Greenhouse Gases, Not Solar Activity, Cause of Global Warming

KATLENBURG-LINDAU, Germany, August 3, 2004 (ENS) - Solar activity affects the climate but plays only a minor role in the current global warming, a German-Finnish team of scientists has found.

Since the middle of the last century, the Sun has been in a phase of unusually high activity, shown by frequent occurrences of sunspots, gas eruptions, and radiation storms.

The influence of the Sun on the Earth was believed to be one cause of the global warming observed since 1900, along with the emission of the greenhouse gas, carbon dioxide, from the combustion of coal, gas, and oil.

But Professor Sami Solanki, solar physicist and director at the Max Planck Institute for Solar System Research, is not convinced that the increased activity of the Sun is responsible for global warming.

Solar physicist Dr. Sami Solanki is director of the Max Planck Institute for Solar System Research (Photo courtesy MPS)

"Just how large this role is, must still be investigated," he says, "since, according to our latest knowledge on the variations of the solar magnetic field, the significant increase in the Earth’s temperature since 1980 is indeed to be ascribed to the greenhouse effect caused by carbon dioxide."

Solanki and other researchers at the Max Planck Institute for Solar System Research (MPS) and at the University of Oulu in Finland reconstructed solar activity based on sunspot frequency since 850 AD.

They found that since 1940 the mean sunspot number is higher than it has ever been in the last thousand years and 2.5 times higher than the long term average, as they report in the scientific journal, "Physical Review Letters."

Then they combined historical sunspot records with measurements of the frequency of radioactive isotopes in ice cores from Greenland and the Antarctic.

In addition, the MPS scientists took the measured and calculated variations in the solar brightness over the last 150 years and compared them to the temperature of the Earth.

"Although the changes in the two values tend to follow each other for roughly the first 120 years, the Earth’s temperature has risen dramatically in the last 30 years while the solar brightness has not appreciably increased in this time," they said.

The Sun affects Earth's climate through several physical processes. For one thing, the Sun's total radiation, particularly that in the ultraviolet range, varies with solar activity.

The Sun as it appeared on August 2, 2004 (Photo courtesy European Space Agency)

On the other hand, the cosmic ray intensity entering the Earth’s atmosphere varies opposite to the solar activity, since the cosmic ray particles are deflected by the Sun’s magnetic field to a greater or lesser degree.

A model proposed by Danish researchers that has attracted much attention from solar and climate scientists says that the ions produced by cosmic rays act as condensation nuclei for larger suspension particles and so contribute to cloud formation.

With increased solar activity, and stronger magnetic fields, the Danish model shows, the cosmic ray intensity decreases, and with it the amount of cloud coverage, resulting in a rise of temperatures on the Earth. Conversely, a reduction in solar activity produces lower temperatures.

To check this idea, Dr. Solanki and fellow MPS scientist Natalie Krivova calculated the Sun’s main parameters affecting climate for the last 150 years using current measurements and the newest models.

The calculated the total radiation, the ultraviolet output, and the Sun’s magnetic field, which modulates the cosmic ray intensity.

They came to the conclusion that the variations on the Sun run parallel to climate changes for most of that time, indicating that the Sun has indeed influenced the climate in the past. Just how large this influence is, is subject to further investigation.

However, they said, "since about 1980, while the total solar radiation, its ultraviolet component, and the cosmic ray intensity all exhibit the 11-year solar periodicity, there has otherwise been no significant increase in their values. In contrast, the Earth has warmed up considerably within this time period. This means that the Sun is not the cause of the present global warming."