2 Abrupt Changes of Primary Concern | Abrupt Impacts of Climate Change: Anticipating Surprises |The National Academies Press

CHAPTER TWO

Abrupt Changes of Primary Concern

The following section describes potential abrupt climate changes that are of primary concern, either because they are currently believed to be the most likely and the most impactful, because they are predicted to potentially cause severe impacts but with uncertain likelihood, or because they are considered to be unlikely to occur but have been widely discussed in the literature or media. As such, the Committee did not attempt to create a comprehensive catalog of potential abrupt changes. As described in the Introduction, this section examines both abrupt climate changes in the physical climate system itself and abrupt climate impacts in physical, biological, or human systems that are triggered by a steadily changing climate.

ABRUPT CHANGES IN THE OCEAN

The Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC)—characterized by warm surface waters flowing northward and cold deep waters flowing southward throughout the Atlantic basin—is defined as the zonal integral of the northward mass flux at a particular latitude. The deep limb of this overturning circulation carries waters that are formed via convection in the Nordic and Labrador Seas (Figure 2.1). Collectively, these waters constitute North Atlantic Deep Water, which is exported to the global ocean at depths between about 1000 and 4000 m. The southward-flowing deep limb of the overturning circulation is compensated by an upper limb of northward-flowing surface waters, which head to the Nordic and Labrador Seas to replenish the regions of convection. Together, the upper and lower limbs of the overturning circulation produce a poleward flux of heat that has strong global and regional impacts. The AMOC also plays an important role in the transport of carbon in the Atlantic. Thus, variability in the AMOC’s strength is of much interest, as a diminishment or strengthening would impact the ocean’s effectiveness as a heat and carbon reservoir.

Examinations of paleoclimate temperatures and other variables recorded in both North Atlantic ocean sediments and Greenland ice cores (e.g., Lehman and Keigwin, 1992; Alley et al., 1993; Taylor et al., 1993) have led to suggestions that the AMOC