Although it looks like a pretty consistent fireball to us, the Sun has its highs and lows. It goes through a regular cycle of maximum and minimum activity over a period of about 11 years, but there are longer term trends as well. New research suggests the Sun might be headed towards a "grand minimum" of a centuries-long cycle, during which the Earth would receive much less ultraviolet radiation. If so, it may even help slow – but not stop – the progression of the climate's general warming trend.
Solar activity is associated with sunspots of increased magnetism, visible on the star's surface as darker areas. The numbers of these sunspots generally rises to a peak, coinciding with the 11-year solar maximum, before they slowly fade away as the Sun heads towards its minimum. Larger amounts of sunspots is also accompanied by events like solar flares and coronal mass ejections.
Historically, astronomers have noted that one of the most drastically low periods of solar activity occurred in the late 17th century. Normally, scientists would expect to observe between 40,000 and 50,000 sunspots over a period of 30 years, yet between 1672 and 1699 less than 50 were recorded. That staggeringly quiet period became known as the Maunder Minimum.
Interestingly, the time frame of the Maunder Minimum lines up with historical records of much colder-than-average temperatures across Europe. Often described as the "Little Ice Age", the winters during this time were apparently so chilly that the Thames River in London regularly froze over, droughts and famines increased, and glaciers in the Alps grew much larger than they had been before or since.
While the Maunder Minimum is the most extreme example on record, it may not really be indicative of what to expect in a similar future event. The new study, headed up by scientists at the Scripps Institution of Oceanography, set out to determine the likelihood of another grand minimum event in the near future, and how much dimmer the Sun would become during that time.
To do so, the researchers reviewed 18 years' worth of data gathered by the International Ultraviolet Explorer. Part of this satellite's mission was to study the activity of stars, and the Scripps researchers identified Sun-like stars that were experiencing their own minima periods. They found that during a grand minimum, a star's UV radiation drops about seven percent lower than the trough at the minimum phase of its regular cycle.
"Now we have a benchmark from which we can perform better climate model simulations," says Dan Lubin, lead researcher on the project. "We can therefore have a better idea of how changes in solar UV radiation affect climate change."
According to the researchers, there's a "significant probability" of a grand minimum event occurring by the middle of the 21st century. Sunspot numbers appear to be trending downwards in recent solar cycles, with solar physicists at NASA even saying in 2009 that "This is the quietest sun we've seen in almost a century."
That might sound like good news for our rapidly-warming world, but unfortunately, it wouldn't be enough to halt the progression of climate change. The researchers say that the warming caused by increasing CO2 levels will far outweigh any cooling effects that a grand solar minimum would bring.
In short, no Earth isn't headed for a global-warming-reversing "mini ice age."
The research was published in the journal Astrophysical Review Letters.
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