evidence: The reasons to disbelieve that humans are causing global warming are many and varied, skeptics say. For example: Natural factors such as long-term variations in solar radiation are causing the rise in worldwide average temperature. The urban heat island effect is skewing modern weather data, so the warming observed in recent decades isn’t real. And besides, not long ago experts all believed the Earth was cooling, not warming. Actually, research has shown that many such ideas are bogus. While changes in solar output have slightly increased global average temperature since the beginning of the Industrial Revolution, the planet-warming effect of man-made greenhouse gases is about 20 times larger ("Heated dispute" letter, SN: 10/27/07, p. 271). And although cities are warmer than neighboring rural areas, that phenomenon doesn’t mask recent warming trends in long-established cities ("Don’t blame the cities," SN Online: 9/5/08). Now, new research also skewers the global warming skeptics’ claim that, in the 1970s, scientists believed that an ice age was imminent. Researchers of the day had discovered that Earth had been cooling since the 1940s. Some believed that continued increases in the amount of planet-cooling aerosols kicked up or emitted by human activity — dust and smog, for example — could easily tip the planet into an ever-deepening cycle of cooling, skeptics have repeatedly pointed out. That wave of concern was obviously a false alarm, the skeptics note, so maybe today’s scientists are equally mistaken about global warming.