Everybody loves trees. They are so popular as offsets they even make Wikipedia’s definition:
But does planting trees reduce global warming? Not in most places on the Earth. The Carnegie Institution’s Ken Caldeira summarized the result of a major 2005 study (detailed below) this way:
To plant forests to mitigate climate change outside of the tropics is a waste of time.
Why? Because forest canopies are relatively dark, compared to what they replace outside the tropics–grass, croplands, or snowfields–and so they absorb more of the sun’s heating rays that fall on them. That negates the “carbon sink” benefit trees have soaking up carbon dioxide. Worse, the study found that planting a large number of trees in high latitudes would “probably have a net warming effect on the Earth’s climate.” Ouch!
So what about an offset project involving tree planting in the tropics where water evaporating from trees increases cloudiness, which keeps the planet cool, according to models? Tropical-tree-planting offset projects suffer from a different problem:
How can we be sure that the project is resulting in a net increase in tropical trees? Imagine planting 1000 acres of trees in Brazil, where the full extent of annual deforestation is not known precisely. How do we know that an extra 1000 acres won’t be chopped down somewhere else in the country?
Until countries with tropical forests join an international greenhouse gas treaty and are subject to rigorous verification strategies, tree-related offset projects will not deliver guaranteed, quantifiable benefits.
So if you are thinking about purchasing offsets, be wary of any company that says it plants trees.
As for the study mentioned earlier, “Climate Effects of Global Land Cover Change,” by the Lawrence Livermore National Laboratory and the Carnegie Institution of Washington, here’s the abstract:
There are two competing effects of global land cover change on climate: an albedo effect which leads to heating when changing from grass/croplands to forest, and an evapotranspiration effect which tends to produce cooling. It is not clear which effect would dominate in a global land cover change scenario. We have performed coupled land/ocean/atmosphere simulations of global land cover change using the NCAR CAM3 atmospheric general circulation model. We find that replacement of current vegetation by trees on a global basis would lead to a global annual mean warming of 1.6 C, nearly 75% of the warming produced under a doubled CO2 concentration, while global replacement by grasslands would result in a cooling of 0.4 C. These results suggest that more research is necessary before forest carbon storage should be deployed as a mitigation strategy for global warming. In particular, high latitude forests probably have a net warming effect on the Earth’s climate.
Offset projects should simply not include tree planting.
RSS
Subscribe by Email
Follow Climate Progress on Twitter
So which organizations do have genuinely beneficial off-set strategies?
That will take some research. I’ll try to get to that after the Rules go up.
Does this suggest that I should put lighter-colored shingles on my roof? That we should use lighter colored concrete instead of black asphalt on roads?
Absolutely.
Trees are largely composed of carbon, hydrogen and oxygen. When trees die, most of the carbon is oxidized into CO2 and is released to the atmosphere.
Limited plant-derived carbon gets preserved. These limited conditions occur when plant materials happen to be deposted in certain anoxic (low oxygen) environments that prevent decay and oxidation of the plant tissue into CO2.
A peat swamp is such an environment where conditions of limited fresh water and decaying plant materials combine to deplete the water column of oxygen. Plant materials accumulate and form peat material in such environments. If the peat swamp is eventually covered with sediments, the organic materials can be preserved in the sedimentary record as organic rich sediments such as coal seams (largely derived from land plants) or kerogen rich oil shales (derived from hydrogen rich algal deposits).
In conclusion, planting trees is an ineffective approach to permanently capturing CO2.
I’m not sure why a single study (which does not even properly account for bodies of water) is repeated to the extend itself. Even the authors have been very modest about these results. It is a single, as of yet incomplete, study. Trees are likely to remain a GOOD way of reducing CO2 levels (there are not that many options for removing CO2).
I do appreciate a good discussion about offsetting, but I would like to see a little less of this one article.
A side note is that I am a medical researcher and am all too familiar with over excitement over one article. Also not meant as a person attack on anyone, just would like more discussion and not simply taking data at face value.
RE: Carnegie Institution´s 2005 study:
The study is probably a realistic simulation of the direct albedo effect. It does not, however, mention at all the impact that forest has on cloudiness. I don´t know if that is included in the model it uses, but the report does not discuss any uncertainties related to clouds. So, perhaps it was not included.
In general, forests increase cloudiness because they reduce rain water runoff and increase evapotranspiration (yes, forest transpires more water than plain water surface, for example). While changing a meadow to a forest decreases the albedo from roughly 0.15 to 0.10, clouds have an albedo of around 0.4 to 0.8. Even a rather small increases in cloudiness offsets the decrease in vegetation albedo.
Additionally, forests can be used for energy and materials, that produce very much less net emissions than fossil energy or alternative materials (concrete, steel, aluminum). Therefore, forests may be a viable alternative to mitigate climate change.
I am afraid that you got it wrong. Don’t feel bad many others did too.
For more information on the topic we invite you to read:
http://ecopreservationsociety.wordpress.com/ 2008/ 02/ 12/ does-reforestation-contribute-to-global-warming-a-second-look-at-the-livermore-study/
I looked through posts that the address you gave led to. I didn’t find anything about clouds. They did mention that they included the cooling effect of evapotranspiration (and it was not enough). That is distinctly different from cloud cover.
In the post that the link points to, there is a strange statement about the albedo effect becoming active only after age 80 of the forests. To me that seems to reflect little knowledge about boreal or temperate forests. The canopy closure typically takes place around age 30-40, and I would think tha albedo effect would be close to maximum already at that time. There is no abrubt change.
Also, the notes about snow cover on trees were very anecdotal. Much of the time the trees in the boreal winter are not those frosty, post card trees. Wind and occasional above freezing temperatures largely remove snow cover, to be recovered at times again but not continuously.
Sorry but I did not notice that I would have got anything wrong!
Has anyone done a study on the difference between carbon “sunk” into a tree over its lifetime and the amount released as it decays?…… on an individual basis or as a pattern of forest growth?….
Just wondering, if the statement above is correct and its a 1/1, then what is the mechanism for carbon removal from the atmosphere and storage in rock or soil?……
(OK,… I’m a newby,…. be gentle……
In the post that the link points to, there is a strange statement about the albedo effect becoming active only after age 80 of the forests. To me that seems to reflect little knowledge about boreal or temperate forests. The canopy closure typically takes place around age 30-40, and I would think tha albedo effect would be close to maximum already at that time. There is no abrubt change.