Miami (AFP) - Levels of the planet's three most dangerous greenhouse gases are rising, and fossil fuels must be taxed to protect children from the turmoil of rising seas and extreme storms, climate scientist James Hansen warned Tuesday.

Otherwise, young people face the "dubious" proposition of somehow sucking carbon dioxide from the air at a price tag of hundreds of trillions of dollars in the next century, said Hansen, who leads the climate science program at Columbia University's Earth Institute.

"The science has become crystal clear," Hansen told reporters on a conference call to discuss his latest research paper, entitled "Young People's Burden: The Requirement of Negative CO2 Emissions."

"We have to phase out carbon emissions over the next few decades," said Hansen, describing the actions of the US government up until now as "grossly inadequate."

Hansen, formerly of NASA, is suing the US government along with 21 youths across the country, including his 18-year-old granddaughter.

The suit alleges that by continuing to burn fossil fuels and causing climate change, the United States is failing to protect essential "public trust" resources such as clean air and water, thereby depriving future generations of their constitutional rights to life, liberty and property.

The paper, published Tuesday in Earth Systems Dynamics Discussion, has not yet been peer-reviewed, but Hansen said he wanted it released now because time is of the essence.

"Some people might object to discussing such a paper before it has gone through the peer-review process. But I am going to do that simply because we are running out of time on the climate issue," Hansen said.

- Leaving 'a mess' for young people -

The paper, authored by Hansen and 11 prominent climate scientists, warns that the global average temperature is already 1.3 degrees Celsius (2.3 Fahrenheit) warmer than pre-industrial times, defined as 1880-1920.

That is perilously close to the level agreed during last year's Paris talks, when global leaders committed to holding the increase of global average temperature to well below 2 degrees Celsius above pre-industrial levels.

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Meanwhile, the heat-trapping gases carbon dioxide, methane and nitrous oxide are all rising, ensuring more global warming in years to come.

Already, fossil fuel burning has unnaturally propelled the Earth to a temperature range last seen 115,000-130,000 years ago, "when sea level was six to nine meters (20-30 feet) higher than today," said the paper.

As a result, glaciers and ice sheets are melting, the oceans are acidifying and rising seas will engulf coastal cities worldwide in the coming centuries.

"That's not fair. Today's adults benefit from fossil fuel burning and leave the waste for young people to clean up," said Hansen's granddaughter, Sophie Kivlehan, in a video message released along with Hansen's paper.

"We should be moving on to clean energy, leaving dirty energy in the ground."

- 'Like a cancer ' -

"The assumption that young (people) will somehow figure out a way to undo the deeds of their forebears has crept into and spread like a cancer through United Nations climate scenarios," said the paper.

In the absence of sharp cuts to emissions, future generations are saddled with figuring out some way to extract CO2 from the atmosphere in order to limit climate change, Hansen argued.

That would require risky, unproven technologies such as bioenergy with carbon capture and storage (BECCS), at a price tag of between $104-570 trillion this century.

"It is a very dubious idea and the cost of it is not negligeable," Hansen said.

- Proposed solutions -

Hansen said the way to reverse course is to place a gradually rising tax on carbon and end government subsidies for polluting fossils fuels like coal, oil and natural gas.

"Make the price of fossil fuels honest. Stop subsidizing them. And make them pay their cost to society," Hansen said.

"If we put a gradually rising fee on carbon emissions, it will spur the business community and entrepreneurs and the public to develop carbon-free energies and energy efficiency, and it will spur the public to change their choices so that we move rapidly to reducing emissions and move to clean energy."

Hansen said he was optimistic that the lawsuit, organized by Our Children's Trust, will go forward.

In April, the case survived an attempt by the fossil fuel industry and US government to get it tossed out of court, and is currently under review by US District Judge Ann Aiken.

She heard oral arguments on September 13 and is expected to announce her decision by mid-November. Then, the case will head either to trial or appeal.