Climate Change

A Fully Loaded, Double-Barreled Forest Climate Solution

September 24, 2014

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Deforestation of in Riau Province, Sumatra. Photo © Riau deforestation 2006" by Aidenvironment, 2006 - flickr:Riau flickr user:Wakx. Licensed under CC BY-SA 2.0 via Wikimedia Commons.

Over the weekend, two op-eds in leading national newspapers brought attention to the role of tropical forests as a solution to climate change — just in time for the UN Climate Summit happening this week in New York.

But their conclusions were a study in contrast. Is forest conservation a near-term solution to climate change, as Ellis and Ellis claim in the Washington Post? Or is forest climate science too complex to risk investments in a forest climate solution, as Nadine Unger claimed in the New York Times?

My view: While Ellis and Ellis propose a fully loaded, well aimed, double-barreled solution for climate action, Unger is firing blanks from one barrel and missing the target with the other.

Scientific Consensus: Tropical Forest Growth is Critical for Fighting Climate Change

Unger would have us believe that tropical forest conservation is controversial in the scientific community. It is not. The Ellis’ argument has broad scientific support.

The most recent International Panel on Climate Change (IPCC) report states that “leveraging the mitigation potential in the [forest and agriculture] sector is extremely important in meeting emission reduction targets (robust evidence; high agreement).”

Translation? One of the lowest-risk, best things we can do to combat climate change is conserve and restore forests. This approach is especially true in the tropics. And what makes that approach fully loaded is that 2,000-plus IPCC scientists and experts are in “high agreement.”

Here’s what their science tells us:

  • CO2 is the primary driver of climate change — now 42% above historic levels.
  • This increase is due to a few centuries of human activities that have released carbon stored by plants — initially in the form of trees (deforestation), and more recently also in the form of fossil plants (fossil fuels).
  • Forest destruction results in about 5 billion tons of CO2 entering the atmosphere every year.  This is what scientists call “net carbon emissions.”
  • In addition, about 10 billion tons CO2 are naturally absorbed by forests every year. The IPCC calls this a “’residual’ terrestrial sink.”
  • Today, halting net deforestation and forest degradation — activities all too common in the tropics — would reduce human CO2 emissions by about 12%.  That’s the single-barreled approach.

But the Ellis’ argument is double-barreled because it points out the added terrestrial sink side of forest conservation

In other words, if we halt all deforestation and degradation (what scientists call gross emissions, around 20% of human-caused CO2) while also accelerating reforestation of degraded lands where forests once stood— these tactics could solve about 30% of the near-term climate problem.

It’s a very ambitious goal. It’s also a goal with all sorts of co-benefits — the ones reforested lands provide people and nature.

The Controversial Science Behind Nadine Unger’s Analysis

Unger, in stark contrast, proposes a not-ambitious initiative: Don’t Plant Trees. The problem is her argument is based on very controversial science.

According to Unger’s latest findings, the volatile organic compounds (VOCs) emitted by trees heat our climate. It’s controversial because it’s a new idea, modeled by Unger, and there are lots of ways to run a model.

This is not to say it’s not good science. But it is frontier research not yet validated by independent analyses, and not at all ready for extrapolation to global climate policy. And it seems to me even more controversial because, according to Unger’s own results, the climate impact of tree VOCs is not significantly different from zero. No kidding – see figure 1 in her paper.

It may be that atmospheric scientists read bar charts and error bars differently from this forest ecologist. But for me, Unger’s main tree VOC argument is not convincing — it’s a chamber that’s firing blanks.

Missing the Target on Albedo Effect

There is another barrel to Unger’s argument, based on the albedo or “whiteness” effect of trees vs. crops or snow.

The argument goes like this: tree leaves are darker than crop leaves, and much darker than snow. So, the same way that dark asphalt gets hot in the summertime, a forest absorbs more heat than a corn field, offsetting the cooling effect of less CO2 in the atmosphere. Maybe fully offsetting that cooling effect if those corn fields would have been covered in snow much of the year.

This barrel sounds reasonable, and has more modeling rigor behind it — multiple studies over a number of years. Only it’s pointed at the wrong target: boreal forests.

As scientists led by Daniel Nepstad and Gregory Asner emphasize in a recent response to Unger on — and Unger herself acknowledges in her op-ed — tropical forests (unlike boreal) have a cooling albedo effect. They pump enough water into the air to make lots of reflective clouds, which more than offsets their dark leaves.

So in the tropics, the albedo effect is a bonus climate benefit of forests — on top of the incontrovertible carbon sequestration. Even if we find that Unger’s VOC argument holds water, it would be offset by this albedo effect in the tropics – leaving us where we started: with large carbon benefits from reforestation.

Let’s remember that the primary proposed target of a UN forest-climate financing mechanism is tropical forests — where all that deforestation is happening, where restoration opportunities abound, and where developing countries need financial assistance to implement these changes.

So, Unger does have a double-barreled argument. It’s just that one argument is firing blanks, and the other is missing the target. Let’s stick with the Ellis and Ellis double-barreled solution. It’s a well-supported, on-target plan to address climate change with tropical forest conservation and restoration.

How cool would it be to solve a big slice of the climate crisis by solving an even bigger slice of the biodiversity crises?

Bronson Griscom

I am an ecological accountant for The Nature Conservancy. What gets me up in the morning is this question: How can nature help us solve climate change? To explain this, let’s start with the fundamentals: CO2 is the largest cause of climate change. Through a magical process called photosynthesis, trees suck CO2 out of the air, turn the C into cellulose (wood), and release the O2 (oxygen) back into the air. In other words, forests, wetlands, and grasslands are a big climate change solution. More from Bronson

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  1. Curious to know what relative role promoting blue carbon conservation would have vis-a-vis forest carbon conservation.

    1. Thanks for the link to Verchot’s rebuttal. One must indeed question the motivation of the NYT for publishing the Unger piece. They seem always to air the musings of people in the early stages of rabies, in order to maintain the high moral ground of presenting all views evenly. But to date, NYT has not published any letters in response to Unger’s article. The overall effect is not helpful.
      I was still prepared to take Unger seriously; I am often finding that widely held assumptions are wrong. But it seemed odd that I had never heard these ideas, especially about the VOCs, before. There was also a strange emphasis on the negative effects of forestation efforts, with little concern for proposing alternatives, and with the merest head-nod to effects on biodiversity. Could this possibly be a plutocrat-bought scientist? So I went straight to the Nature Conservancy for some follow-up, and was happy to see balance restored by Griscom — thanks, Mr. Griscom.
      One shadow remains. The climate initiatives Unger was wrongly attacking involve developing countries in the tropics. But how does the albedo effect interact with boreal forestation, as in the Adirondacks?

      1. Hi Paula, A brief response to your question: how does the albedo effect interact with boreal forestation, as in the Adirondacks?

        I don’t think there is a simple answer. For truly boreal systems (further north than Adirondacks), my understanding is that there is enough evidence for a strong warming albedo effect of forests (counter-acting the cooling effect of C sequestration) that we probably should not attribute carbon offsets to boreal reforestation based simply on carbon accounting of tree biomass. There are also large soil carbon stocks in boreal systems – to the extent that boreal soil carbon flux is linked with land use/ land cover, that’s another important wrinkle to the boreal forest climate story.

        The Adirondacks are within the temperate forest system, where albedo effect is not as strong as in boreal system – but still a complicating variable. I suspect that the more southern portions of temperate region function more like tropical systems, and thus we are on solid footing simply accounting for climate benefits of forests in terms of tree carbon biomass. This leaves us with a question mark about places like the Adirondacks. So, all told, there is a big need for the scientific community to come together to properly answer the question you raised.


        1. Thanks, Mr. Griscom, for your reply, and for reminding me that there is a temperate band between the tropical and the boreal. Laypeople like me need to bone up on basic science if we’re going to be responsible citizens.

  2. Biogenic-VOCs, given the new research on aerosol formation, are an environmental plus. My rebuttle to Unger & Ken Caldera I post bellow;

    My two cents for the trees;
    Dr. Unger “In reality,” is just as guilty about complexity as she accuses others of being.
    “the cycling of carbon, energy and water between the land and the atmosphere is much more complex.”
    Quite so!, has her group at the Yale School of Forestry and Environmental Studies, considered the paleoclimate data for afforestation?

    Anthropogenic activities led to the stabilization of atmospheric CO2 concentrations at a level that made the world substantially warmer than it otherwise would be.
    The Kayopo Indian people with their Terra Preta soils were no carbon Saints, lake sediments show us a 5 Gt Carbon draw down with their demise. Genghis Khan’s Empire also very “green” with a 700,000,000 ton draw down. Now disrupting agriculture by rape and pillage may not be a politically correct form of afforestation, but it works.

    Dr. Jim Hansen’s 100 gigatons of Afforestation will work, as trees have worked time and time again.
    The Black Death increased afforestation in Europe by one third, the mass death of farmers is bench marked across the Paleoclimatic climate records. The Columbian exchange, that Grand reunification of life, was not quite so deliberate, in fact quite unintentional, however the chips of life fell where they may, Losers and winners abound.

    Hansen’s Afforestation accounting for CO2 soil & forest sequestration is understated. Not giving full account for new understandings of the ecological services rendered in light of what we are learning about the Pleistocene and the Aerosol chemistry elephant in the room, way understated.

    Physicist tend to focus on the carbon, oxygen, and nitrogen contents of the organic materials. Pöhlker et al were surprised finding very high soil fungal potassium levels, up to 20 percent, in the 77 Amazonian carbonaceous aerosol samples, in the form of salts, in all but three of them. The samples were on the scale of mere millionths or billionths of a meter. The smaller the aerosol, the greater the proportion of potassium – those collected early in the morning were the smallest and richest in potassium. Larger particles contained more organic material but not more potassium. These facts suggest that potassium salts generated during the night acted as seeds for gas-phase products to condense onto, forming aerosols of different kinds. [1]

    NPP increases CO2 draw-down, sugar exudates pumped deep into soils, if we manage biomass carbon in more recalcitrant compost/humus, and really recalcitrant pyrolitic C, biochar, we moderate the Keeling CO2 curve,. More CO2 inspired, less respired from the breathing biosphere.

    The late Pleistocene to Holocene boundary shows a prestigious pedogenesis, the loess–paleosol sequences of the central and northern Great Plains record a broad peak of high effective moisture, a pedogenesis we can emulate with the bio-remediation techniques we advocate on these lists as the only economic way to reverse climate change..

    The new research concerning the ecologically limiting effects of Phosphorous caused by the loss of the Mega-Fanua means we have never seen the true vigor that forest & grass lands could have. That what we now see as “pristine” systems are but a shadow of their primary production potential. The Pleistocene megafauna extinctions resulted in large and ongoing disruptions to terrestrial biogeochemical cycling at continental scales, switching off this natural nutrient pump by a massive 98%. The megafauna diffused sodium inland and also reduced concentrations in plants near the coast. [2]
    (There is a whole parallel literature developing in the marine literature, with deep diving megafauna playing a key role in nutrient dispersal in the oceans).

    “The science says that spending precious dollars for climate change mitigation on forestry is high-risk:”
    I disagree, and Spending on atmospheric & Biospheric research is imperative.

    “We don’t know that it would cool the planet,”,
    I say historically we do.

    “and we have good reason to fear it might have precisely the opposite effect.”
    We must scrutinize these “reasons” in the light of new biospheric & hydrological cycling data.

    Holocene carbon emissions as a result of anthropogenic land cover change

    The Columbian Encounter and the Little Ice Age: Abrupt Land Use Change, Fire, and Greenhouse Forcing

    Salt Seeds Clouds in the Amazon Rainforest

    How salt in the rainforest becomes clouds

    The legacy of the Pleistocene megafauna extinctions on nutrient availability in Amazonia

    Are Nutrient Limitations a Consquence of the Pleistocene Megafauna Extinctions?

    The Trees that Miss the Mammoths

  3. Very well-written debate. Congrats. To support your discussion, I encourage you to read the latest issue of the Journal of Forestry, a commentary on how deforestation in the southern hemisphere might be a relevant variable leading to global warming. Thanks