Another myth bites the dust - solar activity cannot account for global warming
Direct satellite measurements of solar activity show it has been declining since the mid-1980s and cannot account for recent rises in global temperatures, according to new research.
The findings debunk an explanation for climate change that is often cited by people who are not convinced that greenhouse gas emissions from human activities are causing the Earth's climate to warm.
"If you change the output of the Sun you will undoubtedly change the climate it's just a matter of how much," says Mike Lockwood, of the Rutherford Appleton Laboratory, part of the Science and Technology Facilities Council in the UK.
Sceptics commonly point to climate research's reliance on computer models as a reason for doubting the link between global warming and human greenhouse gas emissions.
"We decided to do a simple and direct analysis of the potential role of the Sun in recent climate change without using any model output," says Lockwood.
Lockwood and colleague Claus Fröhlich, at the World Radiation Center in Switzerland, used direct measurements only for their study. As Lockwood puts it: "This is just what the spacecraft have seen."
U-turn
Looking at data from the past 40 years, the two researchers noticed that solar activity did what Lockwood describes as a "U-turn in every possible way" in the mid-1980s.
"The upshot is that somewhere between 1985 and 1987 all the solar factors that could have affected climate have been going in the wrong direction. If they were really a big factor we would have cooling by now," Lockwood told New Scientist. He adds that he wishes he knew why the Sun's activity had changed in this way.
The number of sunspots peaked twice during the 20th century, once in 1960 and then again in 1985 (see graphs, right), but have been dropping since.
Sunspots are used as indicators of solar activity, and people have tried to link the growing number of sunspots during the 20th century with rising global temperatures (see Global warming: Will the Sun come to our rescue?).
Others have suggested that cosmic rays help generate clouds, which would cool the atmosphere. But Lockwood and Fröhlich's results show that cosmic rays reached a minimum around 1985 and have risen since.
Correspondingly, the magnetic field that shields Earth from cosmic rays also reached a maximum at about the same time, in 1987.
Negligible role
Measurements of the Sun's brightness - which indicates of the amount of energy coming from the sun - only began in 1977. Yet here too the data suggests solar activity is playing a negligible role in current global warming: irradiance rose between 1977 and 1985, but has been dropping since.
Lookwood says the only way of reconciling the data with the idea that solar activity is causing global warming is to propose that there is a time lag between the Sun's activity changing and those changes affecting the Earth's climate. But even with a lag, climatologists would have noticed a slow-down in the rate at which temperatures are rising around the globe, says Lockwood.
"We have had 20 years of the cosmic rays and the irradiance going in the wrong direction, and yet we've not yet seen any effect on temperatures," says Lockwood. "It would have to be an extremely long lag - at least 50 years - which would invalidate a lot of the previous sun-climate proposals."
Lockwood and Fröhlich's results suggest that even the Intergovernmental Panel on Climate Change has overestimated the Sun's influence on the Earth's climate. In February, the IPCC published a report stating that the Sun had roughly 10% of the warming effect of human activities.
Journal reference: Proceedings of the Royal Society A (DOI:10.10.98.rspa.2007.1880)
The findings debunk an explanation for climate change that is often cited by people who are not convinced that greenhouse gas emissions from human activities are causing the Earth's climate to warm.
"If you change the output of the Sun you will undoubtedly change the climate it's just a matter of how much," says Mike Lockwood, of the Rutherford Appleton Laboratory, part of the Science and Technology Facilities Council in the UK.
Sceptics commonly point to climate research's reliance on computer models as a reason for doubting the link between global warming and human greenhouse gas emissions.
"We decided to do a simple and direct analysis of the potential role of the Sun in recent climate change without using any model output," says Lockwood.
Lockwood and colleague Claus Fröhlich, at the World Radiation Center in Switzerland, used direct measurements only for their study. As Lockwood puts it: "This is just what the spacecraft have seen."
U-turn
Looking at data from the past 40 years, the two researchers noticed that solar activity did what Lockwood describes as a "U-turn in every possible way" in the mid-1980s.
"The upshot is that somewhere between 1985 and 1987 all the solar factors that could have affected climate have been going in the wrong direction. If they were really a big factor we would have cooling by now," Lockwood told New Scientist. He adds that he wishes he knew why the Sun's activity had changed in this way.
The number of sunspots peaked twice during the 20th century, once in 1960 and then again in 1985 (see graphs, right), but have been dropping since.
Sunspots are used as indicators of solar activity, and people have tried to link the growing number of sunspots during the 20th century with rising global temperatures (see Global warming: Will the Sun come to our rescue?).
Others have suggested that cosmic rays help generate clouds, which would cool the atmosphere. But Lockwood and Fröhlich's results show that cosmic rays reached a minimum around 1985 and have risen since.
Correspondingly, the magnetic field that shields Earth from cosmic rays also reached a maximum at about the same time, in 1987.
Negligible role
Measurements of the Sun's brightness - which indicates of the amount of energy coming from the sun - only began in 1977. Yet here too the data suggests solar activity is playing a negligible role in current global warming: irradiance rose between 1977 and 1985, but has been dropping since.
Lookwood says the only way of reconciling the data with the idea that solar activity is causing global warming is to propose that there is a time lag between the Sun's activity changing and those changes affecting the Earth's climate. But even with a lag, climatologists would have noticed a slow-down in the rate at which temperatures are rising around the globe, says Lockwood.
"We have had 20 years of the cosmic rays and the irradiance going in the wrong direction, and yet we've not yet seen any effect on temperatures," says Lockwood. "It would have to be an extremely long lag - at least 50 years - which would invalidate a lot of the previous sun-climate proposals."
Lockwood and Fröhlich's results suggest that even the Intergovernmental Panel on Climate Change has overestimated the Sun's influence on the Earth's climate. In February, the IPCC published a report stating that the Sun had roughly 10% of the warming effect of human activities.
Journal reference: Proceedings of the Royal Society A (DOI:10.10.98.rspa.2007.1880)