Next ideas of negative emissions

Worries about anthropogenic climate change are not new. As far back as 1896, Swedish chemist Svante Arrhenius, who would later win the Nobel Prize in Chemistry for other work, suggested that if we burned enough fossil fuels to double the atmospheric level of carbon dioxide from its then 295 parts per million (ppm), the planet would warm by an average of 5°C.

His estimate was a bit high by modern standards, but not off the charts. And even if he didn’t nail it, he got close enough to recognise a looming problem long before anyone else was even thinking about it.

Today, the problem is no longer looming. Carbon dioxide levels are now 419 ppm – 42% higher than in Arrhenius’s day – and closing in on the 430 ppm level the Intergovernmental Panel on Climate Change (IPCC) sees as a critical threshold – above this level, the globe will warm by more than 1.5°C, widely viewed as the difference between being problematic and being a massive catastrophe. (Arrhenius’s 5°C would be horrific.)

“Climate change is occurring, right now,” says Bill Collins, director of the Climate & Ecosystems Division at America’s Lawrence Berkley National Laboratory (LBNL), in Berkeley, California, and a lead author of the IPCC’s latest assessment. “We have run out of [room] to kick the can down the road.”

At a meeting of the American Geophysical Union in New Orleans, Louisiana, last December, Collins said that it was no longer adequate to merely think in terms of reducing emissions. What’s needed are “negative emissions” – the removal of some of the carbon dioxide that is already in the atmosphere.

If that sounds pie in the sky, think again. Collins says his lab has already launched a number of programs examining quick, efficient ways to do just that. Doing so on a large-enough scale will be a major challenge, he says, “but fortunately several of the more promising technologies have been intensively researched”.

One method is to improve on something nature has been doing for millions of years: using soils to store carbon pulled out of the atmosphere by growing plants.

One simple way of enhancing this process, says Whendee Silver, a biogeochemist and ecosystem ecologist at the University of California, Berkeley, is by large-scale applications of compost. She says field tests have found that this can sequester the carbon equivalent of five to seven tonnes of CO₂ per hectare annually. And while that’s a drop in the bucket compared to the 10 to 20 gigatons she says climate scientists believe need to be removed from the atmosphere each year, there is a lot of cropland and rangeland to work with. Worldwide, she says, compost has the potential to sequester two gigatons of carbon dioxide annually — somewhere between 10% and 20% of the overall need.

It’s also a win for agriculture. “It increases plant growth and water-holding and decreases erosion,” says Silver. “There’s not a lot of work that needs to be done to convince ranchers and farmers to do this.”

Another “soil amendment” that could have even more dramatic effects is biochar.

Biochar is partially burned organic material that has been converted into a form of carbon known as black carbon. It’s a bit like charcoal, though it can also be made from a lot of materials, including manure. It is highly resistant to decay (meaning the carbon in it will remain in the soil for a long time) and has long been used to enrich agricultural soils. It was used for thousands of years by indigenous peoples of the Amazon Basin, for example, to convert the area’s normal, infertile red soils into highly prized terra preta (dark earth) croplands that still exist today.

It’s only recently that biochar’s value to the quest to go carbon negative has been fully recognised. Overall, says Silver, it has the potential to remove twice as much carbon dioxide from the atmosphere as does compost.

And that’s not the only way it could be used. “There are people looking at ways to put it into concrete and net-negative building materials,” says Blake Simmons, biological systems and engineering director at LBNL. “So, there is a whole gamut of opportunities out there for this material.”

Another soil-enhancement technology is crushed rock (see “Cleaning up the Greenhouse” in Cosmos Weekly #5).

“[That] can sequester up to two billion tonnes of carbon dioxide per year,” Deng says. Better yet, this too can enhance agricultural productivity. “[It] helps soil health, like liming,” Deng adds.

But soil amendments won’t get us all the way to where we need to be, even if done across the world, as scientists hope. Ultimately, it will also be necessary to start directly removing carbon dioxide from the air, either at point sources such as power plants, cement plants and steel mills, or more diffusely from ambient air at whatever location is convenient. That carbon dioxide can then be transferred via the type of pipelines used for natural gas to locations where it can be more permanently disposed of.

This is already being done, says Jens Birkholzer, a hydrogeologist at LBNL, but only at a scale that captures about 40 megatons of carbon dioxide per year. By 2050, he says, we are going to need to increase it by a factor of nearly 150, to 5.6 gigatons per year.

It’s a two-step process. The first step is to extract the carbon dioxide from the air. Currently, that’s very expensive — in the order of $US600 per tonne, according to Matthew Dods, a graduate student at the University of California, Berkeley. “But there is reason to believe that we can approach $US100,” he says.

The second step, Birkholzer says, is injecting that carbon dioxide “in a pressurised form almost like a liquid” deep into the ground – possibly thousands of meters down – where geologic formations can trap it and prevent it from getting back to the atmosphere.

Birkholzer says there are currently 26 facilities around the world doing this. “Each has been operating safely and without any carbon dioxide escaping its designated storage,” he adds. To increase by the required factor of 150, we may need to create thousands of such facilities, grouped into regions where the geology provides the best opportunities for long-term carbon dioxide storage. “We call these carbon-storage hubs,” says Birkholzer. “Scientists like myself and others are going into overdrive to remove the roadblocks [to this].”