Because it’s apparently too hard to cut the carbon emissions heating up the planet, many countries plan to sweep much of their pollution under the rug instead. This might be fine, except the rug will have to be comically, unrealistically large — the size of the entire U.S., according to a new study.

The net-zero promises made by 140 countries will require turning 990 million hectares of land, or about 3.8 million square miles, into a giant carbon-dioxide sponge, according to a new study in the journal Nature Communications. That’s almost exactly the size of all the land and water within U.S. borders. It’s about two-thirds of all the cropland on the planet.

And 44% of that land, or about 435 million hectares — roughly the area of Alaska, Texas, California, Montana, New Mexico, Arizona, Nevada and Colorado combined — would have to be repurposed, the study estimates. That would mainly mean planting trees where crops or shops or other non-tree things used to be. Some of it would be dedicated to bioenergy crops, or plants burned for fuel, with facilities to capture the carbon emitted. The other 56% of the land would be restored forests, mangrove swamps and other tree-friendly terrain.

There are a couple of troubling things about this finding. For one thing: all those trees.

Obviously, trees are great. They cool people down, provide housing for creatures and look beautiful. But they are suboptimal as carbon-removal machines. It takes years for them to reach their full carbon-drinking potential. And because they’re destined to either catch fire, fall over or die within a century or so, their carbon removal isn’t permanent. Carbon pumped into the atmosphere by burning fossil fuels is, on human timescales, permanent (unless we suck it out of the air and lock it up forever).

Simply paying people to plant trees has long been a predominant feature of the carbon-offsets market, which is one reason offsets are widely considered to be so sketchy. Entire countries relying on planting trees to absolve their own emissions is arguably worse, given the scale of the pollution involved. It’s a flimsy crutch for avoiding real change. That’s even more alarming when you consider the four countries with the biggest carbon-sponge plans to balance their net-zero books are all huge fossil-fuel producers: Russia, Saudi Arabia, the U.S. and Canada, respectively.

“While we will probably need to capture and store carbon in the future to reach net-zero emissions, we clearly need to stop relying so heavily on tree planting and bioenergy crops,” study co-author Wim Carton, a senior lecturer at Lund University in Sweden, said in a release.

The even bigger practical problem with these schemes is the ludicrous amount of land involved. Earth simply isn’t going to cough up another half a continent to serve as a global arboretum. That means vast amounts of land would have to be quickly converted to meet net-zero goals; the study estimates 13 million hectares a year between now and 2060. And this is a conservative estimate, the authors suggest, because the space requirement will likely grow as more countries make net-zero promises.

To get a taste of the political and social upheaval that might ensue from such a vast transformation, you need only look to the recent past. Soaring food prices in 2007 and 2008 led to a global land rush, with private investors and sovereign wealth funds snatching up cropland across the Global South. Between 2007 and 2014, 7 million hectares a year were bought from local owners and transformed, the study estimates, mostly to industrial farming to grow food that was shipped overseas. Local farmers and natural habitats suffered, while food insecurity, inequality and political instability rose. The land grab necessary to meet net-zero goals would be twice as large and go on for decades.

And yet some of the same countries ravaged by the recent land rush plan to dedicate huge swaths of their territory to carbon capture, the study points out. Several countries in sub-Saharan Africa plan to use 20% of their land for such purposes, risking more misery and upheaval for their people.

In contrast, renewable energy’s physical footprint is relatively tiny. Solar panels can be placed on existing buildings, while land dedicated to solar and wind farms can serve other purposes at the same time, including agriculture. The renewable energy sources and transmission lines necessary to decarbonize U.S. electricity by 2035 would take up just 19,700 square miles, or a little bit more than all the land currently dedicated to railroads, according to the National Renewable Energy Laboratory. Clean power is not only cheaper than fossil-fuel power, it’s a way to help countries meet their decarbonization goals without crowding out their people.

At the moment, the world’s net-zero promises aren’t nearly enough to limit global heating to 2 degrees Celsius above preindustrial averages, the goal of the 2015 Paris Agreement. The fuzzy carbon-capture accounting on which they rely makes these promises even weaker. Fortunately, getting real is easier than it might seem.

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This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

Mark Gongloff is a Bloomberg Opinion editor and columnist covering climate change. He previously worked for Fortune.com, the Huffington Post and the Wall Street Journal.

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