More than 100 world leaders have promised to end and reverse deforestation by 2030, in the COP26 climate summit's first major deal.
Brazil - where stretches of the Amazon rainforest have been cut down - was among the signatories on Tuesday.
The pledge includes almost £14bn ($19.2bn) of public and private funds.
Experts welcomed the move, but warned a previous deal in 2014 had "failed to slow deforestation at all" and commitments needed to be delivered on.
Felling trees contributes to climate change because it depletes forests that absorb vast amounts of the warming gas CO2.
UK Prime Minister Boris Johnson, who is hosting the global meeting in Glasgow, said "more leaders than ever before" - a total of 110 - had made the "landmark" commitment.
"We have to stop the devastating loss of our forests," he said - and "end the role of humanity as nature's conqueror, and instead become nature's custodian".
The two-week summit in Glasgow is seen as crucial if climate change is to be brought under control.
The countries who have signed the pledge - including Canada, Brazil, Russia, China, Indonesia, the Democratic Republic of the Congo, the US and the UK (the full list is here) - cover around 85% of the world's forests.
Some of the funding will go to developing countries to restore damaged land, tackle wildfires and support indigenous communities.
Governments of 28 countries also committed to remove deforestation from the global trade of food and other agricultural products such as palm oil, soya and cocoa.
These industries drive forest loss by cutting down trees to make space for animals to graze or crops to grow.
More than 30 of the world's biggest financial companies - including Aviva, Schroders and Axa - have also promised to end investment in activities linked to deforestation.
And a £1.1bn fund will be established to protect the world's second largest tropical rainforest - in the Congo Basin.
Prof Simon Lewis, an expert on climate and forests at University College London, said: "It is good news to have a political commitment to end deforestation from so many countries, and significant funding to move forward on that journey."
But he told the BBC the world "has been here before" with a declaration in 2014 in New York "which failed to slow deforestation at all".
Hope and challenges ahead
There are reasons to be cheerful about the proposed plan to limit deforestation, specifically the scale of the funding, and the key countries that are supporting the pledge.
It is also very positive that it will try to reinforce the role of indigenous people in protecting their trees. Studies have shown that protecting the rights of native communities is one of the best ways of saving forested lands.
But there are significant challenges.
Many previous plans haven't achieved their goals. In fact, deforestation has increased since a similar pledge was launched in 2014.
There are often disputes between donors and recipients - Norway suspended funding for an Amazon fund in 2019 in an argument with Brazil's president.
There are also major questions over how a major financial pledge could be effectively policed.
How can funders verify that forests are actually being protected without spying from satellites or challenging national sovereignty in some way?
And question marks also hang over a key plank of the new plan, which is to try to remove the link to deforestation from consumer goods sold in developed countries.
One aspect is eating meat from animals, raised on imported soy grown on cleared lands. Will governments push companies and consumers to eat less meat to save the world's most important forests?
Ecologist Dr Nigel Sizer called the agreement "a big deal" - but that some will find the target of 2030 disappointing.
"We're facing a climate emergency so giving ourselves another 10 years to address this problem doesn't quite seem consistent with that," said Dr Sizer, a former president of the Rainforest Alliance.
"But maybe this is realistic and the best that they can achieve."
What was the failed 2014 agreement?
- The New York Declaration on Forests was a voluntary and legally non-binding agreement on deforestation in 2014
- It aimed to half deforestation by 2020, and halt it by 2030 - and 40 governments eventually signed up. But some key countries like Brazil and Russia weren't among them
- But the agreement failed, a report in 2019 found, saying deforestation was still continuing at an alarming rate
The deal's signatories include a number of key countries.
Indonesia is the world's largest exporter of palm oil, a product found in everything from shampoo to biscuits. Production is driving tree destruction and territory loss for indigenous people.
Meanwhile, Russia's huge natural forests, with more than one fifth of the planet's trees, capture more than 1.5 billion tonnes of carbon annually.
In the planet's biggest rainforest, the Amazon, deforestation accelerated to a 12-year high in 2020 under Brazilian President Jair Bolsonaro.
Asked whether leaders like Brazil's Mr Bolsonaro could be trusted to abide by the pledge, the UK's Environment Secretary George Eustice said "we should be really positive when countries engage".
"Last time there was an attempt at getting such a commitment on forests [in 2014], Brazil didn't take part, neither did Russia, neither did China.
"Brazil, they've really engaged with us on this agenda. It's a big step for them."
But pressed on whether the agreement will be enforceable, Mr Eustice said: "It doesn't go as far into talking about enforcement mechanisms and so forth, that's not the nature of these agreements."
He said what was different about this pledge in particular is that there is "the finance to back [it] up".
US President Joe Biden said he was "confident" the global pledge could be met, telling world leaders: "All we need to do is summon the will and do what we know is right. We can do this."
He said the US would lead by example, and announced it would spend $9bn (£6.6bn) to conserve and restore forests.
COP26 climate summit - The basics
- Climate change is one of the world's most pressing problems. Governments must promise more ambitious cuts in warming gases if we are to prevent greater global temperature rises.
- The summit in Glasgow is where change could happen. You need to watch for the promises made by the world's biggest polluters, like the US and China, and whether poorer countries are getting the support they need.
- All our lives will change. Decisions made here could impact our jobs, how we heat our homes, what we eat and how we travel.
Ana Yang, executive director at Chatham House Sustainability Accelerator, who co-wrote the report Rethinking the Brazilian Amazon, said: "This deal involves more countries, more players and more money. But the devil is in the detail which we still need to see."
But many people living in the Amazon, including in its urban areas, depend on the forest for their livelihoods and they need support in finding new incomes, she added.
Prince Charles told COP26 delegates that nations had to "honour" the rights of indigenous people as they were "experienced custodians" of their habitats. And speaking to members of the Global Alliance of Territorial Communities - representing 24 countries with tropical rainforests - he said: "We've got to work harder to save your forests, for all our sakes."
Tuntiak Katan, from the Coordination of Indigenous Communities of the Amazon Basin, welcomed the deal, saying that funds should be invested in supporting indigenous communities who are able to manage and protect forests.
Mr Katan, an indigenous Shuar from Ecuador, told the BBC indigenous communities globally protected 80% of the world's biodiversity but faced threats and violence.
"For years we have protected our way of life and that has protected ecosystems and forests. Without us, no money or policy can stop climate change," he said.
One of the biggest causes of forest loss in Brazil is to grow soy beans, much of which goes to China and Europe for animal feed for pigs and chickens, said Dr Sizer.
"We all end up consuming that, unless we're strict vegetarians and we don't eat soy. It's a very serious problem that we're all connected with."
Trees are one of our major defences in a warming world. They suck carbon dioxide out of the atmosphere, acting as so-called carbon sinks. They absorb around one third of global CO2 emitted each year.
Under present targets, the world is on track for warming of 2.7C by 2100 - which the UN says would result in climate catastrophe.
Currently an area of forest the size of 27 football pitches is lost every minute.
Depleted forests can also start to release CO2. If too many trees are cut down, scientists are worried that the planet will reach a tipping point that will set off abrupt and unpredictable climatic change.
Also on the second day of the two-week climate summit:
- The UK and India launched a fund to help small island states to prepare themselves against climate change disasters. Mr Johnson said the difference between a 2C and 1.5 temperature rise would mean "the difference between life and death" for many people living there
- The US and EU are launching an initiative that aims to drive global efforts to cut emissions of methane
- Mr Johnson apologised to an Israeli minister who said she could not attend the summit on Monday because it was not wheelchair accessible
The COP26 global climate summit in Glasgow in November is seen as crucial if climate change is to be brought under control. Almost 200 countries are being asked for their plans to cut emissions, and it could lead to major changes to our everyday lives.
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