Abrupt Climate Change
Earth's climate has changed many times and in many different ways over geologic history. These changes have most often played out over many thousands of years.
Today, however, human activity—primarily in the form of increasing concentrations of greenhouse gases in the atmosphere—is putting slow, inexorable pressure on the planetary system that governs Earth's climate. This is raising concerns that the climate could respond in abrupt and unexpected ways, shifting temperature or precipitation patterns in ways that may make it difficult for human society to adapt.