Think of Japan 100 years from now. The average global temperature has risen by up to 6 degrees, and here is no exception. Just as the cherry blossom wave passes up the country each spring, the frontier of many species, both plant and animal, has been moving steadily northward for a century.

Tropical fruits are no longer limited to Okinawa -- sugar cane and pineapples now grow in Kyushu, and bananas in Tokyo; apples have become more plentiful in Hokkaido than in Tohoku.

The picture may seem an appealing one, but the ecological consequences are potentially devastating. Most species are settled into a "comfort zone," a preferred operational temperature range in which they thrive. Sometimes the niche is highly specialized -- as with the diminutive madarasuzu cricket that lives in geothermally warmed leaf litter in hollows on the Wakoto Peninsula in eastern Hokkaido.

Just as the slightest temperature change would see those crickets fry or freeze, so too will many of Japan's life forms have to move when the country's climate is 6 degrees hotter. If they don't, they'll die.

Now imagine this settlement pattern repeated endlessly, defining the distributions of plant and animal species the world over. Think big. Think all plants, all animals, all crops, all domestic animals, all people -- every species inhabiting a preferred temperature regime that perfectly suits it. What, then, will happen when global warming really heats up? How will species survive and thrive -- and what are the consequences of failure?

There are signposts along our way, for global warming is really nothing new -- it has simply been newly noticed by humans, with our short perspective on life. Seventeen significant periods of climatic warming have occurred over the last 2.4 million years. Even over the last 10,000 years the world's average temperature has gone up by 5 degrees. However, within that period, rapid warming occurred as the Great Ice Age ended. Thereafter, temperatures rose to higher than current levels.

Even during the relative climatic stability of the last 6 millennia, scientists have identified a Medieval Warm Period. This was followed in turn by the so-called Little Ice Age, from 1300-1850, a cool period marked by climatic unpredictability and storms.

Increasing unpredictability is central to any picture of Japan in the future. Already prey to typhoons and storms, if weather changes affecting Europe are anything to go by, Japan may well suffer these with greater severity than ever before. Farmers will have to select crops for cultivation under conditions that may see a balmy summer one year and frigid conditions the next -- though a second annual rice crop may become possible throughout Japan.

But while humans may be adaptable -- farmers can dig up orchards and foresters may replant forests in new areas -- Japan's plant and animal life will not be so fortunate. Flora and fauna move at slower rates, dependent on seed dispersal or their own ability to crawl and fly. Change in itself is not the issue -- it is the speed of change that terminally stresses species.

There will be success stories, of course. In Japan, warmth-loving species will spread northward as higher temperatures reach northern Honshu and Hokkaido. The common broad-leaved evergreen laurels of Kyushu and western Honshu, and the towering southern bamboos, will spread slowly northward through Kanto into Tohoku. Southern Japanese species, such as the beach lily and the red-rumped swallow, previously clinging to coasts washed by warm currents, will extend their ranges into northern Honshu.

Summer-loving insects will spread, too -- though many may be unwelcome. That summer scourge, the cockroach, which once survived Hokkaido's icy winter only in cities, will range throughout the island. Tiger mosquitoes will likewise move northward, while more southerly mosquito species will arrive in Japan carrying Dengue fever, West Nile virus and malaria through Kyushu into Honshu. Medical bills for the treatment of tropical diseases will soar.

The advance of warmth-adapted species will be at the expense of cold-loving ones, which will be displaced ever higher. Thus species requiring a cool "comfort zone" will climb altitudinally, displacing sub-alpine and alpine species of plants, birds and butterflies. Those species already isolated high in the mountains will withdraw steadily upward, into an ever diminishing land area, as far as soil exists -- beyond which retreat is impossible.

Species seeking cooler climes will advance northward and upward. In Japan, a century on, the beautiful native white-barked birches (and the insects depending on them) will likely have gone from Honshu, retaining a toehold only in Hokkaido.

However, retreating species will reach uncrossable physical barriers: highways, massive conurbations and the deep-water channels separating the Japanese islands. Any northward shift of the beautiful beech forests now covering the Shirakami Mountains in Tohoku, like that of the Japanese macaque and badger, will be blocked by the Tsugaru Straits separating Honshu and Hokkaido.

Such species are doomed, trapped in shrinking habitat inside which conditions will change beyond their ability to endure. Gradual climatic change is something that can be met with gradual adaptations -- the movement of plant species, for example, is achieved by the tiny incremental distances traveled by dispersing seeds.

But climatic oscillation -- mercurial changes around the global average -- favors organisms that thrive amid disturbance. Tough, opportunistic species, widespread and adaptable -- weeds and pests, in other words -- will do best; species dependent on narrow or limited conditions will suffer.

Many of our efforts at conservation will have been in vain, for man-made nature reserves, sanctuaries and national parks occupy fixed locations that many of their plant and animal inhabitants will, one day soon, no longer be able to inhabit.

And our own habitat, too, will be very different 100 years hence. Imagine future winters in which snow rarely falls in the lowlands and skiers have to crowd the heights of Hokkaido, while the Sapporo Snow Festival will be a thing of the past and the sea ice along the Okhotsk Sea merely a memory. As well, the massive shoals of fish that came south each winter, ahead of the ice-cold water, may no longer crowd toward Japan's shores.

By then, all efforts will be too late: Many species and habitats will have been lost irrevocably in a Japan that experienced, in just one century, the same extent of warming as has now occurred over the last 10,000 years.