Solar fluctuations and climate change

The sun is the main source of energy to the Earth, yet the effect of variability in solar energy on the Earth’s climate is not clearly recognized. The Sun activity varies on a different length of natural cycles that extend from 11 to 200 years. The mechanisms behind these cycles are still unknown. A new study through cooperation between Uppsala University (department of Earth Sciences and the Tandem Laboratory) and Lund University has, for the first time, reconstructed solar activity during the last ice age (Adolphi et al., Nature Geoscience, 17 Aug. 2014 | DOI: 10.1038/NGEO2225). The research shows that the variability in the activity of the sun was rather constant during the last 20000 years.

During the ice age (20000 to 11000 years ago), North Europe was covered by a thick ice sheet and half of the North American continent was ice covered. The global sea level was more than 100 meters lower than today because of the water stored in the enormous ice caps. The new data indicate that the solar activity has influenced the climate in a similar way as during the last decades.

The solar contribution to the warming during the last 150 years is still discussed and it has been suggested that the decreasing solar activity during the last two decades contributed to less-than-expected global warming during this period. However, the mechanism behind the solar influence on climate is unclear since the variations of the energy output from the Sun have been measured to be quite small over a solar 11-yr cycle.

Alternative explanations have suggested that the solar UV radiation has an influence on the atmospheric circulation leading to, for example, colder winters in Northern Europe with low solar activity (as we have experienced in during the last solar minimum in 2009/2010). Interestingly, the same process leads to winter warming over Greenland with increased snowfall due to stronger southerly winds in that area.

Furthermore, the comparison of the solar proxies with climate proxies from the ice cores shows a similar linkage as can be seen during the past decades. Low solar activity is accompanied with more winter snow accumulation in Greenland and more storminess transporting sea spray to Central Greenland. The global impact of such a climate in the northern pole needs to be further modelled for the mid and tropical latitudes.

Finding these sun-climate linkages during two very different climate regimes, Holocene and glacial conditions, supports the hypothesis that the solar variability is a general driver for, at least, regional climate change. It indicates that indirect effects on atmospheric circulation might be more important than the direct effects via the variable energy output from the Sun.