Msa

5 10 15 20 25 30 35 40 45 Thousands of Years Before Present

the dependence of ice-sheet melt and calving rates on elevation changes caused by isostatic response of the Earth's crust, which is rate-limited by creeping flow of the mantle (Peltier, 1987).

Another feature of the coupled Earth system exhibited in the ice-age experiment is that the global environment has modes, or relatively stable states that it maintains despite continual changes in external forcings. A manifestation is that extremes of climate have generally consistent values; the temperature minima attained during each of the past four glacial cycles are all very similar, as are each of the interglacial temperature maxima, despite the changing character of the insolation forcing. Climate modes likely reflect, in part, multiple stable configurations for ocean circulation. The consistency of extrema despite variable forcings, and the constancy of some climate periods despite continual changes in forcings (as during the Holocene) both indicate internal regulation of the Earth's environmental systems. Self-regulation generally results from feedbacks (see Chapter 17).

18.4.2 Some Large and Abrupt Climate Changes

Despite gradual changes in forcings, some very large climate changes have occurred in very little time - on time scales that are short even compared to human lives and to cultural changes. The clearest example is the abrupt warming and increase in precipitation at the final termination of the last glacial period (the end of the Younger Dryas), which is recorded at high resolution in Greenland ice and shown by coincident methane changes and by southern hemisphere geologic records (Denton and

Fig. 18-22 The last 50 000 years of environmental history, recorded in central Greenland (GISP2 and GRIP ice cores), plus the C02 record from Vostok, Antarctica, and global ice volume measured as sea level depression. (From top to bottom, references are: Shackleton, 1987; Cuffey et al, 1995 and Grootes et al, 1993; Cuffey and Clow, 1997; Chapellaz et al., 1990; Brook et al, 1996; Mayewski et al, 1997; Saltzman et al, 1997.)

Fig. 18-23 Observed correlation of isotopic composition of precipitation with ground temperature (gray diamonds; Jouzel et al., 1987), and predictions of simple isotopic models. A, prediction with constant a; B, prediction with temperature-dependent a.

Temperature (°C)

Fig. 18-23 Observed correlation of isotopic composition of precipitation with ground temperature (gray diamonds; Jouzel et al., 1987), and predictions of simple isotopic models. A, prediction with constant a; B, prediction with temperature-dependent a.

Hendy, 1994) to involve a significant fraction of the globe. This particular change involved a sudden strengthening of the thermohaline circulation and consequent heat transport to the North Atlantic, and thus directly resulted from coupling of the oceans to other climate systems. A major question for research is whether such abrupt changes could be triggered by anthropogenic warming. There is at this point no reason to discount the possibility. Some modeling efforts (Stacker and Schmittner, 1997) have, for example, suggested that an increase in the freshwater flux to the North Atlantic from the continents could cause the thermohaline circulation to abruptly cease, plunging Europe into a cold climate unlike any in recent millennia.

18.4.3 Polar Amplification

A clear characteristic of global climate changes is their amplification in the polar regions. In glacial-interglacial cycling, temperature changes in the Arctic and Antarctic have been two to three times larger than the corresponding changes at low and mid-latitudes. This results, in part, from large albedo changes due to changing land and sea ice cover, and seasonal snowlines, and also from changes in ocean to atmosphere heat transfer as insulating sea-ice cover waxes and wanes. Polar amplification is very interesting because it implies the polar ice sheets (Greenland and West Antarctica) are particularly vulnerable to melt in a warmed climate, and because the melting of permafrost in tundra regions may

Fig. 18-24 Observed correlation (the Meteoric Water Line) of the two most important isotopic ratios in precipitation (gray diamonds; Jouzel et til., 1987 and Dahe et al., 1994), and predictions of simple isotopic models. A, prediction with constant a; B, prediction with temperature-dependent a.

release significant quantities of the powerful greenhouse gas CH4. The increase of global mean temperature over the past century has, however, been only modestly amplified over the North Atlantic, Scandinavia and Greenland (Overpeck et al., 1997), suggesting something is very different about the modern environment from past ones. Indeed there is now a substantial and negative anthropogenic radiative forcing at the mid-latitudes of the northern hemisphere, resulting from industry-generated particles and gases which form aerosols (see Chapter 7). Because these will change through time as technologies and economies change, it would be incorrect to view the modern Arctic as safe from amplified warming based on the past century.

18.4.4 Global Climate Sensitivity

One of the primary goals of "Earth systems" research is to learn how sensitive global climate is to changes in forcings, for instance how much temperature change results from a given increase in atmospheric C02 content. A particular difficulty in learning the sensitivity to a given forcing is that the magnitude of climate change strongly depends on coincident changes in other forcings as a result of feedbacks and couplings. For the particular case of C02, an initial increase in temperature due to C02 change may increase atmospheric H20 and decrease polar albedo, both of which are strong climate forcings themselves. Subsequent changes in biogenic emissions and ocean circulation may be very important too. Thus the net sensitivity due to C02 change is very difficult to quantify from physical models alone, due to the great complexity of system components and their couplings. Current models, for instance, do not attempt to predict changes in planetary albedo resulting from changes in marine biogenic emissions.

The information in paleoenvironmental records therefore is very valuable for better constraining sensitivities. The use of paleoenvi-ronmental records is either through general circulation models (physical models of climate processes combined on a numerical imitation of our planet), or through robust but simplistic "state-variable" analyses.

18.4.5 General Circulation Models (GCMs)

Insofar as paleoenvironmental records reveal histories of both forcings and environments, the accuracy of GCMs may be tested by efforts to replicate these histories. Successful replication would suggest the models capture the essential behavior of the Earth and therefore have predictive ability. Further, GCM simulations of past climates may allow partitioning of net climate changes into components due to various forcings. GCM simulations of the ice-age Earth are very much works in progress.

Recent revisions to the boundary conditions (ice-sheet topography and sea surface temperatures) have added uncertainty to many of the GCM calculations of the past two decades. Moreover, all of these calculations use prescriptions for at least one central component of the climate system, generally oceanic heat transport and/or sea surface temperatures. This limits the predictive benefit of the models. Nonetheless, these models are the only appropriate way to integrate physical models of diverse aspects of the Earth systems into a unified climate prediction tool.

A consistent result from GCM simulations of ice-age climate is that the global cooling of 5 to 8°C can not result solely from the direct radiative forcings (Hansen et al., 1984; Webb et al., 1997) of insolation, greenhouse gas, ice cover and terrestrial albedo changes. To explain the ice ages, there must be a strong net feedback that magnifies temperature changes by a factor of 2 to 4. This feedback is most likely a combination of atmospheric water vapor and clouds. The global climate sensitivity implied by these models is at least 0.5°C/(W/m ), and plausibly greater than l°C/(W/m2).

18.4.6 State-Variable Analyses

An alternative approach to assessing climate sensitivity is to assume that some or all important elements of the Earth's environmental systems are so strongly coupled (as one may infer from Figs 18-21 and 18-22) that the behavior of the whole system may be represented by the behavior of a few variables, which are related using feedback factors or simple dynamic models. Though such analyses cannot substitute for process models operating on realistic geographic domains, they distill the essential lessons of environmental history without creating an illusory aura of physical completeness and accuracy.

Lorius et al. (1990) performed a simple multivariate analysis in which they correlate the temperature changes of the past 160 kyr (as recorded in the Vostok 3D record) with changes in five forcings: atmospheric C02 plus CH4, ice volume, aerosol loading (dust and SO|~ separately), and insolation. These analyses establish that greenhouse gas variations correlate with temperature variations better than do any other forcings, and that these gas variations account for 40 to 60% of the climate change (and most likely 55 to 65%), in the correlative sense. This suggests that gas changes account for more than 2°C of the global deglacial warming. As the gas changes themselves can account directly for only 0.7°C , a feedback factor of greater than 3 is necessary (feedback factor being defined as the ratio of a temperature change to the temperature change due only to direct forcings).

A broader view was explored by Hoffert and Covey (1992), who used estimated forcings and geographically broad data from both the last glacial maximum and the mid Cretaceous to calculate two estimates of bulk climate sensitivity (change in temperature per change in radiative forcing) necessary to explain the paleotemperatures for these climates. Net forcings relative to the pre-industrial Holocene are -6.7 W/m2 and +15.7 W/m2 for the glacial and the Cretaceous, respectively, which yield sensitivities of 0.45cC/(W/m2) and 0.57°C/(W/m2). The recent revisions of last glacial maximum reconstructed temperatures (both low-latitude and polar) to lower values increases Hoffert and Covey's LGM sensitivity to 0.9°C/(W/m2), and Lorius et al.'s feedback factor to 3.5 or 4. Regardless, all these sensitivities are comfortably within the range predicted for modern climate by GCMs. To infer a substantially lower sensitivity of Earth's climate to forcings, one would have to invoke physically implausible large additional forcings for both Cretaceous and glacial periods. It is thus a reasonable suggestion (Hoffert and Covey, 1992) that these paleoclimate records preclude a low global sensitivity.

18.4.7 Perspective on Anthropogenic Forcings

Finally, paleoenvironmental records demonstrate conclusively that anthropogenic changes of C02, CH4, S04, N03, and N20 are very large relative to their natural variability during the entire Holocene, and the modern polluted environment has no analog in at least the past 400 kyr. The magnitude of this human "experiment" is of the same magnitude as the natural experiment that buried the sites of Boston, Chicago, London and Stockholm beneath more than a kilometer of glacial ice, and produced the moraines and lakes that stimulated Agassiz's revelations.

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