United Nations Industrial Development Organization Director General Kandeh Yumkella said annual investments in renewable energy, especially hydroelectric projects, soared more than fourfold to $155 billion between 2004 and 2008, but have dropped in 2009.
"Due to the current economic crisis we have seen almost a 40 percent decline in 2009 alone in these investments," Yumkella told the opening of a global conference on green energy in the central Mexican city of Leon.
The conference of government officials and business executives coincides with a September 28-October 9 climate change meeting of delegates from 180 nations in Bangkok.
Those talks are the last major negotiating session before environment ministers from around the world meet in Copenhagen in December to seal a tougher global pact to replace the Kyoto protocol.
Mexico has proposed the creation of a green fund based on nations' historical and current greenhouse gas emissions, economic growth and population as a way to unlock hundreds of billions of dollars in potential spending and help mitigate the drop in renewable energy investments.
"It is important that there is financing in place. That poor countries know they will be assisted in their financing strategies," Yumkella told Reuters after his speech.
He said a key sticking point for the Copenhagen talks would be where that money could come from.
Mexican President Felipe Calderon plugged his green fund proposal and said Mexico is trying to increase the pace of new projects, like its huge new wind farm in the southern state of Oaxaca.
Mexico has set a goal of having renewable power sources generate 25 percent of its electricity by 2012.