Planting trees to reduce the amount of carbon dioxide in the atmosphere seems like a no-brainer in the fight against climate change.

But until recently it's not been clear how much land we'd need to make a tangible difference to warming, and whether we'd need to reclaim farm and residential land to do it.

Key points: Researchers identified 0.9 billion hectares of land that is available to be reforested

Researchers identified 0.9 billion hectares of land that is available to be reforested That could buy us a 20 year pause in climate warming

That could buy us a 20 year pause in climate warming Coastal ecosystems are capable of storing carbon up to 40 times faster than forests and should also be considered, expert says

Now new research published today in Science estimates there's enough suitable unused land on the globe for reforestation to store around 205 gigatonnes of carbon.

That's enough to buy us about 20 years in the fight against climate change, according to researcher Jean-Francois Bastin from the Institute of Integrative Biology in Zurich.

"This would definitely help to keep us at that maximum of 1.5 degrees by 2050," Dr Bastin said.

The researchers started by modelling the amount of tree cover the earth could sustain under current environmental conditions, if there were no humans on the planet.

They considered local climate factors like rainfall and temperature in their modelling.

Then they worked backwards, subtracting existing tree cover, urban environments and agricultural land.

They were left with 0.9 billion hectares of degraded land, which could be returned to canopy cover ranging from open savannah to dense forest.

The land they identified had been affected by things like logging, slash-and-burn fire regimes, intensive ongoing burning and clearing for grazing.

Other technological methods for combatting climate change, such as injecting sulphate aerosols into the stratosphere, have been proposed but most carry significant risks or are yet to be proven at scale.

The beauty of reforestation, according to Dr Bastin, is that it's win-win.

"When you think about tree restoration, it's not only about fighting climate change it's also about maintaining ecosystems," he said.

"The forests of the world protect 80 per cent of the biodiversity that exists on land."

Reforestation needs to happen before 'tipping point'

Deforestation in Australia is undermining efforts to rejuvenate habitat. ( Supplied )

The researchers also modelled the effect that climate change will have on how much of the earth's surface will be able to support trees in future.

Their findings suggest that reforestation needs to happen soon if it is to be effective.

Under current forecasts, they project that global tree canopy cover will shrink by up to 223 million hectares by 2050.

The impacts will be most significant at the tropics, according to Dr Bastin.

"The tropics will be under a lot more climatic stress. There will be more severe droughts," he said.

"Forests are a little bit resilient. But at some point we are afraid that it might shift, hit a tipping point and we are going to lose a lot."

An argument often espoused by climate-change sceptics is that more carbon in the atmosphere will equal more plant growth.

But David Ellsworth, an expert in the response of forests to climate change from the University of Western Sydney, said that wasn't the case.

Instead, tree growth is limited by a range of factors, including water availability and how nutrient-rich the soil is.

"What we know is that phosphorous is very limiting [in places like Australia]," Professor Ellsworth said.

He has conducted experiments where plants are exposed to the levels of CO2 predicted in the future, and observed no significant changes in growth rate.

While today's research points to exciting possibilities, he warned that the amount of CO2 that could be absorbed should be treated with some caution.

He said that by not factoring in soil nutrients or the full range of carbon densities of different vegetation types, there was room for error.

Most potential reforestation land in Australia and five other countries

Brush-tailed bettongs or woylie's are one species that could benefit from increased forest habitat. ( Supplied: Zoos Australia )

More than half of the land available for what the researchers call "tree restoration potential" was identified in just six countries.

Australia ranked fourth on the list, behind Russia, the United States and Canada, and was followed by Brazil and China.

Twenty million trees are expected to be planted in Australia by 2020 under a federal government program.

But critics say any reforestation efforts in Australia are being undermined by land clearing.

Deforestation in Australia in recent years has spiked, drawing comparisons with tree-clearing hotspots like the Amazon rainforest in Brazil.

In 2017 around 1,000 football fields were being cleared each day in Queensland, and more than a million hectares were cleared in that state between 2012 and 2016.

The Nature Conservation Council (NCC) claims around one football field of bushland was cleared in New South Wales every 10 minutes, in 2017-18.

"We are in an extinction and climate emergency. We must stop destroying wildlife habitat if we are going to stop more species disappearing," NCC CEO Kate Smolski said in a statement last week.

A million species worldwide are now under threat of extinction, according to a UN-backed report published in May this year.

In Australia, 121 species are listed as critically endangered by the IUCN, with 41 species having gone extinct.

A further 239 are endangered.

Numerous critically endangered and endangered species, including Leadbeater's possum, swift parrots and bettong could benefit from forest restoration in Australia.

'Armpits of the ocean' can store carbon 40 times faster

Blue carbon habitat has an image problem, according to Dr Macreadie. ( ABC News: Nick Hose )

But forests aren't the only ecosystems that can help fight climate change.

While trees are able to draw down carbon dioxide from the atmosphere and store it for the duration of their lives, blue carbon ecosystems can do it up to 40 times faster, and can potentially lock away carbon for 1,000 years or more.

Blue carbon is the term referring to stored carbon in coastal ecosystems like mangroves, seagrass and salt marshes.

But according to Peter Macreadie from the Blue Carbon Lab at Deakin University, these systems which he calls the "armpits of the ocean" have an image problem.

"That's a big, big problem," Dr Macreadie said.

"I'm working on a program in the Maldives where they're ripping out seagrass because people don't like the look of the dark patches in the water."

Getting public support to conserve and restore habitats that might be muddy and smelly and home to mosquitoes and sand flies is a difficult ask.

But in terms of a carbon sequestration investment, blue carbon is better value for money than tree planting, according to Dr Macreadie.

"If you're a carbon offset provider you're going to think, 'I can store the same amount of carbon in a fortieth of the land area'," he said.

Like planting trees, restoring coastal ecosystems has other benefits as well like boosting fish stocks and buffering coastlines from storm surges and sea-level rise.

While we focus on decarbonising our economies, restoring natural ecosystems may buy us precious time, according to Dr Bastin.

"We cannot be too picky. We need to use every good idea we can develop to fight climate change."