Official government measurements show that the world’s temperature has cooled a bit since reaching its most recent peak in 1998. That’s given global warming skeptics new ammunition to attack the prevailing theory of climate change. The skeptics argue that the current stretch of slightly cooler temperatures means that costly measures to limit carbon dioxide are unnecessary. Many scientists agree, however, that hotter times are ahead. A decade of level or slightly lower temperatures is only a temporary dip to be expected as a result of natural, short-term variations in the climate system, they say. John Christy, director of the Earth System Science Center at the University of Alabama in Huntsville, who often sides with the skeptics, agreed that the cooling won’t last.