The rise of two degrees centigrade in global temperatures – the point considered to be the threshold for catastrophic climate change which will expose millions to drought, hunger and flooding – is now "very unlikely" to be avoided, the world's leading climate scientists say now.
The latest study from the United Nation's Intergovernmental Panel on Climate Change (IPCC) put the inevitability of drastic global warming in the starkest terms yet, stating that major impacts on parts of the world – in particular Africa, Asian river deltas, low-lying islands and the Arctic – are unavoidable and the focus must be on adapting life to survive the most devastating changes.
For more than a decade, EU countries led by Britain have set a rise of two degrees centigrade or less in global temperatures above pre-industrial levels as the benchmark after which the effects of climate become devastating, with crop failures, water shortages, sea-level rises, species extinctions and increased disease.
Two years ago, an authoritative study predicted there could be as little as 10 years before this "tipping point" for global warming was reached, adding a rise of 0.8 degrees had already been reached with further rises already locked in because of the time lag in the way carbon dioxide – the principal greenhouse gas – is absorbed into the atmosphere.
The IPCC says that the effects of this rise are being felt sooner than anticipated with the poorest countries and the poorest people set to suffer the worst of shifts in rainfall patterns, temperature rises and the viability of agriculture across much of the developing world.
In its latest assessment of the progress of climate change, the body said: "If warming is not kept below two degrees centigrade, which will require the strongest mitigation efforts, and currently looks very unlikely to be achieved, the substantial global impacts will occur, such as species extinctions, and millions of people at risk from drought, hunger, flooding."
Under the scale of risk used by IPCC, the words "very unlikely" mean there is just a one to 10 per cent chance of limiting the global temperature rise to two degrees centigrade or less.
Professor Martin Parry, a senior Met Office scientist and co-chairman of the IPCC committee which produced the report, said he believed it would now be "very difficult" to achieve the target and that governments need to combine efforts to "mitigate" climate change by reducing CO2 emissions with "adaptation" to tackle active consequences such as crop failure and flooding.
Speaking at the Royal Geographical Society, he said: "Ten years ago we were talking about these impacts affecting our children and our grandchildren. Now it is happening to us."
"Even if we achieve a cap at two degrees, there is a stock of major impacts out there already and that means adaptation. You cannot mitigate your way out of this problem... The choice is between a damaged world or a future with a severely damaged world."
The IPCC assessment states that up to two billion people worldwide will face water shortages and up to 30 per cent of plant and animal species would be put at risk of extinction if the average rise in temperature stabilises at 1.5C to 2.5C.
Professor Parry said developed countries needed to help the most affected regions, which include sub-Saharan Africa and major Asian river deltas with improved technology for irrigation, drought-resistant crop strains and building techniques.
Rajendra Pachauri, the chairman of the IPCC, said that 2015 was the last year in which the world could afford a net rise in greenhouse gas emissions, after which "very sharp reductions" are required.
Dr Pachauri said the ability of the world's most populous nations to feed themselves was already under pressure, citing a study in India which showed that peak production of wheat had already been reached in one region.
Campaigners said the IPCC findings brought added urgency to the EU's efforts to slash emissions. John Sauven, executive director of Greenpeace, said: "The EU needs to adopt a science-based cap on emissions, ditch plans for dirty new coal plants and nuclear power stations that will give tiny emission cuts at enormous and dangerous cost, end aviation expansion and ban wasteful products like incandescent lightbulbs."
Plus two degrees: the consequences
Arica: Between 350 and 600 million people will suffer water shortages or increased competition for water. Yields from agriculture could fall by half by 2020 while arid areas will rise by up to 8 per cent. The number of sub-Saharan species at risk of extinction will rise by at least 10 per cent.
Asia: Up to a billion people will suffer water shortages as supplies dwindle with the melting of Himalayan glaciers. Maize and wheat yields will fall by up to 5 per cent in India; rice crops in China will drop by up to 12 per cent. Increased risk of coastal flooding.
Australia/New Zealand: Between 3,000 and 5,000 more heat-related deaths a year. Water supplies will no longer be guaranteed in parts of southern and eastern Australia by 2030. Annual bleaching of the Great Barrier Reef.
Europe: Warmer temperatures will increase wheat yields by up to 25 per cent in the north but water availability will drop in the south by up to a quarter. Heatwaves, forest fires and extreme weather events such as flash floods will be more frequent. New diseases will appear.
Latin America: Up to 77 million people will face water shortages and tropical glaciers will disappear. Tropical forests will become savanna and there will be increased risk of coastal flooding in low-lying areas such as El Salvador and Guyana.
North America: Crop yields will increase by up to 20 per cent due to warmer temperatures but economic damage from extreme weather events such as Hurricane Katrina will continue increasing.
Polar regions: The seasonal thaw of permafrost will increase by 15 per cent and the overall extent of the permafrost will shrink by about 20 per cent. Indigenous communities such as the Inuit face loss of traditional lifestyle.
Small islands: Low-lying islands are particularly vulnerable to rising sea levels with the Maldives already suffering land loss.