Scientists are “95% certain” the cause of climate change over the past 50 years is man-made.
And they warn by the end of the century sea levels will rise between 10 and 32in and polar ice caps will melt as average temperatures rise between 0.3C and 4.8C.
It means the more extreme weather with frequent flooding and longer heatwaves, the United Nations’ Intergovernmental Panel on Climate Change says.
IPCC co-chairman Thomas Stocker said: “Heatwaves are very likely to occur more frequently and last longer.
As the Earth warms, we expect to see currently wet regions receiving more rainfall, and dry regions receiving less, though there will be exceptions.”
The IPCC landmark report – the first for six years – also concluded that it was likely the past 30 years were the hottest since 700AD and that greenhouse gases and CO2 in the atmosphere was unprecedented in the past 800,000 years.
Levels of carbon dioxide are 40 per cent higher than in the pre-industrial era mainly due to the burning of fossil fuels and land changes, including deforestation.
The IPCC experts said even if emissions of carbon dioxide were to stop, the result of emissions so far means the world is locked into some level of climate change, with effects persisting for centuries.
The report, presented today in Stockholm, Sweden, to officials from 195 countries, was prepared with the help of 800 scientists using 9,000 studies.
Countries have agreed to negotiate a legally-binding global deal to tackle climate change at a meeting in France in 2015.
Foreign Secretary William Hague said: “Unless we act now to reduce carbon emissions, all this will continue to worsen.”
Greenpeace’s Doug Parr said: “Those taking action against climate change by peacefully protesting against oil drilling in the Arctic are in custody in Russia while those responsible – the fossil fuel industry – are courted by governments.
"It should be the other way round.”
Q&A on IPCC climate change report
Q: Who is the IPCC?
A: The IPCC was established in 1988 by the United Nations Environment Programme (UNEP) and the World Meteorological Organisation (WMO) to assess climate change science and information. It does not carry out its own research.
This is the fifth assessment it has conducted, six years on from the last report in 2007.
Q: So what has the report found?
A: It has found that it is extremely likely, or 95% certain, that humans are responsible for the majority of climate change, through activities such as burning fossil fuels and deforestation which release greenhouse gases.
As a result, temperatures are rising, sea levels are also going up, ice sheets and glaciers are melting and extreme weather is becoming more severe and frequent, impacts which are set to continue.
Q: Can't the scientists be 100% certain humans are causing climate change?
A: Scientists say very few things in science can be described as 100% certain, because there is always the possibility of issues such as measurement errors, but 95% is a very high probability.
Q: How has the IPCC reached its findings?
A: For this first section of the report alone, hundreds of authors and reviewers drew on more than 9,200 scientific publications, as well as observations and climate models to reach their findings.
Today a summary for policy-makers for this section of the report, which assesses the evidence for climate change and its causes, was published after government representatives and scientists went through the summary line by line to agree it.
Q: What comes next?
A: Next year, the IPCC will publish the second and third parts of the report. Part two looks at the impacts of climate change and how people can adapt to it, and the third part will look at measures that can be taken to mitigate the problem.
Q: What about the slowdown in global warming?
A: The report addresses the issue of a slowdown in temperature rises over the past 15 years, putting it down to natural variation and the impact of volcanoes and the sun which have had a cooling effect.
Scientists say the slowdown is temporary and substantial warming will be back in the coming decades.
Q: Is there any way we can avoid climate change?
A: No, we are already committed to a certain amount of climate change, but taking "ambitious" action now could limit the problem.
The new report outlines four future scenarios, including a low emissions scenario where temperature rises would be limited. But this scenario would require carbon dioxide emissions to stop rising now, start to decline by 2020 and become negative by 2100.