THIS is how we will hold off disaster. To help us avoid dangerous climate change, we will need to create the largest industry in history: to suck greenhouse gases out of the air on a giant scale. For the first time, we can sketch out this future industry – known as geoengineering – and identify where it would operate.
The Intergovernmental Panel on Climate Change now considers geoengineering to be respectable. The reason is simple. Unless our greenhouse gas emissions start falling soon, Earth will probably warm this century by more than 2 °C, at which point things get nasty – because human society might not be able to adapt. But emissions are still rising. The upshot is we urgently need ways to suck CO2 out of the air. This was the subject of the Oxford Conference on Negative Emission Technologies, held last month in the UK.
In conjunction with scientists attending that meeting, we've assessed the effectiveness – and cost – of the most likely methods. They include planting trees, shovelling crushed rock into the ocean, and building millions of chemical "sponges" to pull gas out of the air (see diagram: read about each technique below).
Alone, such CO2-suckers can only handle a fraction of emissions. So we will need several. "If we don't employ some of these technologies, we will go above 2 °C," says Richard Lampitt of the UK National Oceanography Centre in Southampton. "A programme of multiple negative emissions technologies could perhaps store a few billion tonnes of carbon per year by mid-century, and conceivably as much as 5 or 10 billion tonnes," says John Shepherd, also at the UK National Oceanography Centre.
So by 2100, CO2-suckers might just mop up the equivalent of what our annual emissions are now. But there are big costs, even putting money to one side. The biological approaches will cover vast areas of land, pressurising farms and wildlife, while the high-tech approaches will burn lots of energy.
There are other problems. It is hard to verify if plants and oceans are trapping CO2. We need underground space to store the gas. And many techniques could harm ecosystems. Worse, pulling CO2 from the air makes it progressively harder to suck out more, so the methods get less efficient over time (Carbon Management, doi.org/dbkhqh). And models show that removing CO2 from the air affects land plants, too. "They grow more slowly and take up less carbon," says Andreas Oschlies of the Helmholtz Centre for Ocean Research in Kiel, Germany.
That means we have to start soon: with immediate research to assess safety, and roll-out within 20 years. Some methods, like tree-planting, can begin now; others may be decades away (Process Safety and Environmental Protection, doi.org/n33). On top of all this, CO2-suckers are pointless unless paired with dramatic emissions cuts. If not, even a major programme of ocean liming beginning in 2020 would have little effect.
It ain't cheap. As a rough estimate, it will cost several trillion dollars a year – a few per cent of global GDP. And it only makes economic sense with serious incentives, such as a high carbon price. But with a combination of CO2 reduction and geoengineering, we might just miss the worst of climate change. The bottom line is that CO2-suckers are essential, but we also need to ditch fossil fuels quickly. It's that or climate havoc.
This article appeared in print under the headline "Transforming Earth"
T: Plant trees
Plant forests and regularly harvest them. Trees are a carbon sink as long as they are growing, and not allowed to rot.
Location: unused farmland
BE: BECCS (Bioenergy with carbon capture and storage)
Suck out atmospheric CO2 by growing biofuel crops like sugar cane, burn them for energy, capture the resulting CO2, and bury it.
Location: the tropics, where growth is fastest
B: Biochar
Burn plant material without oxygen to make charcoal-like "biochar". This carbon store can then be buried in soil, where it acts as a fertiliser.
Location: anywhere with rich plant growth
DA: DAC (Direct air capture)
Build shipping-container-sized boxes full of a chemical "sponge" that sucks CO2 out of the air, ready for burial. You may need 100 million of them.
Location: windy and dry areas. More wind means more air is driven through the boxes, increasing uptake
IF: Iron fertilisation
Trigger photosynthetic plankton blooms in the ocean by dumping iron into areas that don't have much. If the plankton sinks, carbon is stored.
Location: iron-depleted regions of the ocean
OL: Ocean liming
Throw lime into the ocean. It reacts with dissolved CO2 to form carbonates. This may also help corals by reducing ocean acidification.
Location: coral habitats
EW: Enhanced weathering
Crush common minerals like olivine to powder to increase surface area for reacting with CO2 and water.
Location: proceeds fastest in warm, wet conditions, so areas such as humid coasts and rivers are best
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