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Fast versus slow response in climate change: implications for the global hydrological cycle

Govindasamy Bala, Ken Caldeira, and Ramakrishna Nemani · Climate Dynamics 35, 423-434 · 2010

Key finding. The apparent difference in hydrological sensitivity between solar and carbon dioxide forcing originates entirely in their fast responses; once fast responses are excluded from the definition, the hydrological sensitivity is the same for both, so the slow response of the water cycle is independent of the forcing mechanism.

Eight scatter panels in two columns, each plotting a climate variable against the change in global- and annual-mean surface temperature in kelvin. The variables are top-of-atmosphere net longwave and net shortwave flux, top-of-atmosphere and surface net flux, surface latent and sensible heat flux, precipitation, and precipitable water. Each panel carries two fitted lines — a solid line with star markers for the doubled carbon dioxide case and a dashed line with circles for the solar case. In the precipitation and latent heat panels the two lines are close to parallel but clearly offset from one another; in the net flux panels they very nearly coincide.
In each panel the slope is the slow response and the intercept the fast one. The slopes are almost identical between carbon dioxide and solar forcing — the water cycle's slow response does not care what warmed it — while the intercepts are markedly different, and for precipitation and the heat fluxes that offset is entirely fast adjustment. Figure 3 from Bala, Caldeira and Nemani (2010), Climate Dynamics 35, 423-434. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Changes in global mean precipitation had been found to be larger for solar forcing than for carbon dioxide forcing of the same magnitude, which suggested the hydrological cycle responds differently depending on what drives the warming.

This paper asked where that difference actually lives — in the climate's slow feedback response, or in the fast adjustment that happens before the surface temperature has moved at all.

What did we find?

Adjusted radiative forcing and fast response are estimated with Hansen's fixed-sea-surface- temperature method, and the total climate system response with mixed-layer simulations in the same model, so the two components can be separated cleanly.

The fast response is not a correction term. For precipitation and evaporation it is almost 40% of the total response.

With fast responses removed, solar and carbon dioxide forcing give the same hydrological sensitivity — the change in global mean precipitation per unit of warming.

That makes the slow feedback of the hydrological cycle a property of the climate system rather than of what disturbed it, while everything distinctive about each forcing agent sits in the fast adjustment.

The authors recommend that multi-model intercomparisons compare fast and slow responses separately, since a combined figure mixes a universal response with an agent-specific one.

Why does it matter?

It resolves an apparent inconsistency into a clean decomposition. Two forcings that looked like they drove different water cycles turn out to drive the same one, differing only in the adjustment that precedes it.

That is what makes solar geoengineering's hydrological problem structural. The mismatch between reduced sunlight and elevated carbon dioxide is established in the fast response, before any warming, so no amount of tuning the intervention removes it.

The methodological recommendation has outlasted the particular result: separating fast from slow is now standard practice in comparing forcing agents.

Citation

Govindasamy Bala, Ken Caldeira, and Ramakrishna Nemani (2010). Fast versus slow response in climate change: implications for the global hydrological cycle. Climate Dynamics 35, 423-434.

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