From STOCKHOLM UNIVERSITY and the department of negative feedbacks comes this surprising finding that says not only do contrails add reflectivity for incoming solar radiation, they also increase reflectivity for other nearby clouds.
Clouds may have a net warming or cooling effect on climate, depending on their thickness and altitude. Artificially formed clouds called contrails form due to aircraft effluent. In a cloudless sky, contrails are thought to have minimal effect on climate. But what happens when the sky is already cloudy? In a new study published in the journal Nature Communications, scientists at ACES and colleagues from the UK show that contrails that are formed within existing high clouds increase the reflectivity of these clouds, i.e. their ability to reflect light. The researchers hope that their discovery offers important insights into the influence of aviation on climate.
“Normal contrails are the stripes you sometimes see behind high-flying aircraft. Lots of times these contrails disappear fairly quickly. Other times they stick around for a while, and even spread out, sometimes considerably. There has been a lot of work done to find out how these form, and what kind of climatic effect they have – which is estimated to be rather small. Figuring out what kind of effects airplanes have while flying through clouds that are already present in the atmosphere has been much more difficult,” says Kevin Noone, Professor at ACES.
The researchers used a combination of flight tracking data and satellites equipped with sensitive lasers for detecting small changes in cloud optical thickness, i.e. the degree to which a cloud prevents light passing through it. When they looked at flight tracks from Honolulu to LA and Seattle to San Francisco, they found a significant increase in the optical thickness of the clouds close to the flight tracks compared to those further away. In other words, the clouds close to flight tracks were more reflective or “brighter.”
” Such effects only occur in certain latitude bands, so aircraft flying on polar routes in the Northern Hemisphere and close to the Equator are unlikely to produce these sorts of clouds. The most important areas are in the Northern and Southern mid-latitudes. Work is in progress to calculate the climatic effects of the changes we’ve observed,” says Kevin Noone.
Aviation effects on already-existing cirrus clouds
Matthias Tesche, Peggy Achtert, Paul Glantz & Kevin J. Noone
Determining the effects of the formation of contrails within natural cirrus clouds has proven to be challenging. Quantifying any such effects is necessary if we are to properly account for the influence of aviation on climate. Here we quantify the effect of aircraft on the optical thickness of already-existing cirrus clouds by matching actual aircraft flight tracks to satellite lidar measurements. We show that there is a systematic, statistically significant increase in normalized cirrus cloud optical thickness inside mid-latitude flight tracks compared with adjacent areas immediately outside the tracks.
Air traffic is known to have an immediate and noticeable effect on clouds in the upper troposphere. New clouds that form due to aircraft effluent are called contrails1, 2, and may develop into more persistent and widespread contrail cirrus. Boucher3 was the first to realize that aviation might have a strong influence on the occurrence rate of cirrus clouds. Previous studies of contrail optical properties are either based on passive remote sensing in which contrails are identified as linear features in scenes of brightness temperature differences4, 5, 6 or modelling studies in which contrails are formed when favourable meteorological conditions are reached7. The life cycle of contrails and aviation-induced cirrus, their radiative forcing and feedback on natural clouds have been studied by treating them as an independent cloud class in a climate model8. The study by Iwabuchi et al.9 is the only one so far that has used height-resolved observations from space-borne lidar measurements to investigate the physical and optical properties of contrails. In their approach, the authors used passive MODIS (moderate resolution imaging spectroradiometer) observations to identify contrails for a subsequent detailed analysis of CALIOP (cloud-aerosol lidar with orthogonal polarization) observations.
In general, aviation-induced clouds (that is, contrails and contrail cirrus) have been found to be optically thin10, 11, and their climatic effects have been estimated to be minor4, 12, 13, 14, 15 even when considering their entire life cycle8, 16. The effect of contrails embedded in natural cirrus is a mechanism that currently has neither been studied nor assessed for its radiative effect on climate8,15, 16, 17.
While optically thick cirrus clouds have a net cooling effect on surface temperature, optically thin cirrus clouds, like greenhouse gases, can have a warming effect15, 18. Aircraft emissions and contrails at cirrus altitudes have the potential to either cause optically thin cirrus clouds to form (that would have a warming effect on surface temperatures) or increase the optical thickness of existing clouds (or induce new optically thick clouds), thus, causing a net cooling effect. Enhanced observations of the effects of aircraft on cirrus cloud properties are needed to help bound and quantify these possible effects.
The aim of this study is to test the hypothesis that contrails formed within natural cirrus clouds have no measurable immediate effect on cirrus optical depth inside and outside flight tracks in the upper troposphere. We combine data of aircraft flight tracks with spaceborne lidar observations to investigate the effect of aviation on the optical thickness of already-existing cirrus clouds. We detect a statistically significant 22% increase in normalized cirrus optical thickness in mid-latitude air traffic flight tracks compared with adjacent areas outside the flight tracks.
Figure 1 illustrates our approach. Typical flight tracks for connections between Seattle (KSEA), San Francisco (KSFO), Los Angeles (KLAX) and Honolulu (PHNL) are shown as thick coloured lines. CALIPSO orbits are indicated as thin grey lines in the figure. The inset shows normalized COT (nCOT; see below) at 532 nm across each of the flight corridors between Los Angeles, San Francisco, Seattle and Honolulu. In cases 1 and 2 aircraft had passed the area <30 min before the CALIPSO overpass. In case 3, CALIOP observed the location of the flight track before the passage of the aircraft. For these cases cirrus clouds were present at the flight level of the aircraft. For the cases 1 and 2 where the aircraft arrived before the satellite overpass, CALIOP nCOT was clearly larger for the inner part of the flight track compared with clouds present on either side—creating a ‘plane track’ signature caused by an embedded contrail or another effect on the cloud caused by the aircraft.
Air traffic corridors are far more prevalent in the northern hemisphere than in the southern hemisphere, so we anticipate any climatic effects these embedded contrails may have will be more pronounced there. Even though cloudiness may already be changed by earlier aircraft, we can isolate the effect of a single aircraft on cloud properties. Since the effect of aircraft on cloud properties may well last longer than 30 min the overall effect of air traffic on cloud properties may be larger than the values estimated here. Estimating the climatic effects of embedded contrails is beyond the scope of this paper; however, given the broad coverage of air traffic corridors in the northern hemisphere, embedded contrails as identified in this study are potentially an important and not yet considered contributor to the non-CO2 effects of aviation on climate17.
Further work will be needed to quantify the effect identified in this study. Initially, detailed radiative transfer modelling is needed to assess the impact of an increase in COT on the Earth’s radiative budget. From the modelling perspective, future studies will need to estimate the magnitude of the observed effect on a global scale and assess its contribution to the overall non-CO2 effects of aviation on climate. The increase in cirrus optical depth may result from the emitted soot in the first few seconds within the plume. Soot particles are not efficient ice nuclei. They rather form droplets when water saturation is reached in the plume and freeze subsequently20. Hence, the effect on the microphysics of the cirrus is an open question, and will require detailed microphysical modelling to address.
Full paper (open access) here: http://www.nature.com/ncomms/2016/160621/ncomms12016/pdf/ncomms12016.pdf
NOTE: Under no circumstances will we be discussing “chemtrails” on this thread. Any such reference to that subject will be immediately deleted. – Anthony