The Little Ice Age, following the historically warm temperatures of the Medieval Warm Period, which lasted from about AD 950 to 1250, has been attributed to natural cycles in solar activity, particularly sunspots. A period of sharply lower sunspot activity known as the Wolf Minimum began in 1280 and persisted for 70 years until 1350. That was followed by a period of even lower sunspot activity that lasted 90 years from 1460 to 1550 known as the Sporer Minimum. During the period 1645 to 1715, the low point of the Little Ice Age, the number of sunspots declined to zero for the entire time. This is known as the Maunder Minimum, named after English astronomer Walter Maunder. That was followed by the Dalton Minimum from 1790 to 1830, another period of well below normal sunspot activity.