British misery has nothing to do with the threat level

No one seems terribly concerned about the home office's warning. Londoners look miserable, but they always do in the middle of a cold and soggy British winter.

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I arrived in London just after the British home secretary Alan Johnson raised the terrorist threat level in Britain from "substantial" to "severe", meaning an attack is highly likely. A "severe" threat warning is just one rung below "critical", which means an attack is expected imminently. The government will not give any more information about why Britain is "highly likely" to be hit.

The public is advised to look for suspicious bags or any other "potential signs of terrorist activity". Suspicious bags, fine, I'll watch out for those. But what on earth does the rest mean? At Heathrow airport I saw some young, heavily bearded Asian men wearing shalwar kameez. They were huddled together. Were they planning to set off a bomb? Maybe they were talking about where to go for lunch. There were more young men on the Tube yesterday talking loudly in Urdu and looking rather agitated. Perhaps one of them was late for a dentist's appointment.

No one seems terribly concerned about the home office's warning. Londoners look miserable, but they always do in the middle of a cold and soggy British winter. This week there are two conferences on Afghanistan and Yemen and the US secretary of state Hillary Clinton will be in town. On Friday, the former British prime minister Tony Blair faces the Iraq inquiry and will have to explain why he claimed that intelligence proved beyond a doubt that Saddam Hussein had weapons of mass destruction. Thousands of anti-war protesters are expected. The week will provide a perfect storm of events for anyone wishing to make a deadly statement. But we are meant to simply carry on with our daily routine, go to work, take the train, whatever. Yet we must be ever vigilant for a sign that the guy next to us isn't just rummaging through his rucksack for a pen but is about to blow himself up.

The last time London hosted an international conference on Afghanistan in 2006 it was optimistically called "Building on Success". If there was little success to build upon then, there is even less now. Still, representatives from about 60 countries donating financial aid, military or diplomatic assistance to Afghanistan will converge on Thursday to discuss the next five years and set benchmarks for the Afghan president Hamid Karzai's progress.

For the British public, a timetable for withdrawing troops in an increasingly unpopular war is paramount and the prime minister Gordon Brown has said soldiers will begin leaving next year. But a leaked document reportedly says British soldiers will be fighting in Afghanistan for five years. A total withdrawal of all foreign troops for good will hinges on the ability of the Afghan government to protect its population from insurgents, secure its borders and police Afghan cities and towns.

More than 300,000 soldiers and police are expected to be trained over the next year or so. How a poor country like Afghanistan will afford such a vast security force has not been answered satisfactorily. Two years ago, the Afghan National Army Trust Fund was set up by Nato. The organisation estimated that sustaining a 134,000-strong force will cost about $2 billion a year. Countries were asked to donate. As of October, $312 million was pledged. So far, $35 million has actually arrived. Nations are very good at making gestures of solidarity and no doubt more will be offered on Thursday. Translating that into action is another thing.

Meanwhile, a conference of another worldly sort is taking place in London. This time about aliens from outer space. Respected scientists and academics are discussing at the Royal Society what the consequences will be for science and society if extraterrestrials were to visit earth.

One of the issues to be raised is whether earthlings should make a concerted effort to send signals into space that we are trying to contact alien civilisations. Aliens must know we are here. Over the last 100 years humans have been sending electromagnetic signals into space as a result of radio and television. Images and sounds of the First and Second World Wars, for example, are floating around the universe as we speak.

Professor Simon Conway Morris, a Cambridge University evolutionary biologist, told the Sunday Times newspaper that if aliens have picked up such footage they may have decided to keep hush. "I'm not sure I'd answer the telephone," he said. I agree with the good professor.