Cleaning up the greenhouse

To stave off dangerous levels of global warming in the next 80 years, scientists warn, it won’t be enough just to reduce today’s levels of greenhouse gas emissions, or even cut them to zero.

Instead, says climate scientist Bill Collins of America’s Lawrence Berkeley National Laboratory (‘Berkeley Lab’), we will eventually need to start removing 10–20 gigatons of carbon dioxide from the atmosphere every year.

To put that in perspective, Collins says, that’s about 5–10 times as much CO₂ as is currently emitted annually by all the world’s ships and planes, combined.

Another way to look at it, he says, is to realise that since 1780, humanity has put 1,000 gigatons of CO₂ into the air – a mass “equal to the entire mass of everything we’ve built: all our roads, all our buildings, all our homes”.

So – here’s the hair-raising moment – to achieve the goal of removing 10–20 gigatons of carbon dioxide a year from the air, we’ve got to remove a mass equivalent to 1–2% of the entire built environment, every year, until we get the problem under control.

One way to do this is by harnessing the way geological processes remove CO₂ from the air.

That works by the interaction of dissolved CO₂ in rain with rocks. The chemistry is complex, but in essence, the dissolved CO₂ makes the rain slightly acidic. This acid reacts with rocks and, in a process known as weathering, breaks them down into carbonate minerals that form sediments that eventually get buried in the ocean.

According to the Berkeley Lab’s Hang Deng, the Intergovernmental Panel on Climate Change (IPCC) has estimated that natural weathering removes about 1.1 gigatons of CO₂ from that atmosphere each year.

These natural processes occur largely in mountains, where cliffs, boulders, and talus (rocky debris) are exposed to whatever falls from the sky, and – as you’d expect – there’s not much we can do to speed them up.

But, Deng says, we could mimic the processes by tilling powdered rock into farmlands and fields. There, the material can much more rapidly weather than it normally would, partly because soils are often inherently acidic, and partly because kilogram for kilogram, pulverized rock has a lot more surface area with which to interact with acid than do giant boulders.

As a side effect, Deng adds, the process can even help provide nutrients for plant growth. “It could benefit agriculture as well,” she says.

A 2020 paper in Nature, she adds, projected that if this were done on half the croplands in the world, that alone could sequester 2 gigatons a year of carbon dioxide from the atmosphere—a nice start toward the total that Collins says we’ll need to reach.

Another approach is to use microbes genetically engineered to convert greenhouse gases into useful materials.

We all know that trees do this by using carbon dioxide, sunlight, and nutrients to make wood. But it may be possible to use related biological processes to make things as diverse as biofuels and plastics, and building supplies other than wood.

One person working on this is Deepika Awasthi, a Berkeley Lab molecular biologist on Collins’s team who is particularly interested in microorganisms that grow on methane.

Like carbon dioxide, methane is a greenhouse gas whose atmospheric levels have been substantially increased by human activities. It’s also 80 times more potent that carbon dioxide, making it a major target in the fight against global warming, even if there’s a good deal less of it in the air.

Methane-eating microorganisms, Awasthi says, can help fight this by being engineered to not just consume methane, but convert it to useful products, such as surfactants – important for detergents, emulsifiers, fabric softeners, cosmetics, and other products – which are currently made from petroleum products.

So far, it’s just a pilot study. But, she says, it is demonstrating two things: “One, we are finding bio-based replacements for petroleum-derived chemicals. And secondly, we are reducing the concentration of greenhouse gases in the environment.”

The big-ticket technology, however, is finding ways to remove carbon dioxide (and perhaps other greenhouse gases) from the air, rather than relying on natural processes, however enhanced, to do it for us.

Carbon dioxide absorbers have been around since the 1930s, says Eugene Kim, a graduate student in chemistry at the University of California, Berkeley. (They are vital to keep submariners from suffocating on the buildup of carbon dioxide from their own exhalations.)

The traditional methods work by using amine compounds to pull carbon dioxide out of the air and bind it into nongaseous form.

But they have low capacity, are corrosive (amines are derivatives of ammonia), and require a lot of energy per kilogram of CO₂ captured. Using them to capture power plant emissions, Kim says, would siphon off 35% of the plant’s entire energy output. So they’re not the most feasible way to go, especially if the goal isn’t just to reduce emissions, but to pull CO₂ out of the ambient air.

To solve this, Kim is developing metal organic frameworks (MOFs) – honeycomb-like structures with metallic corners connected by organic linkers containing the type of amines traditionally used to absorb carbon dioxide.

Part of their potency is that they’re porous, and can expose a great deal of surface area to the air flowing through them. “You can imagine a teaspoon of MOFs as having the same area as a football field,” Kim says.

That lets them rapidly and effectively capture whatever carbon dioxide is present. But, equally importantly, he says, MOFs can as easily release the trapped carbon dioxide when the time comes to dispose of it (perhaps by injecting it into underground geological formations that can trap and hold it for millennia).

Currently, Kim says, it costs $US600 (about AUD770) per ton to capture carbon dioxide from the air. “We think [MOFs] have the ability to bring it to below $US100 [about AUD128] per ton.”

But the bottom line is that all of these technologies eventually need to work together.

“We’re talking about creating a whole new industry from scratch,” Collins says. “The win-win from doing so,” he adds, “[is] we would help to launch a $US100 billion to $US1 trillion [AUD128 billion to 1.28 trillion] a year global green industry.”