Since 1880 temperatures have gone up and down like a yo-yo.
In the last 15 years the temperature has gone down, but that has happened before: the IPCC says it's part of natural variability and it won't continue.
I think it's ironic that at the time when scientists say they are at their most confident what they're saying is correct, climate change is slipping down the political agenda.
Part of the reason for that is people are now more concerned about how big their electricity bills are than where that electricity comes from.
Foreign Secretary William Hague said the IPPC's report "confirms that climate change is already happening, as a result of human activity".
"The IPCC's report makes clear that unless we act now to reduce carbon emissions, all this will continue to worsen in coming decades," he added.
US Secretary of State John Kerry called the findings "a wake-up call".
"Those who deny the science or choose excuses over action are playing with fire," Mr Kerry said, referring to sceptics who question the need for urgent action on climate change.
The United Nations Environment Programme's executive director said today's IPCC report on climate change "is not about ideology".
Achim Steiner told a press conference the report is a "dramatic reminder of both the significance, the pace, and also our ability to increasingly understand what is happening to our planet".
Mr Steiner said, "This is not about ideology. This is not about self-interest. This is about the common interest of the international community, the planet, and ultimately our economies and society."
Questions have been raised about the slowdown in temperature rises in the past 15 years, with climate "sceptics" claiming it undermines the theory of climate change.
The Intergovernmental Panel on Climate Change's (IPCC) report acknowledges that there has been a reduction in the rate of warming between 1998 and 2012, and suggests it is the result of natural variation and the impacts of volcanoes and changes in the strength of the sun.
Over the long term, from 1951, climate models have matched what has happened to global temperatures, the report said.
One of the scientists leading the first section of the IPCC study said climate change "is the greatest challenge of our time."
Thomas Stocker said: "Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system.
"Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.
"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, although there will be exceptions."
The Intergovernmental Panel on Climate Change said carbon dioxide concentrations have increased by 40% since pre-industrial times, primarily from fossil fuels such as coal and petroleum.
The UN's Environment Programme wrote on Twitter:
The IPCC's report predicts that temperatures are set to rise by more than 1.5C above pre-industrial levels by the end of the century without ambitious action to tackle emissions, and could rise by over 4C if emissions continue to increase.
The UN panel of experts said ice sheets are losing mass, glaciers are shrinking, sea ice cover has reduced in the Arctic and the permafrost is thawing in the northern hemisphere as a result of global warming.
Storms will become more intense and frequent, sea levels will rise by between 26cm (10in) and 82cm (32in) by the end of the century and the oceans will become more acidic, their assessment projects.
Scientists are more certain than ever that humans are causing the majority of climate change, a key UN report has shown.
The first part of the Intergovernmental Panel on Climate Change's (IPCC) fifth assessment report shows that warming in the climate system is "unequivocal" and human influence on the climate is clear.
The report, which has been published after line-by-line scrutiny by scientists and policymakers, said they are 95% certain that the majority of the warming since the 1950s is down to human activity.
The likelihood is up from a 90% certainty in the last IPCC study in 2007.
A UN panel of climate scientists said 2001-2010 was the warmest decade on record.