If Hurricane Gustav had struck New Orleans with full force, what would that have told us about the scale and speed of climate change? If more of the sea-ice cover in the Arctic Ocean is lost in this year's summer melting season than last year (which was the worst on record), will that convince people that global warming is a real and present threat? What should people accept as evidence? And what will they accept in practice?

For scientists, the most persuasive evidence that global warming is happening faster than the models predict is the accelerating loss of Arctic sea ice. The National Snow and Ice Data Center at the University of Colorado, which tracks the summer melt season each year, calculates that the loss of ice cover in the Arctic Ocean has already exceeded that of 2005, the second-worst year since observations began, and may surpass last year's record low.

This is not only bad news for polar bears, since an ice-covered Arctic Ocean reflects most incoming sunlight back into space while open water, being darker, absorbs most of the sun's heat instead. An ice-free Arctic Ocean changes the world's heat balance and causes faster warming.

In the last 20 years of the 20th century, the ice cover shrank each year from an average of 14 million square kilometres (5.4 million square miles) in late winter to about seven million square kilometres (2.7 million square miles) in late September. Last year's low was only four million square kilometres (1.55 million square miles), and this year looks likely to be about the same. This is the kind of evidence that grabs scientists by the throat––but it barely gets anybody else's attention at all.

Only a couple of years ago, the climate models suggested that we might see a completely ice-free Arctic Ocean in late summer by the 2040. Now some experts are speculating that we might get there as soon as 2013. But a thousand stories have been written about Hurricane Gustav for every one that is written about what is happening in the Arctic.

That's understandable, because not one in a thousand human beings has ever seen the Arctic Ocean close up. Nobody is being evacuated because of this accelerating disaster, and so the media virtually ignores it. Whereas for a few days early this week, we were inundated with stories about the threat posed to New Orleans by Hurricane Gustav only three years after the city was devastated by Hurricane Katrina.

If you include all the "missing" people whose bodies were never found, about two and a half thousand Americans were killed by Katrina. The incompetence of the federal government's response made the event even more shocking to a nation that had come to think that this kind of natural disaster only happened to places like Honduras or Bangladesh, so it's not surprising that President Bush cancelled his planned speech at the Republican National Convention at the last minute. The last thing John McCain's campaign needed was a living reminder of that blunder.

However, the main impact of Katrina was to break a great many people out of their denial that climate change was a problem. The big shift in American public opinion over the following 18 months owed much to Al Gore's film An Inconvenient Truth, but for many Americans who would never believe a word Al Gore said, Katrina was the moment when the denial stopped.

Yet the truth is that Hurricane Katrina could have happened at any time in the past 50 years. In any of those years it would have produced the same results, assuming the same degree of human incompetence, because the flood defences of New Orleans had been inadequate for a long time. The climate change models predict more intense hurricanes, but not necessarily more of them––and Katrina was only category three on a scale that goes up to five.

Katrina hit in just the right place, and exposed the vulnerability of New Orleans. Hurricane Gustav, another category three storm, missed it and struck less populated areas which, this time, had been mostly evacuated. But if it had been Katrina II, it would have done more than a thousand stories about shifting rainfall patterns, acidifying oceans, and melting ice to persuade people that climate change is a real threat to their well-being. Even though it was just a hurricane, and may have had nothing to do with global warming.

The regrettable reality is that there will not be a critical mass of people willing to act decisively on cutting greenhouse gas emissions in the developed countries––where most of the cuts must be made––until some really big natural disaster kills a lot of people in one of those countries.

It doesn't necessarily have to be a disaster caused by climate change (although it probably will be), because most people don't understand enough about the climate to know what is valid evidence for climate change and what is not. Katrina helped to move Americans from denial to acceptance that global warming is a problem, but it will take an even bigger disaster to persuade them to act decisively.