At a July global climate gathering in London, South African environment minister Barbara Creecy presented the world’s wealthiest countries with a bill: more than $750 billion annually to pay for poorer nations to shift away from fossil fuels and protect themselves from global warming.

The number was met with silence from U.S. Climate Envoy John Kerry, according to Zaheer Fakir, an adviser to Ms. Creecy. Other Western officials said they weren’t ready to discuss such a huge sum.

For decades, Western countries responsible for the bulk of greenhouse-gas emissions have pledged to pay to bring poorer nations along with them in what is expected to be a very expensive global energy transition. But they have yet to fully deliver on that promise. Now the price of the developing world’s cooperation is going up.

At the end of the month, negotiators from nearly every country will meet in Glasgow, Scotland, for a two-week climate summit, the first major gathering since governments signed the Paris accord in 2015. The goal is to strike a deal to keep the climate targets of the Paris agreement within reach.

Before signing on, poorer countries are demanding a big increase in funding from the developed world to adopt cleaner technologies and adapt to the effects of climate change such as rising sea levels and more powerful storms.

Bangladesh says it needs cyclone-resistant housing. Kenya wants its countryside dotted with solar farms instead of coal or natural gas-fired plants. India says its climate-change plan alone will cost more than $2.5 trillion through 2030.

“We cannot be talking about ambition on the one hand, and yet you show no ambition on finance,” said Mr. Fakir who is coordinating climate finance policies for the Group of 77, a coalition of developing nations.

Developed nations say it is unrealistic to put them on the hook for such a large sum without also getting middle-income countries—China in particular—to provide funds. In Paris in 2015, the U.S., Europe and a few other wealthy nations committed to funding poorer countries to the tune of $100 billion a year from 2020 through 2025. They have so far fallen short.

Climate ControlRich countries have increasingly channeled​funds to the developing world for climate-​change projects, but the Paris agreement calls​for even more money.Climate-change funding from developed​countriesSource: Organization for Economic Cooperation and​DevelopmentNote: Private contributions not available in 2015
PublicPrivate2013201420152016201720182019$0 billion$100$25$50$75

Developing-world negotiators say the money isn’t financial aid. Rather, they say wealthy countries have a responsibility to pay under the U.N. climate treaties because most of the Earth’s warming since the industrial era is the result of emissions from the rich world. Moreover, poor nations now face the task of raising living standards without burning fossil fuels unchecked as the U.S. and other rich nations did for almost two centuries.

“If you’re going to ask a much poorer country to forgo that option, then there is a moral claim that they need support to go on a lower emissions development pathway,” said Joe Thwaites, a climate-finance expert at the World Resources Institute, an environmental think tank.

Even developed countries are struggling with the transition to renewables. A surge in demand for power from nations recovering from the pandemic has forced governments to lean on fossil fuels; though investment in renewables has increased, it accounts for only about a quarter of the world’s power.