Declining solar activity linked to recent warming : Nature.com

An analysis of satellite data challenges the intuitive idea that decreasing solar activity cools Earth, and vice versa. In fact, solar forcing of Earth's surface climate seems to work the opposite way around — at least during the current Sun cycle.

Joanna Haigh, an atmospheric physicist at Imperial College London, and her colleagues analysed daily measurements of the spectral composition of sunlight made between 2004 and 2007 by NASA's Solar Radiation and Climate Experiment (SORCE) satellite. They found that the amount of visible light reaching Earth increased as the Sun's activity declined — warming the Earth's surface. Their unexpected findings are published today in Nature¹.

The study period covers the declining phase of the current solar cycle. Solar activity, which in the current cycle peaked around 2001, reached a pronounced minimum in late 2009 during which no sunspots were observed for an unusually long period.

Sunspots, dark areas of reduced surface temperature on the Sun caused by intense magnetic activity, are the best-known visible manifestation of the 11-year solar cycle. They have been regularly observed and recorded since the dawn of modern astronomy in the seventeenth century. But measurements of the wavelengths of solar radiation have until now been scant.

Radiation leak

Haigh's team compared SORCE's solar spectrum data with wavelengths predicted by a standard empirical model based mainly on sunspot numbers and area, and noticed unexpected differences. The amount of ultraviolet radiation in the spectrum was four to six times smaller than that predicted by the empirical model, but an increase in radiation in the visible wavelength, which warms the Earth's surface, compensated for the decrease.

Contrary to expectations, the net amount of solar energy reaching Earth's troposphere — the lowest part of the atmosphere — seems to have been larger in 2007 than in 2004, despite the decline in solar activity over that period.

The spectral changes seem to have altered the distribution of ozone molecules above the troposphere. In a model simulation, ozone abundance declined below an altitude of 45 kilometres altitude in the period 2004–07, and increased further up in the atmosphere.

The modelled changes are consistent with space-based measurements of ozone during the same period.
"We're seeing — albeit limited to a very short period — a very interesting change in solar irradiation with remarkably similar changes in ozone," says Haigh. "It might be a coincidence, and it does require verification, but our findings could be too important to not publish them now."


More: Read the paper at Nature.com.........Declining solar activity linked to recent warming