April 19, 2004

Leaving a Trail

Airplane contrails, the condensation trails formed in the wake of high-altitude jets, may be responsible for all warming observed in the United States during the past 25 years, a new study suggests.

Land-use change. The urban heat island effect. Local surface warming in industrialized regions. More and more observational evidence emerges indicating that local and regional processes such as these are important players in recent global temperature changes. As these other influences emerge, anthropogenic greenhouse gas concentrations resulting from fossil fuel combustion appear less important than previously stated in reports such as those by the Intergovernmental Panel for Climate Change (IPCC, 2001).

Still other influences are emerging. Airplanes, for example. Indeed, a new study attributes all of the warming observed in the United States over the past 25 years to air traffic. Published in the Journal of Climate, the paper outlines this often overlooked, observational explanation for regional and potentially global warming unrelated to increasing concentrations of CO2, as articulated by Patrick Minnis, a senior research scientist at NASA Langley, and colleagues Kirk Ayers, Rabindra Palikonda, and Dung Phan of Analytical Services & Materials.

Why airplanes? A climatologically important byproduct of air traffic is the condensation trail, or contrail, formed in the wake of high-altitude jet aircraft. These contrails are basically man-made cirrus clouds, or high-altitude ice clouds. They form when hot and humid air from jet exhaust mixes with the cold and drier air of the upper troposphere, where jets travel. If the relative humidity of the high-level air is very low, contrails dissipate quickly. If the air is moist, however, the contrails spread horizontally and form a thin layer of cirrus clouds that persists for many hours.

Cirrus clouds and therefore contrails are climatologically important because they are net warmers of the atmosphere. Since they are very thin and composed entirely of ice crystals, they allow most of the incoming solar radiation to pass through them. But water vapor (frozen or liquid) is the most important and abundant greenhouse gas in the atmosphere, and therefore traps the outgoing longwave radiation from the surface. As a result, cirrus clouds produce a net positive radiative forcing, or a net warming effect, on the atmosphere and the surface.

Worldwide jet air traffic is expected to grow up to 5% per year over the next 50 years, therefore careful assessment of the effect of contrails on atmospheric and surface temperatures is critical. To date, only the young, linear contrails easily distinguished from other cirrus clouds have been studied before in relation to climate change.

Their effect must not be discounted. In just a few hours, contrails that start out only a few meters wide can spread to cover more than 20,000 square kilometers, Pat Minnis reported at a spring 2002 American Meteorological Society meeting. Therefore, investigating only the young and linear contrails severely underestimates the true radiative forcing of these cirrus clouds. In this new study, Minnis et al. account for that spreading factor and provide an improved long-term assessment of the effect of contrails by using 25 years of surface and satellite cloud observations, in conjunction with air traffic data.
Minnis first determines the trend in mean annual cirrus coverage for various regions of the globe and finds that cirrus coverage—both in terms of amount and frequency— has indeed increased significantly over the United States. Furthermore, the greatest increases are observed in well-traveled air traffic routes (Figure 1). The seasonality in cirrus coverage corresponds to the seasonality in contrails.

Contrails

Figure 1. Estimated linear contrail coverage for air traffic routes across the globe; black boxes denote air traffic over land, and white boxes are for ocean regions. (From Minnis et al., 2004.)

Since cirrus cloud formation is very dependent on the relative humidity in the upper troposphere, potential long-term changes in humidity could account for the observed changes in cirrus clouds. However, Minnis et al. find no changes in the high-altitude relative humidity over the U.S. The combination of the significant positive trends in cirrus clouds in both surface and satellite data, the seasonal correspondence between cirrus and contrails, and the lack of other explanations such as trends in relative humidity all lead Minnis to conclude that air traffic most likely accounts for the observed significant growth in cirrus clouds over the U.S.

Contrails

Figure 2. Correspondence between seasonal trends in cirrus and contrail amounts and frequencies. CC and ISCCP correspond to trend in cirrus for two different data sources; ECON represents the seasonal contrail coverage from Figure 1, and “satellite” and “surface” correspond to mean linear contrail coverage derived from satellite data and surface observations, respectively. (From Minnis et al., 2004.)

Having established that contrails from air traffic are indeed increasing in both amount and frequency, Minnis et al. next determine the warming effect of these contrails. Using well-established values for cirrus clouds’ reflective (cooling) and absorbing (warming) properties, the net temperature change associated with the increase in contrails over the United States is calculated to be a warming of 0.2° to 0.3°C per decade. Based on comparisons with observed trends in surface and atmospheric temperatures over the United States, Minnis finds that the increases in cirrus coverage related to air traffic account for nearly all of the surface and tropospheric warming observed over the last 25 years.

These findings provides further evidence that suggests that the observed warming in recent decades is not the sole product of increasing anthropogenic greenhouse gas concentrations. Though Minnis et al. point out that such region-specific calculations should not be extrapolated to conclude that contrails are responsible for warming in other regions, they do point out that since contrails are already found to have a substantial regional warming effect where air traffic is heavy, as air travel continues to increase globally, the impact of contrails may also become globally significant.

References:

Intergovernmental Panel for Climate Change (IPCC), 2001. Climate Change 2001: The Scientific Basis, Cambridge Univ. Press, New York.

Minnis, P., J. K. Ayers, R. Palikonda, and D. Phan, 2004. Contrails, Cirrus Trends, and Climate. Journal of Climate, 17, 1671–1685.

Minnis, P., L. Nguyen, D. P. Duda, and R. Palikonda, 2002. Spreading of isolated contrails during the 2001 air traffic shutdown. Proceedings of the American Meteorological Society 10th Conference on Aviation, Range, and Aerospace Meteorology. Portland, Oregon, 33–36.




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