LONDON/BRUSSELS - Governments, business leaders and development banks have two years to take action to avert far worse climate change, the U.N.'s climate chief said on Wednesday, in a speech that warned global warming is slipping down politicians' agendas.

Scientists say halving climate-damaging greenhouse gas emissions by 2030 is crucial to stop a rise in temperatures of more than 1.5 Celsius that would unleash more extreme weather and heat.

Yet last year, the world's energy-related CO2 emissions increased to a record high. Current commitments to fight climate change would barely cut global emissions at all by 2030.

Simon Stiell, Executive Secretary of the United Nations Framework Convention on Climate Change said the next two years are "essential in saving our planet".

"We still have a chance to make greenhouse gas emissions tumble, with a new generation of national climate plans. But we need these stronger plans, now," he said.

Speaking at an event at the Chatham House think-tank in London, Stiell said the Group of 20 leading economic powers - together, responsible for 80% of global emissions - urgently needed to step up.

The main task for this year's U.N. climate negotiations in Baku, Azerbaijan, is for countries to agree a new target for climate finance to support developing countries struggling to invest in shifting away from fossil fuels and fighting climate change.

U.N. climate summits have swelled in size in recent years, with thousands of lobbyists and business representatives attending alongside the government delegations directly involved in the negotiations.

Nearly 84,000 people attended last year's COP28 summit in Dubai, drawing criticism from campaigners after more than 2,000 fossil fuel lobbyists registered to attend.

Stiell said he would like to see future COP meetings reduced in size, while prioritising strong negotiation outcomes. He said he was in talks with Azerbaijan and Brazil - host of the next two U.N. climate summits - about this.

He called for more climate finance to be raised through debt relief, cheaper financing for poorer countries, new sources of international finance such as a tax on shipping emissions, and reforms at the World Bank and International Monetary Fund.

"Every day finance ministers, CEOs, investors, and climate bankers and development bankers, direct trillions of dollars. It's time to shift those dollars," Stiell said.

In a bumper year for elections around the world - with voters going to the polls from India, to South Africa and the United States - Stiell warned too often climate action was "slipping down cabinet agendas".

Politicians from Republican frontrunner Donald Trump in the United States, to far-right parties seeking gains in the EU's upcoming election, have pushed back on climate policies as they court voters.

(Reporting by Simon Jessop, Kate Abnett Editing by Ros Russell)