OSLO, Norway - Scientists are sucking carbon dioxide from the air with giant fans and preparing to release chemicals from a balloon to dim the sun's rays as part of a climate engineering push to cool the planet.

Backers say the risky, often expensive projects are urgently needed to find ways of meeting the goals of the Paris climate deal to curb global warming that researchers blame for causing more heat waves, downpours and rising sea levels.

The United Nations says the targets are way off track and will not be met simply by reducing emissions for example from factories or cars - particularly after US President Donald Trump's decision to pull out of the 2015 pact.

They are pushing for other ways to keep temperatures down.

Swiss company Climeworks began to suck greenhouse gases from thin air in May with giant fans and filters in a $23 million project that it calls the world's first "commercial carbon dioxide capture plant".

Worldwide, "direct air capture" research by a handful of companies such as Climeworks has gained tens of millions of dollars in recent years from sources including governments, Microsoft founder Bill Gates and the European Space Agency.

If buried underground, vast amounts of greenhouse gases extracted from the air would help reduce global temperatures, a radical step beyond cuts in emissions that are the main focus of the Paris Agreement.

Climeworks reckons it now costs about $600 to extract 1 metric ton of carbon dioxide from the air and the plant's full capacity due by the end of 2017 is only 900 tons a year. That's equivalent to the annual emissions of only 45 US citizens.

The Paris Agreement seeks to limit a rise in world temperatures this century to less than 2 C, ideally 1.5 C above pre-industrial times.

Many experts fear that pinning hopes on any technology to fix climate change is a distraction from cuts in emissions blamed for heating the planet.

"Relying on big future deployments of carbon removal technologies is like eating lots of dessert today, with great hopes for liposuction tomorrow," Christopher Field, a Stanford University professor, wrote in May.

Jim Thomas of ETC Group in Canada, which opposes climate engineering, said direct air capture could create "the illusion of a fix that can be used cynically or naively to entertain policy ideas such as 'overshoot'" of the Paris goals.

But governments face a dilemma. Average surface temperatures are already about 1 C above preindustrial levels and hit record highs last year.

Faced with hard choices, many experts say that extracting carbon from the atmosphere is among the less risky options. Leaders of major economies, except Trump, said at a summit in Germany this month that the Paris accord was "irreversible".

Raymond Pierrehumbert, a professor of physics at Oxford University, said solar geoengineering projects seemed "barking mad".

By contrast, he said "carbon dioxide removal is challenging technologically, but deserves investment and trial".

Reuters