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Felipe Fernandez-Armesto: It's been a long, cold, lonely winter but, here comes the Sun. (And yes, it's all right)

The last time solar activity slowed this much, whole empires fell. But the past few days' activity may augur well

No wonder people treat the Sun as a god, distributing life and death with a capricious hand. When he flares, as if with anger, he burns forests and shrivels crops. When he hides his face, we shiver and suffer.

Although we now use secular language as we speak of vitamin D, ultraviolet radiation, squamous cell carcinoma, seasonally adjusted disorder and the greenhouse effect, we still cannot escape the Sun's blessings and curses. The latest anxiety to beset Sun-watchers is that it is "going quiet" – generating less energy and radiating less heat. The predicted consequences range from instant apocalypse to mild and favourable adjustments in global temperatures. But if history is anything to go by, it's as well to take note that similar periods of solar quiescence have in the past portended – and probably helped to cause – economic and social disasters, and political upheaval.

Anyone can measure levels of solar activity impressionistically, by squinting at the sky. Sometimes, especially near the northern and southern extremities of the Earth, you can see the evidence in spectacular form, as charges of energy from the Sun light the sky with flashing, brilliant, colourful auroras. Sunspots, up to 50,000 miles across, which sporadically appear to darken the Sun, are even more reliable indicators, visible from anywhere on the surface of our planet. The Sun is a heaving, seething wreath of gases: the faster they writhe, the greater the strain on the force that keeps them together. Sunspots are the signs of that stress. When solar activity speeds up, sunspots multiply. When it slows, they fade. Right now, the sunspots are in retreat, and have been for an alarmingly long time.

Typically, episodes of frenzied activity and quiescence follow a cycle. Astronomers have kept reliable records for hundreds of years, revealing that sunspots begin to build up, on average, every 22 years. In mid-cycle they are visible daily, then diminish until they emerge only sporadically, before starting to increase again.

If the system were totally regular, a new cycle would have started towards the end of 2006, but we are still waiting for it. Measured by the number of days on which sunspots appeared, 2008 was the quietest year on record for solar activity since 1913 (the final full year of a normal cycle, when low levels of intensity were in any case predictable). So far, 2009 has been quieter still – there have been even fewer days' sunspots than in 1913. Such sunspots as have been visible in recent years seem, by most measurements, to be warmer and less charged with magnetism than normal – a sign that the activity producing them has been relatively feeble, even for the quiet phase of a typical cycle. The Sun has not been this lazy for this long since Napoleon was the scourge of Europe.

The effects are equivocal. Sunspots impede the Sun's rays, so that intense solar activity can cause temperatures on Earth to drop in the short term. So when sunspots fail, the world can get hotter, at least for a while. Satellites monitoring the effects of sunspots on solar irradiation in the last couple of decades of the 20th century suggested that energy reaching Earth from the Sun was increasing, making global warming worse. On the other hand, solar quiescence literally makes the Sun less hot, and so threatens Earth with cooling. We may be about to feel the chill. Global temperatures were low, on average, over 2008 as a whole, perhaps as much as 0.7 degrees lower than normal – surprisingly low for a world convulsed by fears of warming.

Still, one simple formula encompasses all this baffling complexity: unusual activity in the Sun means unusual weather on Earth, and unusual weather on Earth helps to cause freak political and economic events.

The historical evidence is disquieting. The most stunning interruption on record in the normal pattern of sunspots occurred between the 1640s and about 1715, when the sequence of cycles came to a halt. As sunspots disappeared, the temperature fell: the winter scenes that Brueghel painted so beautifully captured the intensity of cold that Europeans experienced. So did the lines of diarists recalling skating on the Thames. In fact, the river froze hard enough for Londoners to go skating on 14 occasions during that period. In 1658 the Kattegat, a seawater strait that ought to be unfreezable, was solid enough to support an army of invasion marching across it from Jutland to Zealand.

The early years of that "little ice age", as some historians call it, coincided with a spell of dramatic political instability in Eurasia, the so-called "general crisis of the 17th century". In the 1640s and 1650s, England, France, the Spanish monarchy, Sweden (a great power at the time), the Holy Roman and Ottoman empires and the empires of China and India all collapsed or experienced violent upheavals. The severity of climate change seems at least to have been a contributing factor, causing hardship and stirring revolutions.

A period even more like the present one occurred in the late 18th and early 19th centuries, when the onset of a normal solar cycle was also delayed for a few years. Global temperatures fell as a long period of low sunspot counts coincided with the French Revolution and the revolutionary and Napoleonic wars. There were changes of regimes and ferocious political reactions all over Europe. The oldest and biggest world empire – Spain's – collapsed. The British Raj effectively replaced the Mughal empire in India. The First World War, which ushered in the end of the romantic era and the fragmentation of some of the world's most powerful states, began after a spell of exceptional solar inertia, with more than 1,000 sunspotless days.

With precedents such as these, it will surely not be long before bloggers start attributing the global economic crisis to a stalling Sun. Already the web is vibrating with predictions of the end of the world and the dimming or death of the Sun. Fanatics who wish to deny global warming are invoking the Sun to talk up the prospect of global cooling. Loony augurs see a new ice age coming, or threaten us with prophecies of apocalypse in 2012 – the Doomsday year selected by ancient Mayan soothsayers. But are we really heading for an experience like that of the late 18th century, or a "general crisis" like that of the 17th?

Before we succumb to the oracles of doom, we should formulate a new scientific law: small fluctuations in scientific data produce disproportionate panic in the populace. Climate scares have become like health scares: there is a new one almost every week, and they get less persuasive as they multiply.

In truth, the Sun may well be about to resume normal service. Last Thursday a solar storm of unusual ferocity erupted, exciting hopes of a turnaround. Meteorologists have debated what the next cycle, if it happens, will be like. Does the long delay mean the Sun is storing up, as it were, fury for the future? Or will the return of sunspots be feeble and few?

So far, the evidence suggests a freakish interruption of a normal cycle, rather than its suspension or reversal. If we are in for a cool spell it will probably be a short lurch, whereas warming is the long-term trend. Anxiety, however, is salutary in one respect: it reminds us that we rely absolutely on the Sun. The future of the world really does depend on what happens next 93 million miles away.