Their state-of-science report says that burning fossil fuels and destroying forests have released enough greenhouse gases into the atmosphere to also boost the frequency and intensity of many floods, droughts, wildfires and tropical storms.

"Every heatwave in the world is now made stronger and more likely to happen because of human-caused climate change," says the report authored by Friederike Otto, a scientist at Imperial College London's Grantham Institute, and Ben Clarke of the University of Oxford.

Climate change is a huge game changer when it comes to extreme heat, it says. Extreme hot spells such as the heatwave that gripped South Asia in March and April are already the most deadly of extreme events.

The heatwave that scorched India and Pakistan last month is still under review, Otto tells AFP, but the larger picture is frighteningly clear. What we see right now in terms of extreme heat will be very normal, if not cool, in a 2-degree to 3-degree Celsius world, she says. The reference here is to average global temperatures above preindustrial levels.

The world has warmed nearly 1.2C so far. That increase made record-setting rainfall and flooding last July in Germany and Belgium that left more than 200 dead up to nine times more likely, the WWA found.