Money will likely be the central tension in the U.N.'s COP27 climate negotiations

As the United Nations climate conference opens in Egypt, the most critical talks will likely focus on the soaring costs of limiting — and adapting to — global warming, especially in the world's most vulnerable countries. It's a contentious conversation more than a decade in the making.

In 2009, industrialized countries pledged to provide developing nations with $100 billion a year in climate financing by 2020. The agreement is rooted in the fact that developed countries generated most of the heat-trapping pollution now in the atmosphere, while developing nations have already begun to bear a disproportionate share of the harm from extreme weather.

That 2020 deadline came and went without rich countries delivering on their promises.