By Carlyn Kranking
To control Earth’s mounting climate emissions, we really have our work cut out for us.
First, greenhouse gas emissions must be reduced to nearly zero by 2050 to prevent massive sea level rise, drought and heat waves. But that isn’t enough, since already-emitted carbon dioxide lingers in the atmosphere for millennia. To meet climate goals, this harmful gas must be removed from the air in a process called “negative emissions,” according to scientists at the virtual Comer Climate Conference this fall.
“There’s no scenario that keeps us under even 2 degrees [Celsius of warming] without negative emissions,” said physicist Raymond Pierrehumbert of the University of Oxford.
That is, no realistic scenario, he added.
“If you wave a magic wand” to erase CO2 emissions, that would prevent catastrophic warming, Pierrehumbert said in an email.
Though there’s no magic wand, the Earth does have a few defenses against changing climates. Some natural chemical reactions take carbon dioxide from the atmosphere and store it away. On their own, these methods take many thousands of years, but they would remove CO2 much more quickly if humans could accelerate them.
To succeed, though, the world must dramatically reduce emissions with green energy alternatives such as solar and wind. That way, the negative emissions techniques remove CO2 that’s already accumulated in the atmosphere.
Chemical reactions with rocks
If it weren’t for silicate weathering, Earth would be a hot, almost Venus-like hellscape. Carbon dioxide mixes with rain and causes the weathering, or breakdown, of rocks. This sets in motion chemical reactions that remove CO2 from the atmosphere and store it as minerals hidden away on the ocean floor.
This process has helped control Earth’s temperature throughout history.
“If it gets hotter, the silicate weathering goes faster, and it pulls CO2 out of the atmosphere. And if it cools down, silicate weathering slows down, so CO2 builds up,” said R.J. Graham, a doctoral student at the University of Oxford. “So it sort of acts as a thermostat for Earth’s climate.”
But now, human-forced climate change is warming Earth too quickly for it to be controlled by natural silicate weathering, which can take hundreds of thousands of years to work. To have a larger impact, Graham said we’d have to use enhanced weathering to speed up the process.
A silicate rock called olivine is particularly good for removing CO2 through weathering. If people mined olivine, broke it into tiny pieces to increase its surface area and distributed it on land or in the oceans, it would get weathered at high rates and remove lots of CO2.
“[This idea] hasn’t really had as much attention paid to it as I think it deserves,” Graham said.
Enhanced weathering has the potential to lower atmospheric CO2 concentrations — which currently sit at the unsafe level of over 410 ppm — by 30 to 300 ppm by 2100, according to a report in Nature Climate Change.
A nonprofit called Project Vesta, unaffiliated with the Comer conference, hopes to create beaches with the ‘sand’ made of crushed olivine. Ocean waves will weather the rock quickly, and at scale, this could remove a lot of carbon dioxide from the air.
But it’ll take a lot of rock — to neutralize one year of carbon emissions, it takes a cube of olivine 6.34 miles on each side. Though it’s not as much as the coal mined each year, that cube would be taller than Mt. Everest, which is just under 5.5 miles high. And that’s not the only challenge.
“It’s an interesting concept, but it also might be dangerous, because you’re dumping a lot of rocks into the ocean that’ll have a lot of other trace metals and things that might screw with the biology in the oceans,” Graham said. “It’s something that needs to have a lot more research done.”
Silicate weathering does affect the oceans — it ultimately makes them more basic, or alkaline. Making the oceans more alkaline helps take CO2 out of the atmosphere, and other scientists are studying how to make that happen.
The big, blue carbon sink
Earth’s oceans help to slow climate change by absorbing carbon dioxide, but as they do, the water becomes more acidic. If compounds were added to the oceans to make them less acidic and more alkaline, the oceans would pull even more CO2 out of the air.
Some scientists explore ways this process called ocean alkalinity enhancement could remove CO2 from the atmosphere. Sophie Gill, a final-year Ph.D. student at the University of Oxford, wants to know what effect this has on marine life.
When the oceans become more alkaline, some organisms react to it. Gill studies a kind of plankton called Emiliania Huxleyi, which has a shell made of calcium carbonate plates. In response to increased alkalinity, these plankton may create more plates in a process that releases carbon dioxide.
If these plankton release enough carbon dioxide, it might outweigh the amount that gets removed from the atmosphere through ocean alkalinity enhancement. This could make the entire process ineffective.
That’s what Gill is researching. She’s done lab experiments to increase alkalinity in sea water by adding compounds — sodium bicarbonate or sodium carbonate — and measured the reaction of these shell-forming plankton.
What she found, tentatively, was that adding these compounds does make the plankton release more carbon dioxide, but it’s not enough to outweigh the amount of CO2 that would be removed from the atmosphere.
Gill said she has to perform repeated experiments with different species to know for sure if this is true, but her findings are “tentative good news for CO2 drawdown.”
Limestone could create carbon-neutral shipping
Jess Adkins, a geochemist at the California Institute of Technology, explores how weathering of limestone can be sped up to rapidly neutralize carbon dioxide emissions.
Adkins said that weathering a cube of limestone that’s 2 miles long on every side would neutralize the world’s carbon emissions from one year.
When CO2 from the atmosphere naturally reacts with limestone, the process takes tens of thousands of years. But if it reacted with higher concentrations of CO2, like “in a smokestack or a tailpipe, it goes very, very fast,” Adkins said.
To make this work, sea water and limestone would be placed in a reactor. Then, a stream of gas full of carbon dioxide collected from tailpipes or smokestacks would be bubbled into the water. The carbon dioxide would react with the limestone, and in the end, it would produce a solution that can be safely added to the oceans without affecting ocean acidity.
Adkins studies how this process could work to neutralize emissions from the global shipping industry. If the ships carried a reactor with limestone and pumped sea water through it, they could use this to react away the carbon dioxide produced from running their diesel engines.
At CalTech, Adkins and others are getting ready to build a prototype that would try to neutralize the emissions from diesel engines on campus before attempting the process with a cargo ship.
“We think we can make a reactor that keeps up with the emissions of the ship,” Adkins said. “We would be making the industry completely carbon neutral, that is to say, eliminating 3% of the global carbon emissions.”
‘Humans are more than a force of geological proportions’
One way or another, humans are going to have to figure out a way to stabilize, minimize then offset greenhouse gas emissions. The world currently emits nearly 50 billion tons of carbon dioxide equivalent per year.
“We’ve really clobbered that natural carbon cycle,” Pierrehumbert said. “Humans are more than a force of geological proportions.”
Carbon drawdown is no silver bullet, despite its potential. These methods would take a lot of investment and resources to do at scale.
That’s why we can’t count on carbon drawdown alone to save us from climate change. But it still has a role in climate policy.
“I think that we’ve already burned enough, that we’re beyond the point of just limiting how much we put in the atmosphere,” Adkins said. “We’re going to have to remove CO2 in addition, of course, to moving from fossil fuels to renewable.”
Carlyn Kranking is a Health, Environment and Science reporter at Medill. You can follow her on Twitter at @carlyn_kranking.