CO₂ and forests - deforestation compared to human emissions as a cause of global warming

I decided to do some math to test my theory that the destruction of forests is more significant than human emissions.  If I've calculated correctly, it turns out that they are similar in significance.

Human emissions are 26 GT (billion tonnes) / year, about half of which is absorbed and half is accumulating.

My calculations below suggest that the missing forest and grassland that has become desert might have absorbed 21.5 GT / year, which is more than the 13 GT / year which is accumulating.

I thought the forests were a much stronger sink that this, but I still think it is interesting that this largely overlooked aspect is as significant as or more significant than the human emissions.

from wikipedia deforestation:

  "It has been estimated that about half of the earth's mature tropical forests - between 7.5 million and 8 million sq km (2.9 million to 3 million sq mi) of the original 15 million to 16 million sq km (5.8 million to 6.2 million sq mi) that until 1947 covered the planet - have now been cleared."

Let's say 7.5 million sq km, that is 750 million hectares.  This is only considering mature tropical forests, not other forests, grasslands, etc.

According to "Old-growth forests as global carbon sinks", Sebastiaan Luyssaert et. al., 2008:

  "Our NEP estimates suggest that forests 200 years old and above sequester on average 2.4+-0.8 tC / ha / yr (tC, tonnes of carbon)."

  nature journal forests carbon sinks

This means that the 750 million hectares of missing mature tropical forest would have sequestered 1.8 GT of carbon each year.

1 tonne of carbon corresponds to (12 + 2 * 16) / 12 or 3.667 tonnes of CO₂, so the missing forests would have sequestered 6.6 GT of CO₂.

In fact, this is an underestimate.  Luyssaert's group studied temperate and boreal forests.  Tropical forests sequester at least 50% more CO₂.  We can therefore increase the estimate to 10 GT / year.

Let's consider all of the forests, instead of just mature tropical forest.

  globalchange university of michigan

  "About one half of the forests that covered the Earth are gone."

  "Today, forests cover more than one quarter of the world's total land area, excluding polar regions."

The total land area of the earth is 149 million sq km, the area of the polar regions is approximately 26 million sq km.  This leaves 123 million sq km of land, one quarter of which is still covered by forest.

This tells us that about 31 million sq km of land is currently covered by forests, and about 31 million sq km of forest has been cleared by people (assuming half has been cleared).

The previous calculation was for 7.5 million sq km of missing tropical forests, we calculated at least 6.6 GT of CO₂ would be absorbed.  For the whole 31 million sq km of missing forests, 27 GT of CO₂ would be sequestered per year, or perhaps 31 GT / year if we consider that tropical forests absorb more CO₂.

Global emissions of CO₂ due to industry are 26 GT of CO₂ per year, therefore the missing forests if restored would be able to absorb all of our CO₂ emissions.  If we also include the forests that remain, forests can sequester 62 GT of CO₂ per year, which is more than twice the human emissions.  There are also other emissions, however, and another sink, the photosynthesis and CO₂ fixing that occurs in the ocean.

I did think that the forests would absorb more CO₂ than that, but I was right that the missing forests could have absorbed more CO₂ per year than the present human emissions.

I haven't considered that many forests were replaced with grassland or farmland, which is also a (smaller) CO₂ sink.  Farmland might sink half as much CO₂ as temperate forests.  This would reduce the impact of the missing forest to 17 GT / year.

Let's also consider desertification.

  ifad desertification

  "1035 million hectares (ha) are affected by human-induced soil degradation."

That is 10 million sq km, a somewhat larger area than our missing tropical forests.  This area might have sequestered perhaps 4.5 GT of CO₂ per year.

Accounting for these factors, human land use and abuse has led to perhaps a 21.5 GT / year reduction in the CO₂ sink.

We know that about half of the 26 GT / year human emissions are currently sequestered.  The 21.5 GT / year which missing forests and grasslands could have sequestered is 1.65 times greater than the remaining 13 GT of emissions.

It is known that forests and any photosynthesis will absorb CO₂ more quickly as the CO₂ concentration in the atmosphere increases, and more slowly as the CO₂ concentration decreases.  Obviously the forests cannot continue extracting CO₂ at the same rate forever if little remains in the atmosphere to extract.  An equilibrium would be reached.

In conclusion, my calculations have shown that the forest destroyed by people and the grassland that has become desert might have absorbed 21.5 GT / year of CO₂ (in addition to what is absorbed by that land now).  This is more than the 13 GT / year of CO₂ that is accumulating due to human emissions.

The remaining forests are able to absorb 1/2 of current human CO₂ emissions.  The forests and other lands we have destroyed, if restored, would be able to absorb all our emissions and an additional 30% more.  If we stopped 50% or more of our emissions, the remaining forests would cope.  The forests are able to absorb and fix CO₂ more rapidly as CO₂ concentration increases in the atmosphere, at least up to a point.



Sam Watkins
2009-07-31

sam.nipl.net
sam@nipl.net