“On The Misconception That Planting Trees Worsens Global Warming” by Lianhong Gu

The following is a guest weblog by Dr. Lianhong Gu of  the Terrestrial Water – Carbon Cycles Group Environmental Sciences Division at Oak Ridge National Laboratory:

Forests typically have lower albedo than other land surface types. This has led to the popular view that planting trees may worsen global warming (see Trees may warm the EarthSnowy forests ‘increase warming’ and How trees might not be green in carbon offsetting debate). However, forests and climate influence each other through an interwoven web of energy, mass and momentum flows. In this web, no factor can act alone. Recently, Bonan (2008) provides an excellent review on the complex interactions between forests and climate. I don’t want to repeat the review here. However, there are several forest – climate feedbacks left out in that review. These feedbacks are important to understand why it is a misconception to suggest that planting trees worsens global warming.

  1. Biomass heat and biochemical energy storages reduce daytime land surface temperatures. While it is well-known that forests can cool surface through transpiration, less attention has been paid to the fact that forests store solar energy physically through enthalpy change and chemically through photosynthesis and thus contribute to cooling land surface during daytime. In Gu et al. (2007), the authors find that even for a relatively sparse forest, the sum of the physical and chemical energy storages can reach up to 100 W/m2 and reduce land surface radiative temperature by up to 1 degree Celsius (2 degrees Fahrenheit).
  2. Forest canopy protects soil carbon and thus reduces terrestrial carbon dioxide emission to the atmosphere. Forests not only sequester carbon through photosynthesis but also protect organic carbon that is already stored in the soil. The protection is achieved mainly through two ways. First, forest canopy blocks sunlight from soil and thus reduces soil temperature. On a bare ground during daytime, the highest temperature is found at the soil surface. In contrast, the highest temperature is found at the top of the canopy in a forest, away from the soil (Gu et al. 1999). Second, forest transpiration quickly dries up the moisture of the soil surface during the daytime. The quick drying occurs because trees concentrate their fine roots at the surface where most soil carbon is stored. Soil carbon release to the atmosphere increases with temperature and moisture. Thus forest canopy protects soil carbon by keeping soil temperature and moisture low.
  3. Forests in coastal regions serve as a springboard in the transport of moisture from ocean to continental interior and thus contribute to continental-scale terrestrial productivity and carbon sequestration. Without forests to provide continuous input of latent heat flux to the atmosphere, landfalling hurricanes decay rapidly once over land (Kimball 2008). Forests also influence monsoon strength. Studies have shown that in East Asia, the conversion of forested land to agriculture may have weakened the summer monsoon and reduced precipitation in northern China (Fu 2003). Thus the moist level in continental interior may very well depend on the presence of forests in coastal regions.

The processes discussed above and those articulated in Bonan (2008) are crucial for a complete picture of the roles of albedo in the forest – climate relationships. They show how much is betrayed by the catchy storyline ‘planting trees worsen global warming’. There is no question that the public needs to know about the forest – climate relationships. But it is the responsibility of climate researchers to convey the wholeness of the picture to the public.

Bonan, G.B., 2008. Forests and climate change: Forcings, feedbacks, and the climate benefits of forests. Science, 320: 1444-1449.

Fu, C.B., 2003. Potential impacts of human-induced land cover change on East Asia monsoon, Global and Planetary Change, 37: 219-229. 

Gu, L., T. Meyers, S. G. Pallardy, P. J. Hanson, B. Yang, M. Heuer, K. P. Hosman, Q. Liu, J.S. Riggs, D. Sluss, S. D. Wullschleger, 2007. Influences of biomass heat and biochemical energy storages on the land surface fluxes and radiative temperature. Journal of Geophysical Research – Atmosphere112, D02107, doi:10.1029/2006JD007425.

Gu, L., H.H. Shugart, J.D. Fuentes, T.A. Black and S.R. Shewchuk, 1999. Micrometeorology, biophysical exchanges and NEE decomposition in a two-story boreal forest – Development and test of an integrated model. Agricultural and Forest Meteorology, 94, 123-148.

Kimball, S.K., 2008. Structure and evolution of rainfall in numerically simulated landfalling hurricanes. Monthly Weather Review, 136: 3822-3846.

 

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