When was the last time you heard an environmentalist say something nice about Christmas? For those of us trying to live greenish lifestyles, this time of year, with its commercialism, its excess of consumption and its religious mumbo-jumbo, has always been a trial. In my family we never watched the Queen's Speech, we ate nut-loaf instead of turkey, and one year tried out my father's paper-saving idea of wrapping our presents in old copies of Guardian Weekly (it didn't catch on).
But what if, in rejecting the modern awfulness, we are missing out on some of the most significant traditions of all? Delve a little into the origins of the December rituals and you soon find yourself in a world of nature spirits, of Celtic mythology and of sun-worship, when the changing of the seasons was still the most important event in the calendar. That these traditions survive is a testament to our continuing - if increasingly remote - ties to the landscape and ecology of these ancient northern islands. It is also a window into a history that still binds us to the ancestors who walked these lands hundreds and even thousands of years ago.
Hardly any of the Christmas traditions we observe has anything to do with the birth of Christ. That much is well known, as is the fact that many of the rituals have their origins in older, pagan traditions. But the true significance of those traditions has been lost even as their superficial trappings have been absorbed into the commercial age: Christmas pudding, the decorated tree and Santa Claus are central to every child's idea of Christmas, yet their original meanings have faded away. Perhaps it is time to rediscover their ancient power, and to let nature back in.
It was Pope Saint Julius I, in the 4th century AD, who decided that Christ's official birthday would fall on 25 December. There are few clues to the date in the Bible, and earlier guesses had placed it in the autumn (shepherds rarely tend flocks on hillsides at night in midwinter). But the pope's decision had less to do with scripture than with a wish to co-opt pagan festivities: the Roman Saturnalia reached a high point on 25 December, which was the day - in the Roman calendar - that the sun was at its lowest ebb. The birth of the Persian sun god Mithra was also celebrated on that day, as was the coming of Odin, the Norse god who roamed the skies on an eight-legged horse. Pope Julius saw a chance to turn all these pagan festivals into a Christian one, and he took it.
The Saturnalia - Saturn was the god of agriculture and his feast was a time of great revelry - reminds us that there is nothing modern about our Christmas excesses of eating and drinking. In a world where the harshness of winters could still mean life or death, feasting on the winter solstice made sense: with assurance that the sun was about to reawaken and spring would follow, there could be a temporary relaxation of midwinter rationing.
Other Christmas traditions reflect the same concern about winter food. The Celts kept the harvest god Dagda happy by stirring a pot of porridge for him. This was later livened up with prunes to become plum porridge; then it turned into plum pudding, and finally today's Christmas pudding. In theory, the pudding should be stirred by each member of the family in turn to show that everyone has done their bit, and only in an east-to-west direction, to mimic the sun's path across the sky. Flames from the brandy symbolise the heat of the returning sun and the sprig of holly - with its unique winter berries - evokes the fruit of the harvest.
The seasons also underpin the myth of Father Christmas. The Vikings chose someone to represent Old Winter and he was dressed up, fed and made as welcome as possible in an attempt to appease the gods and bring on a mild winter. This figure later became confused with Santa Claus and they are now considered one and the same. Santa, however, came to Britain from America, where he was introduced by Dutch immigrants. Their "Sinter Klaas" was Saint Nicholas, a Turkish bishop famed for acts of kindness (including putting a bag of gold into the stocking of a girl who needed a dowry - the origin of the Christmas stocking). The story that Santa was popularised by the Coca-Cola company may sound like a paranoid fantasy from the anti-globalisation movement but it is horribly true: he never wore red and white until he appeared in Coke advertisements in the 1930s, wearing the company colours. However, his reindeer, prominent in Clement Moore's 1822 poem "A Visit from St Nicholas", do come from old legend: the Lapps believed that Old Man Winter brought his reindeer down from the mountains at the same time as the first snow.
Vegetation - especially evergreen vegetation - is central to much of the Christmas ritual today. Holly, ivy and mistletoe are some of the few plants that keep their leaves during the European midwinter, and were revered as symbolising everlasting life. Mistletoe in particular was seen as sacred by the Celts, and druids would hold a special ceremony for any mistletoe found growing on a similarly sacred oak. Such a plant could be cut only with a golden sickle on to a white sheet - if it touched either earth or iron the magic powers would be lost. The resulting potion was associated with fertility, which is why the sprig of mistletoe causes such trouble at modern office parties.
The tradition of bringing evergreen vegetation into a dwelling is more ancient still. Sprigs of green leaves were brought inside to provide a temporary haven for the tree-dwelling spirits that were seen to have deserted the rest of the forest for the winter. The belief that bad luck will follow if you leave Christmas decorations up past Twelfth Night also has its origins here: if the spirits were not released once midwinter had passed, the forests and fields might not awaken again. And if 6 January seems early to release your tree spirits, given that the coldest winter days are still likely to be ahead, this is because the original pagan festivities went on until February. (They were shortened in feudal times to get the peasants back into the fields sooner.)
It was to provide extra encouragement to the tree spirits that people attached painted stones and coloured cloth to branches in midwinter - hence Christmas decorations. Particularly common in Germany in the Dark Ages, this practice originally centred on oaks, but Christian missionaries transferred the focus to firs, whose triangular shape they linked to the Holy Trinity. Christmas trees in the modern sense did not arrive in Britain until the middle of the 19th century, when Prince Albert brought one from his native Germany to decorate Queen Victoria's palace.
Other aspects of the Christmas tradition also celebrate our interdependence with nature. One of these was the rural habit of providing extra food to domestic animals (which were often seen as part of the family) as a gesture of gratitude for their role in keeping their human companions alive. Such rituals have been lost because of our disconnection from the land and the disastrous Christian notion that humankind was given "dominion" over nature, to exploit and abuse as it found convenient.
Although the Christmas traditions and rituals carry only faint echoes of their previous significance, their survival tells us something about the strength of our human need for connections both to those who have gone before and to the natural environment that still nurtures and nourishes us despite all the trappings of modern life. If we could use this time of year to recall the meanings and purposes of these ancient ties, perhaps there would again be something worth celebrating about Christmas after all.
Mark Lynas's book High Tide: news from a warming world will be published by Flamingo on 1 March. www.marklynas.org