Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeBox 4.3. Past Mechanisms for Freshwater Forcing of the AMOCIce sheets represent the largest readily exchangeable reservoir of freshwater on Earth. Given the proximity of modernand former ice sheets to critical sites of intermediate and deep water formation (Fig. 4.1), variations in their freshwaterfluxes thus have the potential to induce changes in the AMOC. In this regard, the paleorecord has suggested four specificmechanisms by which ice sheets may rapidly discharge freshwater to the surrounding oceans and cause abrupt changes inthe AMOC: (1) Heinrich events, (2) meltwater pulses, (3) routing events, and (4) floods.1. Heinrich events are generally thought to represent an ice-sheet instability resulting in abrupt release of icebergsthat triggers a large reduction in the AMOC. Paleoclimate records, however, indicate that Heinrich events occurafter the AMOC has slowed down or largely collapsed. An alternative explanation is that Heinrich events are triggeredby an ice-shelf collapse induced by subsurface oceanic warming that develops when the AMOC collapses,with the resulting flux of icebergs acting to sustain the reduced AMOC.2. The ~20-m sea-level rise ~14,500 years ago, commonly referred to as meltwater pulse (MWP) 1A, indicates anextraordinary episode of ice-sheet collapse, with an associated freshwater flux to the ocean of ~0.5 Sv over severalhundred years (see Chapter 2). Nevertheless, the timing, source, and the effect on climate of MWP-1A remainunclear. In one scenario, the event was triggered by an abrupt warming (start of the Bølling warm interval) inthe North Atlantic region, causing widespread melting of Northern Hemisphere ice sheets. Although this eventrepresents the largest freshwater forcing yet identified from paleo-sea-level records, there was little responseby the AMOC, leading to the conclusion that the meltwater entered the ocean as a sediment-laden, very densebottom flow, thus reducing its impact on the AMOC. In another scenario, MWP-1A largely originated from theAntarctic Ice Sheet, possibly in response to the prolonged interval of warming in the Southern Hemispherethat preceded the event. In this case, climate model simulations indicate that the freshwater perturbation in theSouthern Ocean may have triggered the resumption of the AMOC that caused the Bølling warm interval.3. The most well-known hypothesis for a routing event involves retreat of the Laurentide Ice Sheet (LIS) that redirectedcontinental runoff from the Mississippi to the St. Lawrence River, triggering the Younger Dryas cold interval.There is clear paleoceanographic evidence for routing of freshwater away from the Mississippi River at the startof the Younger Dryas, and recent paleoceanographic evidence now clearly shows a large salinity decrease in theSt. Lawrence estuary at the start of the Younger Dryas associated with an increased freshwater flux derived fromwestern Canada.4. The most well-known flood is the final sudden drainage of glacial Lake Agassiz that is generally considered to bethe cause of an abrupt climate change ~8400 years ago. For this event, the freshwater forcing was likely large butshort; the best current estimate suggests a freshwater flux of 4–9 Sv over 0.5 year. This event was unique to thelast stages of the LIS, however, and similar such events should only be expected in association with similar suchice-sheet configurations. Other floods have been inferred at other times, but they would have been much smaller(~0.3 Sv in 1 year), and model simulations suggest they would have had a negligible impact on the AMOC.the near-collapse earlier in the deglaciationat ~17.5 ka, for example, Pa/Th ratios suggestonly a partial reduction in the AMOC duringthe Younger Dryas (Fig. 4.9). Sediment grainsize (Manighetti and McCave, 1995) alsoshows evidence for reduced NADW input intothe North Atlantic during the Younger Dryasevent (Fig. 4.9). Radiocarbon concentration inthe atmosphere rises at the start of the YoungerDryas, which is thought to reflect decreasedocean uptake due to a slowdown of the AMOC(Hughen et al., 2000). Radiocarbon-depletedAABW replaced radiocarbon-enriched NADWbelow ~2500 m, suggesting a shoaling ofNADW coincident with a reduction of theAMOC (Keigwin, 2004). The δ 13 C valuesalso suggest a return to the LGM water massconfiguration (Sarnthein et al., 1994; Keigwin,2004), as do other nutrient tracers (Boyle andKeigwin, 1987) and the Nd isotope water masstracer (Piotrowski et al., 2005).141