As delegates at the COP29 climate conference in Baku try to land a hard-fought deal to boost funding for climate action in developing countries, Lagarde said "the 'financing gap' is widening".

She pointed to figures that suggest that financing needs are "50 percent higher than previously estimated and up to 18 times greater than current commitments".

"The energy transition alone requires investment in clean energy to triple by 2030," she said in a column published by the Financial Times and on the ECB's website.

She cited estimates by the UN Environment Programme (UNEP), according to which up to $11.7 trillion has to be spent on climate change mitigation annually by 2035 in order to meet the Paris agreement target to keep warming "well below" two degrees Celsius compared to pre-industrial levels.

This represents around ten percent of global economic output.

Lagarde, who has made climate protection a central goal of her tenure at the head of the ECB, said that events including the "horrific" recent floods in Spain and storms in North America showed that "the steep price of our inaction... is rising by the day".

Such extreme weather events are "ruining the foundations of our economies and, ultimately, the basis of our economic survival," she wrote.

Also on Tuesday UN Secretary General Antonio Guterres urged the nearly 200 countries taking part in the COP29 talks to reach a deal on funding from richer nations to developing economies to help them adapt to climate change.

Developing nations "must not leave Baku empty-handed" at the end of the conference on November 22, he said, adding: "A deal is a must."