The Younger Dryas was one of the more dramatic climatic transitions ever recorded. How these types of climatic shifts are expressed in continental interiors is of primary scientific interest and of vital societal concern. Here we present a speleothem-based absolutely dated record (using uranium-series data) of climate change for the southwestern United States from growth chronology of multiple speleothems. The stalagmite growth represents the onset of wetter climate (12,500 yr B.P.) soon after the start of the Younger Dryas; the wetter climate persisted a millennium beyond the termination of the Younger Dryas. This wet cycle is likely related to a more southern positioning of the polar jet stream in response to cooler Northern Hemisphere climate. The end of the wet period coincides with the peak of the Holocene summer insolation maximum ca. 10,500 yr B.P. The Å llerød (prior to the Younger Dryas), which corresponds to Clovis occupation in the southwestern United States, was drier in comparison and seems in line with a climatic contribution to megafauna extinction.

The mechanisms driving late Holocene drought cycles in the western United States are not well known due to the general scarcity of long‐term, high‐resolution, absolutely dateable proxies for precipitation in continental interiors. Here we show that late Holocene precipitation variability in the southwestern United States has been caused by changes in the Pacific Ocean. We present a stalagmite‐based, annually resolved moisture record that indicates large shifts from pluvial to drought conditions alternating with periods of dampened, near‐average precipitation over the last 3000 years. Significant spectral peaks at decadal‐scale (∼20–50, 70–80 year) frequencies likely correspond to modern frequencies of the Pacific Decadal Oscillation or a low‐frequency component of the El Niño‐Southern Oscillation, and are dominant during episodes of large precipitation shifts. Overall pluvial conditions punctuated by severe droughts may have challenged the adaptive capacities of emerging agrarian communities of ancestral Americans.

Late Holocene climate in western North America was punctuated by periods of extended aridity called megadroughts. These droughts have been linked to cool eastern tropical Pacific sea surface temperatures (SSTs). Here, we show both short-term and long-term climate variability over the last 1,500 y from annual band thickness and stable isotope speleothem data. Several megadroughts are evident, including a multicentury one, AD 1350–1650, herein referred to as Super Drought, which corresponds to the coldest period of the Little Ice Age. Synchronicity between southwestern North American, Chinese, and West African monsoon precipitation suggests the megadroughts were hemispheric in scale. Northern Hemisphere monsoon strength over the last millennium is positively correlated with Northern Hemisphere temperature and North Atlantic SST. The megadroughts are associated with cooler than average SST and Northern Hemisphere temperatures. Furthermore, the megadroughts, including the Super Drought, coincide with solar insolation minima, suggesting that solar forcing of sea surface and atmospheric temperatures may generate variations in the strength of Northern Hemisphere monsoons. Our findings seem to suggest stronger (wetter) Northern Hemisphere monsoons with increased warming.

Stalagmites can host numerous potential climate proxies (stable and radiogenic isotopes, trace elements, annual and non-annual banding, grayscale, growth hiatuses, mineral assemblage). Reproducibility and/or integration of proxy results between one or more stalagmites will become increasingly important, and ideally, climate records generated by multiple stalagmites from the same cave or cave room are expected to be near-identical. The reality is that stalagmites from the same cave room can yield differing results to some degree, especially in cave environment zones that are evaporative. Our drip water study in an evaporative shallow-depth cave environment in Carlsbad Cavern shows that adjacent drip sites produce differing drip rate behavior, but share some similarities. Drip water collected from four sites in this evaporative cave environment shows Ca, Mg, Sr, Ba, U, and Th elemental concentrations and 234 U/ 238 U (expressed as δ 234 U) to vary seasonally, and all but U have higher values during the winter months when the instrumented cave site exhibited slower drip rates and was drier. Results from our Carlsbad Cavern drip sites indicate that increased relative humidity in the cave and decreased surface and cave atmospheric pressure combined with increased precipitation (rain and snow) are responsible for faster drip rates. Changes in atmospheric pressure play an essential role, although less directly during summer months. We therefore conclude that stalagmites within the same cave room may not record and produce identical proxy records in these more evaporative cave environments, but that differing records are simply recording the same climate signals expressed uniquely by the individual proxies, and that each stalagmite simply has differing sensitivities to the climate signals. Integrated, these proxy differences serve as important past climate indicators. Our drip sites respond to seasonal variations in climate more so than individual rain/snow events, and we favor the interpretation that seasonal changes reflect regional as well as local climate changes. cave, drip water, Carlsbad Cavern, climate, uranium isotopes

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