You had very few sunspots [markings on the face of the sun that indicate heightened solar activity] between 1650 and 1715, and for example, in Sweden in 1696, it caused the harvest to go wrong. (discovermagazine.com)
Long slow cycles with fewer sunspots result in cooling. (newclimatemodel.com)
Fewer sunspots meant fewer faculae, so less heat from the Sun. (slate.com)