A study has confirmed that there are no grounds to blame the Sun for recent global warming. The analysis shows that global warming since 1985 has been caused neither by an increase in solar radiation nor by a decrease in the flux of galactic cosmic rays (M. Lockwood and C. Fröhlich Proc. R. Soc. A doi:10.1098/rspa.2007.1880; 2007). Some researchers had suggested that the latter might influence global warming through an involvement in cloud formation.

“This paper is the final nail in the coffin for people who would like to make the Sun responsible for present global warming,” says Stefan Rahmstorf, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany.

Claims that the Sun, rather than raised levels of greenhouse gases, has been responsible for recent warming have persisted in a small number of scientists and in parts of the media. Mike Lockwood, a physicist at the Rutherford Appleton Laboratory in Chilton, UK, says he was “galvanized” to carry out the comprehensive study by misleading media reports. He cites 'The Great Global Warming Swindle', a television programme shown in March by Britain's Channel 4, as a prime example.

Hot topic: solar activity peaked too early to have caused the current trends in climate change. Credit: W. BURGESS/REUTERS

Together with Claus Fröhlich of the World Radiation Center in Davos, Switzerland, Lockwood brought together solar data for the past 100 years. The two researchers averaged out the 11-year solar cycles and looked for correlation between solar variation and global mean temperatures. Solar activity peaked between 1985 and 1987. Since then, trends in solar irradiance, sunspot number and cosmic-ray intensity have all been in the opposite direction to that required to explain global warming.

In 1997, Henrik Svensmark, a physicist at the Danish National Space Center in Copenhagen, suggested that cosmic rays facilitate cloud formation by seeding the atmosphere with trails of ions that can help water droplets form (H. Svensmark and E. J. Friis-Christensen J. Atmos. Solar-Terrest. Phys. 59, 1225–1232; 1997). He proposed that, as a result of this, changes in the Sun's magnetic field that influence the flux of cosmic rays could affect Earth's climate. This led to claims that cosmic rays are the main influence on modern climate change.

Even in the face of the new analysis, Svensmark insists that solar theories should not be dismissed. “If you look at temperatures in the troposphere, there is a remarkable correlation with solar activity,” he says. Lockwood insists that none of the tropospheric data show the trend that the solar theory would need.

Nir Shaviv, an astrophysicist at the Hebrew University of Jerusalem who has championed a Sun–climate link, argues that there may be a lag in Earth's reaction to the Sun because of the thermal inertia of the oceans.

But other climate researchers find the idea of a 'hidden' time lag unconvincing. “With each year, and with each new set of data that comes in, a time lag becomes ever more unlikely,” says Urs Neu, deputy head of ProClim-, the climate and global change forum of the Swiss Academy of Sciences in Bern.

On other timescales, however, Sun–climate links may remain worthy of study. “Climate change is a cocktail of many effects,” says Jasper Kirkby, a physicist at CERN, the European particle-physics laboratory near Geneva in Switzerland, who is leading an experiment aimed at simulating the effect of cosmic rays on clouds. “Past climate changes have clearly been associated with solar activity. Even if this is not the case now, it is still important to understand how solar variability affects climate.”

Ken Carslaw, an atmospheric scientist at the University of Leeds, UK, points out that solar effects might still be possible. They might have acted to cool the climate in recent decades, but been overwhelmed. If so, the climate could be more sensitive to greenhouse gases than is generally thought, and future temperature increases might be greater than expected if a countervailing solar effect comes to an end.