Solar activity drops to 100-year low, puzzling scientists

LONDON: Predictions that 2013 would see an upsurge in solar activity and geomagnetic storms disrupting power grids and communications systems have proved to be a false alarm. Instead, the current peak in the solar cycle is the weakest for a century.

 Subdued solar activity has prompted controversial comparisons with the Maunder Minimum, which occurred between 1645 and 1715, when a prolonged absence of sunspots and other indicators of solar activity coincided with the coldest period in the last millennium.

 The comparisons have sparked a furious exchange of views between observers who believe the planet could be on the brink of another period of cooling, and scientists who insist there is no evidence that temperatures are about to fall.

 New Scientist magazine blasted those who predicted a mini ice age, opening a recent article on the surprising lack of sunspots this year with the bold declaration: "Those hoping that the sun could save us from climate change look set for disappointment".

 "The recent lapse in solar activity is not the beginning of a decades-long absence of sunspots, a dip that might have cooled the climate. Instead it represents a shorter, less pronounced downturn that happens every century or so," ("Sun's quiet spell not the start of a mini ice age" July 12).

 The unusually low number of sunspots in recent years "is not an indication that we are going into a Maunder Minimum" according to Giuliana DeToma, a solar scientist at the High Altitude Observatory in Colorado.

 But DeToma admitted "we will do not know how or why the Maunder Minimum started, so we cannot predict the next one."

 Many solar experts think the downturn is linked a different phenomenon, the Gleissberg cycle, which predicts a period of weaker solar activity every century or so. If that turns out to be true, the sun could remain unusually quiet through the middle of the 2020s.

 But since the scientists still do not understand why the Gleissberg cycle takes place, the evidence is inconclusive. The bottom line is that the sun has gone unusually quiet and no one really knows why or how it will last.

Counting sunspots

 Solar flares and coronal mass ejections (CMEs), when billions of tonnes of solar plasma erupt from the surface of the sun and are flung out into space at speeds up to 3,000 kilometres per second, pose the biggest risk to power grids and communications systems.

 Sunspots are less dramatic, but because they are easy to count and closely correlated with flares, mass ejections and other indications of solar activity, astronomers and scientists have used them for centuries to monitor variations in the sun's activity.

 Careful observation has revealed the number of sunspots rises and falls in a regular cycle that repeats every 11 years.

 Variations in the amount of heat and light reaching the planet's surface as a result of the cycle are tiny. Total solar output reaching the surface varies by just 1.3 Watts per square metre (0.1 percent) between the maximum and minimum phases of the cycle.

 Even this variation has profound impacts on climate and weather. Rainfall, cloud formation and river run-off are all strongly correlated with the sun's 11-year cycle.

 The impact is far smaller than the warming associated with man-made climate change. Solar activity cannot explain long-term trends in global temperatures such as those associated with global warming. But it may have a noticeable impact over shorter timescales.

Maunder Minimum

 Not all solar cycles are the same. Cycles in the 1940s and 1950s were especially strong. Those at the end or the 19th century and the start of the 20th were much weaker.

 More profoundly, in the 1890s, Walter Maunder of the Royal Observatory at Greenwich, identified a "prolonged sunspot minimum" between 1645 and 1715 in which hardly any sunspots were observed by contemporaries.

 At times, whole years passed without any sunspots being recorded. Sunspots became so rare that in 1684 Britain's Astronomer Royal John Flamsteed was moved to write: "these appearances, however frequent in the days of ... Galileo have been so rare of late that this is the only one I have seen ... since December 1676".

 John Eddy of the High Altitude Observatory confirmed Maunder's findings in an article published in the peer-reviewed journal Science in 1976 ("The Maunder Minimum: The reign of Louis XIV appears to have been a time of real anomaly in the behaviour of the sun").

 Eddy found convincing evidence for an actual absence of sunspots, not just an absence of observations. Maunder's prolonged sunspot minimum correlates well with other evidence of unusually low solar activity at the time, including few sightings of the Northern Lights, no mention of the sun's normally spectacular corona during eclipses, and the carbon-14 record in tree rings. Continue reading

No comments:

Post a Comment