Book 2.indb - US Climate Change Science Program

Abrupt Climate ChangeResults similar to those of Wood et al. (1999)were found by Hu et al. (2004), although Hu etal. (2004) also noted a significant increase inGreenland-Iceland-Norwegian (GIN) Sea convectionas a result of enhanced inflow of salineNorth Atlantic water, and reduced outflow ofsea ice from the Arctic. Some coupled models,on the other hand, found significant reductionsin convection in the GIN Sea in response to increasingatmospheric greenhouse gases (Bryanet al., 2006; Stouffer et al., 2006). A cessationof LSW formation by 2030 was also found inhigh-resolution ocean model simulations of theAtlantic Ocean driven by surface fluxes fromtwo coupled atmosphere-ocean climate models(Schweckendiek and Willebrand, 2005). Cottet-Puinel et al. (2004) obtained similar results toWood et al. (1999) concerning the transientcessation of LSW formation and further showedthat LSW formation eventually reestablishedupon stabilization of anthropogenic greenhousegas levels. The same model experiments ofWood et al. (1999) suggest that the fresheningNorth Atlantic surface waters presentlyobserved (Curry et al., 2003) is associated witha transient increase of the AMOC (Wu et al.,2004). Such an increase would be consistentwith findings of Latif et al. (2006), who arguedthat their analysis of ocean observations andmodel simulations supported the notion of aslight AMOC strengthening since the 1980s.The best estimate of sea level rise from 1993 to2003 associated with mass loss from the Greenlandice sheet is 0.21 ± 0.07 mm yr –1 (Bindoffet al., 2007). This converts to only 0.0015 to0.0029 Sv of freshwater forcing, an amount thatis too small to affect the AMOC in models (seeWeaver and Hillaire-Marcel, 2004a; Jungclauset al., 2006). Recently, Velicogna and Wahr(2006) analyzed the Gravity Recovery andClimate Experiment (GRACE) satellite datato infer an acceleration of Greenland ice lossfrom April 2002 to April 2006 correspondingto 0.5 ± 0.1 mm/yr of global sea level rise. Theequivalent 0.004–0.006 Sv of freshwater forcingis, once more, too small to affect the AMOCin models. Stouffer et al. (2006) undertook anintercomparison of 14 coupled models subjectto a 0.1-Sv freshwater perturbation (17 timesthe upper estimate from GRACE data) appliedfor 100 years to the northern North AtlanticOcean. A simple scaling analysis (conductedby the authors of this assessment report) showsthat if over a 10-year period Arctic sea ice wereto completely melt away in all seasons, NorthAtlantic freshwater input would be about halfthis rate (see Box 4.1 for a discussion of observedand projected Arctic sea ice change). Inall cases, the models exhibited a weakening ofthe AMOC (by a multimodel mean of 30% after100 years), and none of the models simulated ashutdown. Ridley et al. (2005) elevated greenhousegas levels to four times preindustrialvalues and retained them fixed thereafter toinvestigate the evolution of the Greenland Icesheet in their coupled model. They found a peakmelting rate of about 0.1 Sv, which occurredearly in the simulation, and noted that thisperturbation had little effect on the AMOC.Jungclaus et al. (2006) independently applied0.09 Sv freshwater forcing along the boundaryBox 4.4. Would a Collapse of the AMOC Lead to Cooling of Europe and North America?One of the motivations behind the study of abrupt change in the AMOC is its potential influence on the climates of NorthAmerica and western Europe. Some reports, particularly in the media, have suggested that a shutdown of the AMOC inresponse to global warming could plunge western Europe and even North America into conditions much colder than ourcurrent climate. On the basis of our current understanding of the climate system, such a scenario appears very unlikely.On the multidecadal to century time scale, it is very likely that Europe and North America will warm in response toincreasing greenhouse gases (although natural variability and regional shifts could lead to periods of decadal-scale coolingin some regions). A significant weakening of the AMOC in response to global warming would moderate that long-termwarming trend. If a complete shutdown of the AMOC were to occur (viewed as very unlikely, as described in this assessment),the reduced ocean heat transport could lead to a net cooling of the ocean by several degrees in parts of the NorthAtlantic, and possibly 1 to 2 degrees Celsius over portions of extreme western and northwestern Europe. However, evenin such an extreme (and very unlikely) scenario, a multidecadal to century-scale warming trend in response to increasinggreenhouse gases would still be anticipated over most of North America, eastern and southern Europe, and Asia.159