It is one of 35 indicators which have been officially chosen to detect the initial signs of climate change.
Without needing to be prompted by the weekend's collapse of Beachy Head, which the Environment Agency said was climate change-induced, Government scientists have quietly started to monitor and bring together a mass of small and large events, both in the natural world and in society, which may be the first signs of a hotter planet.
Some of these are already strongly suggesting that climate change is no longer a theoretical calamity predicted by the supercomputers of the Met Office, but is already with us.
The indicators they have chosen range from the arrival dates of swallows in spring and the leafing dates of oak trees, to the number of possible skiing days in Scotland, the number of insurance claims for "major weather perils" and the number of human cases of Lyme Disease, caused by a tick which flourishes in warmer weather.
The first indications, certainly from the evidence assembled about the behaviour of birds, plants and insects, is that the greenhouse effect is now here. Over the past twenty years many events in the natural world have started to take place much earlier in the spring, in a way entirely consistent with a warmer climate
Much of this evidence is startling and when it is assembled, as the Government is doing, it is compelling. The orange tip butterfly, for example, is now emerging about 11 days earlier than it was 20 years ago. The leaves on oak trees at a monitored site in Surrey are emerging about three weeks earlier than when records began in 1947 and the swallow is arriving earlier at eight different bird observatories. Twenty species of birds have shifted their egg-laying dates an average of 8.8 days earlier in the 25-year period from 1971 to 1995.
The importance of the Government's new initiative to bring all this together is that for the first time it puts official emphasis on monitoring, as well as prediction.
Until now most of the effort and funding in the fight to combat global warming - hundreds of millions of pounds - has gone into climate prediction. Scientists with complex computer models could happily tell you what was likely to happen in 50, 75 or 100 years, but were unable to tell you what was happening now, as noting that the egg-laying date of the chaffinch was weeks earlier than before was regarded as a suitable occupation only for types in anoraks.
But official recognition has finally come of the fact that dramatic trends may suddenly emerge from large numbers of small observations, once they are plotted, and the need to observe the many minute changes which may be the first signs of a dramatically warmer world is now accepted.
The mass of information is being brought together in a single database, coordinated by Professor Melvin Cannell of the Institute of Terrestrial Ecology in Edinburgh.
His unpublished pilot study, "Indicators of Climate Change in the UK," which is now being reviewed by officials at the Environment Department, contains an initial 35 indicators chosen to register global warming's effect. They begin with climate and temperature itself. Obvious measures such as the number of hot and cold days and the amount of rainfall are joined by subtler indicators like soil moisture amounts, amounts of groundwater stored in chalk and the number of times a year the Thames Barrier is closed (an indicator of the rising sea-levels global warming is predicted to cause).
Social indicators include the value of annual domestic claims for subsidence (which increases in very hot dry summers), amount of gas consumed in winter, and the number of holidays taken within the UK. Agricultural indicators selected include areas of vineyards in the UK, yields of non-irrigated potatoes and the amount of late summer hay yields. These latter two categories are more likely to show changes once climate change is firmly established. But it is the changes in wildlife behaviour that are pointing to global warming's arrival already.
Two of the scientists helping coordinate the Government's data, Tim Sparks, of the Institute of Terrestrial Ecology at Monk's Wood near Peterborough, and Humphrey Crick, of the British Trust for Ornithology, gave remarkable information of their own on climate-related changes in plant life last week when they published the first report of their new monitoring network.
They compared the flowering of four different plants in 1998 with records made a century ago and found all to be much earlier.
Mr Sparks, who is an environmental statistician, holds a number of similar remarkable records not yet in the Government database. He quotes the Hertfordshire Natural History Society, whose record of the dates of arrival of six migrant species of birds show the swallow is now arriving in Hertfordshire on about March 26 as against April 8 in the 1890s.
"All these records show changes in behaviour which is entirely consistent with a warmer climate," Tim Sparks said.
"Soon," said Humphrey Crick, "We shall be waiting for the first swallow of the winter."