"We found that if BECCS is available, atmospheric temperature increases can be limited to 1.5 degrees over preindustrial levels by 2150, after having peaked at just over 2 degrees around 2100," Niclas Mattsson told environmentalresearchweb. "Even if climate sensitivity turns out to be quite high (4.5 degrees for a doubling of carbon-dioxide concentration), this 'relaxed 1.5 degree target' is still feasible, albeit at high cost. Put simply, we find that BECCS allows global warming to begin to be reversed within 100 years."

BECCS could produce electricity, heat or liquid fuels such as ethanol at the same time as generating sustained negative carbon-dioxide emissions, the researchers said.

To come up with the results, the team used GET-Climate, an integrated energy-climate model that merges a technology-oriented energy system model with a reduced-complexity climate model. Previously the team had modelled carbon-dioxide concentration, rather than temperature, targets. Temperature targets take into account the inertia of the climate system.

According to Mattsson, it is interesting that ambitious targets are physically and economically possible to reach. "The implication is that there is a way for low-temperature targets to remain on the table in international climate negotiations, despite the recent lack of progress – if targets are relaxed to allow for a temporary overshoot," he added.

The economic benefit of BECCS almost vanishes if temperature target overshoots are not allowed, the team discovered. The model assumed that 200 EJ of bioenergy was available worldwide each year along with carbon-storage capacity of 2000 Gt of carbon dioxide, and that climate sensitivity was 3°C per doubling of carbon-dioxide concentration.

"We must emphasize that these optimistic scenarios require first reducing global carbon-dioxide emissions to nearly zero, and then going even further by implementing BECCS on a very large scale to reach net negative global emissions," said Mattsson. "Needless to say, this is an extremely difficult challenge. It also requires large areas of land for bioenergy plantations, although marginal lands not suitable for agriculture and forest residues can be used to some extent."

Using large amounts of bioenergy may adversely affect global food prices, biodiversity and water availability, the researchers said. Producing 200 EJ of bioenergy per year could require around 500 Mha of land, roughly one-third of global crop land. If BECCS is severely limited by low biomass availability, other negative-emission technologies such as direct air capture of carbon dioxide could conceivably play a role similar to that of BECCS, wrote the team in their paper in Environmental Research Letters (ERL).

The possibility of achieving negative emissions in the future should not be perceived as a carte blanche for delaying emission-abatement efforts, the team stressed. What's more, the rate of temperature decline feasible with BECCS is too slow to act as an "emergency brake" on short timescales. "Overshoot targets [are] contentious, although they may be necessary in order to reach low temperature levels," the researchers wrote.

One issue to study next is abatement strategies and the role of BECCS in revising long-term climate targets. "There are many uncertainties in the climate system and as we get more observations and better knowledge our view on long-term climate targets may change," said Mattsson. "BECCS may be critical technology for rapidly changing the future emission pathway."

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