Living at risk, it has been said, is akin to jumping off the cliff and building your wings on the way down. Not everyone would be content with such a strategy. Some would not venture close to the edge, even if that meant missing a discovery or a thrill. Others would insist on constructing wings in advance and keeping them nearby just in case they might be needed at short notice.

Sir David King, the Government's outgoing Chief Scientist, is clearly and understandably in the second category. He has spent much of a tenure that is probably best remembered for the foot-and-mouth outbreak, evangelising inside Whitehall on the prospect of global warning. He pulls few punches on this score in his valedictory interview with The Times this morning.

His other theme, and no less dramatic, is the possibility of a global flu pandemic — triggered by the emergence of a strain of bird flu which could easily be transferred to humans. This, he gives warning, would spread rapidly from South-East Asia on to Europe. While he has convinced ministers to invest heavily in stockpiling antiviral drugs, Sir David assumes such an outbreak would involve 300,000 British fatalities and massive economic dislocation.

Despite this chilling thought, his view of risk remains optimistic. Sir David believes in what Donald Rumsfeld famously labelled “known unknowns”. Climate change and a bird flu pandemic are dangers that officials appreciate could happen. The policy argument consists of how large a risk they constitute and what should be done and at how much cost to alleviate the chance of their occurring. In a similar spirit, financial institutions can hardly complain that there were no grounds for thinking that “sub-prime” loans in the US mortgage market could return to haunt them. The risk might have been, as Gordon Brown claimed this week, “underpriced” but it manifestly existed.

There are others, however, who would maintain that it is “unknown unknowns” that are the far greater menace to humanity. In one of the most interesting works of recent times, Nassim Nicholas Taleb makes the case that “black swans” have been the dominant feature of history. These are events of enormous impact that are rare and virtually impossible to predict, not least because they come from beyond the realm of conventional expectations. The most potent “black swan” of recent times (the phrase comes because it was thought universally that all swans were white until a black variety was found in Australia) was September 11, 2001, but Taleb puts forward the theory that there have been others, all notable for their bolt-from-the-blue quality. If so, then logically, while it may seem a sound precaution to focus on bird flu, if a pandemic comes it will not be the one that Sir David expected.

Where on earth does this leave the concerned citizen? What should he or she be disturbed by most in the year ahead? Another terrorist atrocity? Chinese inflation? Assertive Russian nationalism? American protectionism? The price of food and other soft commodities? Oil moving relentlessly on to $150 a barrel? The collapse of several leading global banks? The possibility that Led Zeppelin are actually serious about a 2008 world tour? Or if Taleb is correct and the black swan is the ultimate cuckoo in the nest, is it even worth attempting to plan for the future?

The difficulty with the black swan concept is that it can acquire nihilistic characteristics. It can be taken to mean that climate change and bird flu should not be addressed because something else will arrive in their place. Risk becomes an Act of God rather than an area of life in which calculations of probability have any utility. This is too sweeping. Unknown unknowns may be more devastating than known unknowns because of their shock value. This cannot mean that known unknowns have never manifested themselves or been prevented because they were anticipated. Would we be comfortable if Sir David revealed that he had not bothered to commission antiviral drugs because they might not be effective against the pandemic that emerges? Those who leap off cliffs would be wise, as far as possible, to have those home-made wings at close quarters.