It has long been estabished that gas and fine ash from large equatorial explosive eruptions can spread globally, and that the sulphuric acid that is consequently produced in the stratosphere can cause a small, but statistically significant, cooling of global temperatures,. Central to revealing the ancient volcano-climate connection have been studies linking single eruptions to features of climate-proxy records such as found in ice-core and tree-ring chronologies. Such records also suggest that the known inventory of eruptions is incomplete, and that the climatic significance of unreported or poorly understood eruptions remains to be revealed. The AD 1600 eruption of Huaynaputina, in southern Peru, has been speculated to be one of the largest eruptions of the past 500 years; acidity spikes from Greenland and Antarctica ice, tree-ring chronologies, along with records of atmospheric perturbations in early seventeenth-century Europe and China,, implicate an eruption of similar or greater magnitude than that of Krakatau in 1883. Here we use tephra deposits to estimate the volume of the AD 1600 Huaynaputina eruption, revealing that it was indeed one of the largest eruptions in historic times. The chemical characteristics of the glass from juvenile tephra allow a firm cause-effect link to be established with glass from the Antarctic ice, and thus improve on estimates of the stratospheric loading of the eruption.