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3 Processes that Cause Abrupt Climate Change
Pages 73-106

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From page 73...
... Two key components of the climate system are oceans and land ice. In addition, the atmospheric response is a crucial ingredient in the mix of mechanisms that might lead to abrupt climate change because the atmosphere knits together the behavior of the other components.
From page 74...
... Other major events of interest in the paleoclimatic records include decadal Holocene droughts, the dry-moist cycles of North Africa, the Little Ice Age, and the cooling event that occurred about 8,200 years ago (the "8.2K event". Abrupt shifts in the dominant modes of the modern climate are also being documented, and the active mechanisms in these shifts might be relevant to larger abrupt changes in the distant past or in the future.
From page 75...
... In addition to multiple equilibria in particular ranges of parameters, these models exhibit self-sustained oscillations and chaotic behavior. A system oscillates near one of two preferred centers; abrupt change occurs when the system switches from one mean of oscillation to the other.
From page 76...
... The high salinity of the Atlantic waters allows them to sink into the deep ocean when they cool, and warmer waters flowing along the surface then replace them. This yields a net heat transport into the high northern latitudes of the Atlantic and northward heat transport throughout the South Atlantic, carrying heat into the North Atlantic (Ganachaud and Wunsch, 2000; see also Plate 4b.)
From page 77...
... The implications of fluctuation in heat transport by the Atlantic THC have received particular attention, especially as a mediator of Younger Dryas and Dansgaard/Oeschger abrupt change. Deep water forms only in the North Atlantic and around the periphery of Antarctica, where extremely cold, dense waters occur.
From page 78...
... Furthermore, the density of surface waters in the North Atlantic is not determined by purely local processes, in as much as the North Atlantic salinity is affected by mixing with transported subtropical Atlantic waters, whose salinity in turn is affected by tropical winds, which can systematically transport moisture out of the Atlantic basin. The freshwater balance of the Atlantic is further affected by melting of glaciers, transport of freshwater by sea ice, and land-surface processes that determine runoff patterns.
From page 79...
... The site of North Atlantic deep-water formation is roughly where a substantial fraction of the heat transported northward in the Atlantic Ocean is deposited. Changes in the location affect the sea-ice margin and can have a net effect on the planetary radiation budget.
From page 80...
... The primary although by no means the only effect of oceans on air temperature derives simply from the heat capacity of the oceans and has little to do with horizontal ocean heat transport. Being composed of a fluid that can mix heat vertically, oceans are slow to coo!
From page 81...
... Even such a major forcing as the introduction of Northern Hemisphere ice sheets of the last glacial maximum into models has little influence south of the equator (Manatee and Broccoli, 1985; Broccoli and Manabe, 1987~. In models generally, North Atlantic cooling affects the strength of the tropical
From page 82...
... This is an unsettling possibility, in that it suggests that models could also fail to anticipate the threat of surprising and abrupt changes, which might occur in connection with global warming, as indicated in Plate 7 and discussed extensively in Chapter 4. Ocean dynamics could help to extend northern extratropical influences to the tropics and to the Southern Hemisphere, and indeed ocean circulation experiments show some global changes in response to North Atlantic freshwater pulses.
From page 83...
... , without the reduced greenhouse effect arising from low carbon dioxide in glacial times, the Southern Hemisphere would have experienced little cooling despite the massive growth of Northern Hemisphere ice sheets. However, carbon dioxide fluctuations are not radiatively significant on the time scale of the Younger Dryas or Dansgaard/Oeschger events, and up to industrial times carbon dioxide had only insignificant fluctuations throughout the Holocene.
From page 84...
... Mode! results suggest that changes in North Atlantic temperature associated with cutoff of the THC cause substantial changes in the Hadley circulation, propagating the influence of THC shutdown into the
From page 85...
... In simulations, imposed North Atlantic cooling causes enhanced wind-driven oceanic upwelling in tropical and extratropical regions, which would bring colder waters to the surface and thus might contribute to additional cooling (Agustsdottir et al., l999~. There are many possibilities for regime switches lurking in the collective behavior resulting from coupling the Hadley cell to ocean dynamics.
From page 86...
... These forcings vary too slowly to be prime movers of abrupt change, but if the climate system exhibits discontinuous response to continuous variations of some forcing parameters,
From page 87...
... On longer time scales, there are no direct observations of the fluctuation of solar output. Observations and proxies for solar activity going back centuries or more do indicate long-term fluctuations in activity, as measured by sunspot number or solar-wind effects; it is not known how much fluctuation in solar brightness goes with such fluctuations in activity.
From page 88...
... It had been hoped that better understanding of dynamic ocean heat transport would solve the problem, but recent work on Cretaceous and Eocene ocean dynamics does not support this idea. Moreover, even in simulations with increased carbon dioxide, continental interiors become too cold in the winters to reconcile with the equable climate that the fossil record demands.
From page 89...
... In the North Atlantic, where much of the deep sinking occurs (Gordon, 1986; Ganachaud and Wunsch, 2000) , the THC is responsible for the unusually strong northward heat transport; part of this heat is imported from the Southern Hemisphere.
From page 90...
... A crucial question is which density contrast one should consider the one between the equator and the poles, or the one between North and South Atlantic, or perhaps even between North Atlantic and North Pacific. The choice matters in assessing what order of magnitude of change in surface density it might take to change the THC drastically.
From page 91...
... Slow changes of the surface freshwater balance constitute one possible mechanism to induce abrupt change. Using an ocean-only model, Mikolajewicz and Maier-Reimer (1994)
From page 92...
... Following the first, by zonally averaging the equations of motion in the ocean (e.g., Marotzke et al., 1988; Wright and Stocker, 1991) , a very efficient ocean model component is obtained, which can be coupled to an energy balance model of the atmosphere (Stocker et al., 1992)
From page 93...
... A given perturbation (indicated by the horizontal arrows) in the freshwater balance of the North Atlantic (precipitation plus runoff minus evaporation)
From page 94...
... For some values of the freshwater balance of the North Atlantic, the THC can be either in a strong or in a collapsed state. Numerous studies with a variety of ocean models coupled to simple representations of the atmosphere have demonstrated the existence of hysteresis (e.g., Mikolajewicz and Maier-Reimer, 1994; Rahmstorf and Willebrand, 1995~; this is a robust property of such models.
From page 95...
... and, in contrast with the version with multiple equilibria, it exhibits only reversible changes (Plate 6~. A different way of illustrating this behavior is depicted in Figure 3.4, which shows the time evolution of the THC in response to a slow increase in freshwater forcing followed by an equally slow decrease.
From page 96...
... Is there a possibility that a temporary perturbation in, say, the freshwater forcing can induce a permanent change in the THC? This would
From page 97...
... Two cases are considered here. The "standard" case is the classical THC box model with only very weak diffusion; the other case will be designated "diffusive." In the example shown, the diffusive case has a different proportionality factor relating density differences to flow strength, such that with the same freshwater forcing, the two cases have very similar strengths of the "normal" North Atlantic THC.
From page 98...
... Depending on the strength of the horizontal mixing (diffusion) the model exhibits multiple eqilibria for a limited range of freshwater fluxes (bottom curve)
From page 99...
... Results that depend crucially on a specific shape of the hysteresis are likely not to be robust findings at this stage. Whether the nonlinearities giving rise to abrupt change in the simplified models are an artifact of the simplifications or carry over to more complex and realistic systems needs to be investigated.
From page 100...
... to a slow increase and subsequent slow decrease in freshwater forcing for the two cases in Figure 3.3. concerns the response to a sudden external perturbation, such as a disintegrating ice sheet.
From page 101...
... . The complete, temporary shutdown of the THC causes a massive cooling in the North Atlantic and weak warming in the South Atlantic.
From page 102...
... enhanced the East Greenland Current, and brought freshwater into the northern North Atlantic. Only then did the THC respond with a weakening that amplified the initial perturbation.
From page 103...
... has argued that the results from single-hemisphere models should be viewed as applying to the global integral of the various THC branches and that the magnitude of the global integral obeys different laws from the distribution of the grand total over various competing deepwater formation sites. A number of idealized and more realistic ocean GCMs have shown that varying the freshwater flux forcing leaves the global integral of deep sinking nearly constant but the strength of North Atlantic sinking considerably changed (Tziperman, 1997; Klinger and Marotzke, 1999; Wang et al., 1999~.
From page 104...
... There is a huge gap in our conceptual understanding linking changes in convective activity, in the North Atlantic or elsewhere, to the THC and the northward heat transport. Regionally, changes in properties appear to occur rapidly but are poorly understood (e.g., Sy et al., 1997~.
From page 105...
... In lieu of establishing true forecast skill, the models used for future climate-change scenarios should pass all the tests posed by available data. For example, one would reasonably have more confidence in climate models of future evolution that were able also to simulate the complicated climate history, including the rapid changes, of the last glacial interval.
From page 106...
... A dedicated supercomputer could be used to test whether the THC shows the possibility of threshold behavior, abrupt change, and hysteresis in a climate mode! that in its ocean components resolves the most important features of the ocean circulation.


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