Now the two most famous scientific institutions in Britain and the US agree: 'Climate change is more certain than ever'

A boat is seen seen among the icebergs that broke off from the Jakobshavn Glacier in Ilulissat, Greenland (Getty Images)

Climate change is one of the defining issues of our time and the indisputable warming of the world over the past century is largely the result of human activities, according to the two most august science bodies in Britain and the United States.

The speed of global warming is now 10 times faster than at the end of the last ice age, which represents the most rapid period of sustained temperature change on a global scale in history - and there is no end in sight if carbon emissions continue to increase, the Royal Society and the US National Academy of Sciences have warned.

Concentrations of carbon dioxide in the atmosphere are the highest for at least 800,000 years and 40 per cent higher than they were in the 19th century. They are set to increase even further without a binding global agreement on significant cuts in industrial emissions, the scientists said.

Average global surface temperatures have increased by 0.8C since 1900 and the last 30 years have been the warmest in 800 years. On the current carbon dioxide trajectory, global warming could increase further by between 2.6C and 4.8C by 2100, which would be about as big as the temperature difference between now and the last ice age, they said.

“Detailed analyses have shown that the warming during this period is mainly the result of the increased concentrations of CO2 and other greenhouse gases. Continued emissions of these gases will cause further climate changes in regional climate,” says a joint report by the two academies.

In a foreword to “Climate Change Evidence and Causes”, Ralph Cicerone, president of the National Academy of Sciences, and Sir Paul Nurse, president of the Royal Society, say that climate change is now more certain than ever and that many lines of evidence point to human activity as the cause.

“The evidence is clear. However, due to the nature of science, not every single detail is ever totally settled or completely certain. Nor has every pertinent question yet been answered,” the two presidents say.

“Some areas of active debate and ongoing research include the link between ocean heat content and the rate of warming, estimates of how much warming to expect in the future, and the connections between climate change and extreme weather events,” they say.

The aim of the joint report, written as a series of answers to 20 questions, is to make a clear statement to policy makers and the wider public about the scientific basis of climate change and its uncertainties, which should not distract from the main message about what needs to be done, said Professor Tim Palmer of Oxford University, one of the report’s main authors.

“Every day we are putting more CO2 into the atmosphere and for all practical purposes once we put it there it is there forever. So we can wait 10 or 20 years to get a better estimate of the science but of course in that time you’ll have 20 years of CO2 emissions that are going to be impossible to reverse,” Professor Palmer said.

“We are talking about changes where there will be a distinct risk by the next century of something that could in terms of global temperatures be as big as that between today and the last ice age. Do we want to take that risk?” he asked.

“Every day we do nothing is another load of carbon in the atmosphere that we’ll never get rid of in many generations and it’s effectively there forever. We are not trying to promote policy or be endorsers of government policy, we’re just trying to give our best estimates of the science,” he added.

The report says there is no “pause” in global warming only a temporary and short-term slowdown in the rate of increase in average global surface temperatures in the non-polar regions which is likely to start accelerating again in the near future.

“Globally averaged surface temperature has slowed down. I wouldn’t say it’s paused. It depends on the datasets you look at. If you look at datasets that include the Arctic, it is clear that global temperatures are still increasing,” Professor Palmer said.

Climate change Q&A

Is the climate warming?

Yes, by about 0.8C in terms of average surface temperatures since 1900. The period between 1983 and 2012 was probably the warmest 30 years in more than 800 years. The rate of warming over the past 15 years has slowed but not stopped.

Are humans to blame?

Carbon dioxide concentrations in the air have increase by about 40 per cent from 1800 to 2012, largely due to the burning of fossil fuels. A range of scientific analyses suggest this is largely responsible for the increase in global temperatures.

Isn’t CO2 naturally present in the air?

Yes, but the evidence is overwhelming that human activity has disturbed the natural carbon cycle by burning fossil fuels. CO2 levels are higher than for at least 800,000 years and possibly higher than for the past few million years.

Has climate change always happened?

Yes, but the speed at which it is happening now is unprecedented. The current climate is changing about 10 times faster than at the end of the last ice age, which is the most rapid sustained change in temperatures on a global scale.

Why is Arctic ice disappearing but Antarctic ice growing?

The sea ice in the Arctic appears to be responding directly to the rapid rise in regional temperatures, which have seen some of the fastest increases in the world. The sea ice in the Antarctic is influenced more by changes in the patterns of winds and ocean currents – in some areas it has decreased.

How fast are sea levels rising?

Over the past two decades, global average sea levels are estimated to have increased by about 3.2mm (0.12inches) per year and the overall observed increase since 1900 is about 20cm. Sea level may increase further by between 0.5-1 metres by 2100, and continue throughout the next century.

Why are the oceans becoming more acidic?

About a third of the CO2 emitted from fossil fuels have ended up being absorbed by the oceans. This results in the production of carbonic acid, which lowers pH levels making the seawater less alkaline and more acidic – causing lethal problems for some marine species with shells.

How confident are we that global warming will continue?

Very confident. On the current carbon dioxide trajectory, global temperatures are almost certain to increase by between 2.6C and 4.8C. Even if all emissions are stopped tomorrow, the climate will continue to warm. But cutting emissions will reduce the total amount of warming.

Steve Connor