Can we all stop worrying about global warming? According to a recent rash of stories in the media, the “climate sensitivity” – the extent to which temperatures respond to more carbon dioxide in the atmosphere – is lower than expected, and thus that the world won’t get as hot as predicted. One story, in The Economist, based on leaked information from a draft of the next assessment by the Intergovernmental Panel on Climate Change, claims the IPCC will revise its sensitivity estimate downwards when they release their official report this September.

The sceptics have mounted a concerted campaign to persuade journalists and politicians that climate scientists now think that climate sensitivity is lower, says Bob Ward of the Grantham Research Institute on Climate Change and the Environment, London. But is there any truth to the claims?

Climate sensitivity refers to how much the world will warm if carbon dioxide levels double. But this apparently simple concept is slippier than a Turkish wrestler. As the planet warms in response to rising CO₂ levels, a whole series of feedbacks kick in over the following decades, centuries or millennia. Depending on which feedbacks are included and what the timescale is, there are many competing ways of defining sensitivity. To add to the confusion, there are also dozens of ways of calculating it.

Pollutants

One way is to look at how much warming there has been in response to rising CO₂ levels over the past century. But this approach has all kinds of problems. For starters, we have been pumping out all kinds of pollutants, some of which may be masking the effect of CO₂.

What’s more, in the last decade, CO₂ levels have continued to rise but with little surface warming. Such lulls are expected and the latest is probably a blip, due to the oceans soaking up more heat than usual. Unsurprisingly, this means that any sensitivity estimate that includes the past decade will produce a lower value than any calculated without taking the last decade into account, says Reto Knutti of the Institute for Atmospheric and Climate Science in Zurich, Switzerland. He co-authored one such study published earlier this year, which concluded that “equilibrium sensitivity” – usually taken to mean the warming expected after several decades of doubled CO₂ – is between 1° C and 5° C, most likely 2° C.

Another way to calculate sensitivity is to look at how global temperatures changed thousands or millions of years ago in response to changing CO₂ levels. Such studies point to a higher value for equilibrium sensitivity, closer to 3° C, says Knutti, who reviewed the evidence last year. But there are all kinds of problems with this approach too, such as the uncertainties about what the world was like in the past.

A third way to calculate sensitivity is to use climate models, which point to even higher values for equilibrium sensitivity, between 2.2 and 4.7° C, says Knutti: “Above 3 °C at least.” But there’s a lot missing from the models. For one thing, most only include fast feedbacks such as the effect of water vapour. They don’t include slower feedbacks such as increasing vegetation, or the risk of a sudden methane “belch” as the Arctic warms.

No new consensus

The bottom line is that there is no new consensus that climate sensitivity is lower than previously thought, says Knutti. The observed trend points to lower values because of the recent slowdown, but other evidence continues to support higher values.

The last IPCC report stated that equilibrium climate sensitivity was between 2 and 4.5 °C, mostly likely 3 °C. The Economist claims the IPCC’s next report will give a figure between 1.5 and 4.5 °C, with no most likely value. The IPCC won’t confirm or deny it, but it’s not a huge change if it is true.

“What matters for avoiding dangerous climate change is the upper end, and that hasn’t changed,” says Knutti. Ward makes the same point. “We can’t afford to gamble on sensitivity definitely being low,” he says.

But will it all be a huge waste if sensitivity does turn out to be low? Far from it. If we don’t cut emissions, Knutti points out, all low sensitivity means is that it will take a decade or two longer for the planet to warm as much as it would if sensitivity was high.

Link to video: Santiago train crash: Spanish PM visits site of derailmentAs Spain mourned the 80 dead in Europe’s worst rail crash this century, questions were being asked about how the train had been able to hit a tight curve at such a speed that it spun off into a concrete security wall.

Analysis of video of the accident in the northern city of Santiago de Compostela suggested the train was going faster than 85mph on a bend where drivers are supposed to slow down after a straight stretch that allows them to reach up to 125mph.

“We were going strongly when we got into the curve,” one driver was reported to have admitted shortly after surviving the accident on Wednesday, which killed more than a third of the passengers and left 168 injured.

A spokeswoman for the Galicia supreme court said the driver, who was only slightly injured, was under investigation.

The man, who has been named, is not believed to be under arrest but is expected to face questions from a judge with access to the train’s data recording black box.

While trapped in the cab, the driver was reported to have given an account over the radio to officials at Santiago station. He was quoted saying, “I hope there are no dead because they would fall on my conscience” and having repeated over and over: “We’re human. We’re human.”

Rail safety experts said such accidents are usually the result of more than one failure, and questions will inevitably be asked about how warning signals about the train’s speed were not picked up and acted on.

On Thursday evening the death count looked set to creep up, with 36 of the 95 victims in hospital said to be in a critical condition.

The Foreign Office confirmed that one Briton was among the injured.

The Spanish prime minister, Mariano Rajoy, who is from the crash city, toured the scene alongside rescue workers and went to a nearby hospital to visit the injured and their families.

“For a native of Santiago, like me, this is the saddest day,” said Rajoy, who declared Spain would observe a three-day period of mourning. He said judicial authorities and the Public Works Ministry had launched parallel investigations into what caused the crash.

Link to video: Santiago de Compostela train crash: CCTV captures moment of derailmentEyewitness accounts backed by security camera footage of the disaster suggested the eight-carriage train was travelling at high speeds as it took the pronounced left-hand curve through a deep culvert.

An estimate by Associated Press of the speed at the moment of impact using the time stamp of the video and the estimated distance between two pylons gives a range of 89mph to 119mph. Another estimate calculated on the basis of the typical distance between railroad ties gives a range of 96mph to 112mph.

The speed limit on that section of track is 50mph, and locals said that trains often creep through, as the station is just a short way down the tracks.

The leaked video footage, which railway authority Adif admitted must have come from one of its cameras, shows the front engine and train carriages buckling as they enter the turn.

Professor Roger Kemp, of the Royal Academy of Engineering, said a derailment would be expected at high speeds on such a curve.

“The big question is why the train was running at more than twice the speed limit. There must have been at least prominent visual warnings to reduce speed, if not audible warnings and an electronic speed supervision system,” he said.

Francisco Otero, who lives near the crash site and is a relative of a woman who was a passenger, told the Guardian that she had said the train was going too fast.

A firefighter carries an injured girl from the wreckage of the train crash near Santiago de Compostela. Photograph: AFP/Getty Images”Until then I thought that it had been terrorist attack,” Otero said. “But it was one of the first things she said.”

One of the survivors, Sergio Prego, told Cadena Ser radio station that the train “travelled very fast” just before it derailed and the cars flipped upside down, on their sides and into the air.

“I’ve been very lucky because I’m one of the few able to walk out,” he said.

Forensic scientists are still trying to identify the most mutilated corpses. Groups of families and friends gathered at the city’s Cersia hospital waiting for news of loved ones – though there was little chance they were alive as all survivors had been identified and their families informed.

“It’s a major challenge to identify the people who have died,” Rajoy said. “Unfortunately, in many cases, this isn’t easy, but we are very conscious that the families cannot live in a state of uncertainty.”

The Alvia 730 series train started from Madrid and was scheduled to end its journey at Ferrol, about 60 miles north of  Santiago.

Alvias do not go as fast as Spain’s AVE bullet trains, but still reach 155mph on AVE tracks and travel at a maximum of 137mph on normal gauge rails.

The accident came a day before a public holiday in Galicia: the feast of St James, after whom the region’s capital Santiago is named.

“24 July will no longer be the eve of a day of celebration but rather one commemorating one of the saddest days in the history of Galicia,” said Alberto Núñez Feijóo, the region’s president.

Residents of the semi-rural neighbourhood by the accident site struggled to help victims out of the toppled cars on Wednesday night. Some passengers were pulled out of broken windows as rescuers used rocks to try to free survivors from the wreckage.