Why is CO2 Rising – Part 2
By Ed Caryl
How much biosphere has been destroyed? Deforestation is a large loss to the biosphere. From Wikipedia:
Global deforestation sharply accelerated around 1852. It has been estimated that about half of the Earth’s mature tropical forests—between 7.5 million and 8 million km2 (2.9 million to 3 million sq. mi.) of the original 15 million to 16 million km2 (5.8 million to 6.2 million sq. mi.) that until 1947 covered the planet—have now been destroyed .”
In the 24 years from 1988 to 2012, Brazil alone destroyed 400,000 square kilometers of Amazon rainforest, though that estimate may be low. The good news is that this forest loss has now subsided from over 26,000 sq. kilometers in 2004, to a mere 5000 or so sq. kilometers last year. The forest burners have now moved to Indonesia to make room for oil palms for “green” energy.
The estimate is that 500 million metric tonnes of carbon each year are released into the atmosphere from the Amazon alone from slash fires and decomposition of slash piles. (Slash: The leftover stumps, limbs, and other waste from forest clearing, including selective logging waste.) The Amazon is half of the tropical rain forest. Each hectare of intact Amazonian rain forest contains 190 metric tonnes of carbon, so that 500 million tonnes represents 26,300 square kilometers of forest, the amount destroyed in 2004, so the figures match.
500 million tonnes represents about half the tropical rain forest destroyed, so the total tropical forest annual destruction may be 1 gigatonne of carbon. Russia destroys about as much temperate forest as Brazil does tropical forest, and Russia represents about half the temperate forest destroyed per year. The total is the 2 gigatonnes of carbon each year that is mentioned on the NOAA chart as “land use changes”.
The problem with destroying forest to convert the area to some other use, is that forest soils, whether tropical or temperate, are very poor. Amazonian rain forest soils, when cleared, typically can grow a crop for a year or two, if the burned vegetation ash is returned to the ground. when the forest is removed erosion can wash away the thin topsoil, further depleting fertility. Then the best use of the ground is as cattle pasture, but then the soil turns into a hard, compacted laterite clay surface that cannot be plowed. Reversion to forest is very slow if it happens at all. After 25 years, the average carbon content of soil and above surface vegetation is about 26 metric tonnes per hectare, about 14% of the original carbon. Details here. A tropical forest destroyed is destroyed for a very long time.
The problem with this subject is that there is little accurate data on how much forest is being destroyed per year. The overall uncertainty is a factor of two. Is the contribution to annual carbon emissions 10% or 20% of the total? Oh, and who is doing the cutting? The mafia (or similar), pirates, organized crime. Now think about oil palms, government subsidies…
In the long term, forest destruction has two impacts on the biosphere: It emits a large fraction of the total carbon emitted, and that area destroyed no longer contributes very much to carbon sequestration.
I return to the Wikipedia quote above. If half the primary forest on earth has been lost, then the 120 gigatonnes carbon on that diagram above is changing over time. If that 120 gigatonnes of carbon returning to vegetation has been shrinking by two percent per year, that would account for all the increase in carbon dioxide in the last 50 years. The good news is that it is possible to turn that situation around very quickly with improved forest management, law enforcement, and replanting.