If snow and ice covered areas begin melting,

Ifsnowandicecoveredareasbeginmelting,________

producesmuchmoreglobalwarmingthanacarbon-dioxideforcingofthesamemagnitude

contr ibutedto25percentofobservedglobalwarming

winterandspring

thewarmingeffectsincrease

stillsurpasssootwarmingtheworld'sclimateduringthelastcentury

canbeseenmostlyintheNorthernHemisphere

答案及解析：本题定位到第三段的第二句话,可知一旦冰雪融化,温室效应就会加剧。