Agence France-Presse: Big volcanic eruptions have had a cooling effect on Earth’s climate, but the Icelandic event is too small to provide any such respite from manmade global warming, scientists said on Friday. The benchmark cooling event of the past 20 years was in 1991, when Mount Pinatubo erupted in the Philippines. It cooled Earth’s surface by 0.5 degrees Celsius (0.9 degrees Fahrenheit) over the next year, enough to offset the impact of greenhouse gases from 1991 to 1993. A smaller …