February 2004
New insights
into solar output
CGD’s Caspar Ammann has found that changes in the Sun’s output over the past millennium may be lower than previously thought. This finding comes as a growing body of evidence shows that natural variations in solar energy and volcanic emissions were major players for climatic fluctuations in earlier times but cannot explain the climate changes of the last few decades of the 20th century.
At a press briefing at the American Geophysical Union in December, Caspar used improved climate reconstructions and advanced climate model results to share new insights into the impact of solar output on Earth’s climate.
Scientists have long suspected that changes in the Sun’s activity played an important role in past climatic variations. However, useful measurements of solar energy are limited to the last 25 years of satellite data. To estimate solar energy further in the past, scientists have used tentative connections between the measured solar activity and sunspots or the production of certain particles in the Earth’s atmosphere (such as carbon-14 and beryllium-10).
When Caspar applied these solar estimates in simulations by the NCAR coupled Climate System Model (CSM), he found that the climate system indeed contained a clearly detectable signal from the Sun. However, smaller rather than larger background trends in the Sun’s emitted energy generated climate variations that are in better agreement with the long-term climate history, as obtained from proxy climate records, such as tree ring data. Although some research had suggested that the Sun’s output during the Little Ice Age (1300–1850 AD) declined by as much as 10 watts per square meter (W/m2), Caspar’s work indicates that such a decline would lead to a Little Ice Age much cooler than observed. In the CSM, solar irradiance decreases between 1.4 and 4 W/m2 were large enough to contribute substantially to the climate variations.
Reconstructions of total solar irradiance (solar output): Sunspot-based reconstructions containing either no background trend or a modest one (Judith Lean of the Naval Research Laboratory and colleagues)
are shown in black, and 10Beryllium-based reconstructions (Edouard Bard of European Center for Research and Teaching in the Geosciences and colleagues, updated) are shown in blue. In long Community System Model runs, only Beryllium-based reconstructions have been used so far. The smaller amplitude cases generated results that are in better agreement with the proxy climate record while the large solar variation led to an overly cold Little Ice Age (1300-1850). (Courtesy Caspar Ammann).
Because the climate model was able to reproduce the climate of the past millennium to a reasonable degree, this research also shows that the influence of the Sun on Earth’s climate shifted from being one of the major natural regulators of past climate to a much more minor role during the predominantly human-modified climate of the last few decades. Although some skeptics of climate change research have suggested the recent warming trend is merely part of a natural cycle in which solar output is rebounding from lower levels of a couple of centuries ago, models indicate that natural variation alone cannot account for the increasing temperatures.
“When we subtracted human activities in the form of increasing greenhouse gases in the 20th century, we found the greatest discrepancies between the model results and observations over the past millennium increased dramatically at the end of the century,” Caspar explained at the briefing.
•David Hosansky and Anatta
Also in this issue...
Tom Windham soars to Washington
CG auditorium draws large crowds
