July 19, 2011

Earth Getting Greener, not Browner

Filed under: Adaptation, Plants

A new study in Scienceexpress (Science magazine’s pre-paper-publication outlet) by Yude Pan of the U.S. Forest Service and colleagues finds that the net carbon sink in terrestrial forest systems across the globe has been expanding, taking up ever more carbon dioxide from the earth’s atmosphere. (A “sink” is a place where something—carbon dioxide, heat, water, etc…winds up.)

The net carbon sink in the world’s forests is made up of carbon uptake less carbon loss. Carbon (C) uptake is expressed as bigger trees and more dense forests, storage in leaf litter, dead wood, wood products, and in the soil. Carbon loss occurs through deforestation and burning. By analyzing “recent inventory data and long-term field observations coupled to statistical or process models” Pan et al. conclude that “the global net forest C sink was 1.0 ± 0.8 and 1.2 ± 0.9 PgC yr–1 for 1990-1999 and 2000-2007”—indicating that the terrestrial forest sink has been at least consistent, if not expanding, over at least the past 18 years (1990-2007). A “Pg” is a Petagram, which is 1 followed by 15 zeroes worth of constant grams. For comparative purposes, our federal deficit is 14 followed by 12 zeroes worth of inflating $$$.

In fact, if it were not for tropical deforestation, the world’s forests would be taking up a huge percentage of the carbon dioxide emitted from anthropogenic activities. Pan et al explain:

Notably, the total gross C uptake by the world’s established and tropical regrowth forests is 4.0 PgC y–1, equivalent to half of the fossil fuel C emissions in 2009 [emphasis added]. Over the period studied (1990-2007), the cumulative C sink into the world’s established forests was ~43 PgC, and for the established plus regrowing forests was 73 PgC; the latter equivalent to 60% of cumulative fossil emissions in the period (i.e., 126 PgC). Clearly, forests play a critical role in the Earth’s terrestrial C sinks, and exert strong control on the evolution of atmospheric CO2.

The researchers find that even though the greatest annual carbon flux is occurring in tropical forests, those fluxes nearly balance out with the result being that tropical forests are largely carbon neutral. That’s because the annual carbon sink from tropical forest growth and regrowth (after logging), is offset by continued deforestation. Temporal and boreal forests, on the other hand, prove to be net carbon sinks (Figure 1).


Figure 1. Carbon sinks and sources (Pg C yr–1) in the world’s forests. Down-direction represents sink, while up-direction represents source. Light and dark purple colors are for global established forests (boreal, temperate and intact tropical forests), dark brown and orange colors are for tropical regrowth forests from deforested lands; and yellow and yellow green colors are for tropical gross deforestation emissions. (source: Pan et al., 2011).

Pan et al. describe the situation in the United States as follows:

The U.S. forest C sink increased by 33% from the 1990s to 2000s, caused by increasing forest area, growth of existing immature forests that are still recovering from historical agriculture, grazing, harvesting, and environmental factors such as CO2 fertilization and N deposition. However, forests in the western United States have shown significantly increased mortality in the past few decades, related to drought stress, and increased mortality from insects and fires.

Basically, the bottom line is that the world’s forests systems are subject to a number of difference influences, many of which are rooted in human activities (logging, CO2 emissions, nitrogen emissions, climate change), but overall, are expanding their carbon reserves—a pretty good sign that the world’s forests are thriving.

When it comes to the actual numbers forwarded by Pan et al., there is not a lot that is particularly surprising. The net annual forest sink determined by the Pan et al. team was 1.04 PgC in the 1990s increasing to 1.20 in the 2000s (through 2007). The Fourth Assessment Report of the Intergovernmental Panel on Climate Change reported the annual net terrestrial carbon sink to be about 1.0 Gt/year for the 1990s, a value pretty similar to Pan et al.’s numbers.

And the good news reported by Pan et al. that the terrestrial carbon sink continues to expand, is also not particularly shocking. After all, we have known (and reported here at World Climate Report) for some time that the percentage of anthropogenic CO2 emitted each year that actually stays in the atmosphere has remained pretty constant for several decades, despite ever-rising CO2 emissions from human activities. In order for that to be the case, the earth’s total (land + ocean) carbon sink must be expanding. And as we know that CO2 makes plants grow faster, better, stronger, more nutritious, more water use efficient, etc., is seems only reasonable to expect that the terrestrial carbon sink in the world’s forest systems is expanding.

We thank Pan et al. for work further confirming that (with all due respect to the departed Climate Czarina) the world is getting greener, not browner.

Reference:

Pan, Y., et al., 2011. A large and persistent carbon sink in the world’s forests. Sciencexpress, July 14, 2011, doi:10.1126/science.1201609.




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