Small temperature benefits provided by realistic afforestation efforts (planting trees isn’t enough)

[OKIsItJustMe]

http://dx.doi.org/10.1038/ngeo1182

Small temperature benefits provided by realistic afforestation efforts

Vivek K. Arora & Alvaro Montenegro

Nature Geoscience
…
19 June 2011

Afforestation, the conversion of croplands or marginal lands into forests, results in the sequestration of carbon. As a result, afforestation is considered one of the key climate-change mitigation strategies available to governments by the United Nations¹. However, forests are also less reflective than croplands, and the absorption of incoming solar radiation is greater over afforested areas. Afforestation can therefore result in net climate warming, particularly at high latitudes^{2, 3, 4, 5}. Here, we use a comprehensive Earth system model to assess the climate-change mitigation potential of five afforestation scenarios, with afforestation carried out gradually over a 50-year period. Complete (100%) and partial (50%) afforestation of the area occupied at present by crops leads to a reduced warming of around 0.45 and 0.25 °C respectively, during the period 2081–2100. Temperature benefits associated with more realistic global afforestation efforts, where less than 50% of cropland is converted, are expected to be even smaller, indicating that afforestation is not a substitute for reduced greenhouse-gas emissions. We also show that warming reductions per unit afforested area are around three times higher in the tropics than in the boreal and northern temperate regions, suggesting that avoided deforestation and continued afforestation in the tropics are effective forest-management strategies from a climate perspective.

…

[NickB79]

1. If we reforest ALL of our current cropland, we only cut 0.45C off the temp. rise?

Crap, so much for that idea. With global food prices skyrocketing, we aren't reforesting ANY cropland in the near future.

[bananas]

2. Reforest the right places; reforest the wrong places makes it worse

http://www.grist.org/article/the-first-rule-of-carbon-offsets-no-trees

The first rule of carbon offsets: No trees
A good reason we shouldn’t love trees, at least not in this case
by Joseph Romm
2 Jul 2007

<snip>

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 (PDF) 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 1,000 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.

<snip>

Done right, reforestation can provide 2 wedges:

http://thinkprogress.org/romm/2011/01/10/207320/the-full-global-warming-solution-how-the-world-can-stabilize-at-350-to-450-ppm/

The full global warming solution: How the world can stabilize at 350 to 450 ppm
By Joe Romm on Jan 10, 2011 at 4:32 pm

<snip>

2 of forestry “” End all tropical deforestation. Plant new trees over an area the size of the continental U.S.

<snip>

[Gregorian]

3. I have doubts about this.

I know that global dimming is masking global warming. But I'm not convinced from what I read in that paragraph of an article that what they are talking about is fully understood. It feels like an onion that someone is trying to put back together. Which layer went where. When we look at the planet before we cut down the Black forests of the world, it was in equilibrium. I think the problem the article is realizing that we're up against is that we don't have 200 years to wait for forests to become fully diverse. We're trying to patch up an emergency now. I fear the article may have hit on something important. We have altered the climate in a way that is irreversible. The way back is to use less fossil fuel, and to stop draining nitrates into the oceans. Or in other words, fewer humans. The alternative is a band-aid. Unfortunately that may be all we have at our disposal now. Pretty awful.