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Schlagwörter:
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Zusammenfassung:
The global hydrologic cycle is likely to increase
in strength with global warming, although some studies indicate
that warming due to solar absorption may result in a
different sensitivity than warming due to an elevated greenhouse
effect. Here we show that these sensitivities of the
hydrologic cycle can be derived analytically from an extremely
simple surface energy balance model that is constrained
by the assumption that vertical convective exchange
within the atmosphere operates at the thermodynamic limit
of maximum power. Using current climatic mean conditions,
this model predicts a sensitivity of the hydrologic cycle of
2.2%K−1 to greenhouse-induced surface warming which is
the sensitivity reported from climate models. The sensitivity
to solar-induced warming includes an additional term, which
increases the total sensitivity to 3.2%K−1. These sensitivities
are explained by shifts in the turbulent fluxes in the
case of greenhouse-induced warming, which is proportional
to the change in slope of the saturation vapor pressure, and in
terms of an additional increase in turbulent fluxes in the case
of solar radiation-induced warming. We illustrate an implication
of this explanation for geoengineering, which aims
to undo surface temperature differences by solar radiation
management. Our results show that when such an intervention
compensates surface warming, it cannot simultaneously
compensate the changes in hydrologic cycling because of the
differences in sensitivities for solar vs. greenhouse-induced
surface warming. We conclude that the sensitivity of the hydrologic
cycle to surface temperature can be understood and
predicted with very simple physical considerations but this
needs to reflect on the different roles that solar and terrestrial
radiation play in forcing the hydrologic cycle.
