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FIGURE 7.4 Earth's climate history during the previous 500,000 years based on atmospheric, marine, and terrestrial records that continuously cover the last five glacial-interglacial transitions (identified by dashed vertical lines with Roman numerals and the duration of the period in thousands of years, ka). Temperature profile from the Vostok ice core from Antarctica has been calculated in relation to the oxygen and hydrogen isotope content of the snow. Close correlation between atmospheric temperatures and greenhouse gases is further revealed by the carbon dioxide (CO2) concentrations in the Vostok ice core, which are higher today than at any previous period during the last four climate cycles. The calcium carbonate vein from Devils Hole (DH11) in North America and the SPECMAP composite of calcareous planktonic foraminifera in 17 sediment cores from the Atlantic Ocean reveal coupled climate shifts and interacting dynamics of air-sea-land reservoirs in the Earth system. All of these profiles are compared to variability of the Earth's insolation, which is forced by the orbital relationship with the Sun (Figs. 7.2 and 7.3). Units are ppmv (parts per million by volume), %o (parts per thousand), W m~2 (watts per square meter), and °C (degrees Celsius). Modified from published information on the Vostok ice core (Petit et al., 1999), Devils Hole core (Winograd et al., 1992) and SPECMAP composite core (Imbrie et al., 1989).

Atmospheric gases trapped within the Vostok ice core reflect changes in global ice volumes and the overall hydrological cycle (Fig. 1.5). In addition, hydrogen and oxgyen isotopes in water (H2O) vapor and snow can be calibrated directly to atmospheric temperatures. When global temperatures cool and there are expanding volumes of ice on Earth (Fig. 6.5), the ocean becomes relatively enriched in 18O while atmospheric precipitation becomes relatively enriched in 16O (which is easier to evaporate). Consequently, based on the notation for 18O/16O ratios (Eq. 6.1) glacial <518O values are relatively negative in ice sheets and terrestrial carbonates and relatively positive in marine carbonates during glacial periods. Conversely, during interglacial periods, when global temperatures are warm and there is more seawater from melting ice sheets, <518O values become more negative in marine carbonates while becoming more positive in terrestrial carbonates and ice masses (Chapter 6: Spreading Planet).

As the longest high-resolution record of Earth's climate, the Vostok ice core indicates over the last four climate cycles that the amplitude of glacial-interglacial temperature change is around 8°C (Fig. 7.4). In addition, the 10,000-year duration of the current interglacial climate (Holocene) is the longest stable warm period recorded in Antarctica during the past 420,000 years. The sawtooth temperature profile from the Vostok ice core also shows the dominance of the 100,000-year cycle along with strong imprints of the 41,000-year and 23,000-year periodicities associated with Milankovitch changes in Earth's insolation (Figs. 7.2 and 7.3).

To evaluate the global synchrony of changes in the hydrological cycle during the past half million years, <518O climate records also have been compiled from marine and terrestrial reservoirs (Fig. 7.4). Like marine sedimentary strata through the Cenozoic (Fig. 6.5), <518O shifts in the calcium carbonate of planktonic foraminifera from the SPECMAP composite of marine sediment cores in the Atlantic Ocean reflect the same climate cycles as in the Vostok ice core. These climate shifts are reproduced again by the <518O in a calcium carbonate vein that precipitated continuously in the Devils Hole groundwater discharge area in Nevada, North America.

Together, these independent and synchronous records of <518O variability during the last four glacial-interglacial climate periods (Fig. 7.4) demonstrate global cycling of water between air, sea, land, and ice reservoirs in the Earth system. Moreover, these climate records indicate that the Earth system has tended to be in a glacial condition most of the time during past half-million years, with only punctuated periods of interglacial warmth.

The Vostok ice core also reveals shifts in the carbon dioxide (CO2) concentrations in the atmosphere (Fig. 7.4). In contrast to the external forcing of the Earth system by the sun, CO2 changes indicate that global climate also is influenced internally by life on our planet [Eq. (1.1), Fig. 2.2]. During every glacial-interglacial transition, the atmospheric CO2 increased from about 180 to 300 parts per million, respectively. Methane (CH4) concentrations, which also were measured in the Vostok ice core, increased in phase with CO2 from about 320 to 770 parts per billion by volume during each climate transition.

FIGURE 7.5 Illustration of a generalized ice-sheet retreat sequence. (a) Ice sheets extend through ice shelves into the ocean, covering and submerging coastal areas. (b) Retreating ice sheets add meltwater to the ocean, which raises sea level and alters seawater chemistry. (c) Afterward, moraines (piles of rocks and boulders) remain that represent changes in the margin of the ice sheets over time. With the diminished weight of the ice sheets, coastal areas begin rebounding and producing raised beaches that have emerged with marine fossils above sea level (Plate 4). Modified from Berk-man etal. (1992).

FIGURE 7.5 Illustration of a generalized ice-sheet retreat sequence. (a) Ice sheets extend through ice shelves into the ocean, covering and submerging coastal areas. (b) Retreating ice sheets add meltwater to the ocean, which raises sea level and alters seawater chemistry. (c) Afterward, moraines (piles of rocks and boulders) remain that represent changes in the margin of the ice sheets over time. With the diminished weight of the ice sheets, coastal areas begin rebounding and producing raised beaches that have emerged with marine fossils above sea level (Plate 4). Modified from Berk-man etal. (1992).

tually, as the massive weight of the overlying ice sheets disappeared, coastal areas began rebounding above sea level to produce raised beaches with fossils that constrain the timing and magnitude of ice-sheet retreat (Fig. 7.5c).

In the Arctic, raised beaches have elevations that exceed 100 meters above sea level with fossils that have radiocarbon ages older than 14,000 years before present. Around Antarctica, however, raised beaches have elevations less than 35 meters above sea level with marine fossils with radiocarbon ages younger than

With a view toward understanding future climate changes, stratigraphic records of terrestrial, marine, and atmospheric variability (Fig. 7.4) reveal a strong linkage between the pace of glacial-interglacial cycles and the predictable orbital dynamics between the Earth and the Sun (Figs. 7.2 and 7.3). However, the underlying relevance to human civilization is related to more immediate changes in the weather patterns that ultimately are integrated over time in the global climate (Fig. 6.1).

What is the difference between weather and climate?

sea-level seesaw

Over geological time scales, sea-level rise and fall are related directly to the temperature history of the Earth system (Fig. 1.6). During cold periods, as evaporated water from the sea is locked into glaciers and ice sheets, sea level drops. Conversely, when climate warms and glacial meltwater flows back into the ocean, sea level goes up. Interpreting this coupling between ice sheets and sea level is fundamental to understanding impacts associated with global climate change that are relevant to the world we live in.

At present, nearly 90% of the ice on the planet is locked in the Antarctic ice sheets. The vast majority of the ice occurs in the land-based East Antarctic Ice Sheet, which if it melted entirely would raise sea level more than 50 meters. In contrast, ice is in contact with the ocean in the marine-based West Antarctic Ice Sheet, which has about the same ice volume as the Greenland Ice Sheet and could raise sea level around 5 meters if it entirely melted.

Because of its connection to the ocean, the West Antarctic Ice Sheet is considered to be particularly responsive to climate warming that elevates seawater temperatures and sea level. Melting or floating basal areas of the ice sheet that are grounded on the sea floor may remove critical ''pinning points'' that anchor the West Antarctic Ice Sheet. In the absence of these terminal restraints, relatively fast-flowing streams of ice in the interior of the West Antarctic Ice Sheet would be free to discharge into the sea. Although the future dynamics of this marine-based ice sheet and its ''ice streams'' are still largely unknown, there are marine sedimentary records that suggest the West Antarctic Ice Sheet has collapsed at least once during the past million years.

At the end of the Last Glacial Maximum, which ended around 17,000 years ago, the Earth's ice sheets were more extensive than they are today (Fig. 7.5a)— particularly since the Laurentide Ice Sheet in the Arctic has completely vanished. Regions in the middle of North America, for example, were covered by more than a kilometer of ice. As these ice sheets retreated, meltwater began gushing down streams and rivers back into the ocean, causing sea level to rise (Fig. 7.5b). Even-

Between 13,000 and 9000 years ago, when sea level was rising most rapidly (Fig. 7.6), there also were two major pulses of glacial meltwater into the North Atlantic. At their peaks, these two meltwater pulses discharged 14,000 and 9500 cubic kilometers of water per year—far greater than the Amazon (the largest river system on Earth today), which discharges around 6300 cubic kilometers per year.

At least 90 meters of the sea-level rise since the Last Glacial Maximum was associated with the northern hemisphere ice sheets, indicating that the past ice volume over the Arctic was nearly 50% larger than across all of Antarctica today. Moreover, the Arctic ice sheets melted in less than 20,000 years, signifying that glacial-interglacial climate shifts can occur over relatively short periods. In fact, transitions in the climate system can be downright abrupt, as illustrated by the termination of the Younger Dryas (named after a tundra flower that was living in northern Europe at the time) when Arctic temperatures warmed nearly 7°C within a couple of decades 11,640 years ago. The timing of this climatic event is well constrained by annual layers of snow accumulation that were counted in ice cores from Greenland. The Younger Dryas also is exhibited by the interval between the two major meltwater pulses into the ocean after the Last Glacial Maximum (Fig. 7.6).

In addition, the Younger Dryas suggests that there are ocean-atmosphere feedbacks in the Earth system that affect global climate dynamics. After the Last Glacial Maximum, with massive volumes of meltwater flowing from North America through the Gulf of St. Lawrence, a buoyant lid of freshwater floating on the denser seawater in the North Atlantic would have been produced. It has been hypothesized that such a meltwater lid would have changed oceanic and atmospheric circulation patterns—reversing the warming trend of the Earth system that began after the Last Glacial Maximum.

At the end of the Younger Dryas, climate warming and ice-sheet melting abruptly switched on again—shifting the Earth system into the current interglacial period. This current climate regime, which began 10,000 years ago, is known as the Holocene.

It is speculated that the second pulse of meltwater was initiated from Antarctica, possibly from the marine-based West Antarctic Ice Sheet in response to the earlier sea-level rise from the retreating ice sheets in the Arctic. Nonetheless, subsequent sea-level rise during the Holocene must have been influenced by Antarctic melting because the northern hemisphere ice sheets had already vanished. Following the ''climate optimum'' around 6000 years ago, when temperatures were warmer than today by 1 or 2°C, sea level effectively stabilized in concert with reduced variability of the interglacial climate.

During this period, as reflected by calendars that have been updated continuously for nearly 6000 years, diverse human cultures also began emerging. Apparently, along with bristlecone pines and other biotic assemblages (mentioned in Chapter 6: Spreading Planet), our civilization has been flourishing under relatively stable climate warmth since the mid-Holocene (Plate 5).

9000 years before present (Plate 8). These data alone demonstrate that climate impacts are asymmetric around the Earth with ice sheets retreating earlier and more massively in the northern hemisphere than around Antarctica after the Last Glacial Maximum.

The combined effect of this last deglaciation across the Earth is reflected by sea-level records extracted from coral reefs (Fig. 7.6), in tropical seas far from any direct ice-sheet impacts or tectonic movements. This paleoenvironmental record shows that global sea level has risen more than 120 meters since the Last Glacial Maximum.

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