For those of us in the Northern Hemisphere, it's winter. Hold on. Let me rewind that glib phrase and start with the backstory:

Think of the universe. Think all the time it has existed, and think of all the matter in it. Imagine it expanding and spinning. Think of the orbits of planets around the sun. Think of the light emitted from the sun falling on the surfaces of planets and moons.

It's hard to imagine that all that stuff is really happening, even if you lay out apples, oranges, melons, tennis and basketballs balls as planets with marbles for moons, and shine a torch in place of the sun. On earth, all we see are the side effects: Day and night. Warm or cold. Summer or winter. Our experience of the universe is comparable to an ant walking across a page of novel: Patches of dark and light with no way to piece them together. Before we knew why and how the seasons changed, the mystery started making us act strangely.

The significance of winter first came from our rumbling stomachs. Seasonal plants had died, and the animals who ate those plants hibernating or not reproducing. Food was scarce.

We pushed stones vertical on the Wiltshire plains to mark significant risings and settings of the sun: mid summer and mid winter. Significant because having a calendar meant agriculture could be managed. Perhaps they were also significant as part of a ritual attempting to bring back the life force of nature at the dead centre of winter. The reason we built things like Avebury and Stonehenge wasn't because we were at one with nature, but exactly the opposite: We couldn't understand it, we felt removed and alienated.

These days, we are just as alienated. We've tried as hard as possible to equalise the inconveniences of seasonal difference. We can regulate internal temperatures and eat seasonal produce all year long. You can get a better tan indoors in January than on the beach in August. Technology shrinks winters discontents. It's harder to actually significantly experience winter.

It's unlikely we'll miss it though. Even in the depths of a shopping mall, where the temperature stays constant all year round, cardboard snowflakes will hang from suspended ceilings. German markets will appear in lots of places that aren't Germany. Ice skating rinks will be erected in city squares. Fir trees will be put in places fir trees can't grow. Aerated plastic foam will be sprayed onto the windows of pubs and offices - simultaneously conjuring the ghosts of Dickensian christmas' and low grade graffiti (perhaps the decorative equivalent of Run DMCs 'Christmas in Hollis' - sample lyric: 'The rhymes you hear are the rhymes of Darryl's/But each and every year we bust Christmas carols'). There will be musak rattling with sleigh bells everywhere.

If you wondered exactly where you were, you might guess a pagan Germanic forest with touches of the middle east and the north pole, enjoying some apres-ski, sometime between 1550 and 1850.

All those wintry symbols have been handed down over thousands of years, from pagan to Christian to consumer. Their meanings and purposes submerged, subverted, hidden and hijacked till they form a cultural white noise: Ancient druid-magick fertility rights wired up to the mains.

Tinsel recalls strings of ivy only made sparkling, diffuse, the glistening of frost abstracted to metallic sheen. Patterns recalling foliage have passed from pagan ritual to church and are now punched into film-thin novelty.

Trees were once brought inside as a winter solstice offering. They were decorated with tree sprites, which became Christianized as angels. Now, trees are laced with LEDs, or remade from nylon and fibreoptics - as white as spacesuits. Imagine the quantities of artificial nature stockpiled in distribution centres, ready for shipping. Imagine if hillsides were planted with colour cycling synthetic forests. Imagine birds nests lit in washes of magenta and cyan tinted rabbit warrens. What kind of electric fairy stories would emerge from this unnatural forest?

We extend our artificial winters: Harrods opens its Christmas shop in august, mince pies are on the supermarket shelves in September, Christmas singles are plotted in spring. Meanwhile, winter itself is shrinking. Research into climate change shows how autumn and spring are eroding either end of winter. Many European plants flower a week earlier than they did in the 1950s and lose their leaves 5 days later. Biologists report that many birds and frogs are breeding earlier in the season. An analysis of 35 non-migratory butterfly species showed that two-thirds now range 2 to 150 miles farther north than they did a few decades ago. The spring ice thaw in the Northern Hemisphere occurs 9 days earlier than it did 150 years ago, and the autumn freeze now typically starts 10 days later.

Maybe it's because there are less real snowflakes that we need to make decorative ones.

Climate change is altering iconic winter landscapes. North of forty degrees north latitude, the growing season for vegetation has increased by several days. The artic is becoming greener. The ice cap is thinning. There are predictions that the arctic could be completely ice-free during summer months by 2060.

In the Alps, average temperatures have risen by up to 3C over the last century during the winter months at 1800 meters. A lack of snowfall in some regions has exacerbated the problems to the skiing industry. Even the snow canons, which blast artificially created snow over the pistes to augment natural precipitation can't fix it when temperatures are above zero.

Our representations of these wintry scenes become stronger, denser and more hysterical just as their reality is threatened.

Each winter, you can, like at many other places, skate on a temporary ice rink at Somerset House. What's different here is that it is just yards away from the old banks of the Thames. Since the construction of the new London Bridge and the Embankment, the rivers increased speed has stopped it freezing over. 200 years previously, it was the site of the Frost Fairs, which took place on the frozen river. The idea still persists, wearing rented ice skates, standing on ice frozen by a grid of pipes pumped through with brinewater cooled to -9 degrees by evaporating Freon.

Landscapes are as much about imagination as they are geography. The landscapes we draw and make of winter are not quite illustrations. These winter wonderland scenes freeze moments when the world looks new and fresh: coated with overnight snow; icicles glistening in the winter sun; the crystalline patterns of snowflakes. That freshness is a brief glimpse of what we imagine nature to be: an idealised version of nature before man. The myths of Eden and Arcadia served previous generations as visions of the world before the fall. Frosty the Snowman does the same thing for us.

What we are seeing now is the result of the greenhouse gases emitted up till 1968, because the climate takes about 30 years to catch up. If its true the effect of todays pollution will not become apparent till about 2030. Culturally, the lag is less predictable.

If the environment and landscape is shifting and changing, it's perhaps no wonder that we attempt to invoke certainty through other means: nativity scenes, shop window displays, and seasonal blockbusters. But it's not just these obviously ephemeral things. The line between strictly decorative items and real, proper objects becomes blurred. Think of the winter coats you might find yourself wearing: This winter might find you in some kind of military inspired trench coat - a perverse nostalgia for the certain horror of the First World war. Vogue tells us that other autumn/winter 2005 trends include other wintry visions: Victorian and Imperial Russian. Alternatively, you might well find yourself sporting a high tech jacket featuring breathable fabrics, with coatings, laminates, insulation, even Bluetooth control of your phone or MP3 player buried deep in your pockets. These items invoke the security of protection though engineering. Certainty through technology.

These things aren't just protecting you from the environment. They are protection against what you imagine winter to be.

Simulations of future climate change are running on the computer systems of university research departments. Equations represent the physical processes of the climate, and the maths charts possible futures versions of the earth. As they cycle through seasons, winters change. The United Nations' Intergovernmental Panel on Climate Change projects that global temperatures will rise an additional 1.6 to 5.5 degrees Celsius by the century's end. What these winters will look like is probably not part of these simulations. What new meanings will we attach to winter? What new significance will it gain?

As actual winter shrinks and technology reduces its impact, we have expanded its simulation - an artificial Narnia applied to a warming planet. We have found ourselves redesigning winter, both intentionally and accidentally. Winter has become biggest, most ambitious design project we've ever undertaken.