"The sooner you start socking away part of your paycheck for retirement, the easier it is to hit your goals. If you wait until late in your career to start, your goal could be simply out of reach unless you take a second job or win the lottery." That axiom is a bit like how we've treated carbon emissions: when governments started negotiating to limit greenhouse gases in the early 1990s, the agreed-upon goal of limiting global warming to 2 degrees Celsius was pretty doable. The continued rise of emissions since then has turned that into a tall task, while the 1.5 degree Celsius aspiration added to the recent Paris agreement is even taller.

At this point, hitting those targets very likely requires more than just shifting away from fossil fuels. We’ll also need “negative emissions”—activities that absorb carbon dioxide from smokestacks or directly from the atmosphere—in order to squeeze under the line. It’s a simple concept but a very big challenge. Some nascent negative emission technologies exist, like capturing CO 2 from smokestacks and pumping it into underground reservoirs. Other proposals, however, sound pretty wild and may not work—like dumping iron dust into the oceans to spur plankton growth.

One tempting strategy is to imitate nature’s longterm carbon control—the weathering of fresh rock. When certain minerals react with CO 2 in rainwater, they turn into different minerals, and the CO 2 turns into bicarbonate that ends up in groundwater, rivers, and ultimately the ocean.

The problem is that this natural process is very slow. To make a dent in the concentration of atmospheric CO 2 , we'd need thousands of years that we simply don't have. But what if we could greatly accelerate the process?

A recent study from the University of Sheffield’s Lyla Taylor explored one scheme to do just that, implemented on an absolutely massive scale. The idea is to mine igneous rock, pulverize it, and spread it over land in the tropics where it could rapidly weather in the soil. To assess the idea in greater detail, the researchers started with models that simulate weathering, soil processes, and plant growth. These were fed a couple of climate model scenarios—the business-as-usual scenario that results in more than 4 degrees Celsius warming compared to pre-industrial times by 2100 and a lower scenario where significant action keeps warming to around 2.5 degrees Celsius.

In the model, they “limit” the area where this pulverized rock is spread to the most effective portions of the tropics, which includes the rainforests. That still involves fully 20 million square kilometers—an area larger than Russia. Crushed basalt, or a faster-weathering igneous rock called harzburgite, is applied at a rate of 1 to 5 kilograms per square meter of ground per year.

In the simulations, spreading basalt reduced atmospheric CO 2 at the end of the century. It went from 930 parts per million in the business-as-usual scenario down to about 880 or 760 ppm, depending on the amount of rock. For the lower emissions scenario, CO 2 was reduced from 540 ppm down to 500 or 400 ppm. Crushed harzburgite could have an even bigger impact than basalt, reducing atmospheric CO 2 an additional 60 to 120 ppm. That translates to 0.2-0.9 degrees Celsius less warming in 2100 for basalt spreading, and 0.5-2.2 degrees Celsius for harzburgite.

There are also the oceans to consider, though. Rising concentrations of CO 2 in the atmosphere cause ocean acidification as well as warming, reducing the pH of seawater to the detriment of critters that build skeletons out of calcium carbonate—like coral and some plankton. In the lower emissions scenario, basalt spreading could return ocean surface pH to its current level at the end of the century, rather than see it continuing to drop. In the high emissions scenario, it could at least lessen the acidification.

There’s a bonus, too. Remember the bicarbonate produced by the weathering reaction that washes out to sea? That could raise the amount of carbonate available for organisms in much of the tropical ocean, keeping corals significantly healthier.

Those are the pros. The cons, on the other hand, are significant. To employ the scheme at this scale, you would have to mine more rock than our current global coal production. That comes at the price of landscape and ecosystem disruption. Then you have to crush, transport, and distribute (somehow) the stuff. That all requires energy. With current technology, that means greenhouse gas emissions involved in running this scheme would offset something like 8 to 33 percent of your CO 2 drawdown. Worse yet, the researchers ballpark the cost of removing the first 50 parts per million of CO 2 at hundreds of trillions of dollars—a number that exceeds the combined GDP of every country on Earth by a significant amount.

The researchers characterize the idealized scheme in their simulation as “the maximum potential [carbon dioxide removal] capacity of the approach” for a host of reasons. They conclude, “These issues support calls for the alternative of a rising international carbon fee. We proffer enhanced weathering not as a panacea for erasing impacts of fossil fuel burning, but as a sobering indication of actions that may be required if fossil fuel emissions are not phased down rapidly.”

Nature Climate Change, 2015. DOI: 10.1038/nclimate2882 (About DOIs).