Contrails may be causing the recent warming.

The reference to this claim is here:

Minnis, Patrick, J. Kirk Ayers, Rabindra Palikonda, and Dung Phan, 2004. Contrails, Cirrus Trends, and Climate. Journal of Climate Vol. 17, No 8, pp. 1671-1685, April 15, 2004, full text available online <http://enso.larc.nasa.gov/sass/pub/journals/Minnis.etal.JClim.04.pdf>

Abstract

Rising global air traffic and its associated contrails have the potential for affecting climate via radiative forcing. Current estimates of contrail climate effects are based on coverage by linear contrails that do not account for spreading and, therefore, represent the minimum impact. The maximum radiative impact is estimated by assuming that long-term trends in cirrus coverage are due entirely to air traffic in areas where humidity is relatively constant. Surface observations from 1971 to 1995 show that cirrus increased significantly over the northern oceans and the United States while decreasing over other land areas except over western Europe where cirrus coverage was relatively constant. The surface observations are consistent with satellite-derived trends over most areas. Land cirrus trends are positively correlated with upper-tropospheric (300 hPa) humidity (UTH), derived from the National Centers for Environmental Prediction (NCEP) analyses, except over the United States and western Europe where air traffic is heaviest. Over oceans, the cirrus trends are negatively correlated with the NCEP relative humidity suggesting some large uncertainties in the maritime UTH. The NCEP UTH decreased dramatically over Europe while remaining relatively steady over the United States, thereby permitting an assessment of the cirrus–contrail relationship over the United States. Seasonal cirrus changes over the United States are generally consistent with the annual cycle of contrail coverage and frequency lending additional evidence to the role of contrails in the observed trend. It is concluded that the U.S. cirrus trends are most likely due to air traffic. The cirrus increase is a factor of 1.8 greater than that expected from current estimates of linear contrail coverage suggesting that a spreading factor of the same magnitude can be used to estimate the maximum effect of the contrails. From the U.S. results and using mean contrail optical depths of 0.15 and 0.25, the maximum contrail cirrus global radiative forcing is estimated to be 0.006 to 0.025 W m^--2 depending on the radiative forcing model. Using results from a general circulation model simulation of contrails, the cirrus trends over the United States are estimated to cause a tropospheric warming of 0.2 to 0.3 ̊C decade^--1, a range that includes the observed tropospheric temperature trend of 0.27 ̊C decade^--1 between 1975 and 1994. The magnitude of the estimated surface temperature change and the seasonal variations of the estimated temperature trends are also in good agreement with the corresponding observations

Patrick Michaels disccusses these results at http://www.greeningearthsociety.org/wca/2004/wca_16c.html

His concluding paragraph states:

“The preconceived notion that carbon dioxide emissions from fossil fuel burning is responsible for all the warming that is observed oftentimes deflects attention away from other local and/or regional influences on the climate. Recent studies such as this one by Minnis implicating air travel, and others that suggest large-scale urbanization (e.g. Kalnay and Cai, 2003) and industrialization (e.g de Laat and Maurellis, 2004) have a non-negligible impact on the climate are becoming commonplace in the scientific literature. The continued failure to recognize this type of work by those organizations that are “concerned” about the effects of climate change makes their true agenda clear — eliminate use of fossil fuels.”

References:

de Laat, A. T. J., and A. N. Maurellis, 2004. Industrial CO2 emissions as a proxy for anthropogenic influence on lower tropospheric temperature trends. Geophysical Research Letters, 31, L05204, doi:10.1029/2003GL019024.

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

Kalnay, E., and M. Cai, 2003. Impact of urbanization and land use change on climate. Nature, 423, 528–531.

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.