By Richard Black
Environment Correspondent, BBC News website
Predictions vary from the catastrophic to the cataclysmic.
Glaciers: Coming to a sticky end?
Glaciers are melting, the ice caps disappearing into the oceans. Sea levels may rise by many metres as a consequence.
Indigenous Arctic peoples will find their food stocks gone, while fresh water supplies in Asia and south America will disappear as the glaciers which provide them melt away; penguins, polar bears and seals will find their habitats gone, their traditional lives unliveable.
But how realistic is this picture? Is the world's ice really disappearing, or is it unscientific hot air?
A European satellite named Cryosat was designed to provide definitive answers to some of these questions.
A launcher fault destroyed the mission in October 2005, but the European Space Agency has approved a replacement. In the meantime, here is our global snapshot.
Huge, pristine, dramatic, unforgiving; the Antarctic is where the biggest of all global changes could begin.
2002: The Larsen B ice shelf shatters and drifts out to sea
There is so much ice here that if it all melted, sea levels globally would rise hugely - perhaps as much as 80m. Say goodbye to London, New York, Sydney, Bangkok, Rio... in fact, the majority of the world's major cities.
But will it happen? Scientists divide the Antarctic into three zones: the east and west Antarctic ice sheets; and the Peninsula, the tongue of land which points up towards the southern tip of South America.
"Everybody thinks that the Antarctic is shrinking due to climate change, but the reality is much more complex," says David Vaughan, a principal investigator at the British Antarctic Survey in Cambridge, UK.
"Parts of it appear to be thickening as a result of snowfall increases. But the peninsula is thinning at an alarming rate due to warming.
"The West Antarctic sheet is also thinning, and we're not sure of the reason why."
On the up
Temperatures in the Peninsula appear to be increasing at around twice the global average - about 2C over the last 50 years. Those figures are based on measurements made by instruments at scientific stations.
Earlier this year, David Vaughan's group published research showing that the vast majority of glaciers along the Peninsula - 87% of the 244 studied - are in retreat.
The ice dumped into the ocean as the glaciers retreat should not make much difference to global sea levels - perhaps a few cm.
More worrying, potentially, are the vast ice sheets covering the rest of Antarctica.
Making temperature measurements for the continent as a whole is difficult; it is a vast place - more than 2,000km across - there are few research stations, and temperatures vary naturally by 2-3C from year to year.
But measurements indicate that in the west, melting is underway.
"About one-third of the West Antarctic ice sheet is thinning," says Dr Vaughan, "on average by about 10cm per year, but in the worst places by 3-4m per year."
The rock on which the West Antarctic ice rests is below sea level - and British Antarctic Survey researchers believe the thinning could be due to the ice sheet melting on its underside.
"It may be that the ocean is warming and that's causing the ice to melt, but there may be other reasons as well; for example, there's lots of volcanism in that area and so that could change how much heat is delivered to the underside of the ice sheet."
Cryosat should help to pin down what is happening at the West Antarctic fringe. The radar altimeters on board its predecessors ERS1 and ERS2 have been unable to map the steep slopes at the coast, whereas Cryosat's instrument should be able to cope.
If the entire West Antarctic ice sheet did melt, sea levels globally would rise, by around 5m. But at the moment, there is no sign of that happening.
One recent scientific paper attempted to calculate probabilities for how much West Antarctic melting would contribute to global sea-level rises during this century.
The conclusions: a 30% probability of a 20cm rise, and a 5% chance of a 1m rise.
And what of the big monster, the much larger east Antarctic sheet?
A recent study using altimeter data suggested it is getting thicker, by about 1.8cm/yr; another, using the gravity satellite mission Grace indicates its mass remains stable.
But could rising temperatures in time drain the ice away?
"It is not going to happen on any realistic human timescale," says David Vaughan.
"It's so cold that you could raise temperatures by 5-10C without having much of an impact; it's on rock above sea level, so warming in the ocean can't affect it."
Largely insulated from global trends and so big as to generate its own climatic systems, most of Antarctica appears to be immune to the big melt for now, though answers to what is happening in the west are eagerly awaited.
At the top of the world, the Arctic is a region built on water.
Around the North Pole is ocean, with ice floes crowding in each winter and thinning again in the summers.
In September, we learned from scientists at the US National Snow and Ice Data Center that the extent of ocean covered by ice is getting smaller each year; the current rate of shrinkage they calculate at around 8% per decade.
Their projection is that within about 60 years, there will be no summer ice at all on the Arctic Ocean.
"Overall, the extent has been declining, with some oscillations, since the 1970s when satellites were able to map it," comments Peter Wadhams, Professor of Ocean Physics at Cambridge University, UK, and currently at the Laboratoire Océanographique in Villefranche-sur-mer, France.
"There's been a slow decline, but now the thinning appears to be more rapid.
"In the last two decades, not only has the area shrunk but the ice has got thinner by about 40%; the prediction is that it will vanish altogether during summers in the second half of this century."
Measurements of thickness come mainly from military submarines, which spent long periods under the Arctic ice during the Cold War.
Peter Wadhams was one of the scientists who afterwards persuaded the authorities in Britain and the United States to de-classify their data.
But as a method of measurement, it is far from perfect; and satellites have given only limited help.
Annual Arctic air temperatures relative to the 1961-1990 average
The existing satellite fleet gives good measurements of ice extent, but is not so good at detecting thickness, partly because the orbits of satellites with radar altimeters do not cover every portion of the ocean
This data deficit has led to a rival theory; that the ice is not melting at all, it is simply piling up in another part of the ocean, perhaps along the north Canadian coast.
Peter Wadhams believes he has now disproved this idea.
"We did an experiment where we installed a set of buoys in that region which measure the thickness of the ice and transmit it back via satellite," he says.
"The buoy sits on the ice, and as waves pass under it they make it rise and fall, just by a millimetre or two; measuring this allows you to calculate the thickness of the ice."
The preliminary results, announced at a scientific meeting in April 2005, show that the extra ice is not there; it really has melted away.
Temperatures, meanwhile, show a similar pattern to that seen along the Antarctic Peninsula; an average warming of about 2C in the last 50 years, about twice the global average, albeit with significant variations between different parts of the Arctic.
This is reflected in changes to ice cover on land as well as on sea.
The Greenland ice sheet is, after Antarctica, the second biggest expanse of ice in the world.
Its fringes expand and contract with the seasons; but images show it is melting more each summer now than a decade ago.
In February 2006 researchers discovered glaciers in Greenland were moving much faster than before, meaning that more of its ice was entering the sea.
In 1996, Greenland was losing about 100 cubic km per year in mass from its ice sheet; by 2005, this had increased to about 220 cubic km.
A complete melt of the ice sheet would cause a global sea level rise of about 7m; but the current picture indicates that while some regions are thinning, others are apparently getting thicker.
Other land masses surrounding the Arctic, such as Siberia and Alaska, are also beginning to feel the impact of rising temperatures.
Much ground here is permafrost - ground that is, or at least should be, permanently frozen.
Construction methods have been adapted to the rock-hard ground. Buildings can be erected with minimal foundations until the ground begins to melt.
That it is now melting deeper than before in some areas is beyond dispute. Projections indicate that the permafrost line - its southern boundary - will migrate northwards.
"As permafrost melts more rapidly," says Peter Wadhams, "a lot of methane and methane hydrates will be released to the atmosphere; and these are greenhouse gases."
So melting here would create a positive feedback, stirring the atmospheric mix to generate further warming.
To people living in the region, the melting brings mixed news.
Current lifestyles and staples foods will almost certainly change. But the open ocean may permit new opportunities for trade and agriculture.
A bigger question is what it means for the rest of the planet.
Ice reflects the Sun's radiation; water absorbs it.
More water and less ice - a lower albedo - mean that the pace of warming could increase.
In this scenario, the Earth would be losing one of its "natural checks and balances" against warming - another positive feedback mechanism.
The Arctic is intimately tied to the global climate system, and disruptions here have the potential to create worldwide changes - albeit over long timescales.
Possibly the most powerful link is via the thermohaline circulation, the global conveyor taking warm water along ocean surfaces and returning colder water at depth.
"One very sensitive place is the middle of the Greenland sea," says Peter Wadhams.
"That has been ice free in the summer, but usually in winter it would be covered by a lobe of ice growing out from the Greenland coast. As it formed, it rejected salt back into the water, making the water heavier and helping it to sink.
"Since 1997, the ice tongue has never formed. That will be having an impact on the thermohaline circulation."
Back in geological history, about 55 million years ago, the Arctic was a warm (possibly 20C) shallow sea that would have been ice-free without the intervention of a human-enhanced greenhouse effect.
Natural variations may be playing a role in the picture seen now; but, as with other parts of the planet, it is the speed of change which alarms many researchers as much as the change itself.
THE TEMPERATE ZONES
Glaciers snake over many of the world's high regions - the Himalayas, the Andes, the Alps, Alaska.
The recent signs are that these, like the Arctic, are feeling the impact of rising temperatures.
Over the last five years, various teams have reported glaciers shrinking in Peru, Kazakhstan, Nepal and Alaska.
"There is a global pattern of melting in most of the world's mountain glaciers," says Michael Hambrey, director of the Centre for Glaciology at Britain's University of Aberystwyth.
"There are exceptions - some glaciers are advancing - but overall the state of mountain glaciers is a dramatic shrinking since the 1970s.
"Some have disappeared completely, and most could be gone by the end of this century."
Snow and snouts
The data on this comes principally from a number of glaciers - perhaps less than 100 - which scientists consider make up a representative sample.
These are studied in some detail, though many more will be surveyed periodically to measure the length of their snout.
Satellite measurements are also of some value.
Glaciers have lost an average 6m in height in 20 years
Two years ago, a study in South America found that rates of thinning in the Patagonian ice fields were double in the period 1995-2000 compared with what they had been in the previous 25 years.
The World Glacier Monitoring Service (WGMS), supported by the United Nations Environment Programme (Unep), collates records from across the globe and issues regular bulletins of area and volume changes.
Two years ago, they concluded that 30 major glaciers - assessed as being a representative global sample - had thinned by an average 6m between 1980 and 2001.
None of this is without its impact on human society.
"It will have a major impact," says Professor Hambrey, "mainly through reductions in the fresh water supply.
"Cities like La Paz in Bolivia and Lima in Peru rely heavily on glacial meltwater from the high Andes brought down into dry arid areas.
Will iconic mountains like the Matterhorn become ice-free?
"Switzerland, by contrast, uses meltwater for hydroelectric power generation. If the glaciers disappear, their generating capacity will be very much reduced."
Melting glaciers can form lakes on mountainsides - lakes which can suddenly burst, creating torrential streams which threaten life and property.
Michael Hambrey says these incidents have already caused about 30,000 deaths in Peru. They are also a major issue in the Himalayas, where a programme of work is in place to make the lakes safe.
If ice comes off the land and ends up as water in the sea that should, logically, have some impact on sea level.
Its scale is minute compared with the potential melting of the West Antarctic or Greenland ice sheets - perhaps half a metre globally if all the mountain glaciers disappeared.
But that could happen before the end of the century - much earlier than projected for Greenland and Antarctica.
There seems little doubt that the changes seen in mountain glaciers, however, are going to have an impact on human societies in coming decades, as populations expand and the supply of fresh water shrinks with the ice.
A little under 70% of the world's fresh water is locked up in ice