New Paper “Two Opposing Effects Of Absorbing Aerosols On Global-Mean Precipitation” By Ming Et Al 2010

There is a new paper

Ming, Y., V. Ramaswamy, and G. Persad (2010), Two opposing effects of absorbing aerosols on global-mean precipitation, Geophys. Res. Lett., 37, L13701, doi:10.1029/2010GL042895

which further documents the complexity and diversity of human climate forcings.  While it still, unfortunately, focuses on a global average (in this case precipitation), it does add to the understanding of the role of aerosols in climate.

The abstract reads

“Absorbing aerosols affect global-mean precipitation primarily in two ways. They give rise to stronger shortwave atmospheric heating, which acts to suppress precipitation. Depending on the top-of-the-atmosphere radiative flux change, they can also warm up the surface with a tendency to increase precipitation. Here, we present a theoretical framework that takes into account both effects, and apply it to analyze the hydrological responses to increased black carbon burden simulated with a general circulation model. It is found that the damping effect of atmospheric heating can outweigh the enhancing effect of surface warming, resulting in a net decrease in precipitation. The implications for moist convection and general circulation are discussed.”

There are some interesting findings in this paper. These include

Although some particular aspects of the issue (e.g., reduced surface solar flux, atmospheric heating, stabilization of the troposphere and reduced precipitation) have been discussed, often in the context of the surface energy budget and on the regional scale, still missing is a theoretical framework in which one is able to quantify all the processes essential for determining the change in global‐mean precipitation, and thus to devise an a priori measure of the ability of a particular climate perturbation to alter precipitation, analogous to what radiative forcing is for surface temperature. Such a measure would be highly desirable for purposes like model intercomparison and attribution of observed and model‐simulated changes in precipitation.

This study approaches the issue from the angle of energy balance constraint on the hydrological cycle. We argue that despite the large uncertainty in the current physical understanding and model representation of the radiative and/or microphysical effects of aerosols on individual precipitation events [Khain, 2009, and references therein], the global‐mean precipitation has to vary under such a constraint. This would generate valuable insights into the robustness of model simulations. The same methodology has been utilized successfully to study decadal‐scale hydrological response to greenhouse gases [Allen and Ingram, 2002; Held and Soden, 2006].


“We propose a concept of hydrological forcing (HF), which would provide a means to quantify the ability of a climate perturbation to modify global‐mean precipitation without performing expensive coupled model simulations.”

The proposal for new metrics on hydrologic forcing was introduced in

National Research Council, 2005: Radiative forcing of climate change: Expanding the concept and addressing uncertainties. Committee on Radiative Forcing Effects on Climate Change, Climate Research Committee, Board on Atmospheric Sciences and Climate, Division on Earth and Life Studies, The National Academies Press, Washington, D.C., 208 pp

where we wrote

“Despite all these advantages, the traditional global mean TOA radiative forcing concept has some important limitations, which have come increasingly to light over the past decade. The concept is inadequate for some forcing agents, such as absorbing aerosols and land-use changes, that may have regional climate impacts much greater than would be predicted from TOA radiative forcing. Also, it diagnoses only one measure of climate change—global mean surface temperature response—while offering little information on regional climate change or precipitation.”

We have proposed specific hydrologic metrics in our presentation

Pielke, R.A. Sr., and T.N. Chase, 2003: A Proposed New Metric for Quantifying the Climatic Effects of Human-Caused Alterations to the Global Water Cycle. Presented at the Symposium on Observing and Understanding the Variability of Water in Weather and Climate, 83rd AMS Annual Meeting, Long Beach, CA, February 9-13, 2003.

As research papers such as Ming et al continue to appear, the realization that the human influence on the climate system is significant beyond CO2, as we discussed in Pielke et al 2009, will become better realized.

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