The Publication of the Gu et al JGR Paper On Two Important Largely Neglected Climate Processes

The very important new research paper on vegetation dynamics, that was introduced on September 28 2006 on Climate Science, has appeared;

Gu L., et al. (2007), Influences of biomass heat and biochemical energy storages on the land surface fluxes and radiative temperature, J. Geophys. Res., 112, D02107, doi:10.1029/2006JD007425.

As was discussed in an earlier Climate Science weblog and highlighted at the begining their abstract,

“The interest of this study was to develop an initial assessment on the potential importance of biomass heat and biochemical energy storages for land-atmosphere interactions, an issue that has been largely neglected so far.”

and at the end of their abstract,

“From these simulation results, we concluded that biomass heat and biochemical energy storages are an integral and substantial part of the surface energy budget and play a role in modulating land surface temperatures and must be considered in studies of land-atmosphere interactions and climate modeling.â€?

This study demonstrates that models without these two processes will have significant errors in their accurate representation of surface heat and moisture fluxes, and in radiative temperatures, that are measured when vegetation is present. As they write,

“…..the radiative forcing of greenhouse gases (CO2, CH4, N2O, and halocarbons together) is about 2.43 W m−2 above the preindustrial level [IPCC, 2001]; this value could be smaller in the current atmosphere since some of the earlier imbalance presumably has already warmed the climate system. Thus at least at regional scales, biochemical energy storage is on the same order of magnitude as the radiative forcing of atmospheric greenhouse gases. Therefore, for long-term climate system modeling which includes vegetation processes, biochemical energy storage could be important, particularly at regional scales.”

We have shown in an observational study that the incorporation of the amount of transpiring vegetation does significantly affect maximum and minimum temperatures;

Hanamean, J.R. Jr., R.A. Pielke Sr., C.L. Castro, D.S. Ojima, B.C. Reed, and Z. Gao, 2003: Vegetation impacts on maximum and minimum temperatures in northeast Colorado. Meteorological Applications, 10, 203-215.

The Gu et al study presents evidence of two climate processes that would contribute to such differences.

Their work also means that the assessment of multi-decadal surface air temperature trends which do NOT factor in changes in vegetation at an observing site over this time period, must have errors in any attribution of temperature trends. They write on this subject,

“The diurnal temperature range (DTR) on land has been decreasing since the middle of the 20th century [Easterling et al., 1997]. The cause of this trend is not completely understood even though there have been many studies on this topic [e.g., Hansen et al., 1995; Dai et al., 1999]. Collatz et al. [2000] suggested that changes in vegetation cover may have contributed to this trend through controls on latent heat flux and atmospheric stabilities and feedbacks on atmospheric processes. We suggest that changes in biomass heat and biochemical energy storages may be another mechanism for vegetation to influence DTR. Biomass heat and biochemical energy storages act to reduce daytime surface temperature and increase nighttime temperature, thus leading to decreased DTR. Globally, vegetation productivity has been increasing [Myneni et al., 1997; Boisvenue and Running, 2006] and therefore should contribute to dampening DTR. We emphasize that our estimate of influences of biomass heat and biochemical energy storages on DTR (0.5°C) is conservative because we did not consider the feedback from changes in biomass temperature on the atmospheric forcing temperature. If this feedback is considered, the effect of biomass heat and biochemical energy storages on DTR might be even larger.”

They also conclude that,

“Finally, biomass distribution is spatially heterogeneous, which means that biomass heat and biochemical energy storages must be also spatially heterogeneous. This heterogeneity is in essence a form of gradient radiative forcing [Matsui and Pielke, 2006]. In conjunction with spatial variations in evapotranspiration, albedo, and surface roughness associated with vegetation cover, it can influence horizontal pressure gradients and mesoscale atmospheric circulations and therefore regional climates. More studies are needed in this area.”

These two climate processes are yet additional sources of uncertainty in the assessment of climate system heat changes (global warming or cooling) using land surface air temperatures as the selected metric.

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