A lot has changed since the world’s leading climate scientists last gathered in the name of the UN Intergovernmental Panel on Climate Change (IPCC) in 2007, to put the finishing touches to their fourth landmark assessment of the state of global warming.

Since the release of that report, the world has been through a prolonged and continuing economic downturn. This was arguably good for the environment in the short term, because the resulting slump in manufacturing had the effect of curbing the growth in damaging carbon emissions.

But in the longer term, the recession has been profoundly damaging, because it has knocked green issues firmly off the political agenda. While renewable technologies such as wind and solar power can benefit from free sources of energy, the initial investment is huge and the rewards will not be felt until much further down the line.

Download the new Independent Premium app Sharing the full story, not just the headlines

The cause of reducing emissions was dealt a further significant blow at the annual UN climate change conference in Copenhagen in December 2009, when the world’s governments failed to agree on legally-binding targets to reduce their CO2 emissions.

This failure to forge a treaty to replace the 1997 Kyoto protocol, which was rejected by the US and which placed no obligations on big developing countries such as China, was hugely disappointing to many and took much of the wind out of the sails of the campaign to reduce emissions.

Climate sceptics have effectively exploited the opportunities provided by the recession and the disappointing Copenhagen summit to push their case, arguing that the last thing people need is expensive and unnecessary renewable energy pushing up their utility bills.

According to a recent survey from the UK Energy Research Centre, they have been effective. The proportion of people living in Britain who do not believe in climate change has more than quadrupled since 2005 – from 4 per cent to 19 per cent.

But the prospect of the world’s leading governments agreeing dramatic and co-ordinated action to tackle climate change has brightened considerably in recent months.

China and the US, the world’s biggest emitters of CO2 by far, have lately made a series of positive noises about the prospect of cutting their carbon footprint and have even agreed to team up to tackle the matter.

It is against this backdrop that the fifth IPCC assessment will be published in Stockholm tomorrow. With its synthesis of thousands of peer-reviewed papers put together by hundreds of leading scientists, it will be the most authoritative document ever produced about climate change.

It will therefore play a central role at the next significant UN Climate Change Conference in Paris in 2015. This is when governments have agreed to try and negotiate legally-binding emissions reductions targets that, they hope, will be sufficient to keep global warming to 2C – the level beyond which the consequences become increasingly devastating.

1. Atmospheric observations

The report will say there is a 95 per cent chance – which it defines as “extremely likely” – that humans are responsible for the majority of climate change through their greenhouse gas emissions. This compares with the 90 per cent figure given by the previous IPCC assessment in 2006. This, in turn, was a significant increase on the 66 per cent certainty reached in the 2001 assessment and just over 50 per cent in 1995.

The report will say that the “global combined land and ocean temperature data” show an increase of about 0.8C between 1901 and 2010 and of about 0.5C between 1979 and 2010.

It is also expected to slightly reduce the minimum temperature increase that is “likely” (defined as a greater than 66 per cent chance) to result from climate change in the long term, to 1.5C, compared with 2C in 2006. The upper end of the “likely” temperature increase remains at 4.5C.

The report is also like to reference the fact that, in May, the concentration of CO2 in the atmosphere breached the symbolic level of 400 parts per million for the first time in five million years, after rising at its fastest rate since records began.

The elevated carbon emissions reading harks back to the Pliocene period, of three to five million years ago, when global average temperatures were 3C or 4C hotter than today; the Arctic was ice-free, sea levels were about 40m higher than today, and jungles covered northern Canada.

2. Ocean observations

The report will say “it is virtually certain that the upper ocean has warmed since 1971 and that ocean warming dominates the change in the global energy content”. This is because warming of the ocean accounts for more than 90 per cent of the extra energy stored by the Earth between 1971 and 2010, it will say.

The largest warming is found near the sea surface, with the top 75 metres recording a 0.1C increased per decade, between 1971 and 2010, decreasing to about 0.015C per decade by 700 metres. In terms of sea levels, the report will find it to be “virtually certain” that over the 20th century the mean rate of increase was 1.4-2mm a year, rising to 2.7-3.7mm a year from 1993.

It will also forecast that, by the end of the century, sea levels could rise by up to 81cm – largely as a result of an expansion in the volume of the oceans as they warm, and from the melting of glaciers.

It will say it is “virtually certain” that the oceans have absorbed huge amounts of carbon dioxide that have resulted in the gradual acidification of seawater. Estimates of the global oceanic content of anthropogenic (human-sourced) carbon range from 93bn to 137bn tonnes in 1994 to 125bn-185bn in 2010.

3. Cryosphere

This term collectively describes those parts of the Earth’s surface where water is in solid form, including sea, lake and river ice, snow cover, glaciers and frozen ground, or permafrost.

There’s not much good news here. The overarching conclusion of the draft report is: “More comprehensive and improved observations strengthen the evidence that the ice sheets are losing mass, glaciers are shrinking globally, sea ice cover is reducing in the Arctic, and snow cover is decreasing and permafrost is thawing in the Northern Hemisphere.”

It notes that global glacier mass has declined by between 210bn and 371bn tonnes since 2003. It also finds that the Greenland ice sheet is diminishing at an accelerating rate – the average annual ice loss from Greenland was between 101bn tonnes and 145bn tonnes between 1993 and 2010, rising to 174bn to 282bn tonnes a year in the period 2005-2010.

The report will also show that the Antarctic ice sheet is losing mass at an accelerating rate, and that the overall decrease in Arctic sea ice between 1979 and 2011 has been about 3.9 per cent per decade.

The state of the cryosphere is seen as a barometer for the extent and fallout from climate change because it is so clearly (though not exclusively) influenced by global temperatures. Its demise also has the added effect of speeding up climate change because snow and ice curb warming by reflecting the sunlight back into space.

4. Carbon allowance

The report is likely to say that the world has already burned more than half the maximum amount of fossil fuel that can be consumed if catastrophic levels of global warming are to be avoided. Scientists estimate that if global warming is to have an above-average chance of remaining below the crucial 2C level, the total amount of carbon burnt since the industrial revolution must not exceed one trillion tonnes.

This is because CO2 can remain in the atmosphere for more than 200 years, giving the greenhouse gas a cumulative impact. Scientists calculate that about 570bn tonnes, or 57 per cent of that one trillion tonnes maximum, has been burnt in the past three centuries and these, or similar, figures are expected to be included in the final report.

The report is also expected to warn that we are on course to use up our entire remaining global carbon allowance of 330bn tonnes within 30 years, unless drastic action to curb emissions is taken. This is because rapid economic growth in developing countries is accelerating the increase in carbon emissions.

Experts have blamed much of the increase on rising emissions from China and India, which still rely heavily on coal for their energy, although it is not known whether the latest IPCC assessment will single them out. Reduced absorption by shrinking forests is another factor.

About 50bn tonnes of CO2 is emitted globally each year, with the average global citizen producing seven tonnes, compared with about ten tonnes per person in the UK. Scientists estimate global emissions need to come down to about 20bn tonnes a year if the world is to have a fair chance of limiting global warming to 2C.

5.The warming hiatus

The most contentious part of the report will probably be how it deals with the “hiatus” in warming, with signs that the rate of increase in global temperatures has slowed dramatically. In a development that climate sceptics have seized upon as evidence that the threat of climate change is greatly exaggerated, the temperature rise has slowed from 0.12C per decade since 1951, to 0.05C per decade in the past 15 years. It is understood that some governments are pressing the report authors to be clearer about the reasons behind the slowdown in the report, in a bid to head off the climate change sceptics.

While the precise wording and prominence of the “hiatus” section are still being hammered out, the gist of it is that it is unlikely to last. It will say factors such as a haze of volcanic ash and a cyclical dip in the energy emitted from the sun are likely to have contributed to a slower warming trend since 1998. It will also point out that the decade to 2012 was the warmest since records began in the mid-19th century.

The IPCC forecasts a resumption of the higher level of warming that it will say is likely to cause ever more heatwaves, droughts, floods and rising sea levels. “Fifteen-year-long hiatus periods are common” in both historical records and computer models, the report will say. “Barring a major volcanic eruption, most 15-year global mean surface temperature trends in the near-term future will be larger than during 1998-2012.”

Heated debate: Publishing an IPCC report

The publication of any Intergovernmental Panel on Climate Change (IPCC) report is always preceded by several days of intense debate, as hundreds of scientists descend on the launch city – in this case Stockholm – to agree the final wording of the first instalment of a three-part report that is likely to weigh in at more than 3,000 pages in total, released over the next 14 months.

One rather prominent member of the proceedings in particular, the IPCC’s vice-chair, Francis Zwiers, could put a few noses out of joint.

While most climate scientists are likely to be wrangling over just how bad climate change is – and why it seems to have slowed down recently – Mr Zwiers, the director of the Pacific Climate Impacts Consortium at the University of Victoria in Canada, is likely to be focusing more on the inadequacy of established climate models, for failing to predict the warming “hiatus”.

Published in the journal Nature Climate Change, the study he recently co-authored was scathing about these models. It found they have “significantly” overestimated warming over the past 20 years – and by even more over the past 15 years, the period which coincides with the warming hiatus.

“Recent observed global warming is significantly less than that simulated by climate models. This difference might be explained by some combination of errors in external forcing, model response and internal climate variability,” the study found.

However, Lord Stern, the author of the influential Stern Review into the financial implications of climate change, said that climate-change models significantly underestimated the extent of global warming.

Mr Zwiers’s stance on the effectiveness of climate models will feed into the reasons for the slowdown in climate change in the past 15 years.

IPCC panellist Shang-Ping Xie, a professor of climate science at the Scripps Institution of Oceanography at the University of California, San Diego, said: “It’s contentious. The stakes have been raised by various people, especially the sceptics.”