Trees, Water & Climate

On the U.K. based website The Independent, their science section published a report on March 19th entitled “Dams: Deep Trouble.” Below this ominous title, the tag line read “Are vast dams around the world masking the full extent of sea level rise?”

The story goes on to state “over the past 50 years, new dams and reservoirs have held back some 10,800 cubic kilometers of water, which would have been enough to raise global sea levels by about 30mm.” We crunched the numbers and came in at 32mm, and while the 10,800 km3 of water in reservoirs seems a bit high, let’s go with it. But the implication – that we’ve stopped building dams and therefore we’ll see a sea level rise of somewhat more than an inch – is underwhelming.

The Independent can be commended for noting other factors that may have been equally significant, such as draining of wetlands and aquifer depletion. But the 800 pound gorilla is deforestation, alluded to in the Independent’s article, but the focus of this post.

Rabbits will find the tall grass.
(Photo: EcoWorld)

We believe if there is climate change – which is manifested regionally and consists of three only indirectly related phenomena; drought, extreme weather, and global warming – it is the result of deforestation far more than the result of human CO2 emissions. And with respect to deforestation and sea level rise, it is clear that deforestation, at least theoretically, has had a far more dramatic impact on sea level rise than construction of large reservoirs.

There is an extremely interesting website called “Ten Billion Acres,” that advocates “reforesting planet earth for the sake of human survival.” They take the position, with detailed arguments that are at the very least thought provoking, that “were there enough Trees in the equation, Climate Change would not be occurring other than that which would be normal for the Earth’s and Oceans’ cycles during this Era.” “Ten Billion Acres” refers to the amount of deforestation experienced on earth in the last 500 years – accelerated in the last 150 years. We’ve verified these numbers – ten billion acres is approximately 15 million square miles or 40 million square kilometers – so how much land-based water was lost when these trees were cut down?

It is shockingly difficult to get online data on the water content of trees, but thanks to Google Books, there is a 1896 study available that documents the water content of a variety of representative species of trees through the cycle of seasons. The study is entitled “On the variation of water content in trees,” by James Barkley Pollock of the University of Wisconson. And it is clear from the data presented that the water content of trees is at least 50%, averaged across all trees and all seasons.

If you assume, for a global average, a forest has one tree for every five square meters, and that each tree has 10 cubic meters of mass (you can roll that around, this average assumes a rather dense forest of rather small trees, but overall these are probably somewhat conservative assumptions), then with a 50% water content, land based water that’s been lost to the oceans through deforestation would total 40,000 cubic kilometers, 4x the water volume sequestered in large reservoirs. And this number is grossly understated, since trees also sequester water underground as well as play a crucial role in replenishing aquifers. So why aren’t sea levels much higher?

At the least, these calculations indicate we still understand very little regarding the global hydrologic cycle. The volume of subsurface water, and the impact of depleting these aquifers still requires significant investigations. Our conclusion is that once again, the emphasis on CO2, or reservoirs for that matter, is misplaced. We should be figuring out how to increase forest canopy, particularly in the tropics where deforestation has the most significant impact on rainfall, aquifer health, climate, and global atmospheric quality. And we should be figuring out how to restore positive inflow to every aquifer on earth, where negative drawdowns have been grossly unsustainable ever since the invention of the mechanized pump.

Related Links:
Hydraulic Redistribution
Ogalalla Aquifer (Wikipedia)
CO2 & Global Warming
Aquabirds & Aquabuoys

3 Responses to “Trees, Water & Climate”
  1. Chris Greacen says:

    Seems to me that 1 tree per 5 square meters is a bit high. 5 square meters is a square 2.23 meters on a side. I don’t see trees, on average, growing that close together. I’d guess something closer to 5 meters per tree or so, which means 1 tree per 25 square meters.

    I think your estimate of tree volume is also high. 10 cubic meters is awfully big on average… Assuming your average tree is 10 meters high, that would assume a tree with a cross-section area of 1 sq meter from the trunk up to the top. That’s a radius of square root(1 / pi) = 0.564189584, or an araea of about 1.1 meters. While some trees are bigger, most aren’t this big in diameter, especially all the way to the top. I’d say something closer to 1 or 2 cubic meters would be correct.

    If these two corrections are right then you’ve over-estimated by a factor of about 5 in each — or total about 25 times in total. That puts the answer at something closer to 2,000 cubic km. Still a lot of water, but would have little effect on sealevel. Oceans cover around 350 million square km (about 70% of the earth’s surface). Using volume = area x height, it’s easy to calculate that 2,000 cubic km spread over the earth’s oceans is about 0.05 meters, or about 5 cm.

  2. Chris Greacen says:

    oops… instead of “araea of 1.1 meters” I meant “diameter 1.1 meters”.

  1. [...] which is mostly water, even trees, averaged over variety, climate, and season, still consist of roughly 50% water. Why waste all the space underground (presumably in old mines, empty oil and natural gas [...]

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