IT HAS long been known by police officers on the beat that cold winter nights keep criminals off the streets and crime levels down. But what happens when winter nights are no longer cold?
Criminologists and police officers are now beginning to speculate that one of the hidden consequences of global warming will be an increase in street crime during mild winters.
They are quick to point out that the picture is complex and largely unknown, as the vast demographic and social changes that will accompany climate change will also have a large impact on crime patterns.
But as Britain warms over the next century, law enforcement experts are beginning to worry that they will soon lose the help of two of their key winter allies - Frost and Snow.
There are also concerns that warmer summers will encourage increased alcohol consumption, which is strongly linked to criminal activity.
Ken Pease, visiting professor of crime science at University College London and one of Britain's leading criminologists, said: "We know that more people on the streets, larger crowds, and alcohol consumption are all linked to increases in crime. And it stands to reason that warmer weather will encourage all three.
"The question really is not whether global warming will lead to an increase in street crime, but by how much?"
Several prominent, peer-reviewed studies undertaken in the 1990s found strong evidence of a positive effect of temperature on most types of property and violent crime.
One report, The Effect of Temperature on Crime, published in the British Journal of Criminology in 1992, looked at more than 40 years of data and concluded that there was a strong relationship between warmer temperatures and "most types of property and violent crime... independent of seasonal variation".
Clive Murray, president of the Scottish Association of Police Superintendents, said it was very difficult to predict the impact of changing climate conditions because scientists are predicting wetter, as well as warmer, winters.
He added: "There's an old expression that the best firefighter and police officer in Scotland is rain, and that's regardless of temperature.
"If it's raining, the streets tend to stay quiet. But it's also true that if it's cold and wet the streets clear quicker."
Many police officers said a bigger concern would come during hot summers when pubs fill up. There is a biological basis for this concern; animal studies have suggested that warmer temperatures boost aggression hormones such as epinephrine and testosterone.
Prof Pease, who has published studies on crime seasonality, said pretty crime would increase at a higher rate than violent crime.
He added that there would be more thefts and assaults as more people crowd onto trains and buses. There could also be more white-collar crime as a black market emerges with the implementation of carbon-trading schemes.
Prof Pease also warned that police would likely have to tackle vigilantism against those who are not environmentally friendly. He added: "We've already seen attacks against so-called Chelsea tractors in London."
Scientists project a global temperature increase of between 1.4 and 5.8 C in the next century. However, some crime experts point to a coinciding dip in crime rates over the last five years, despite the increase in temperatures, to argue that the effect of global warming on crime remains unknown.
Kenneth Scott, director of Scottish Centre for Police Studies, said: "There have been recent drops in crime over the last few years, despite an increase in temperatures, which would seem to run contrary to this thesis. The relationship will be quite complex. It does seem clear there will be changes to many aspects of life, so it would be reasonable to expect some change to criminality. In many ways, we'll just have to wait and see."