Sabziparvar: Aviation and Global Climate Change, 2nd International Conference on Environmental Science and Development, Singapore 2011 (pdf).

Contrail, Wikipedia.