Most of us are used to seasons. Each year, spring follows winter, which follows autumn, which follows summer, which follows spring. And winter is colder than summer. But the earth goes through temperature cycles over much longer periods than those that we experience. Between 65,000 and 35,000 years ago, the planet was much colder than it is now. During that time the temperature also changed a lot, with periods of warming and cooling. Ice melted during the warm periods, which made sea levels rise. Water froze again during the cold periods. A new study from Switzerland, sheds light on (使某事清楚明白地显示) where ice sheets (冰川) melted during the ice age. It now seems that the ice melted at both ends of the earth, rather than just in either northern or southern regions (区域). This surprised the researchers from the University of Bern (伯恩大学). Scientists have long assumed (假设) that most of the ice that melted was in the Northern hemisphere (半球) during the 30,000 year long ice age. That belief was held because the North Pole is surrounded by land, while the South Pole is surrounded by the Antarctic Ocean. It is easier for ice sheets to grow on land. If surrounded by sea the ice can easily just slip into the ocean instead of building up. The researchers used a computer model to look at ways the ice could melt and how it might affect sea levels. They compared these results to evidence (证据) of how temperatures and currents actually changed during that time. The model showed that if it was only in the Northern hemisphere that ice melted, there would have been a bigger impact (影响) on ocean currents (流) and sea temperatures than what actually happened. Studies suggest that melting just in the Southern hemisphere would have been impossible, too. The only reasonable conclusion, the scientists could make, was that ice melted equally in the North and the South. It is still a mystery (神秘的事物) as to what caused the temperature changes that caused the ice to melt. The North Pole is surrounded by land, while the South Pole is surrounded by the Antarctic Ocean. So scientists thought that______.