Researchers from the international Past Global Changes (PAGES) project write in the journal Nature Geoscience that they hold identified an unprecedented,long-lasting cooling in the northern hemisphere 1500 years ago. The drop in temperature immediately followed three large volcanic eruptions in quick succession in the years 536, 540 and 547 AD (also known as the Common Era CE). Volcanoes can cause climate cooling by ejecting large volumes of small particles - sulfate aerosols - that enter the atmosphere blocking sunlight.
Source: phys.org