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The climate in changing, faster than we thought - and humans have caused it.
Last night, the Intergovernmental Panel on Climate Change released the most comprehensive report on climate change ever - with hundreds of scientists taking part.
It says human activity is "unequivocally" driving the warming of atmosphere, ocean and land. The report projects that in the coming decades climate changes will increase in all regions.
For 1.5 degrees Celsius of global warming, there will be increasing heat waves, longer warm seasons and shorter cold seasons.
At 2 degrees Celsius of global warming, heat extremes would more often reach critical tolerance thresholds for agriculture and health.
Corin Dann spoke to one of the lead authors of the report, Amanda Maycock.
She told me the report gives governments a range of scenarios on what the world will look like with action - and inaction.