Simulated changes in extreme temperature and precipitation events at 6 ka Noah S. Diffenbaugh a,b, ⁎ , Jason L. Bell a , Lisa C. Sloan a a Department of Earth Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA b Purdue Climate Change Research Center and Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907-2051, USA Received 30 September 2004; accepted 11 November 2005 Abstract We have employed a high-resolution regional climate model (RCM) to test the sensitivity of extreme climate events to 6 ka orbital forcing, using western North America as a case study. Calculated differences (6 ka - Control) in annual extreme precipitation event frequency were mostly positive throughout the RCM domain, as were differences in extreme event contribution to total annual precipitation. These annual mean differences were driven by changes in the seasonal distribution of extreme precipitation events at 6 ka, which was in turn linked to changes in the daily variability of upper-level zonal wind speeds over the northeast Pacific. Positive differences in the frequency of extreme maximum daily temperature values occurred inland in the RCM domain, with peak differences of 28 days/year over the Great Basin. Likewise, days in which the maximum daily temperature exceeded 32 °C were 24% more frequent in the 6 ka integration than in the control integration and heat waves lasted up to 12 days longer. Mean first and last freeze dates were generally earlier in the 6 ka integration along the Pacific coast and over the southern Great Basin, and later over the northern Great Basin and interior of the Pacific Northwest. We propose that, by exerting controls on paleoclimate archives unique from changes in the mean climate state, such changes in the frequency and duration of extreme climate events could have played a substantial role in shaping the mid-Holocene paleoclimate record of the region, and that extreme climate events may have been similarly important in other regions and during other time periods. © 2006 Elsevier B.V. All rights reserved. Keywords: Holocene; Paleoclimatology; General circulation models; Orbital forcing; Climate change 1. Introduction Paleoclimate records at a given time horizon have been shaped by a variety of environmental variables. Most prominently, archives record the mean state of the climate system, whether at mean annual scale (as in the case of mean annual aridity (Kohfeld and Harrison, 2000)) or at mean seasonal scale (as in the case of mean temperature of the coldest month (Thompson and Anderson, 2000)). Considerable insight into the dynam- ics of the climate system has been gained by reconstruct- ing the mean state of past regional and global climates from paleo-proxy records (e.g., Bartlein et al., 1998; Prentice et al., 1998; Kohfeld and Harrison, 2000). In addition, as recognition of the importance of climate variability to modern environments has increased, Palaeogeography, Palaeoclimatology, Palaeoecology 236 (2006) 151 – 168 www.elsevier.com/locate/palaeo ⁎ Corresponding author. Purdue Climate Change Research Center and Department of Earth and Atmospheric Sciences, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907- 2051, USA. Tel.: +1 765 494 0754; fax: +1 765 496 1210. E-mail address: diffenbaugh@purdue.edu (N.S. Diffenbaugh). 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2005.11.037