Short answers, pulled from the story.
Abrupt climate change occurs when the climate system transitions at a rate determined by the climate system's own energy balance, faster than the external forcing causing it. In terms of impacts, it is defined as change so rapid and unexpected that human or natural systems cannot adapt in time. These two definitions are complementary: one explains the mechanism, the other explains why the subject attracts so much scientific research.
Past abrupt climate change events include the Younger Dryas, which ended with a warming of plus ten degrees Celsius in Greenland within just a few years; the Paleocene-Eocene Thermal Maximum, timed at 55 million years ago; the Permian-Triassic Extinction Event, in which up to 95 percent of all species were lost; and the Carboniferous Rainforest Collapse 300 million years ago. Scientists have also identified about 25 Dansgaard-Oeschger cycles in ice core records spanning the last 100,000 years.
Climate models project that under ongoing greenhouse gas emissions, Earth's near-surface temperature could depart from the range of variability observed in the last 150 years as early as 2047. That departure would represent a crossing into territory outside recent historical norms.
Feedback loops such as ice-albedo feedback, soil carbon release, and the burning of forests can amplify warming events once they begin, turning a moderate shift into a rapid one. The probability of abrupt change from any single feedback is relatively low, but it increases with higher magnitudes of warming, faster rates of warming, and warming sustained over longer periods.
RESET, short for Response of Humans to Abrupt Environmental Transitions, was a five-year study completed in 2013 and led by the Oxford School of Archaeology alongside Royal Holloway University of London, the Oxford University Museum of Natural History, and the National Oceanography Centre Southampton. It examined 130,000 years of palaeoenvironmental, archaeological, oceanographic, and volcanic geological records across continents to test whether major human development shifts coincide with abrupt climate changes.
A 2017 study found that around 17,700 years ago, stratospheric ozone depletion triggered atmospheric circulation and hydroclimate changes similar to today's Antarctic ozone hole, accelerating Southern Hemisphere deglaciation. This event coincided with an estimated 192-year series of massive volcanic eruptions attributed to Mount Takahe in West Antarctica.