The climate of North America and the adjacent ocean at 6000 BP was estimated using five independent approaches. Using pollen data, the terrestrial climate was estimated by the movement of ecozone boundaries and by the method of modern analogues. Both analyses indicate warmer temperatures in the western Great Lakes area and the northern Great Plains. A model of Sphagnum-dominated peatland initiation, when forced by Canadian Climate Model 6 ka output projected a cooler and (or) wetter climate for continental western North America. Contrary to this, a reconstruction of the distribution of Sphagnum-dominated peatlands in western Canada indicates that they were located north of their modern distribution, suggesting warmer and (or) drier conditions at 6000 BP. This interpretation is strengthened by observations of lower lake levels at 6000 BP in western Canada. This drier climate may have been associated with warmer conditions as indicated by the quantitative climate reconstructions. In general, eastern North America was drier, while western North America was warmer and drier at 6 ka compared to the present. A model of vegetation and carbon storage, when forced using 6 ka Canadian Climate Model and pollen-based climate reconstructions, showed an increase in area covered by boreal forest, extending north and south of the present location. This was not, however, verified by the fossil data. Additionally, the model showed little total change in carbon storage at 6 ka in the terrestrial biosphere. Estimated sea surface temperatures off eastern Canada suggest warmer surface waters at 6 ka, in agreement with reconstructions based on terrestrial records from the eastern seaboard.