The sun's activity is winding down, triggering fevered debate among scientists about how low it will go, and what it means for Earth’s climate. Nasa recorded no sunspots on 266 days in 2008 - a level of inactivity not seen since 1913 - and 2009 looks set to be even quieter. Solar wind pressure is at a 50-year low and our local star is ever so slightly dimmer than it was 10 years ago.

Sunspots are the most visible sign of an active sun - islands of magnetism on the sun's surface where convection is inhibited, making the gas cooler and darker when seen from Earth - and the fact that they're vanishing means we're heading into a period of solar lethargy.

Where will it all end? Solar activity varies over an 11-year cycle, but it experiences longer-term variations, highs and lows that can last around a century.

"A new 11-year cycle started a year or two ago, and so far it’s been extremely feeble," says Nigel Weiss of the University of Cambridge. With Jose Abreu of the Swiss Federal Institute of Aquatic Science and Technology in Dübendorf and others, Weiss recently predicted that the long-term solar high we've been enjoying since before the second world war is over, and the decline now under way will reach its lowest point around 2020. Their prediction is based on levels of rare isotopes that accumulate in the Earth’s crust when weak solar winds allow cosmic rays to penetrate the Earth's atmosphere.

There's even a chance, says Weiss, that we might be heading for a low as deep as the Maunder minimum of the 17th century. Either side of that trough, Europe shivered through the Little Ice Age, when frost fairs were held on the Thames and whole Swiss villages disappeared under glaciers. So should we expect another freeze?

Those who claim the rise in temperatures we've seen over the last century are predominantly the result of intense solar activity might argue that we should, but they’re in the minority. Most scientists believe humans are the main culprit when it comes to global warming, and Weiss is no exception. He points out that the ice remained in Europe long after solar activity picked up from the Maunder minimum. Even if we had another, similar low, he says, it would probably only cause temperatures on Earth to drop by the order of a tenth of a degree Celsius- peanuts compared to recent hikes.

太陽活動正逐漸下降，這引發了科學家之間關于太陽活動會下降到什么程度以及對地球氣候影響的白熱化爭論。美國航天局記錄2008年一共有266天沒有監測出太陽黑子——這種休眠狀態1913年以來從未見過——2009年似乎將變得更安靜。太陽風的壓力正處于50年來的最低水平，我們的太陽也比10年前稍暗了。

太陽黑子是太陽活動最明顯的跡象——它是太陽表面一種熾熱氣體的巨大漩渦，因為比太陽光球層表面溫度要低，所以從地球看上去像一些深暗色的斑點——事實上它們的消失意味著我們進入了一個太陽活動低迷期。

最終會是什么樣子？黑子的活動周期為11.2年，但也會經歷更長期的變化，頂峰和低估可以持續一個世紀左右。

“一個新的11年周期在一兩年前開始了，目前為止它活動極其微弱，”劍橋大學的 Nigel Weiss說。和位于Dübendorf的瑞士聯邦水產科技學院的Jose Abreu及其他一些人一起，Weiss最近預測，我們自二戰后長期享受的充足太陽能量已經結束，目前能量正在減少，將在2020年左右達到最低點。他們的預測是基于積聚在地殼的罕見同位素的水平，輕微的太陽風使宇宙射線穿透地球的大氣層。

還有一種可能，Weiss說，我們可能會與1645－1715年間發生在整個歐洲、北美和格陵蘭島的降溫過程非常相似。當時，倫敦的泰晤士河、巴黎的塞納河，以及格陵蘭地區的所有河流均結冰凍結，格陵蘭島上的居民還不得不放棄了部分居民點。我們會不會再經歷一次這樣的事情？

這些聲稱主要是由于激烈的太陽活動導致過去一個世紀地球變暖的人可能會主張我們會經歷十七世紀歐洲發生的事情，但這種人是少數。大多數科學家認為人類是全球氣候變暖的罪魁禍首，Weiss也不例外。他指出，那次冰凍時期后太陽活動活躍起來了冰山依然在歐洲保留。即使這種情況會發生，他說，大概也只能導致地球溫度下降0.1攝氏度左右，實在是微不足道。