The fifth assessment report from the IPCC looks at everything from oceans and sea ice to carbon budgets and geoengineering

Global change

The global climate has already changed in many ways that are unprecedented in the past hundreds or thousands of years, the world's scientists and governments concluded in the new report from the Intergovernmental Panel on Climate Change. These changes have affected every region of the globe, on land and at sea. Continued carbon emissions will drive further heatwaves, sea level rise, melting ice and extreme weather. The changes will last for centuries and limiting the effects would require "substantial and sustained" cuts in carbon dioxide, the scientists report.

Temperatures

Scientists are now at least 66% certain that the last three decades are the warmest in 1400 years, with global temperature having risen by 0.9C in the last century. However, more than 90% of the heat trapped by greenhouse gases is being stored in the oceans.

By mid-century, scientists predict a further rise of 1.4-2.6C if carbon emissions continue to rise as they are today. If emissions were halted almost immediately and significant carbon was extracted from the atmosphere, the rise by mid-century would be 0.4-1.6C.

The scientists predict the average temperature between 2080 and 2100 will be 2.6-4.8C higher than today if emissions are unchecked. They are 90% certain that heatwaves will be more frequent and longer.

In the oceans, the strongest surface warming is expected in tropical and sub-tropical regions, up to 2C by 2100 and posing a grave threat to coral reefs which sustain much sealife. Scientists conclude that a collapse of the Gulf Stream that warms western Europe, as dramatised in the film The Day After Tomorrow, is very unlikely this century but cannot be ruled out afterwards.

Oceans

Global sea level has already risen by 20cm in the last century and scientists are now 90% certain that the rate of the rise will increase. The tide line is rising as warming glaciers and ice sheets pour hundreds of billions of tonnes of water into the oceans each year, but an equally big factor is the warming – and therefore expansion – of the seawater itself.

The new projections for the average sea level in the period 2080-2100 are greater than in the 2007 report, ranging from 45-82cm higher than now if nothing is done to curb emissions to 26-55cm if carbon emissions are halted and reversed. In the former case, sea level could have risen by a 98cm by the end of the century, seriously threatening cities from Shanghai to New York and meaning hurricanes and cyclones inflict far worse damage when they hit shorelines.

Sea level projections have been controversial because exactly how fast glaciers and ice sheets will slip into the sea is not well known. A collapse in ice sheets is therefore not included in the estimates and could add tens of centimetres more to the rise. Because the big Greenland and Antarctic ice sheets are slow to melt, scientists predict melting and sea level rise will continue for centuries. If a temperature rise of between 1C and 4C is sustained, the vast Greenland ice sheet will completely melt adding 7m to sea level, scientists predict, but over the course of a millennium.

The acidity of the ocean is also increasing, due the large amounts of carbon dioxide it is absorbing, and this will continue. This will harm shell-forming sealife but scientists are still determining to what extent.

Ice

The impact of warming is crystal clear in the faster rates of melting in virtually all the world's glaciers and the huge ice sheets in Greenland and Antarctica. The ice sheets have been shedding at least five times more water in the 2000s than in the 1990s, the scientists report. Northern hemisphere snow cover has fallen by 11% a decade since 1967 and the temperature of the seasonally frozen ground, or permafrost, has increased by 2-3C in Russia and Alaska.

Arctic sea ice has been melting by 9-14% a decade since 1979, while sea ice around Antarctica has been increasing by 1-2%, probably due to current changes.

Scientists are 90% sure that Arctic sea ice, snow cover and glaciers will continue to shrink. The scientists say a "nearly ice-free" Arctic ocean in September is at least 66% likely before 2050. By 2100 between 35% and 85% of the remaining world glacier volume will have vanished if emissions are not cut. Permafrost is also 99% likely to shrink further.

Extremes

It is 90% certain that the number of warm days and nights has increased globally and heatwaves have become more frequent, lasting longer in Europe, Asia and Australia. Droughts have also become more frequent and intense in the Mediterranean and west African regions.

The number of heavy rainfall events over land has increased in more regions than it has decreased. It is virtually certain that the frequency and intensity of the strongest tropical cyclones in the North Atlantic has increased since the 1970s.

The scientists concluded it is 99% certain that the frequency of warm days and warm nights increases in the next decades, while that of cold days and cold nights to decrease. The frequency and intensity of extreme downpours is very likely to increase in many populous regions.

Pause

The last decade has been the warmest on record but although CO2 concentrations in the atmosphere have continued to accelerate, surface air temperatures have only marginally increased in the past 15 years, leading some to suggest global warming has stopped. The IPCC scientists reject this, reporting that while the warming trend is robust over decades, there is "substantial" variability within decades. They conclude: "Trends based on short records ... do not in general reflect long-term climate trends."

They add that the heat being trapped by global warming in 2011 was 43% more than the estimate for 2005 in their last report and that over 90% of all the heat added enters the oceans.

Carbon budget

Scientists calculate that nearly half of all the carbon dioxide that can be safely emitted without raising temperatures above a dangerous 2C has already been emitted. This, says the IPCC, means governments must act quickly to have a reasonable change of avoiding 2C. It is also very likely that more than 20% of emitted CO2 will remain in the atmosphere longer than 1,000 years after man-made missions have stopped. According to the IPCC, a large fraction of climate change is thus "irreversible on a human time-scale", except if man-made CO2 emissions are sucked out of the atmosphere over a long period.

Geoengineering

The scientists report that "geoengineering" the climate by reducing the amount of sunlight being absorbed by earth or by extracting and storing carbon dioxide and other climate-changing emissions is theoretically possible. But, the IPCC warns, there is insufficient knowledge to assess how effective such methods, such as pumping sunscreen chemical into the stratosphere, would be and warns of "side effects and long-term consequences on a global scale."

Abrupt change

It is "very likely" that the so-called Gulf Stream, which ferries warm water to western Europe, will weaken over the 21st century. But it is "very unlikely" to collapse or undergo a major transition this century. Further warming will lead to significant methane emissions from permafrost over the next century, equivalent to 50 to 250 billion tonnes of CO2. But the IPCC scientists do not assess the possibility of catastrophic releases this century.

Uncertainties

In terms of data, information is still limited in some locations and especially from before 1950s. There is also limited data from oceans below 700m.

Theoretical uncertainties are how pollution affects cloud formation and the planet's overall climate "sensitivity", ie how much it responds to extra CO2 in the atmosphere. The new report slightly reduces the minimum climate sensitivity but at the report's launch event, co-chair Thomas Stocker said that change, if realised would slow the impacts of climate change by just a few years.

There is uncertainty about the contribution of human activity to changes in tropical cyclones and droughts.

Other explanations of warming

The scientists state: "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century." The report rules out any significant contribution from changing solar cycles, volcanoes and cosmic rays.