USGS/NOAA Abrupt Climate Change: Synthesis and Assessment Report

(From Executive Summary)

For this Synthesis and Assessment Report, abrupt climate change is defined as:

A large-scale change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades, and causes substantial disruptions in human and natural systems.

This report considers progress in understanding four types of abrupt change in the paleoclimatic record that stand out as being so rapid and large in their impact that if they were to recur, they pose clear risks to society in terms of our ability to adapt: (1) rapid change in glaciers, ice sheets and hence sea level; (2) widespread and sustained changes to the hydrologic cycle; (3) abrupt change in the northward flow of warm, salty water in the upper layers of the Atlantic Ocean associated with the Atlantic meridional overturning circulation (AMOC); and (4) rapid release to the atmosphere of methane trapped in permafrost and on continental margins.

Based on an assessment of the published scientific literature, the primary conclusions presented in this report are:

1. The regions likely to experience future rapid changes in ice volume are those where ice is grounded well below sea level such as the West Antarctic Ice Sheet or large glaciers in Greenland like the Jakobshavn Isbrae that flow into the sea through a deep channel reaching far inland. Inclusion of these processes in models will likely lead to sea-level projections for the end of the 21st century that substantially exceed the projections presented in the IPCC AR4 report (0.28