Why the Sun Went Quiet for 70 Years

Records were kept regularly by different observers in different countries - by then, telescopes were already standard tools. So the dip was not due to a lack of observations, but to a real absence of sunspots.

There is indirect confirmation, too. The levels of certain isotopes in tree rings and ice cores depend on solar activity - and in those same years, they show the same dip.

Eruptions have a rapid, powerful effect: particles thrown into the atmosphere reflect light and cool the surface for years. Several major eruptions occurred during the same period, making their contributions difficult to separate.

There is also a physical challenge in explaining it through the Sun. When the star is quiet, it is not so much its overall brightness that changes as the ultraviolet part of its radiation and the incoming flow of particles; how this affects atmospheric circulation is still being studied.

Another line of reasoning links the cooling to people themselves. After epidemics and wars, some farmland in the Americas was reclaimed by forests, which may have changed carbon dioxide levels - the hypothesis is still debated but has not been accepted as the main explanation.

The cooling itself was noticeable, but not catastrophic: globally, the average change was a fraction of a degree, with more pronounced changes in some regions of Europe, where chronicles of freezing rivers survived.

It is also important that this is not an argument against today's warming. Changes in solar activity over the past few decades have been measured directly by satellites and do not explain the observed rise in temperature.