Adlerstein said about US$100,000 would be saved by turning the thermostats up 5 degrees to 77 Fahrenheit (25 Celsius) in the UN secretariat building and to 75 F (24 C) in conference rooms, during a trial run in the scorching month of August.

It would also help the environment in New York City, he said. About 4,400 million pounds of steam -- equivalent to several hundred tons of carbon dioxide -- would be saved by reducing air conditioning at the landmark midtown Manhattan building.

He said staff were being encouraged to shed their trademark dark suits and switch to lighter business casual clothing. Adlerstein sported a white shirt with neither jacket nor tie as he addressed reporters at UN headquarters.

"I don't want to get involved in the fashion police of determining exactly what people can wear, but the encouragement of business casual is where we are going," he said.

If the trial is a success, thermostats will be turned down 5 degrees in winter. The year-round changes could save up to US$1 million annually, Adlerstein said.

Japanese diplomats will likely have an easy time making the transition. In 2005, Japan launched a "Cool Biz" campaign encouraging people to dress down in summer to reduce air conditioning use and greenhouse gas emissions. (Reporting by Megan Davies)