(Adjusts terminology in paragraph 9; adds comment)
By Laurie Goering
LONDON, Oct 10 (Thomson Reuters Foundation) - As efforts to cut planet-warming emissions fall short, large-scale projects to suck carbon dioxide out of the atmosphere will be needed by the 2030s to hold the line against climate change, scientists said on Tuesday.
Many new technologies that aim to capture and store carbon emissions, thereby delivering "negative emissions", are costly, controversial and in the early phase of testing.
But "if you're really concerned about coral reefs, biodiversity (and) food production in very poor regions, we're going to have to deploy negative emission technology at scale," said Bill Hare of Climate Analytics, a science and policy institute.
"I don't think we can have confidence that anything else can do this," the Berlin-based chief executive told a London climate change conference.
World leaders agreed in 2015 an aim of holding global warming to 1.5 degrees Celsius above pre-industrial times.
Scientists believe this is key to protecting small island nations from sea level rise, shoring up food production and preventing extreme weather.
Carbon-sucking technologies may even be needed to hold the planet to a less ambitious 2 degrees Celsius of warming, said scientists at Chatham House, a British think tank.
The world has already seen an average of about 1 degree of warming, they said.
"It's something you don't want to talk about very much but it's an unavoidable truth: we will need (negative emissions) by the mid-2030s to have a chance at the (1.5 degree) goal," Hare said.
Ideas for achieving those include planting carbon-absorbing forests across large areas, then harvesting the wood for energy and pumping the emissions produced underground - a process likely to feature in an Intergovernmental Panel on Climate Change report next year.
Machines might also be developed to capture carbon dioxide directly from the air and pump it underground or otherwise neutralise it.
But efforts to store captured carbon underground are "showing no progress... and even backwards steps in some cases", said Corinne Le Quéré, director of the Tyndall Centre for Climate Change Research at the University of East Anglia.
Underground carbon storage has been promoted as part of a push by the United States and other countries to develop "clean coal" technology.
Similarly, planting more forests - a technology known as BECCS, or bioenergy with carbon capture and storage - raises questions about food security and land rights, scientists said.
Le Quéré said BECCS is "probably essential to take us to zero emissions" although "it's really difficult to imagine we can use land at the levels required in the models".
She called for experts to focus on proven approaches, such as improving energy efficiency, promoting cleaner transport, eating less meat and scaling up renewable energies.
Many experts fear that launching costly "negative emissions" technologies could reduce the pressure to act swiftly to cut emissions now.
Hare also warned that other efforts being promoted to try to turn back excessive climate change - such as spraying aerosols into the planet's atmosphere to reflect back some of the sun's energy reaching the earth - would be "unsafe" and would not address key climate change problems such as seas becoming more acidic as oceans absorb more carbon dioxide.
(Reporting by Laurie Goering @lauriegoering; editing by Katy Migiro. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, climate change, resilience, women's rights, trafficking and property rights. Visit http://news.trust.org/climate)
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