Northern-hemisphere temperatures stayed flat from November to March

凛冬不至 Winter is not coming-书迷号

THE MOST commonly cited risks of climate change are natural disasters: fiercer wildfires and hurricanes, bigger floods and longer droughts. But one of the most striking recent effects of global warming has been unusually mild weather in many parts of the world.

The northern-hemisphere winter that ended on March 20th was the second-warmest since records began, and the warmest ever on land. The anomaly was biggest in Europe and Asia, where average temperatures from December to February were 3.2°C (5.8°F) and 3.1°C above the average from 1951-80, and 0.8°C and 0.7°C above those continents’ previous record highs. After a normal autumn, temperatures stayed close to their November levels for months. In Boston, where daily lows in January tend to hover around -6°C, the average minimum this January was 0°C; for Tokyo the figures were 0°C and 5°C. By local standards, the balmiest winter of all was in Russia. Moscow’s average daily low in January was -2°C, far from the customary -13°C.
北半球于3月20日结束的这个冬季是有记录以来气温第二高的冬天,也是陆地上有史以来最温暖的一个冬天。欧洲和亚洲气温异常的情况最显著,两地12月至2月的平均气温分别比1951到1980年的均温高出3.2℃和3.1℃,比各自之前的历史峰值高0.8℃和0.7℃。经过一个正常的秋天后,气温连续几个月与11月的水平相近。在波士顿,1月份的每日低温往年一般都在 – 6℃上下徘徊,而今年1月份的平均最低气温为0℃;东京的这两个数字分别是0℃和5℃。按地方标准衡量,俄罗斯的这个冬天最温暖。莫斯科1月份的日均低温为﹣2℃,远低于通常的﹣13℃。

The winter-that-wasn’t of 2019-20 is not yet a new normal. The main factor determining the severity of northern winters is the “Arctic oscillation”: the relative pressure of Arctic and sub-tropical air. When pressure is higher in the Arctic, cold air from the North Pole pushes south, bringing harsh, dry winters to many places. When pressure is higher towards the sub-tropics, warm air pushes northwards, hemming in cold air around the pole. These two patterns flip back and forth irregularly.

For reasons that are not yet clear, pressure in the sub-tropics this year was much stronger than in the Arctic. And researchers have not yet determined how rising temperatures affect the Arctic oscillation. Until a few years ago, climate models tended to show pressure in the Arctic strengthening, reducing the amount of warming during winter at temperate northern latitudes. The latest models find the reverse.

However, climate change is still responsible for anomalies like this one. At the average global temperature in 1950, a winter this mild was all but impossible. In today’s climate, such reprieves from the cold should occur once every 11 years. And if global warming continues on its current trajectory, winters like this year’s could become standard within a few decades.

Mild winters offer benefits. Heating is cheaper, flu seasons are shorter and fewer people die overall. But problems mount as well. Without hard frosts, pests can survive and multiply to attack crops more harshly. Warmer winters are usually wetter, changing snowfall patterns. This can shrink the snowpack that supplies rivers, and cause floods. Even people who bemoan frigid winters may miss them if they vanish. ■