Solar activity & climate: is the sun causing global warming?

The skeptic argument...

Over the past few hundred years, there has been a steady increase in the numbers of sunspots, just at the time when the Earth has been getting warmer. The data suggests changing solar activity is influencing in some way the global climate causing the world to get warmer (BBC).

What the science says...

Solar activity has shown little to no long term trend since the 1950's. Consequently, any correlation between sun and climate ended in the 1970's when the modern global warming trend began.

As supplier of almost all the energy in Earth's climate, the sun certainly has a strong influence on climate change. Consequently there have been many studies examining the link between solar variations and global temperatures.

The correlation between solar activity and temperature

The most commonly cited study by skeptics is a study by scientists from Finland and Germany that finds the sun has been more active in the last 60 years than anytime in the past 1150 years (Usoskin 2005). They also found temperatures closely correlate to solar activity.

However, a crucial finding of the study was the correlation between solar activity and temperature ended around 1975. At that point, temperatures rose while solar activity stayed level. This led them to conclude "during these last 30 years the solar total irradiance, solar UV irradiance and cosmic ray flux has not shown any significant secular trend, so that at least this most recent warming episode must have another source."

In other words, the study most quoted by skeptics actually concluded the sun can't be causing global warming. Ironically, the evidence that establishes the sun's close correlation with the Earth's temperature in the past also establishes it's blamelessness for global warming today.

Other studies on solar influence on climate

This conclusion is confirmed by many studies quantifying the amount of solar influence in recent global warming:

• Erlykin 2009: "We deduce that the maximum recent increase in the mean surface temperature of the Earth which can be ascribed to solar activity is 14% of the observed global warming"
• Benestad 2009: "Our analysis shows that the most likely contribution from solar forcing a global warming is 7 ± 1% for the 20th century and is negligible for warming since 1980."
• Lockwood 2008: "It is shown that the contribution of solar variability to the temperature trend since 1987 is small and downward; the best estimate is −1.3% and the 2σ confidence level sets the uncertainty range of −0.7 to −1.9%."
• Lockwood 2008: "The conclusions of our previous paper, that solar forcing has declined over the past 20 years while surface air temperatures have continued to rise, are shown to apply for the full range of potential time constants for the climate response to the variations in the solar forcings."
• Ammann 2007: "Although solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years, the impacts of greenhouse gases have dominated since the second half of the last century."
• Lockwood 2007: "The observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability, whichever of the mechanism is invoked and no matter how much the solar variation is amplified."
• Foukal 2006 concludes "The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years."
• Scafetta 2006 says "since 1975 global warming has occurred much faster than could be reasonably expected from the sun alone."
• Usoskin 2005 conclude "during these last 30 years the solar total irradiance, solar UV irradiance and cosmic ray flux has not shown any significant secular trend, so that at least this most recent warming episode must have another source."
• Solanki 2004 reconstructs 11,400 years of sunspot numbers using radiocarbon concentrations, finding "solar variability is unlikely to have been the dominant cause of the strong warming during the past three decades".
• Haigh 2003 says "Observational data suggest that the Sun has influenced temperatures on decadal, centennial and millennial time-scales, but radiative forcing considerations and the results of energy-balance models and general circulation models suggest that the warming during the latter part of the 20th century cannot be ascribed entirely to solar effects."
• Stott 2003 increased climate model sensitivity to solar forcing and still found "most warming over the last 50 yr is likely to have been caused by increases in greenhouse gases."
• Solanki 2003 concludes "the Sun has contributed less than 30% of the global warming since 1970".
• Lean 1999 concludes "it is unlikely that Sun–climate relationships can account for much of the warming since 1970".
• Waple 1999 finds "little evidence to suggest that changes in irradiance are having a large impact on the current warming trend."
• Frolich 1998 concludes "solar radiative output trends contributed little of the 0.2°C increase in the global mean surface temperature in the past decade"

Ocean Thermal Inertia

Usoskin 2005 also found that over 1150 years, temperature lagged solar activity by 10 years. Due to ocean thermal inertia, it takes the climate a decade to catch up to long term changes in solar activity. This is exactly what's observed in the 20th century - in the early decades, solar activity rose sharply with temperature lagging a decade behind. When solar activity leveled out in the 40's, so too did global temperatures.