Throughout its ~4.5 billion year history, the Earth has experienced periods of warming and cooling. However, the current drastic increase in global temperatures is well outside of the Earth's cyclic norms, and evidence for human-caused global climate change is compelling. Paleoclimatology, the study of ancient climate conditions, provides ample evidence for human-caused global climate change by comparing recent conditions with those in the past. Past Periods of Warming and Cooling In the last 650,000 years alone, there have been seven cycles of glacial advance and retreat. Over 11,000 years ago, the most recent ice age abruptly ended-marking the beginning of our modern climate era, the Holocene geological epoch. Most historical climate changes are associated with small variations in earth's orbit that changed the amount of the solar radiation received by the Earth. Paleoclimatology Paleoclimatology is the study of climatic conditions and their causes and effects throughout the Earth's geologic past. Paleoclimatologists employ a variety of scientific methods to deduce the past conditions of Earth's climate and atmospheric system. Ice Core Data Ice core samples taken from thick mountain glaciers and polar ice sheets can reveal information about global climate change. Seasonal pauses in ice accumulation can lead to observable layers in ice cores. These layers can be used to establish a chronology, with the lowest layers being the oldest. Researchers can then analyze properties of the ice and the material trapped inside it (e.g., air bubbles and ratios of oxygen and hydrogen isotopes) to reconstruct the Earth's ancient climate. The European Project for Ice Coring in Antarctica analyzed volcanic ash trapped in ice cores from Antarctica to provide a detailed record of Holocene volcanic history. This record revealed 96 eruptions during the Holocene, with 33 of those eruptions occurring during the last 2,000 years. Dendroclimatology Scientists can also study past climate by examining annual tree rings-a field called dendroclimatology. Essentially, tree rings are wider under favorable conditions and narrower during harsh times. Tree rings can be accurately dated by matching rings from sample to sample-even samples from archaeological digs. Although dendroclimatology has limitations, tree rings are useful climate proxies that can be used to create high-resolution climate records for specific geographic locations.