John Kerry, the U.S. Special Presidential Envoy for Climate, urged the world's governments to speed up efforts against the climate crisis Tuesday, saying the political response to it has "failed" to respond effectively for more than three decades.

In a speech at the Royal Botanic Gardens in Kew, west London, Kerry called for more ambitious emissions reduction pledges, saying that even if all countries delivered on their commitments to the Paris Agreement, global average temperatures would still rise by 2.5 or 3 degrees Celsius (4.5 to 5.4 degrees Fahrenheit), or even more.

"We are forewarned -- everything the scientists have been telling us will happen for 30 years now, is happening -- but bigger and faster than predicted," he said.

Making a comparison with the Second World War, Kerry called for the same show of unity that followed the six-year conflict and established a new world order.

"That journey has always given me a bedrock confidence that we can solve humanity's biggest threats together. The climate crisis is the test of our own times, and while it may be unfolding in slow motion, to some, this test is as acute and as existential as any previous one," he said.

He also pointed to the failure of adults to meet the climate challenge for younger generations, calling it "ironic" that young people were the ones "calling on adults to behave like adults."

"They know the world is not responding fast enough to an existential threat they didn't create, but for which they risk bearing the ultimate burden: uninhabitable communities on an increasingly unlivable planet in their lifetimes," he said, adding it was the onus of adults in politics and business to make change.

"And above all, we need to provide action and action now. Because time is running out. Not a euphemism, not an exaggeration. Time is running out."

The Earth is already, on average, 1.2 degrees Celsius warmer than pre-industrial levels, and if it exceeds a 1.5-degree threshold, scientists say millions more people will be at risk of extreme heatwaves, drought and flooding. Food shortages will worsen and coral reefs will all but disappear.

Kerry is on a tour of several European countries to drum up support ahead of the COP26 climate negotiations in Glasgow, Scotland, later this year.

His message of urgency comes as the U.K. issued its first-ever extreme heat warning on Monday, and as parts of Europe slowly emerge from some of the worst rains and flooding in hundreds of years.

In Germany, where more than 160 people have died in recent floods, Chancellor Angela Merkel gave the same message over the weekend -- that the world needs to speed up its response to climate change. Some scientists are also saying that recent extreme weather events indicate a faster impact of climate change than projected.

Kerry acknowledged in his speech that the world didn't need any new investment in oil, coal or gas production to meet energy needs, and called out countries opening up new coal mines. The U.K. is one country that has such plans.

"By 2040, we should have entirely phased out all unabated coal and oil power plants and sharply reduced reliance on unabated natural gas generation," he said.

But Joe Biden's administration has also come under criticism for issuing a high number of permits for oil and gas drilling on public and tribal lands.

THE ROADMAP

To end his speech, Kerry laid out a global to-do list, based on scientific advice, including cutting global emissions by at least 45 per cent by 2030, to put the world on a path to net zero by the mid-century.

"That makes this a decisive decade. And it makes 2021 a decisive year," he said.

Net zero refers to a state where the amount of greenhouse gases released into the atmosphere is no greater than the amount removed by natural carbon sinks, like forests, or technology that removes carbon from the source or atmosphere, known as carbon capture.

By 2030, the world must speed up its use of clean technologies, Kerry said, and pointed to International Energy Agency assessments to put in perspective what exactly is required to meet net zero: building the equivalent of the world's biggest solar plant every day for the next decade; quadrupling that use of wind and solar every year to 2030; and ensuring 60 per cent of new car sales in 2030 are electric vehicles, among other measures.

Green energy must be scaled up rapidly for heavy industry, shipping and aviation in particular.

"A raft of other technologies -- spanning advanced renewables and nuclear, long-duration energy storage, smart grids, and direct air capture -- will all need to be commercialized and scaled. This is even more of a challenge," he said.

Kerry will travel to Rome on Wednesday and will return to London on Friday for a ministerial meeting over the weekend in the lead-up to COP26, which is starting on October 31.