The Big Question: Officially, this is the first day of spring. So why does it feel like midwinter?

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Why are we asking this question now?

As you may have noticed, it's been cold outside. And the simple answer to the big question at the top of the page is that one swallow does not make a summer. In other words, the weather and the climate are not the same thing, although they are of course related. One is what you get on a day-to-day basis, the other is a long-term trend over many decades, centuries or even millennia.

The current cold spell is due to cold winds streaming down from the frozen Arctic, which is not an unusual event for Britain at this time of year. Global warming does not mean an end to spells of cold weather in winter, but it does make them less likely over time. The computer models of global warming predict that as average worldwide temperatures rise, there will be an increase in the chances of warmer, wetter weather in winter and hotter, drier spells in summer.

Where does 'weather' end and 'climate' begin?

There is no strict point when one becomes the other - it is purely a question of time. Climate is something that can be defined over hundreds or even thousands of years. As a spokesman for the Met Office said: "It would be fair to say the weather becomes climate when observations of the weather are long enough to glean a trend from those records."

For the past 100,000 years or so the Earth has experienced fairly regular swings in climate from freezing ice ages to warmer periods of "interglacial" temperatures. We are currently living in a warm, interglacial period, which began at the end of the last ice age about 10,000 years ago. It is no coincidence that this was the time when we invented agriculture, which led to the rise of civilisation.

We can expect another ice age in a few thousand years, all other things being equal, although current man-made global warming could affect the timing of that fairly inevitable event. Ice ages appear to be triggered by small changes to the angle of the Earth's tilt in its orbit around the Sun.

On a shorter timescale, the weather patterns in any geographic region can be described in terms of that region's local climate. Britain, for instance, has a maritime climate where the seasonal variations are not as extreme as in areas affected by continental air masses, such as central-southern Siberia which is on about the same latitude as Britain yet experiences far colder winters and hotter summers. Nevertheless, the mild maritime climate of north-west Europe can be interrupted occasionally by incursions of cold Arctic air in winter - which is happening now - and hot subtropical air in summer.

How do we know the climate is changing?

This is the key to our understanding of climate change. Being able to discern long-term trends in the apparently random "noise" of day-to-day variations lies at the heart of understanding the scale of global warming.

Essentially, scientists believe that the climate is changing for many reasons but one of the most important is the fact that long-term weather records suggest it is happening. Different countries have gathered records on many different phenomena related to the weather and these individual measurements can be collated and analysed to give a bigger picture of how the weather - and climate - have changed over time.

Good records of course depend on sound instruments and expert monitoring. The longest accurate series of monthly temperatures, for instance, is the Central England Temperature records. These cover a triangular area between Liverpool, Bristol and London with the monthly series of temperatures beginning in 1659 - the oldest continuous temperature record anywhere in the world.

This temperature record shows that the hottest years in the sequence were 1990 and 1999 - with an average annual temperature of 10.63C - and the hottest ever year was 2006, with an average annual temperature of 10.84C. The coldest year was 1740 and the hottest month was July 2006.

But aren't we seeing more flowers in winter?

One of the most interesting ways of looking at the results of long-term climate is called "phenology", which is the study of recurring natural phenomena such as the flowering of the first daffodils in a garden, the appearance of the first migratory bird or the times of ripening of autumn fruits.

According to the Woodland Trust, spring in Britain is coming about two weeks earlier than it would have been about 30 or 50 years ago and autumn is arriving about a week later. Many amateur naturalists can now observe the sort of changes that would be expected if the climate were altering as a result of global warming.

What about birds and animals?

There can be real problems for animals and birds that have been lured into breeding earlier than normal. A warmer-than-average winter, such as we have seen this year, may tempt birds and animals into having their young weeks earlier than they generally would. Then, if the cold returns, the nestlings and the young animals are suddenly very vulnerable, and may die. We have seen this happening recently in southern England, with baby squirrels, baby hedgehogs, even baby grass snakes being found in distress and taken to animal refuges like the St Tiggywinkles wildlife hospital near Aylesbury in Buckinghamshire.

Will it get worse?

It may do, if the weather and wildlife breeding patterns increasingly get out of sync. This year and indeed in previous years there have been warm spells followed by cold snaps which have hit wildlife hard locally and temporarily, but this has been a merely passing phenomenon. It is possible that it could become more systematic and more serious. For example, many woodland birds feed their young on caterpillars and time their nesting to coincide with the caterpillars' appearance. If they appear earlier, resident birds will learn to adapt, but migratory species may arrive in Britain too late for the major food source for their chicks - and their populations may crash as a result.

Has too much been made of particular dramatic events?

Some people claim that, for example, the flooding of New Orleans by Hurricane Katrina in 2005 was caused by global warming; but others say that such events, although rare, are not unknown in the Gulf of Mexico. Others warn that while no one event can be ascribed to global warming, it is the trend that is to be feared. Although it may well be that there are more frequent weather shocks than previously, we just cannot know which of them is caused by global warming and which would have happened anyway.

Should we be surprised by unexpected cold spells?

Yes...

* People have come to think that because global warming is a fact, it must always be warmer than we remember

* As the winters are becoming warmer, it goes against expectation that the early spring suddenly turn cold

* If the Arctic is warming up, one would not think that winds from this region would be so cold at this time of the year

No...

* Natural variability means that the temperature is always going to oscillate over the short term

* What we are seeing here is day-to-day weather, whereas climate change occurs over longer periods

* The turning point between winter and spring is usually a period of unsettled weather

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