New climate models suggest Paris goals may be out of reach

New climate models show carbon dioxide is a more potent greenhouse gas than previously understood, a finding that could push the Paris treaty goals for capping global warming out of reach, scientists have told AFP. Developed in parallel by separate teams in half-a-dozen countries, the models -- which will underpin revised UN temperature projections next year -- suggest scientists have for decades consistently underestimated the warming potential of CO2.

Vastly more data and computing power has become available since the current Intergovernmental Panel on Climate Change (IPCC) projections were finalised in 2013.

"We have better models now," Olivier Boucher, head of the Institut Pierre Simon Laplace Climate Modelling Centre in Paris, told AFP, adding that they "represent current climate trends more accurately". The most influential projections from government-backed teams in the US, Britain, France and Canada point to a future in which CO2 concentrations that have long been equated with a 3C world would more likely heat the planet's surface by four or five degrees.

"If you think the new models give a more realistic picture, then it will, of course, be harder to achieve the Paris targets, whether it is 1.5 or two degrees Celsius," scientist Mark Zelinka told AFP.

Zelinka, from the Lawrence Livermore National Laboratory in California, is the lead author of the first peer-reviewed assessment of the new generation of models, published earlier this month in Geophysical Research Letters.

For more than a century, scientists have puzzled over a deceptively simple question: if the amount of CO2 in the atmosphere doubles, how much will Earth's surface warm over time?

The resulting temperature increase is known as Earth's "climate sensitivity". That number has been hard to pin down due to a host of elusive variables. Whether oceans and forests, for example, will continue to absorb more than half of the CO2 emitted by humanity is hard to predict. But the biggest wild card has always been clouds.

"How clouds evolve in a warmer climate and whether they will exert a tempering or amplifying effect has long been a major source of uncertainty," explained Imperial College London researcher Joeri Rogelj, the lead IPCC author on the global carbon budget the amount of greenhouse gases that can be emitted without exceeding a given temperature cap. The new models reflect a better understanding of cloud dynamics in at least two ways that reinforce the warming impact of CO2.

Zelinka said new research had confirmed high clouds in the bottom layer of Earth's atmosphere boost the Sun's radiation and global heating accentuates that dynamic. "Another big uncertainty has been how low clouds will change, such as stratocumulus decks of the west coast of continents," he said.

"That has been the holy grail of climate modellers for a long time."

Recent observations suggest this type of cloud cover decreases with warming, which means less of the Sun's energy gets bounced back into space by white surfaces. For most of the last 10,000 years -during which time humanity's numbers swelled from a few million to 7.6 billion -- the concentration of CO2 in the atmosphere was a nearly constant 280 parts per million (ppm).

But at the start of the 19th century as the industrial revolution kicked into high gear, fuelled by oil, gas and especially coal, the number of CO2 molecules in the air shot up. Today the concentration stands at 412 ppm, a 45 percent rise half of it in the last three decades. Last year alone, human activity injected more than 41 billion tonnes of CO2 into the atmosphere, some five million tonnes per hour.