Signs of Climate Change

Re (news article, Aug. 7): The new paper by James E. Hansen, a NASA climate scientist, and two colleagues addresses a crucial difficulty many people have in differentiating weather and climate. During hot spells, for example, more people accept that climate is changing, but a colder spell reverses those opinions. Everybody understands weather: its hot, cold, raining, snowing, windy and so on. Fewer people understand that a regions climate is the frequency of such weather events: how often does it rain or get hotter than, say, 90 degrees? The Hansen paper shows that these frequencies have changed substantially, both globally and throughout the Northern Hemisphere. Hot and very hot days have become more frequent; very cold days less frequent. The paper shows that there is more than a perception that climate change is occurring. The data show that change is real and substantial. The frequencies have shifted dramatically over the last half-century, especially for the most extreme weather events. The implication is that even more extreme weather is in the offing unless greenhouse gas emissions are quickly brought under control. ROBERT REPETTO Boulder, Colo., Aug. 7, 2012