Are we Headed for a Neo Oligocene? Global Drying as Part of Climate Change

Robust Hadley Circulation changes and increasing global dryness due to CO2 warming from CMIP5 model projections

Authors:

Lau et al

Abstract:

In this paper, we investigate changes in the Hadley Circulation (HC) and their connections to increased global dryness (suppressed rainfall and reduced tropospheric relative humidity) under CO2 warming from Coupled Model Intercomparison Project Phase 5 (CMIP5) model projections. We find a strengthening of the HC manifested in a “deep-tropics squeeze” (DTS), i.e., a deepening and narrowing of the convective zone, enhanced ascent, increased high clouds, suppressed low clouds, and a rise of the level of maximum meridional mass outflow in the upper troposphere (200−100 hPa) of the deep tropics. The DTS induces atmospheric moisture divergence and reduces tropospheric relative humidity in the tropics and subtropics, in conjunction with a widening of the subsiding branches of the HC, resulting in increased frequency of dry events in preferred geographic locations worldwide. Among various water-cycle parameters examined, global dryness is found to have the highest signal-to-noise ratio. Our results provide a physical basis for inferring that greenhouse warming is likely to contribute to the observed prolonged droughts worldwide in recent decades.

Evidence for a Neo Oligocene for the Near Future Climate

Drought is heating up around the warming world. Particularly hot drought has cost more than US$40 billion and claimed 218 human lives since 2010 in the United States alone. These hot and dry conditions have also contributed to unusually widespread and devastating wildfires, fuelled by wide expanses of weakened and dead trees that were unable to deal with heat stress and subsequent insect attack. Yet, to get a real sense of how this recent change in drought severity might shape the future, one has to look to the past. An analysis of regional and pan-continental North American drought over the past 1,000 years, reported by Cook et al.3 in the Journal of Climate, makes it clear that recent droughts, as costly as they have been, are only a taste of what might lie ahead, independently of any big climate change.

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