Careful and vigilant monitoring, combined with a constantly improving scientific understanding of the climate system, would help society anticipate major changes before they occur. With this goal in mind, the report’s authoring committee summarized the state of knowledge about potential abrupt changes in Table S.1. This table includes potential abrupt changes to the ocean, atmosphere, ecosystems, and highlatitude regions that are judged to meet the above criteria. For each abrupt change, the Committee examined the available evidence of potential impact and likelihood. Some abrupt changes are likely to occur within this century—making these changes of most concern for near-term societal decision making and a priority for research. In other cases, there are still large scientific uncertainties about the likelihood of a potential abrupt change, highlighting the need for further research in these areas. Finally, recent data has revealed that some abrupt changes, widely discussed in the scientific literature because they were once identified as possible threats, are no longer considered likely during this century. This illustrates how focused efforts to study critical climate change mechanisms can also assuage societal concern about potential abrupt changes, in addition to identifying them.
Abrupt Changes Already Underway
The abrupt changes that are already underway are of most immediate concern for societal decisions. These include the disappearance of late-summer Arctic sea ice and increases in extinction rates of marine and terrestrial species.
Disappearance of Late-Summer Arctic Sea Ice
Recent dramatic changes in the extent and thickness of the ice that covers the Arctic sea have been well documented. Satellite data for late summer (September) sea ice extent show natural variability around a clearly declining long-term trend (Figure S.1). This rapid reduction in Arctic sea ice already qualifies as an abrupt change with substantial decreases in ice extent occurring within the past several decades. Projections from climate models suggest that ice loss will continue in the future, with the full disappearance of late-summer Arctic sea ice possible in the coming decades.
The impacts of rapid decreases in Arctic sea ice are likely to be considerable. More open water conditions during summer would have potentially large and irreversible effects on various components of the Arctic ecosystem, including disruptions in the marine food web, shifts in the habitats of some marine mammals, and erosion of vulnerable coastlines. Because the Arctic region interacts with the large-scale circu-