In addition to abrupt changes within the climate system itself, gradual climate changes can cross thresholds in both natural systems and human systems. For example, as air and water temperatures rise, some species, such as the mountain pika or some ocean corals, will no longer be able to survive in their current habitats and will be forced to relocate or rapidly adapt. Those populations that cannot do so quickly enough will be in danger of extinction. In addition, human infrastructure is built with certain expectations of useful life expectancy, but even gradual climate changes may trigger abrupt thresholds in their utility, such as rising sea levels surpassing sea walls or thawing permafrost destabilizing pipelines, buildings, and roads.
Climate is not the only stressor on the Earth system—other factors, including resource depletion and ever-growing human consumption and population, are exerting enormous pressure on nature’s and society’s resilience to sudden changes. Understanding the potential risks posed by both abrupt climate changes and the abrupt impacts resulting from gradual climate change is a crucial piece in advancing the ability of society to cope with changes in the Earth system. Better scientific understanding and improved ability to simulate the abrupt impacts of climate change would help researchers and policymakers with a comprehensive risk assessment. This report, sponsored by the US intelligence community, the National Oceanic and Atmospheric Administration, the National Science Foundation, and the National Academies, examines current knowledge about the likelihood and timing of potential abrupt changes, discusses the need for developing an abrupt change early warning system to help anticipate major changes before they occur, and identifies the gaps in the scientific understanding and monitoring capabilities (the full Statement of task can be found in Chapter 3).
This study differs from previous treatments of abrupt changes by discussing both the abrupt changes in the physical climate system (hereafter called “abrupt climate change”), as well as the abrupt changes in the physical, biological, or human systems that result from steadily changing aspects of the climate system (hereafter referred to as “abrupt climate impacts”). This report focuses on abrupt climate changes and abrupt climate impacts that have (or were thought to possibly have) the potential to severely affect the physical climate system, natural systems, or human systems, often affecting multiple interconnected areas of concern. The primary timescale of concern is years to decades. A key characteristic of these changes is that they can come faster than expected, planned, or budgeted for, forcing more reactive, rather than proactive, modes of behavior.