Four cost-effective methods are ready today to remove substantial amounts of planet-warming carbon dioxide (CO2) from the atmosphere, according to a new report from a panel of top scientists.
All four take advantage of nature's ability to take carbon from the air and store it.
However, fully implementing all of them still would not be enough to prevent potentially catastrophic levels of global warming, according to the report from the National Academies of Sciences, Engineering, and Medicine.
Nearly all nations have signed on to the Paris climate agreement, which pledges to keep global warming to less than a global average of 2 degrees Celsius above pre-industrial levels, and ideally below 1.5 degrees.
Emissions from burning fossil fuels and other sources have already warmed the planet about 1 degree. At the current pace, temperatures will likely top 1.5 degrees by mid-century, according to the latest report from the U.N. Intergovernmental Panel on Climate Change.
Zero emissions technologies such as wind farms and solar panels will not be enough to stop global warming, the U.N. report says. Negative emissions technologies will be needed as well.
Trees are tops
The National Academies panel looked at existing strategies for removing CO2 from the atmosphere and found four that are ready for widespread use.
The first is among the most tried-and-true: planting trees.
"It's even kind of a misnomer to call it a technology," said Princeton University biologist Stephen Pacala, who chaired the panel.
Adding and restoring forests, plus better management of existing forests, are the two cheapest ways to get substantial amounts of carbon out of the atmosphere, the report says.
Farm and ranch lands offer the next biggest and cheapest CO2 removal strategies.
Overused soils lose carbon, as well as nutrients. Rebuilding them increases their fertility and water-holding capacity.
"And you get a negative emission because the carbon comes from the atmosphere," Pacala said.
"We know how to do quite a bit of this," he added, with soil conservation techniques that began after the 1930s Dust Bowl in the U.S. Great Plains.
The fourth ready-to-go approach, the report says, is known as biomass energy with carbon capture and storage, or BECCS. It generally involves burning or fermenting some kind of plant matter to produce electricity, fuel or heat, then capturing and storing the carbon emissions in underground sinks or elsewhere.
The report says BECCS has the biggest CO2 removal potential of the four but is also the most costly.
Applied worldwide, these techniques together have the potential to pull up to 10 gigatons of CO2 out of the atmosphere per year.
The world emits about 50 gigatons per year.
The authors note that devoting more land to CO2 removal would mean diverting land needed to produce food and clothing.
For example, they say removing 10 gigatons of CO2 by BECCS alone would consume nearly 40 percent of the world's cropland.
Even 10 gigatons of CO2 removal is extremely optimistic, the authors note. It assumes all the strategies are used to their fullest extent everywhere.
The panel also considered emerging technology that captures carbon dioxide directly from the air.
Currently, it is too expensive to be practical. But if the costs come down, Pacala said, it "would have essentially unlimited capacity to remove carbon."
Another promising approach would take advantage of the ability of certain naturally occurring minerals to react with CO2 and lock it up. But the authors say the fundamentals are poorly understood.
The report outlines a detailed research agenda to maximize all of the strategies. It notes that while the U.S. federal government may have other priorities, U.S. states, cities, corporations and other countries around the world are investing in fighting climate change. The country where the tools are developed stands to gain an economic boost, it says.