US Southwest expected to dry further as climate warms

Based on a study of seasonal rainfall variations in the desert Southwest between 56,000 and 11,000 years ago as recorded in cave stalagmites, geoscientist Stephen Burns of the University of Massachusetts Amherst, with colleagues at the University of New Mexico, suggest the rapidly growing Southwest could become even more arid as global temperatures rise. Their findings are published in this week's Nature Geosciences and are corroborated by another study presented in the same issue.

Burns is an expert in reading past climate data from the ratio of oxygen isotopes found in calcite, in speleothems - stalagmites, stalactites and other water-deposited cave features. The ratios indicate seasonal precipitation levels. Burns says data from this study covering approximately 45,000 years agree with modern evidence that the polar jet stream shifts northward in response to climate warming. Further, when the polar jet stream retreats toward the pole, winter precipitation in the Southwest decreases, reducing recharge to underground aquifers.

'We believe this cycle is controlled by the position of the polar jet stream, and that lower moisture levels reach the Southwest from the Pacific Ocean when the climate overall is warmer. Likewise, in periods when the Northern hemisphere's climate is cooler, the polar jet stream sinks southward and winter rains increase in the desert Southwest, probably in response to advancing glaciers in Northern latitudes,' he says.

Speleothem records collected by Burns and colleagues in New Mexico for this National Science Foundation-supported study are among the first long, high-resolution records of rainfall ever collected for the region.

For such studies, the researchers collect speleothems, in this case stalagmite slices a few inches long from a cave in New Mexico. Speleothems are formed over tens of thousands of years by water seeping through cracks in bedrock and dissolving calcite and aragonite. Depending on temperature, carbon dioxide level and other cave factors, these mineral deposits can precipitate out as stalagmites, stalactites, ribbons, domes or straws.

Analysis of radioactive isotopes and stable oxygen isotopes in the calcite indicate past rainfall over many centuries. 'We then try to determine what caused the observed variations at various timescales, from just a few years up to tens of thousands,' Burns says. For the current work, they compared the record with baseline data from Greenland ice cores and with speleothem data from a cave in China, halfway around the world. 'This helps to show that the pattern extends across the entire Northern hemisphere,' says Burns.

This relatively new method of oxygen isotope analysis from calcite sampled from ancient speleothems is practised by only a few research teams worldwide, but it offers more chronological control and is more precise than previous methods that used lake bed sediment records. However, some have questioned its reproducibility, Burns acknowledges. That's why it was a very pleasant surprise when he and colleagues learned that without any prearrangement between research teams, another team is reporting very similar conclusions in the same journal issue this week, based on speleothem data from a different cave in the Southwest, but using a different laboratory for isotope analyses.

This coincidental but key validation by a completely separate investigating team should go a long way to answer doubts about the reproducibility of climate records from speleothem analysis, says Burns. 'Results from our two groups reproduce each other incredibly well, which is a quite exciting and satisfying validation of the overall method.'