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 Mongabay.com — 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?