Visualizza wikitesto di New Scientist - “Trees can have a massive impact – if we get it right”

[[Categoria:Ritagli]]
[[Categoria:Ecologia]]
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== Fred Pearce ==
'''January 2020'''

{{libertine|''British ecologist Tom Crowther came under fire for suggesting we should plant a trillion trees. But it may be the best and cheapest way to draw down carbon from the atmosphere, he tells '''Fred Pearce'''.''}}

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{{commentoimg|[[File:2020-01-11-New-Scientist_Tom-Crowther.jpg]]}}

{{capogotico|Two years ago, British ecologist Tom Crowther set up a lab at ETH Zurich in Switzerland with the aim of doing high-impact science to show how and where we can restore the planet. His 30-strong team is already making waves. Crowther’s lab typically starts by counting things – from trees to nematodes – before bringing the numbers together to see global trends and quantify the effects of potential interventions.}}

Last July, his team made headlines around the world for claiming we have space to plant a trillion trees in areas of new forest amounting almost to the size of the US, and that doing so would be one of the most effective ways to address climate change. 

The media loved it, but pushback from Crowther’s fellow academics was huge. Some grassland ecologists feared that the “tree counter” wanted to plant over their savannahs. Others said his proposed forests could end up having a warming effect by altering how sunlight is reflected, or that he had got his carbon numbers wrong. Crowther recognises these concerns, and is working to address them with ambitious new research.

; Fred Pearce - Where did the trillion trees idea come from? 
: '''Tom Crowther''' - A friend of mine was working for an organisation that wanted to plant a billion trees to save the climate. But they didn’t know if that was a lot or a little. So we started collecting data on tree density around the world. In 2015, we published the first ever attempt to answer a seemingly obvious question. We found there were probably 3 trillion trees on the planet today, nearly eight times more than previously thought. 

; How accurate is that figure? 
: Not very. There is a lot of uncertainty. I am sure the error bar probably runs from around 2 trillion to 4 trillion. Within that, we are quite confident, however. 

; And we can have a trillion more trees? 
: Yes, I think so. If you ignore urban and agricultural areas, and places that have climates unsuitable for trees, we estimate that forests would naturally grow on an additional 900 million hectares around the world, which is probably room for 1.2 trillion trees. We also estimate that, when fully mature, these ecosystems could potentially store 100 to 200 billion extra tonnes of carbon. 

; The criticism from other scientists was intense. 
: Yes, and it has been really hard to take. A lot of the responses basically said the land wasn’t there. That we ignored competing land use demands, such as agricultural needs. Or that we ignored non-carbon impacts on climate from forests, such as the potential warming effects of dark foliage changing Earth’s reflectiveness. 
: Fair enough. We did. But we only ever had a simple aim_ to map all the land that was potentially available. We would never say whether the world should or should not restore trees to specific places, only whether they could. So it is only the start of a discussion, not the end. 

; But you were also slammed for suggesting that trees were the answer to climate change. 
: I agree that we got the messaging totally wrong. To call reforestation the most effective climate-change solution, which we sometimes did, was a naive mistake. We only meant that the restoration of natural ecosystems is quantitatively the largest single method available for drawing carbon out of the atmosphere. It is also the cheapest, if we do it right. But it is not a silver bullet. It is one of many solutions to climate change that we need to adopt in combination.

; Would the new forests be plantations or natural growth? 
: Where possible, the latter. Nature always does it better. But it is not always possible. So people need to help out by spreading seeds or planting saplings. We are trying to figure out which regions will restore themselves naturally, and where we need to assist. 

; What other areas are on your agenda? 
: I think that soils are the most important part of the puzzle for carbon storage. Soils are not visible, and assessing how much carbon they contain is hard. But they certainly contain more than the planet’s vegetation and atmosphere combined. So there is huge potential in encouraging the world’s soil to accumulate more. Not just in forests, but beneath grasslands, in peatlands and even on farms. 
: Key to that are soil organisms. We just had a paper out trying to do for soil organisms what we did for trees_ get an idea of how many there are and where they are distributed. We started with nematodes. These worms feed on plants, bacteria and fungi, and are a good, measurable indicator of soil activity and carbon cycling. They are also just big enough to count under a microscope.
: We reckon there are 57 billion nematodes for every human. Interestingly, there are more in far northern latitudes than in the tropics. In these cold areas, they are slow and inactive, but as those areas warm, they could potentially become important for future climate stability. 

; What else are you working on? 
: Clouds. The world needs to know how ecosystems like forests affect climate. We know about their carbon storage, but the production of clouds is important, too. Trees release massive amounts of moisture into the atmosphere every day. That moisture makes clouds that deliver rain to water other trees downwind. These clouds also influence temperature. They reflect a lot of the sun’s radiation away from Earth, but can also have a warming effect by preventing heat from escaping. : Climate modellers say clouds are one of their biggest uncertainties. So we want to quantify how reforesting the planet could make clouds and assess the climate impact of that. 

; Some say you are as much an activist as a scientist. 
: We are extremely rigorous in our research, but it is certainly targeted to what we think matters. I would not want activism to get in the way, but as long as the science is robust, then I want to do all I can to encourage a responsible restoration movement. A lot of people have asked us to help them with ecosystem restoration. So we are setting up a team to do this in a way that helps science too, by allowing us to monitor progress and how much carbon ecosystems absorb. 

; What are the limitations on replanting the planet? 
: There are so many challenges. First, it would take an immense effort to restore the world’s forests, and they would take more than 100 years to reach their full carbon-holding potential. That is why we want to help communities around the world to get going now. 
: But it can be dangerous to restore the wrong areas. If you plant trees in a native grassland, you can destroy local biodiversity, and if you restore forests in the high latitudes that would otherwise be covered with reflective snow, you can warm the planet. So the right ecological information is vital. 
: Then there is the social context. We know that too many reforestation projects don’t last. Organisations just plant trees and walk away. But as soon as the project teams have gone, the trees get cut down for timber or land that is desperately needed for people’s livelihoods. Only when the local communities benefit from ecosystem restoration can the projects be truly sustainable. So this is about science, but it’s about scientists being good citizens, too. ❚
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