Mid-to-Late Holocene packrat midden data from the Shivwits Plateau record an eastward contraction of the North American monsoon

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The North American Monsoon (NAM) brings precipitation to southwestern North America between July and September. The NAM evolved in strength and extent during the Holocene, in response to changes in North Hemisphere insolation and associated sea-surface temperature changes. Today, the strong moisture surges associated with the NAM are concentrated in areas east of 114˚W, the longitude of St. George, Utah. However, in the early Holocene, summer precipitation was considerably stronger than it is today west of 114˚W, influencing Mojave Desert plant and animal communities in southeastern California and southern Nevada. In the mid-Holocene, cooler sea-surface temperatures in the Gulf of California are presumed to have shifted the bulk of NAM precipitation east of 114˚W, out of southern California and southern Nevada, and more or less into its present range. However, the position of the western boundary of the strong NAM belt in mid-Holocene through late-Holocene time is poorly constrained.

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