The glaciers that have long provided water and electricity to this part of Bolivia are melting and disappearing, victims of global warming, most scientists say.

Image The Milluni reservoir has receded as glaciers that provide some of Bolivia's water and electricity have melted and disappeared. Credit... Ángel Franco/The New York Times

If the water problems are not solved, El Alto, a poor sister city of La Paz, could perhaps be the first large urban casualty of climate change. A World Bank report concluded last year that climate change would eliminate many glaciers in the Andes within 20 years, threatening the existence of nearly 100 million people.

For the nearly 200 nations trying to hammer out an international climate accord in Copenhagen, the question of how to address the needs of dozens of countries like Bolivia is a central focus of the negotiations and a major obstacle to a treaty.

World leaders have long agreed that rich nations must provide money and technology to help developing nations adapt to problems that, to a large extent, have been created by smokestacks and tailpipes far away. But the specifics of that transfer  which countries will pay, how much and for what kinds of projects  remain contentious.

Last week, a group of the poorest small countries debated whether they would stage a walk-out in Copenhagen if rich nations failed to provide enough money. Todd Stern, the lead negotiator for the United States, while reiterating that the United States would help pay, bridled at the idea that the money was a “climate debt.” And on Friday, the European Union made an initial pledge to pay $3.5 billion annually for three years to help poor countries cope  though economists project the total cost to be $100 billion or more.