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Why is there a short-term increase in global precipitation in response to diminished CO2 forcing?

Long Cao, Govindasamy Bala, and Ken Caldeira · Geophysical Research Letters 38, L06703 · 2011

Key finding. In the HadCM3L coupled model, instantaneously removing a quadrupled-CO2 forcing raised global mean precipitation by 5.1% within one year, while removing an equal-magnitude solar forcing made precipitation fall immediately — showing the transient increase is a consequence of the fast tropospheric adjustment to CO2 forcing rather than a general feature of reduced radiative forcing.

Four panels of time series from HadCM3L. Panel a shows the change in global mean surface air temperature for the quadrupled-carbon-dioxide step in red and the equivalent solar step in blue; the two curves lie on top of each other, rising to about 5.5 kelvin and dropping together when the forcing is removed at year 70. Panel b shows the percentage change in global mean precipitation for the same two runs: the blue solar curve sits near 11 per cent and the red carbon dioxide curve near 5 per cent, and at year 70 the red curve jumps sharply upward to meet the blue one before both decay. Panel c shows the gradual ramp case, where temperature in red falls as carbon dioxide in dashed black falls, while precipitation in green keeps rising for several decades. Panel d shows ocean heat uptake, nearly identical between the carbon dioxide and solar runs.
The two forcings produce the same warming (panel a) and the same ocean heat uptake (panel d) but different precipitation (panel b). Removing the carbon dioxide forcing at year 70 makes precipitation jump up; removing the solar forcing makes it fall — which is why the temporary increase after a carbon dioxide cut cannot be explained by stored ocean heat. Figure 1 from Cao, Bala, and Caldeira (2011), Geophysical Research Letters 38, L06703. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Cutting atmospheric carbon dioxide had been found to raise global mean precipitation temporarily, for several decades, before precipitation began to fall. The same global mean temperature could therefore correspond to two different precipitation states depending on which way carbon dioxide was moving. That behaviour had been attributed to heat accumulated in the ocean.

This paper asked whether the ocean explanation is right, or whether the temporary increase is instead a property of carbon dioxide forcing specifically — in which case any negative radiative forcing would not produce it.

What did we find?

Two step-function experiments were tuned to produce nearly the same warming — 5.33 degrees Celsius from quadrupling carbon dioxide and 5.25 degrees Celsius from raising solar intensity 4.54%, a difference of 0.08 degrees. Precipitation responded very differently, rising 10.8% under the solar forcing and only 4.7% under the carbon dioxide forcing.

Removing the carbon dioxide forcing raised global mean precipitation by 5.1% between year 70 and year 71, and it took about ten years to return to its prior level. Removing the solar forcing made precipitation fall at once.

Regression separates a fast response, set by the forcing agent, from a slow response, set by surface temperature. The fast term is about -4.2% per carbon dioxide doubling but only about -0.3% per one per cent of solar intensity, while the slow term is nearly identical across experiments at roughly 2.4 to 2.6% per degree of warming.

The asymmetry follows from where the forcing acts. Carbon dioxide absorbs and emits longwave radiation within the atmosphere, so its instantaneous forcing at the surface is much smaller than at the top of the atmosphere; the atmosphere cannot store the difference, and it is resolved within weeks by cutting the latent heat flux, which means cutting precipitation. Solar forcing creates almost no such surface-to-top imbalance.

Ocean heat uptake cannot be the explanation. Its evolution is nearly indistinguishable between the carbon dioxide and solar experiments even though the precipitation responses differ sharply.

A gradual ramp — 2% per year up to quadrupled carbon dioxide, then 2% per year back down — gives the same coefficients, about -4.2% per doubling and 2.6% per degree. Surface temperature begins falling as soon as carbon dioxide does, but precipitation keeps rising for several decades until the cooling effect overtakes the fast adjustment.

Why does it matter?

It separates two things that a global mean temperature alone conflates. Precipitation responds to the forcing agent as well as to temperature, so two worlds at the same temperature but different carbon dioxide concentrations have different hydrological cycles.

This is the mechanism behind the recurring result that solar geoengineering cannot restore both temperature and precipitation at once. Offsetting carbon dioxide warming with reduced sunlight leaves the fast carbon dioxide suppression of precipitation in place with no compensating temperature-driven increase, leaving the world drier than it started.

It also means the transient precipitation increase after emissions cuts is not a paradox to be explained away by ocean heat. It is the direct signature of removing carbon dioxide, and it would appear at realistic rates of change as well as in these idealized step experiments.

Citation

Long Cao, Govindasamy Bala, and Ken Caldeira (2011). Why is there a short-term increase in global precipitation in response to diminished CO2 forcing?. Geophysical Research Letters 38, L06703.

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