The National Snow and Ice Data Center (NSIDC) has just announced that Arctic ice cover has reached its minimum extent for 2012, far below the previous record[1]. Peter Doherty discusses some of the implications here. As far as the broader debate about climate change is concerned, there are some big implications.

* First, this is irrefutable evidence that the climate is changing, and that the idea that climate change stopped or slowed down after 1998 or 1995, as delusionists have regularly claimed, is nonsense. On the contrary, the loss of Arctic ice is accelerating, far ahead of model predictions{2] In this context, I have yet to see any “sceptics” actually accept the evidence proving them wrong. But, with a handful of exceptions, we have silence rather than the usual rash of talking points to explain the evidence away. A notable example is Andrew Bolt, who ran lots of posts claiming there was no problem (most recently here), but hasn’t mentioned the topic since the minimum extent record was broken nearly a month ago.

Update While the blog was off-air, Bolt came up with a snark about the Antarctic, which presumably is supposed to offset the long string of posts he made claiming that there was no problem in the Arctic. As usual, Bolt’s talking point has already been debunked, here at Skeptical Science, but you can do it yourself. Compare Bolt’s graph of the Antarctic, showing a small increase in the winter maximum, to the NSIDC graph of the Arctic showing the summer minimum collapsing. End update

* Second, the “catastrophic” part of the delusionists favorite acronym “Catastrophic Anthropogenic Global Warming” is looking a lot more likely. Not only will an ice-free Arctic produce a bunch of feedbacks that accelerate warming, but it will substantially affect climate conditions in Northern Europe, though exactly how remains to be seen.

On the other side of the coin, there’s one predicted catastrophe that didn’t happen. As elsewhere in the world, the introduction of the carbon tax did not “send a wrecking ball through the economy”. In fact, adverse effects are barely detectable. Of course, a lot more action is needed, but the near-universal view of economists that the cost of stabilising the global climate will be of the order of 1 per cent of income is certainly supported by the evidence from the initial steps in this direction.

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The satellite data showing Arctic ice at record low levels is pretty striking but, as I mentioned, unlikely to change many minds at this stage. One point that can reasonably be made is that satellites have only been around for thirty years – maybe there was less ice further back in history. There is one sort of evidence that goes back many centuries – the repeated failure to find the fabled Northwest Passage, by voyagers going back at least as far as John Cabot in 1497, and arguably as far back as the Vikings in the 11th century. Roald Amundsen managed it in three years in 1903-06 and the voyage was managed with the aid of icebreakers in the middle of the 20th century. in 1984, a specially designed cruise ship managed the voyage, but now it’s become almost routine. “ In 2011 16 private yachts made their way successfully through the once-dreaded passage. ” Here’s an example of the kind of boat that can now manage, within a season, a voyage that defeated the well-equipped expedition of Sir John Franklin, with the loss of the entire crew.

Of course, in a parallel universe, the feat was accomplished by a Chinese fleet in 1421.

As expected, several measures of Arctic ice cover have hit record lows already, and others are likely to do so soon. What’s unexpected is how early this has happened. Melting usually continues until mid-September, so it seems likely that this year’s minimum will be far below the previous record, set in 2007. Those who prefer observational evidence to models will doubtless be pleased to note that the rate of melting far exceeds that predicted by most models. Predictions of when the Arctic might be entirely ice-free at the summer minimum are being revised sharply.

One prediction that seems safe to make is that few if any “sceptics” will treat this unequivocal evidence of warming as a reason to apply scepticism towards the authorities on whom they rely, all of whom have got this wrong. At most they will temporarily shift their ground from “warming has stopped” to “we don’t know what causes it”. However, I’d be glad to be proved wrong on this, so if you see any examples, please let me know. As previously advised, I don’t plan to engage in polemics on this, so if you want to provide confirming evidence for my prediction, feel free, but don’t expect a response from me.

It looks as if 2012 will set a new record low for Arctic ice extent[1]. As a measure of the impact of global warming, this is depressingly clear-cut. There’s no need to go into arguments about trends and variability, or use any kind of modelling – the ice is melting visibly.

Source: National Snow and Ice Data Center.

fn1. Satellite data on ice extent goes back to 1979. There are other measures, arguably more relevant, such as estimates of ice volume, for which the data set is shorter. They tell an even gloomier stories.

Having just given a relatively optimistic view of US policy on climate change, I’ve seen a string of signals that the Obama Administration is set to sell out on this vital issue, presumably on the advice of the same political experts who persuaded him to ignore the unemployment problem for most of his first term in office. The bad news

* An Obama ad attacking Romney for saying (correctly) in 2003, that dirty coal-fired power plants kill people.

* Partial approval of the Keystone pipeline

and, worst of all, the apparent abandonment of the 2 degree target announced by U.S. Special Envoy for Climate Change Todd Stern

The radio is deplorable, but not significant in the greater scheme of things. Whatever the ad might say, the EPA regulations introduced under Obama are closing down lots of the worst coal-fired power stations. As regards the other two points, the best we can say is that nothing is locked in yet. Still, it’s more depressing stuff from someone who once promised Hope.

I was at the Australian Conference of Economists earlier in the week, and had a chat with Roger Jones, who has occasionally commented here. I asked him about his estimates of the impact of emissions mitigation policies in Australia, and was able to confirm that our estimates, although reached in very different ways, are in quite close agreement. Roger is cited here and here, estimating that a 5 per cent reduction in Australia’s emissions would result in a reduction in equilibrium global temperature of 0.0034 degrees. In a blog comment, I made the estimate that a 25 per cent reduction, relative to business as usual (the official target of the carbon price policy and also of the Opposition’s ‘direct action’ alternative) would result in a reduction in equilibrium global temperature of 0.02 degrees.

Unfortunately, Andrew Bolt did not observe the reason for the difference, and suggested that we disagreed by a factor of five. For the second time, a comment I sent correcting the mistake was lost in moderation. I was inclined to give up at this point, but given that Bolt did admit an error in his own estimate that I had pointed out to John Humphreys[1], I thought it would be worth one last try.

Policy disagreements are inevitable, but it would be helpful if we could avoid unnecessary disputes over arithmetic. I’m always happy to check for, and if necessary correct, errors in my calculations. If Bolt and others could do likewise, we would have a better chance of making progress in public debate, or at least of avoiding regress.

Update I appear to have misinterpeted my conversation with Roger, though I need to check on a number of issues before making a final assessment. So, I’m going to withdraw my claim that Bolt and John Humphreys in error on this point, and discuss the estimates with Roger in more detail. I’ll report back when this is complete.

Further update Unsurprisingly, Andrew Bolt has enjoyed a bit of a gloat on the subject, and some of his fans have joined in. So, it’s worth reminding everyone that he was out by a factor of 100 in his own calculations, presenting the impact of one year’s emissions reductions as if it was the total effect over the next 100 years.