Mercury News: Climate change transforms an election campaign. Extreme weather and water restrictions galvanize voters. Both political parties scramble to propose mandatory limits on carbon emissions. That’s happening now, but not in the United States. It’s Australia. There, the impact of global warming has shaken up a national election, with climate change issues a major factor in Saturday’s defeat of longtime Prime Minister John Howard. In the 2008 U.S. presidential campaign, “going …