Will global warming bring a wet or dry world?

A new study of the Paleocene/Eocene boundary in the arctic published two weeks ago in the journal Nature makes the argument that it depends where you live.

The Paleocene/Eocene boundary marks a period in Earth's history where the global mean temperature dramatically increased (5 to 7 degrees C) over a short 60,000 to 120,000 year interval. Understanding the consequences of this 55.5 million year old event helps shed light on the possible outcomes of recent global tends, due to the input of green house gases. The authors of the study examined the level of Deuterium in sediments of deep ocean cores at various latitudes. Deuterium is a heavy isotope of Hydrogen and is an indicator for more intense precipitation. They found higher levels of Deuterium in the arctic indicating more intense rainfall. Furthermore, they looked at fossil dinocysts, which are useful for determining ocean salinity, and found increasing salinity after the global warming event, and decreased salinity at the onset of the event, indicating a stratified ocean. It has also be argued that the Paleocene/Eocene brought increased runoff from the continents due to the increase in clay minerals across the boundary. Taken together it looks like the poles got much wetter across the Paleocene/Eocene boundary, while the mid-latitudes and subtropics dried.

Pagani et al., 2006. Arctic hydrology during global warming at the Palaeocene/Eocene thermal maximum. Nature 442, 671-675(10 August 2006).