THE sun's ability to shield the solar system from harmful cosmic rays could falter in the early 2020s, just in time to threaten the health of NASA astronauts as they return to the moon.
As well as the 11-year cycle of sunspots and solar flares, the sun's activity experiences longer-term shifts lasting several decades. The sun is currently in a long-term high, having been relatively active for nearly a century, but it is not known when this will end.
To find out, a team led by Jose Abreu of the Swiss Federal Institute of Aquatic Science and Technology in Duebendorf analysed 66 long-term highs from the past 10,000 years, as recorded in fluctuating levels of rare isotopes such as beryllium-10 in ice cores from Greenland. These are produced when cosmic rays break down the nuclei of oxygen and nitrogen atoms in the Earth's atmosphere. Production of these isotopes peaks when the sun is inactive, as the weaker solar wind lets more cosmic rays enter the solar system, which hit the Earth.
Based on the duration of past highs, and the fact that the current one has already lasted 80 years, the team has calculated that its most likely total lifetime is between 95 and 116 years, and they suspect the high will probably end at the shorter end of this range (Geophysical Research Letters, DOI: 10.1029/2008GL035442).
Records of the sun's brightness during the 20th century show that it gets slightly dimmer when it is less active, so could a long-term reduction in activity help to offset global warming? No such luck, says Nigel Weiss from the University of Cambridge, who is a member of Abreu's team. While there is a rough correspondence between a period of very low activity from 1645 to 1715 and the middle of a period of lower average global temperatures lasting from the late 16th to mid 19th century, leading some to suggest a causal link, this correlation could be a coincidence, Weiss says.
Weiss also points out that the sun's brightness changes only slightly with variations in activity. If the sun does dim slightly in the coming decades, he says, this would only reduce the warming expected due to human-induced climate change by 0.1 °C. "It might be discernible, but it would be a blip rather than a major change," he says. "It is nothing [compared] with the global warming that is now being produced through pumping of greenhouse gases into the atmosphere."
Those most likely to be affected would be astronauts. Beyond the Earth's protective magnetic field, their exposure to the increased cosmic rays let into the solar system due to a weaker solar wind could cause cancer and fertility loss. One benefit to astronauts would be a decline in the number of solar flares.
David Hathaway of NASA's Marshall Space Flight Center in Huntsville, Alabama, says the evidence for past lulls is strong, but he is sceptical about the team's attempt to predict the arrival of the next one. "This is a little like trying to predict when someone's winning streak will end," he says. "We know that it will happen, but reliable predictions are virtually impossible."