Look out, another knot of tortured researchers just went past. All this time we’ve been pouring money into planting trees and stealing land from farmers because we were sure that trees would cool the world. (Just like solar panels do, yeah?) But life is so complicated — for years now some researchers have been quietly wondering if more trees were actually going to warm the planet instead, but they didn’t want to say much. It turns out that while trees absorb the sacred CO2 (that’s cooling!) they also emit methane (that’s warming!), and terpenes (cooling) and isoprene (warming and cooling!) If that’s not complicated enough, then there is the albedo effect. Trees are dark, they absorb more sunlight than bare ground and snow. So depending on where they are planted, that makes for “warming”. Then some VOCs or volatile organic compounds also seed clouds.
So what’s the net effect? Who knows, it’s not like there are whole industries dependent on it…
Now they ask?
As usual, the debate is based on logic and death threats:
At the same time, some researchers worry about publishing results challenging the idea that forests cool the planet. One scientist even received death threats after writing a commentary that argued against planting trees to prevent climate change.
It doesn’t matter if the planet dies, editors don’t want to look stupid:
“I have heard scientists say that if we found forest loss cooled the planet, we wouldn’t publish it.”
Yay, free speech!
To greener types, it doesn’t matter if trees warm the world on net, because they cool the world as well. And if that makes sense to you, the UN has a job waiting for you:
Although the analysis relies on big assumptions, such as the availability of funding mechanisms and political will, its authors say that forests can be an important stopgap while the world tackles the main source of carbon emissions: the burning of fossil fuels. “This is a rope that nature is throwing us,” says Peter Ellis, a forest-carbon scientist at The Nature Conservancy in Arlington, Virginia, and one of the paper’s authors.
Australians should — in theory — care more about this than almost anyone. We are one of the countries that did count carbon storage. If it turns out that land clearing cools the planet, Australia is stuffed (carbon accounting-wise): Our emissions per person fell 28% since 1990, but the largest single factor there was “land use” — meaning we stopped clearing and let regrowth take over some farms and paddocks. Then we shafted the farmers who owned the land and couldnt use it.
The 1997 climate treaty known as the Kyoto Protocol allowed rich countries to count carbon storage in forests towards their targets for limiting greenhouse-gas emissions. In practice, few nations did so because of the agreement’s unwieldy accounting mechanisms and other factors.
The albedo effect:
Researchers have known for decades that tree leaves absorb more sunlight than do other types of land cover, such as fields or bare ground. Forests can reduce Earth’s surface albedo, meaning that the planet reflects less incoming sunlight back into space, leading to warming. This effect is especially pronounced at higher latitudes and in mountainous or dry regions, where slower-growing coniferous trees with dark leaves cover light-coloured ground or snow that would otherwise reflect sunlight. Most scientists agree, however, that tropical forests are clear climate coolers: trees there grow relatively fast and transpire massive amounts of water that forms clouds, two effects that help to cool the climate.
This chemistry is so complicated, if only we had climate models that worked, we could figure out how much all this mattered:
Atmospheric chemist Nadine Unger, then at Yale University in New Haven, Connecticut, conducted one of the first global studies examining one part of this exchange: the influence of volatile organic compounds, or VOCs, emitted by trees. These include isoprene, a small hydrocarbon that can warm the globe in several ways. It can react with nitrogen oxides in the air to form ozone — a potent climate-warming gas when it resides in the lower atmosphere. Isoprene can also lengthen the lifetime of atmospheric methane — another greenhouse gas. Yet isoprene can have a cooling influence, too, by helping to produce aerosol particles that block incoming sunlight.
Some people are very good at complaining:
[Unger wrote an] opinion piece in The New York Times entitled ‘To Save the Planet, Don’t Plant Trees’, which argued that the large uncertainties around the extent to which forests cool or warm the climate made tree planting a risky strategy for fighting climate change. The article, and especially the headline (which Unger did not write), triggered a tsunami of complaints from researchers, who disputed the science and said the piece threatened to undermine years of research and advocacy. A group of 30 forest scientists wrote a response on the environmental news website Mongabay, saying, “We strongly disagree with Professor Unger’s core message.”
Trees emitting methane? Shame no one knew this before: Ban trees!
The latest findings are piling on even more complexity. Ecologist Sunitha Pangala at Lancaster University, UK, spent much of 2013 and 2014 in the Amazon rainforest, where she placed gas-measuring chambers around the trunks of more than 2,300 trees. “What we were really surprised about was the magnitude at which these trees are emitting methane,” says Pangala. She and Vincent Gauci at the Open University in Milton Keynes, UK, and their colleagues reported in 2017 that trees account for around half of the Amazon’s total methane emissions5.