It is often suggested that lower total solar irradiance (TSI) due to a decline in solar activity (less sunspots) was responsible for the Little Ice Age, a period of colder weather conditions from the end of the Middle Ages to the start of the 20th century.
It is one of two solar myths that new research debunks simultaneously.
During the climate minimum of the 17th century, that saw 1-2 degrees lower temperatures in Western Europe, there were indeed fewer sunspots – as scientists can measure from magnetic records. But during this Maunder Minimum the TSI was of equal strength to the TSI during the exceptional lull in solar activity of recent years, that reached its minimum around 2008-2009.
Solar hypothesists should have shown cooling
This means the 2000-2009 hottest decade on record has nothing to do with solar variations. It also means we may have to consider the 17th century English and Dutch harsh winters as local climate phenomena (as southern hemisphere temperature did not rise) and that especially variations in the thermohaline circulation deserve more attention in explaining these.
Steady irradiance during minimum
The researchers argue there is a minimum solar activity, even during periods where sunspots are totally absent.
“The implied marginally significant decrease in TSI during the least active phases of the Maunder Minimum by 140 to 360 ppm relative to 1996 suggests that drivers other than TSI dominate Earth’s long-term climate change,” write Dutch astronomer Karel Schrijver from Lockheed Martin Advanced Technology Center and colleagues from three other space institutions in the US in their recent publication in Geophysical Research Letters.
Sol Iustitia Illustra Nos
Apart from the theoretical science base there is additional evidence in atmospheric measurements to refute the solar hypothesis as warming force behind the current climate change: record low temperatures in the stratosphere. Stratospheric cooling is in line with increased tropospheric concentrations of greenhouse gases.
© Rolf Schuttenhelm | www.bitsofscience.org