A major new report commissioned by the UK government suggests it is unlikely that "dangerous" climate change can be avoided.
The impacts of climate change will fall heaviest in poorer countries
The BBC News website here dissects the main findings from the 406-page report, which collates evidence presented at an international scientific conference hosted by the UK Meteorological Office in February 2005.
Major consequences of rising temperatures
It is likely that sea levels globally will rise as the climate warms, the report concludes.
Thermal expansion of water is already believed to be raising sea levels by around 1.8cm per decade; but the extent of that rise would be considerably greater if the Earth's major ice sheets, in Greenland and Antarctica, were to melt.
On Greenland, it concludes: "Local warming of more than 2.7C would cause the ice sheet to contract."
As the Arctic is warming faster than the planetary average, a local warming of 2.7C may equate to a global warming of only about 1.5C, which some climate scientists believe is certainly going to happen whatever policies are now set in train.
There is considerable uncertainty, though, with researchers talking in terms of probabilities and refraining from making a definite prediction.
Melting Arctic ice could make polar bears extinct in the wild
The majority of Antarctic ice is in the east of the continent and appears secure; it may even thicken.
The west Antarctic sheet, often described in previous assessments as a "slumbering giant", is another matter.
Much of it rests on rock which is below sea level, and recent research suggests that a warming sea may be starting to melt the ice from below.
"These new insights suggest that the issue of the contribution of Antarctica to global sea level rise needs to be reassessed," writes Chris Rapley, director of the British Antarctic Survey.
"We could characterise the situation as 'a giant awakened'."
It has long been suspected that melting ice in the Arctic region could trigger collapse of the north Atlantic conveyor, sometimes called the Gulf Stream or the Atlantic thermohaline circulation (THC), the ocean current which brings warm water from the Caribbean to European shores.
Again, the discussion is framed largely in terms of uncertainties and probabilities.
But Michael Schlesinger from the University of Illinois at Urbana-Champaign and colleagues caution: "One cannot but be taken by the finding that in the absence of any policy intervention to slow the emission of greenhouse gases, uncertainty in our understanding ... supports a greater than 50% likelihood of an Atlantic THC collapse."
Warming apart, increased concentrations of carbon dioxide in the atmosphere will lead to rising concentrations in the oceans too.
This will make sea water more acidic; not to anything approaching corrosive levels, but to a degree which could affect marine ecosystems.
The impacts are difficult to predict, say Carol Turley and colleagues from the UK's Plymouth Marine Laboratory; but, they caution: "Such dramatic changes in ocean pH (acidity) have probably not been seen for millions of years of the Earth's history."
Impact on humans and nature
Not surprisingly, the report concludes that impacts will change according to the extent of temperature rise.
"Above a one degree Celsius increase, risks increase significantly, often rapidly for vulnerable ecosystems and species," concludes Bill Hare from the Potsdam Institute of Climate Impact Research in Germany, who produced an overview of more than 70 studies of impacts on water resources, agriculture and wildlife.
"In the one to two degree range, risks across the board increase significantly, and at a regional level are often substantial," he writes.
"Above two degrees the risks increase very substantially, involving potentially large numbers of extinctions or even ecosystem collapses, major increases in hunger and water shortage risks as well as socio-economic damages, particularly in developing countries."
A rise of 2C, the report suggests, will be enough to cause:
- decreasing crop yields in the developing and developed world
- tripling of poor harvests in Europe and Russia
- large-scale displacement of people in north Africa from desertification
- up to 2.8bn people at risk of water shortage
- 97% loss of coral reefs
- total loss of summer Arctic sea ice causing extinction of polar bear and walrus
- spread of malaria in Africa and north America
Linking temperature rise and carbon emissions
One of the great unknowns in climate science is just how high temperatures will rise for a given atmospheric level of greenhouse gases.
The European Union has set a policy target of preventing a global rise of more than 2C; but precisely what needs to be done to achieve that target in terms of restraining emissions is unclear.
Once again, probabilities and uncertainties are the langauge of the day.
"For achieving the two Celsius target with a probability of more than 60%, greenhouse gas concentrations need to be stabilised at 450 ppm CO2-equivalent or below," conclude Michel den Elzen from the Netherlands Environmental Assessment Agency and Malte Meinshausen of the US National Center for Atmospheric Research.
A rise of two Celsius could spread malaria in Africa and elsewhere
"A stabilisation at 450 ppm CO2-equivalent requires global emissions to peak around 2015, followed by substantial overall reductions in the order of 30% to 40% compared to 1990 levels in 2050."
This, various researchers note, should be compared against forecasts from the International Energy Agency and others that the global demand for energy will rise by about 60% by the year 2030.
Can it be done? And how much will it cost?
Technologies do exist, the final section of the report concludes, which can change this picture.
Renewables, energy efficiency, nuclear and "clean coal" are all options which can maintain energy supplies and fuel economic growth while lowering emissions.
The report notes that costs have come down and are expected to fall further, while refraining from giving definitive figures for how much it would cost to adopt these technologies globally.
But will they be adopted?
"The biggest problem does not seem to be the technologies or the costs, but overcoming the many political, social and behavioural barriers to implementing mitigation options," conclude Bert Metz and Detlef van Vuuren of the Netherlands Environmental Assessment Agency.
"There is a multitude of potential obstacles, ranging from lack of awareness, vested interests, prices not reflecting environmental impacts, cultural and behavioural barriers to change and, in the case of spreading technologies to developing countries, the lack of an effective enabling environment for new investments."