For the Vikings, a series of abnormally cold winters in the late 1300s spelled doom. Annual layers of snowfall in ice cores can be counted as easily as tree rings, allowing precise dating of events such as volcanic eruptions. Distinct annual layers stand out because, in snow that falls in summer, crystals are larger and acidity higher than in winter snow. In some cases, scientists can even tell seasons apart, by using a laser to measure the concentration of dust particles. In this photograph of an ice core drilled in the Kunlun Mountains of western China, the thick, lighter bands indicate heavy snowfall during the monsoon season in the year 1167 AD, while the thinner, darker strips show layers of dust blown into the snowfield during the dry season.
Temperature has yo-yoed over the ages as wildly as it does through any single year.
Light isotopes have regular hydrogen and oxygen, while heavy isotopes have either hydrogen with an extra neutron or oxygen with one or two additional neutrons. Since heavy isotopes precipitate out of the atmosphere more quickly than light ones, scientists can measure the ratio between the two isotopes to estimate the temperature at any given time. Approximately 73,000 years ago, an Indonesian volcano known as Toba erupted with enough force to send more than 600 cubic miles of volcanic material into the atmosphere. Detected on this graph, which displays volcanic sulfate levels between 20,000 and 110,000 years ago, Toba was the largest eruption of the past 500,000 years. Such violent, so-called caldera eruptions can drastically alter global climate, by spewing so much ash and sulfur compounds into the atmosphere as to block sunlight and lower temperatures worldwide.