This study will address the likelihood of various physical components of the Earth system to undergo major and rapid changes (i.e., abrupt climate change) and, as time allows, examine some of the most important potential associated impacts and risks. This study will explore how to monitor climate change for warnings of abrupt changes and emerging impacts. The study will summarize the current state of scientific understanding on questions such as:

1. What is known about the likelihood and timing of abrupt changes in the climate system over decadal timescales? Are any of the phenomena considered by the committee currently embodied in computational climate models? The committee could consider relevant physical and biological phenomena such as:

• large, abrupt changes in ocean circulation and regional climate;

• reduced ice in the Arctic Ocean and permafrost regions;

• large-scale clathrate release;

• changes in ice sheets;

• large, rapid global sea-level rise;

• growing frequency and length of heat waves and droughts;

• effects on biological systems of permafrost/ground thawing (carbon cycle effects);

• phase changes such as cloud formation processes; and

• changes in weather patterns, such as changes in snowpack, increased frequency and magnitude of heavy rainfall events and floods, or changes in monsoon patterns and modes of interannual or decadal variability.

2. For the abrupt climate changes and resulting impacts identified by the committee, what are the prospects for developing an early warning system and at what lead time scales? What can be monitored to provide such warnings? What monitoring capabilities are already in place? The committee will consider monitoring capabilities that include both direct observations and the use of models in conjunction with observations.

3. What are the gaps in our scientific understanding and current monitoring capabilities? What are the highest priority needs for future research directions and monitoring capabilities to fill those gaps?

Chapter 4 examines the way forward in terms of both research on abrupt changes and their impacts, and monitoring to detect and potentially predict abrupt changes. This chapter examines priorities and capabilities for addressing research knowledge gaps. It also addresses the question of what to monitor to observe that an abrupt change is coming, and how to identify tipping points in various systems.

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