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But forecast global temperature rise of 4C heralds disaster for large swaths of planet with oceans absorbing most global warming

Some of the most extreme predictions of global warming are unlikely to materialise, new scientific research has suggested, but the world is still likely to be in for a temperature rise of double that regarded as safe.

The researchers said that warming was most likely to reach about 4C above pre-industrial levels if the past decade's readings were taken into account.

That would still lead to catastrophe across large swaths of the Earth, causing droughts, storms, floods and heatwaves and with drastic effects on agricultural productivity leading to secondary effects such as mass migration.

Some climate change sceptics have suggested that because the highest global average temperature yet recorded was in 1998, climate change has stalled. The new study, which is published in the journal Nature Geoscience, shows a much longer "pause" would be needed to suggest that the world was not warming rapidly.

Alexander Otto, at the University of Oxford, lead author of the research, told the Guardian that there was much that climate scientists could still not fully factor into their models. He said that most of the recent warming had been absorbed by the oceans, but this would change as the seas heat up. The thermal expansion of the oceans is one of the main factors behind current and projected sea level rises.

The highest global average temperature ever recorded was in 1998, under the effects of a strong El Niño, a southern Pacific weather system associated with warmer and stormy weather, which oscillates with a milder system called La Niña. Since then, the trend of average global surface temperatures has shown a clear rise above the long-term averages – the 10 warmest years on record have been since 1998 – but climate sceptics have claimed that this represents a pause in warming.

Otto said that this most recent pattern could not be taken as evidence that climate change has stopped. "Given the noise in the climate and temperature system, you would need to see a much longer period of any pause in order to draw the conclusion that global warming was not occurring," he said. Such a period could be as long as 40 years of the climate record, he said.

Otto said the study found that most of the climate change models used by scientists were "pretty accurate". A comprehensive global study of climate change science is expected to be published in September by the Intergovernmental Panel on Climate Change, its first major report since 2007.

Jochem Marotzke, professor at the Max Planck Institute for Meteorology in Hamburg and a co-author of the paper, said: "It is important not to over-interpret a single decade, given what we know, and don't know, about natural climate variability. Over the past decade, the world as a whole has continued to warm, but the warming is mostly in the subsurface oceans rather than at the surface."

Other researchers also warned that there was little comfort to be taken from the new estimates – greenhouse gas emissions are rising at a far higher rate than had been predicted by this stage of the 21st century, and set to rise even further, so estimates for how much warming is likely will also have to be upped.

Richard Allan, reader in climate at the University of Reading, said: "This work has used observations to estimate Earth's current heating rate and demonstrate that simulations of climate change far in the future seem to be pretty accurate. However, the research also indicates that a minority of simulations may be responding more rapidly towards this overall warming than the observations indicate."

He said the effect of pollutants in the atmosphere, which reflect the sun's heat back into space, was particularly hard to measure.

He noted the inferred sensitivity of climate to a doubling of carbon dioxide concentrations based on this new study, suggesting a rise of 1.2C to 3.9C was consistent with the range from climate simulations of 2.2C to 4.7C. He said: "With work like this, our predictions become ever better."

• Climate change
• Climate change scepticism
• Climate change

Fiona Harvey
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New temperature norms under climate change will increase weather-related deaths in metropolitan areas in coming decades

New York city could experience up to 22% more deaths from extreme summertime heat in the coming decade under global warming, according to a study of the impact of climate trends.

The higher deaths will be partially offset by a reduction in deaths due to the milder winters predicted in Manhattan.

Overall, however, the net effect of the new temperature norms under climate change would be to increase weather-related deaths in New York city by up to 6.2% a year by the 2020s, according to the scientists.

The study, published in Nature Climate Change, predicted oppressive summer temperatures would exact an increasingly heavy toll on people living in metropolitan areas such as Manhattan in the coming decades.

The numbers would not be significantly offset by milder winters, the study found, and deaths due to extreme temperatures would rise more dramatically in the later decades of this century.

Without bold action to cut greenhouse gas emissions, heatwave deaths in New York city could rise by as much as 91% on 1980s levels by the 2080s, according to the study's projections. The net loss of life would be as much as 31% on 1980s levels, the study said.

"This is the first real study of the seasonal trade-off of climate change," Patrick Kinney, a professor of environmental health sciences at Columbia University and one of the authors of the study, said.

Kinney added: "What our study suggests is that the heat effects of climate change dominate the winter warming benefits that might also come: climate change will cause more deaths through heat than it will prevent during winter."

The findings, based on computer projections of future climate and their impact on deaths, provide a scaled-down version of the potential public health challenges posed by future climate change. The scientists used a set of 16 computer models to arrive at their findings.

The conclusions debunk the popular notion put forward by climate sceptics that warmer temperatures would benefit public health.

As the study notes, even under current conditions, there are more deaths due to extreme heat than to extreme cold in New York city every year.

Last year, the hottest summer since record-keeping began in the US, saw a string of days on which the temperature hit more than 37.7C (100F) in a number of US cities.

The week-long heatwave killed 82 people, according to figures compiled by the Associated Press.

In large metropolitan areas, such as New York, the impact of those temperature extremes are compounded by densely built-up areas. Cities such as Chicago, Cincinnati, Philadelphia and St Louis have also recorded sharp rises in deaths due to heart attacks and strokes during heatwaves, according to the draft of the National Climate Assessment, which was released last year.

"Urban heat islands, combined with an ageing population and increased urbanisation, are projected to increase the vulnerability of urban populations to heat-related health impacts in the future," the assessment said.

Kinney said he hoped the findings would push city planners in New York and other large urban areas to step up preparations for hotter and deadlier summers.

New York city has already begun efforts to cool the city during the summer, encouraging tree-planting programmes and setting new building standards.

Other cities also routinely set up "cooling centres", with cots and air conditioning, to allow people relief from the heat. Kinney said city officials also needed to target poor, elderly or disabled residents who are confined in hot and airless apartments during heatwaves.

"How can we reach out to people who are stuck in their apartments trying to ride out the events? We have to try to target vulnerable people," he added.

• Climate change
• Climate change
• New York
• United States
• Natural disasters and extreme weather

Suzanne Goldenberg
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Prof James Hansen rebukes oil firms and Canadian government over stance on exploiting fossil fuel, which he says would make climate problem unsolvable

Major international oil companies are "buying off" governments, according to the world's most prominent climate scientist, Prof James Hansen. In a visit to London, he accused the Canadian government of acting as their tar sands' salesman and "holding a club" over the UK and European nations to accept their "dirty" oil.

"Oil from tar sands makes sense only for a small number of people who are making a lot of money from that product," he said in an interview with the Guardian. "It doesn't make sense for the rest of the people on the planet. We are getting close to the dangerous level of carbon in the atmosphere and if we add on to that unconventional fossil fuels, which have a tremendous amount of carbon, then the climate problem becomes unsolvable."

Hansen met ministers in the UK government, which the Guardian previously revealed has secretly supported Canada's position at the very highest level.

Canada's natural resources minister, Joe Oliver, has also visited London recently to campaign against current EU proposals to penalise oil from Alberta's tar sands as highly polluting. "Canada can offer energy security and economic stability to the world," he said. Oliver also publicly threatened a trade war via the World Trade Organisation if the EU action went ahead: "Canada will not hesitate to defend its interests."

The lobbying for and against tar sands has intensified on both sides of the Atlantic as the EU moves forward on its proposals, which Canada fears could set a global precedent, and President Barack Obama considers approving the Keystone XL pipeline to transport tar sands oil from Canada to the US gulf coast refineries and ports. Canada's president, Stephen Harper, was met by protesters when he visited New York last week to tell audiences that KXL "absolutely needs to go ahead".

Canada's tar sands are the third biggest oil reserve in the world, but separating the oil from the rock is very energy intensive and causes three to four times more carbon emissions per barrel than conventional oil. Hansen argues that it would be "game over" for the climate if tar sands were fully exploited, given that existing conventional oil and gas is certain to be burned.

"To leave our children with a manageable situation, we need to leave the unconventional fuel in the ground," he said. Canada's ministers are "acting as salesmen for those people who will gain from the profits of that industry," he said. "But I don't think they are looking after the rights and wellbeing of the population as a whole."

"The thing we are facing overall is that the fossil fuel industry has so much money that they are buying off governments," Hansen said. "Our democracies are seriously handicapped by the money that is driving decisions in Washington and other capitals."

The EU aims to penalise oil sources with higher carbon footprints, as part of a drive to reduce the carbon emissions from transport called the fuel quality directive. But Canada, supported by the UK, is fiercely opposed: "We are not saying they should not move to reduce emissions," said Oliver. "But the proposed implementation of the FQD [fuel quality directive] is discriminatory to oil sands and not based on scientific facts." However, Europe's commissioner for climate action, Connie Hedegaard, said the FQD is "nothing more, nothing less" than accurate labelling and putting fair price on pollution.

Hansen, who informed the US congress of the danger of global warming in 1988, has caused controversy before by saying the "CEOs of fossil fuel companies should be tried for high crimes against humanity" and calling coal-fired power plants "factories of death". In April, he stepped down from his Nasa position after 46 years to spend more time communicating the risks of climate change and to work on legal challenges to governments.

Hansen has started a science programme at Columbia University, the first task of which is to produce a report is support suits filed again the US federal government and several state governments. It is being pursued by the Our Children's Trust charity and is based on a legal "trust" principle recognised in US law.

"We maintain that the atmosphere and climate are held in trust by the present generations for the future generations and we do not have the right to destroy that asset," Hansen said. "Therefore the courts should require the government to give a plan as to how they are going to ensure that we still have that asset to pass on to the next generation."

• Oil sands
• Energy
• Fossil fuels
• James Hansen
• Climate change
• Climate change
• Canada
• European Union
• Europe

Damian Carrington
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