If we all get planting, roughly 4.4 billion hectares of our planet's surface could be shaded by trees – enough to pull decades of carbon we've pumped into the atmosphere.

As we reported earlier this year, ETH Zurich researcher Thomas Crowther and his colleagues have made it their mission to determine how we might press rewind on emissions before it's too late.

At a conference in February, he presented his team's findings on how maximising biodiversity will help us lock away carbon in the form of wood and other organic material.

Now they've published details on their research, and the news is kind of bitter-sweet.

Their analysis of the planet's potential to support lush woodlands found if we discount the parts of the globe that already have forests, not to mention cities and agriculture, there's still space left over - just under a billion hectares we can still squeeze some trees onto.

There's one small problem. Our global climate is changing quickly, which could mean in the next few decades this space for potential greenery could shrink considerably.

Considering the Intergovernmental Panel on Climate Change (IPCC) includes growing new forests and renewing old ones in recommendations for avoiding a 1.5 degree Celsius temperature rise by 2050, the new findings are a welcome addition to our data.

While we can crunch the numbers on how many trees we might need to offset emissions, we also need to know if we have the room for all those plants.

Previous studies have provided us with some rough estimates, mostly based on the capacity of current ecological areas. While we know these areas can expand, it doesn't tell us much about the potential of other open areas.

To avoid constraining themselves and get a more accurate range of figures, Crowther's research team looked at a range of protected environments that hadn't been heavily affected by humans.

Based on their analysis of nearly 80,000 measurements taken using Google Earth Engine tools, the researchers got a better idea of the actual amount of real estate we could devote to woodland carbon sinks.

Their calculations revealed 1.7 to 1.8 billion hectares of land areas now covered in sparse vegetation and bare soil qualify as tree-worthy.

If the right kinds of trees could be grown across all of that, Earth would have another 0.9 billion hectares of canopy containing around 200 gigatonnes of carbon.

These numbers could be even higher if we got a little crazy and also planted on agricultural land and throughout urban centres. Roughly 8.7 billion hectares in total could theoretically support trees.

Maybe a little crazy is what we need right now, because the conditions for growing these forests are based on our current climate conditions. If we don't change course, roughly 223 million hectares – especially across the tropics – won't count as potential forest land by 2050 as our temperatures rise.

Encouraging communities to get out wherever they can to plant trees is no doubt the right course of action, especially if it includes the restoration of degraded forests.

"It is vitally important that we protect the forests that exist today, pursue other climate solutions, and continue to phase out fossil fuels from our economies in order to avoid dangerous climate change," says Crowther.

"If we act now, this could cut carbon dioxide in the atmosphere by up to 25 percent, to levels last seen almost a century ago."

While it's easy to be pessimistic in light of reports that tree coverage is getting thinner by the year, there's reason for hope.

Indonesia has shown the way forward by declaring a moratorium on tree clearance several years ago. A decade ago, Norway promised to pay the country if deforestation rates fell.

That's exactly what happened in 2017. So in February, Norway paid up.

It's this kind of international effort we might need if we're to beat the clock. The space for forests is there – we just need to work together to fill it.

This research was published in Science.