Climate Cooling Role of Forests Uncovered

Written by David J. Mildrexler, Maosheng Zhao, Steven W. Running

New study shows climate scientists have previously under-estimated the major cooling role forests play in regulating climate. Forests cover over 21% of the Earth’s surface. forestScientists say their global regulation of surface temperature highlights the important role of forests in local, regional and global climate. 

The new paper [1] published in the Journal of Geophysical Research, shows that the transpiration of forest ecosystems through the growing season dissipates more energy and lowers the Bowen ratio. In other words, this study reinforces the need for government climate researchers to include land use and land cover change when seeking to calculate human impact on the global energy balance.

Abstract

[1] Most global temperature analyses are based on station air temperatures. This study presents a global analysis of the relationship between remotely sensed annual maximumLST (LSTmax) from the Aqua/Moderate Resolution Imaging Spectroradiometer (MODIS) sensor and the corresponding site-based maximum air temperature (Tamax) for every World Meteorological Organization station on Earth. The relationship is analyzed for different land cover types. We observed a strong positive correlation between LSTmax andTamax. As temperature increases, LSTmax increases faster than Tamax and captures additional information on the concentration of thermal energy at the Earth’s surface, and biophysical controls on surface temperature, such as surface roughness and transpirational cooling. For hot conditions and in nonforested cover types, LST is more closely coupled to the radiative and thermodynamic characteristics of the Earth than the air temperature (Tair). Barren areas, shrublands, grasslands, savannas, and croplands have LSTmax values between 10°C and 20°C hotter than the corresponding Tamax at higher temperatures. Forest cover types are the exception with a near 1:1 relationship betweenLSTmax and Tamax across the temperature range and 38°C as the approximate upper limit of LSTmax with the exception of subtropical deciduous forest types where LSTmax occurs after canopy senescence. The study shows a complex interaction between land cover and surface energy balances. This global, semiautomated annual analysis could provide a new, unique, monitoring metric for integrating land cover change and energy balance changes.

Summary and Conclusions

[47] We compared the LSTmax from the Aqua/MODIS sensor to the corresponding site-basedTamax for every WMO station on Earth where Tamax is available. We first examined the relationship irrespective of land cover type, and as expected, a consistent positive correlation was observed between LSTmax and Tamax. Our results show that as temperature increases and more thermal energy is concentrated at the Earth’s surface, LSTmax and Tamax become increasingly decoupled. At the highest temperatures, LSTmax can be as much as 20°C higher than the corresponding Tamax. Tair can significantly underestimate the actual radiative surface temperature, especially at high temperatures and in nonforested areas. Because LST is more tightly coupled to the radiative and thermodynamic characteristics of the Earth’s surface, it may be an improvement to substitute LST for Tair in calculations of the global average surface temperature in the radiative-convective equilibrium concept equation [Pielke et al., 2007].

[48] We found the strength of the LSTmax/Tamax relationship to be land-cover-dependent. At low temperatures, LSTmax and Tamax are well coupled for all land cover types. Forests are the only cover type that maintains a strongly coupled LSTmax/Tamax relationship at highest temperatures and are distinct from the other land cover types because both LSTmax and Tamaxtend to range between the same values. The transpiration of forest ecosystems through the growing season dissipates more energy and lowers the Bowen ratio, and is the key driver for the stronger coupling of LSTmax and Tamax. Forests cover over 21% of the Earth’s surface and span a very large latitudinal gradient. The global regulation of surface temperature highlights the important role of forests in local, regional and global climate.

[49] Humans continue to dramatically influence global land cover through habitation, forest clearing, agriculture, and increasingly through anthropogenic driven climate change. This study reinforces the need to include land use and land cover change in holistic climate change studies and the important role that forests have in the global energy balance. Regarding policies proposed to influence forestry and land management practices for climate change-mitigation, the greatest uncertainties are in the biophysical influences that temperate forests have on climate [Jackson et al., 2008]. This study shows that temperate forests characterized by a seasonal summer drought cycle, such as in western North America, have a similar cooling effect on LSTmax and Tamax as tropical forests. A change to any other land cover type will result in a higher LSTmax, with commensurate impacts on the surface energy balance and hydrologic cycle of the affected area. Temperate forests with moist, humid summers do not have the same cooling effect on the expression of LSTmax and Tamax relative to the surrounding nonforested cover types because water is not limiting in the ecosystem during the time of thermal maxima.

[50] LST provides additional information on energy partitioning at the land surface-atmosphere boundary, and is more sensitive to changes in vegetation density compared toTair. With continuous spatial coverage the satellite-derived LSTmax data set may have value in studying the energy balance heterogeneity of the global land surface. The LSTmax is a particularly robust metric of the canopy temperature because during high Sun around noon when maximum temperatures occur, more short-wave radiation penetrates deep into the canopy of vegetation [Huband and Monteith, 1986]. The multidimensional thermal view of the environment that accurate, satellite-derived LST provides is critical to the actual experience of many organisms.

[51] The unique information provided by LST compared to Tair also enhances the benefits of combining these two variables together. Our findings suggest that the LSTmax/Tamaxrelationship presents new ways to track climate change, especially as these changes impact one climatological variable more than the other. For example, should summers become warmer in the cryosphere, as predicted by climate change, more snow free areas and drier soil conditions would result in the LSTmax rising faster than the Tamax. These long-term trends in the LSTmax/Tamax relationship would need to be tracked for decades. It may be important to further compare these data sets with other satellite based and ground based data sets such as the MODIS Albedo product, and with data from Fluxnet sites.

[1]  ‘A global comparison between station air temperatures and MODIS land surface temperatures reveals the cooling role of forests,’ Authors: David J. Mildrexler, Maosheng Zhao, Steven W. Running; First published: 30 August 2011. Full publication history; DOI: 10.1029/2010JG001486View/save citation

Read the full paper at onlinelibrary.wiley.com/doi/10.1029/2010JG001486/full