Paleoecological Evidence of Short-Term Climate Variations in the Northwestern U.S.

Cathy Whitlock (Dept. of Geography, University of Oregon, Eugene OR 97403; Internet: whitlock@oregon.uoregon.edu)

Paleoecologic records from the western U.S. register a hierarchy of responses to climate variations on different time scales. The strongest response occurs on Milankovitch time scales, when the dominant controls of regional climate are variations in the seasonal cycle of insolation, atmospheric CO2, and associated changes in ice sheet size and SSTs. These controls in turn influence the location of storm tracks and the strength and location of major atmospheric circulation features. The patterns of vegetation on these times scales include regionally coherent changes in vegetation composition, species ranges, and disturbance regime. On millennial time scales, the few available high-resolution records reveal a heterogeneous response. The Younger Dryas event (~11.6-13 cal ka), for example, is registered in Alaska and California in various data sets, but cooling in the Pacific Northwest is weakly expressed and postdates the YD event (Grigg and Whitlock, 1998, Quat. Res.). Sites that show cooling are located in coastal settings implicating changes in SSTs in the northeast Pacific as the proximal cause. Millennial-scale climate changes are also noted in pollen data that span Stages 2 through 5. At Carp Lake, Washington, variations in forest cover and the abundance of spruce correlate well with stadial/interstadial periods in Stage 5 (Whitlock and Bartlein, 1997, Nature 388). The record also shows cold events that align with Heinrich events 1, 2, 3, 5, and 6 in the North Atlantic, and in each case, they are followed by abrupt warming. Similar cold maxima are registered at other times as well, and the paleoecologic expression of Heinrich-type responses changes throughout the record, as the dominant large-scale controls (i.e., insolation, ice sheet size, SSTs) change. Millennial scale variations are also noted in the diatom record from Owens Lake, California, which are attributed to changes in snowpack in the Sierras that in turn is related to variations in SSTs (Bradbury, in press). Holocene pollen records show abrupt shifts towards warming at ~11 ka and cooling at 3-4 ka that appear to be regionally significant. Short-term changes in fire frequency during the Holocene seem to be more sensitive records of climate change than pollen data (Long et al., in press, Can. J. For.Res).