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Late Pleistocene ice-age climates are routinely characterized as having imposed moisture stress on low- to mid-latitude ecosystems1–5. This idea is largely based on fossil pollen evidence for widespread, low-biomass glacial vegetation, interpreted as indicating climatic dryness6. However, woody plant growth is inhibited under low atmospheric CO2(refs. 7,8), so understanding glacial environments requires the development of new palaeoclimate indicators that are independent of vegetation9. Here we show that, contrary to expectations, during the past 350 kyr, peaks in southern Australian climatic moisture availability were largely confined to glacial periods, including the Last Glacial Maximum, whereas warm interglacials were relatively dry. By measuring the timing of speleothem growth in the Southern Hemisphere subtropics, which today has a predominantly negative annual moisture balance, we developed a record of climatic moisture availability that is independent of vegetation and extends through multiple glacial–interglacial cycles. Our results demonstrate that a cool-moist response is consistent across the austral subtropics and, in part, may result from reduced evaporation under cool glacial temperatures. Insofar as cold glacial environments in the Southern Hemisphere subtropics have been portrayed as uniformly arid3,10,11, our findings suggest that their characterization as evolutionary or physiological obstacles to movement and expansion of animal, plant and, potentially, human populations10should be reconsidered.

Climate projections1–3and observations over recent decades4,5indicate that precipitation in subtropical latitudes declines in response to anthropogenic warming, with significant implications for food production and population sustainability. However, this conclusion is derived from emissions scenarios with rapidly increasing radiative forcing to the year 21001,2, which may represent very different conditions from both past and future ‘equilibrium’ warmer climates. Here, we examine multi-century future climate simulations and show that in the Southern Hemisphere subtropical drying ceases soon after global temperature stabilizes. Our results suggest that twenty-first century Southern Hemisphere subtropical drying is not a feature of warm climates per se, but is primarily a response to rapidly rising forcing and global temperatures, as tropical sea-surface temperatures rise more than southern subtropical sea-surface temperatures under transient warming. Subtropical drying may therefore be a temporary response to rapid warming: as greenhouse gas concentrations and global temperatures stabilize, Southern Hemisphere subtropical regions may experience positive precipitation trends.

Relationships among wet-season recharge, hydrochemistry and the distribution of travertines at Louie Creek were investigated from water samples collected during four campaigns between April 1993 and November 1997. pH, temperature and major cation and anion data reveal that the springs feeding the creek rise with high concentrations of CO2 and dissolved calcium carbonate. The pattern of downstream changes in hydrochemistry was similar during each campaign: rapid outgassing of CO2 over the ˜1.3 km of surface flow increases calcite saturation, triggering extensive travertine deposition over ˜1.5 km. At low discharge, the creek sinks then re-emerges at the downstream limit of travertine deposition in a more CO2-rich state. Together with a low stream gradient and high magnesium concentrations, this chemical change arrests the evolution of the waters and inhibits further accumulation of travertine downstream. Significant wet-season recharge dilutes spring water concentrations and retards their downstream evolution, resulting in a downstream shift of the reach of deposition. Following a wet season of low magnitude, the spring waters rise with higher concentrations of dissolved carbonate and evolve to a supersaturated state over a shorter distance. This results in an upstream migration of the travertine reach. These results have significance for the interpretation of fossil Quaternary travertines at Louie Creek.