(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 ± 0.10 m to 0.42 ± 0.16 m rise).

2. Climate model scenarios of future hydro-climatic change over North America and the global subtropics indicate that subtropical aridity will likely intensify and persist due to future greenhouse warming. This drying is likely to extend poleward into the American West, thus increasing the likelihood of severe and persistent drought there in the future. If the model results are correct then this drying is likely to have already begun.

3. The AMOC (Atlantic Meridional Overturning Circulation) is the northward flow of warm, salty water in the upper layers of the Atlantic, and the southward flow of colder water in the deep Atlantic. It plays an important role in the oceanic transport of heat from low to high latitudes. It is very likely that the strength of the AMOC will decrease over the course of the 21st century in response to increasing greenhouse gases, with a best estimate decrease of 25-30%.

4. A dramatic abrupt release of methane (CH4) to the atmosphere appears very unlikely, but it is very likely that climate change will accelerate the pace of persistent emissions from both hydrate sources and wetlands. Current models suggest that a doubling of CH4 emissions could be realized fairly easily. However, since these models do not realistically represent all the processes thought to be relevant to future northern high-latitude CH4 emissions, much larger (or smaller) increases cannot be discounted. Acceleration of release from hydrate reservoirs is likely, but its magnitude is difficult to estimate.

Full Report