* Summer Arctic sea ice could disappear
* Glacier melt could hurt 20 to 25 pct of world population
* Coastal California water already acidic (Adds quotes from UNEP executive director)
By Timothy Gardner
WASHINGTON, Sept 24 (Reuters) - Droughts from Australia to the U.S. Southwest, acidic ocean water and melting glaciers are signs that the pace of climate change is surpassing the worst-case scenarios scientists predicted in 2007, a U.N. report said on Thursday.
Mountain glaciers in Asia are melting at a rate that could eventually threaten water supplies, irrigation or hydropower for 20 percent to 25 percent of the world’s population, the U.N. Environment Program report said.
"We are headed to very serious changes in our planet and we need to appreciate how serious it is in order to lend support to the transformational policy measures that need to be taken," Achim Steiner, UNEP’s executive director, told reporters.
The Climate Change Science Compendium 2009 report analyzed 400 scientific reports released through peer-reviewed literature, or from research institutions, since the U.N.’s Intergovernmental Panel on Climate Change published its last report in 2007.
Global leaders including Chinese President Hu Jintao and U.S. President Barack Obama spoke at a one-day climate change conference at the United Nations this week to try to break a global deadlock on how rich and developing countries will share the burdens of slowing global warming.
Some 190 countries will try to reach an agreement on how to slow global warming at a meeting in Copenhagen in December.
ACIDIFICATION OF OCEANS
An increase in global greenhouse gas concentrations has raised concern among scientists that a rise of between 1.4 and 4.3 degrees Celsius (2.5 to 7.75 F) above pre-industrial temperatures is likely, the report said.
That is above the range of between 1 to 3 degrees C (2 to 5.4 F) many scientists see as a level that could lead to the end of summer Arctic sea ice and the eventual melting of the Himalayan glaciers and the Greenland ice sheet, the report said.
In addition, increased absorption of the main greenhouse gas carbon dioxide by oceans is leading to acidification of sea water faster than expected. For example, water that can corrode a seashell-making substance is "already welling up along the California coast -- decades earlier than existing models predict," the report said.
The acidification of oceans could threaten shellfish and coral reefs, the breeding grounds for many fish species.
Environmentalists hope leaders will pay attention to the UNEP report as the Copenhagen meeting nears but news about how the global recession is affecting emissions may also be a factor. The Paris-based International Energy Agency, said this week the recession has set the stage for the 2.6 percent drop in global carbon dioxide output this year, the sharpest fall in in 40 years.
Still, the UNEP report said that emissions that have already been released into the atmosphere could lead to the loss of ecosystems and increased desertification from Africa to Asia.
The U.S. Energy Information Administration expects U.S. emissions to begin rising again next year as the economy improves.
(Reporting by Timothy Gardner; Editing by Sandra Maler)