Friday, May 20, 2011

New paper: Increased CO2 decreases tropical cyclones

A paper published online today in the Journal of Climate finds climate models predict a 20% decrease in tropical cyclone frequency in response to doubled atmospheric CO2 and global warming. Another recent paper also found global warming decreases the frequency of the strongest wind events. Regardless of the many limitations and biases of climate models, the papers expose the alarmist claims of Al Gore et al that warming causes more hurricanes as baseless.

The response of tropical cyclone statistics to an increase in CO2 with fixed sea surface temperatures

Isaac M. Held1* and Ming Zhao1,2

1 Geophysical Fluid Dynamics Laboratory / NOAA, Princeton, New Jersey, USA
2 University Corporation for Atmospheric Research

Abstract:  The effects on tropical cyclone statistics of doubling CO2, with fixed sea surface temperatures (SSTs), are compared to the effects of a 2K increase in SST, with fixed CO2, using a 50km resolution global atmospheric model. Confirming earlier results of Yoshimura and Sugi (2005), a significant fraction of the reduction in globally averaged tropical storm frequency seen in simulations in which both SST and CO2 are increased can be thought of as the effect of the CO2 increase with fixed SSTs. Globally, the model produces a decrease in tropical cyclone frequency of about 10% due to doubling of CO2 and an additional 10% for a 2K increase in SST, resulting in roughly a 20% reduction when both effects are present. The relative contribution of the CO2 effect to the total reduction is larger in the Northern than in the Southern Hemisphere. The average intensity of storms increases in the model with increasing SST, but intensity remains roughly unchanged, or decreases slightly, with the increase in CO2 alone. As a result, when considering the frequency of more intense cyclones, the intensity increase tends to compensate for the reduced total cyclone numbers for the SST increase in isolation but not for the CO2 increase in isolation. Changes in genesis in these experiments roughly follow changes in mean vertical motion, reflecting changes in convective mass fluxes. Discussion is provided of one possible perspective on how changes in the convective mass flux might alter genesis rates.

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