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Recent warming in the West seems to have human fingerprints

Climate scientists have documented the fact that carbon dioxide levels in the atmosphere are increasing, that this increase comes largely from the burning of fossil fuels, and that carbon dioxide is a primary cause of rising temperatures. That is true for the world in general. For the continental US, in particular, the IPCC’s most recent report includes a breakdown of how natural and human influences have contributed to the rise in temperature from 1906 to 2005 over the Earth's continents.1

The report shows that climate models that include both natural and human factors accurately reproduce the observed temperature increases over the past 100 years, whereas those that include only natural factors do not — further supporting the assertion of a human imprint on warming.

For the west in particular, a study led by Tim Barnett, of the Scripps Institution of Oceanography, shows that trends in several factors (river flow, winter air temperature and snow pack) in the western US are inconsistent with any known explanation other than human-induced climate change.2 Also, a paper co-authored by Celine Bonfils, of Lawrence Livermore National Laboratory, shows that trends in daily minimum and maximum temperatures, frost days and days above 32°F in the West are too rapid to be explained by any known phenomenon other than human-enhanced greenhouse warming.3

The consequences of this warming are already many. A partial list includes earlier snowmelt, longer fire seasons, more outbreaks of insects that harm forests, reduced streamflow and threats to fish habitat.

References
  1. See Figure 1. Hegerl, G.C., F. W. Zwiers, P. Braconnot, N.P. Gillett, Y. Luo, J.A. Marengo Orsini, N. Nicholls, J.E. Penner and P.A. Stott, 2007: Understanding and Attributing Climate Change. (PDF) In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA p 703.
  2. Barnett, Tim P., David W. Pierce, Hugo G. Hidalgo, Celine Bonfils, et al. “Human-Induced Changes in the Hydrology of the Western United States, 2008.” (Abstract) Science Vol. 319. no. 5866, February 2008 pp. 1080 - 1083.
  3. Hidalgo, H.G., T. Das, M.D. Dettinger, D.R. Cayan, D.W. Pierce, T.P. Barnett, G. Bala, A. Mirin, A.W. Wood, C. Bonfils, B.D. Santer, and T. Nozawa, “Detection and Attribution of Streamflow Timing Changes to Climate Change in the Western United States.” (Abstract) Journal of Climate 22, no. 13. July 2009. 3838-3855.