Dryland hydrology in a warmer world: Lessons from the Last Glacial period

It has long been recognized that the tropics were drier and mid-latitude deserts wetter during the Last Glacial Maximum (LGM). Until now there has not been a single, unifying explanation for this pattern. Recently, Held and Soden [34] suggested that ongoing global warming will cause the Earth's drylands to become progressively drier and its tropics to become progressively wetter. Because no suitable "warm world" analogue is available in the paleoclimate record, the best available test of Held and Soden's proposal is to look at records from the last glacial period in which drylands should have been wetter and the tropics drier. Our survey of the recent paleolake literature confirms that closed basin lakes located in the poleward limits (similar to 40A degrees) of the drylands in both hemispheres were far larger during the Last Glacial Maximum and parts of the tropics appear to have been less wet. While these observations are consistent with Held's prediction, evidence from the sub-tropical drylands (15 to 25A degrees) is more complex. As with high-latitude drylands, lakes in subtropical drylands of South America and probably the Kalahari Desert were larger than present during the LGM. By contrast, lakes in the sub-tropical Sahara and Arabian Deserts of the northern hemisphere were largest in the early Holocene, but also apparently larger than today in the early LGM. What paleolake records show are that 1) a strong hemispheric symmetry in lake response occurred during the LGM, 2) a difference in response occurred during the latest-glacial, and 3) lake expansions occurred in response to shifts in the thermal equator related to Heinrich Events and insolation variation as well as to colder temperatures.