U.N. report warns carbon must be pulled from air to limit climate change

Paris – To save the world from the worst ravages of climate change, slashing carbon pollution is no longer enough — carbon dioxide will also need to be sucked out of the atmosphere and buried, a landmark U.N. report is expected to say on Monday.

If humanity had started to curb greenhouse gas emissions 20 years ago, an annual decrease of 2% out to 2030 would have put us on the right path. Challenging, but doable.

Instead, emissions climbed another 20% to more than 40 billion metric tons of CO2 in 2021.

This means an abrupt drop in emissions of 6% or 7% a year is needed to avoid breaching the Paris Agreement’s goal of capping global warming at “well below” 2 degrees Celsius compared to pre-industrial levels.

Staying under the safer aspirational threshold of 1.5 C would mean an even steeper decline.

To put that in perspective, the painful 2020 shutdown of the global economy due to COVID-19 saw only a 5.6% decrease in carbon dioxide emissions.

Hence the need for carbon dioxide removal, or “negative emissions,” is likely to figure prominently in the U.N. Intergovernmental Panel on Climate Change (IPCC) report.

Even under the most aggressive carbon-cutting scenarios, several billion metric tons of carbon dioxide will need to be extracted each year from the atmosphere by 2050, and an accumulated total of hundreds of billions of metric tons by 2100.

As of today, however, carbon dioxide removal is nowhere near these levels. The largest direct air capture facility in the world removes in a year what humanity emits in three or four seconds.

There are at least a dozen removal techniques on the table, with different potentials and costs.

Using bioenergy

Most of the hundreds of models laying out a game plan for a liveable future reserve an important role for a negative emissions solution called BECCS, or bioenergy with carbon capture and storage.

In a nutshell, this is the recipe: grow trees, burn them for energy, and bury the carbon dioxide underground, in an abandoned mineshaft, for example.

But what works on paper (or in so-called integrated assessment models), has not materialized in reality.

One of the few commercial-scale BECCS facilities in the world, in Britain, was dropped last year from the S&P Clean Energy Index because it failed to meet sustainability criteria.

“I don’t see a BECCS boom,” said Oliver Geden, a senior fellow at the German Institute for International and Security Affairs and an expert on carbon dioxide removal.

Planting trees

Restoring forests and planting trees that absorb and stock carbon dioxide as they grow also figure prominently in development scenarios achieving net-zero emissions, whether in 2050 or later.

Many businesses, including fossil-fuel companies, rely heavily on carbon offset programs based on afforestation to compensate for continuing carbon pollution.

But the amount of land needed to put a serious dent in carbon dioxide levels through tree planting — up to twice the size of India — could clash with other priorities, such as growing food and biofuel crops.

Biodiversity could suffer as well, especially in savannahs converted to monoculture tree farms.

Newly planted forests could also fall victim to wildfires made more frequent and intense by rising temperatures, resulting in the release of all their stored CO2.

‘DACCS’

One of the youngest carbon removal technologies is also one of the hottest: direct air carbon capture and storage.

With variations, DACCS is a chemical process that extracts carbon dioxide directly from the atmosphere, converting it into solid form or locking it away underground.

Because carbon dioxide in the air is so sparse — a few hundred parts per million — it is a very energy-intensive and expensive process.

DACCS has benefited from a wave of corporate backing.

Last year, Tesla CEO Elon Musk launched the $100 million X-Prize for an innovative carbon removal technology, and Breakthrough Energy founder Bill Gates unveiled a corporate partnership to turbocharge its development.

How quickly it can scale up, and at what cost, remain open questions.

Enhanced weathering

Enhanced weathering involves mining and crushing rocks rich in minerals that naturally absorb carbon dioxide, and then spreading them over land or sea.

It aims to vastly accelerate a process that normally unfolds on geological timescales of tens of thousands of years.

Silicate rocks with minerals rich in calcium and magnesium but lacking metal ions such as nickel and chromium are the best raw material for the job.

But, again, it’s unclear if enhanced weathering can be scaled up enough, and at what cost.

Ocean-based methods

Oceans already take up more than 30% of humanity’s carbon emissions, and scientists are experimenting with ways to boost that capacity.

One approach is to enhance marine alkalinity, either by directly adding natural or synthetic alkaline minerals, or the electrochemical processing of seawater.

Another approach, known as ocean fertilisation, increases the density of tiny phytoplankton that produce and sequester organic carbon through photosynthesis, like plants on land. Adding nitrogen or iron stimulate phytoplankton growth.

The main concerns here include unintended consequences on ecosystems.