Technologies to remove carbon from the air are no silver bullet - scientists
Technologies to capture carbon emissions from the atmosphere have "limited realistic potential" to help avoid dangerous climate change, scientists have warned.
Schemes such as planting forests, adding iron to oceans to boost plankton absorbing carbon dioxide, and capturing and storing emissions from wood-fuelled power plants have been touted as ways to cut greenhouse gases.
Other potential measures include managing land to increase carbon stored in the soil, directly capturing carbon dioxide from the air and adding certain minerals to land or oceans to absorb the greenhouse gas.
But none of the measures can take carbon out of the atmosphere at the scale and rate of deployment required to meet targets set in the world's first comprehensive climate treaty, the Paris Agreement, a report warned.
There is no "single silver bullet" and the focus needs to be on deep and rapid cuts to the amount of greenhouse gases being put into the atmosphere by human activities such as burning fossil fuels and deforestation.
The Paris Agreement commits countries to holding temperature rises to "well below" 2 Celsius above pre-industrial levels, which means emissions must be cut to net zero by the second half of the century.
But with emissions from some sectors such as aviation and agriculture hard to tackle, "negative emissions technologies" would have a role to play, the scientists from the European Academies Science Advisory Council added.
Some 87% of scenarios by the Intergovernmental Panel on Climate Change (IPCC) for meeting the 2C target require negative emissions technologies, while all models for meeting a more stringent 1.5C limit involve such measures.
Professor Michael Norton, EASAC environment programme director, said: "There's no single technology that offers potential for removing carbon dioxide from the atmosphere at the scale envisaged in IPCC models, which are now Paris Agreement targets."
He added: "We can rule out a single silver bullet."
He warned of "severe drawbacks" to one of the main suggested technologies, which would see renewable energy sources such as wood pellets burned in power stations and the carbon emissions captured and permanently stored.
Even if all the carbon were captured when biomass was burned, there could be significant emissions during the supply chain as a whole, including changing use of land to grow energy crops and transporting them, he said.
A report on negative emissions technologies also warned measures such as putting iron in the ocean and even planting new forests could have impacts on wildlife, while land-based measures could compete with growing food.
Emissions stored in forests were not secure, as the trees could be cut down in the future, and reforesting and planting new woods should not distract from efforts to curb the existing problem of deforestation, the scientists said.
But in the UK, initiatives to plant more woodlands such as the recently announced Northern Forest could play a part in tackling emissions in the country, which is already deforested, over the next few decades.
Measures to increase the amount of carbon in soils are a "win-win" because they could also improve soil productivity and reduce the need for fertilisers which also create greenhouse gas emissions.
But Prof Norton warned: "Overall our message is that we cannot trust technology to come to the rescue and the (European) Commission should intensify its efforts together with member states to reduce emissions as rapidly as possible."
Prof John Shepherd added: "These negative emissions technologies are very interesting but they're not an alternative to deep and rapid emissions reductions, these remain the safest and reliable option that we have for dealing with too-high levels of carbon dioxide."