How does rising atmospheric CO2 affect marine organisms?

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Earth's Climatic History: The Last 2,000,000 Years
Throughout earth's geologic history, major ice ages have occurred over and over again, with ten of them affecting the planet in the past one million years (Erickson, 1990) and another ten in the million or so years before that (Whyte, 1995).  Indeed, they recur almost like clockwork at approximate 100,000-year intervals and persist for about 90,000 years, after which they are followed by approximate 10,000-year interglacials (Suarez and Held, 1976; Berger, 1978; Pollard, 1978; Schneider and Thompson, 1979; Williams et al., 1993). One million year temperature history Their periodicity is clearly discerned in the accompanying figure, which depicts the relative changes in mean global air temperature over several of the most recent cycles, as reported by the Intergovernmental Panel on Climate Change (IPCC) (Houghton et al., 1990).

The glacial epoch about which we know the most is the most recent one, which was at its peak only 18 to 20 thousand years ago.  At that time, approximately one-third of the earth's land area was covered by ice (West, 1977; Erickson, 1990; Williams et al., 1993).  In North America, the southern edge of the glacial ice extended all the way to the Ohio river valley and in some locations was over a mile (1.61 km) thick (Flint, 1971; Bowen, 1978; CLIMAP, 1981).  Other glaciers advanced around the world (Andrews, 1985; Barrett, 1991), extending through portions of England (West, 1977; Preece, 1995), Northern Europe (Coleman, 1969), Asia (Tung-sheng, 1991) and South America (Rabassa, 1983).  Mountain glaciers also formed and expanded in the tropical regions of New Guinea (Bowler et al., 1976), Hawaii (Porter, 1979), eastern Africa (Hamilton, 1982; Mahaney, 1990) and the Andes (Hastenrath, 1985; Fairbanks, 1989). At its peak extent, earth's glacial ice contained approximately 5 percent of the planet's water and reduced sea levels by as much as 120 meters (Fairbanks, 1989; Erickson, 1990; Williams et al., 1993; Andersen and Borns, 1994).

In North America, there were two major ice sheets: the Laurentide ice sheet and the Cordilleran ice sheet.  The Laurentide ice sheet was the larger of the two, covering five million square miles (16 x 106 km2) (Williams et al., 1993) and stretching from the Arctic down though eastern Canada to the northern half of the Midwestern United States (Erickson, 1990).  The Cordilleran ice sheet emanated from the Canadian Rockies and engulfed western Canada, Alaska and portions of the northwestern United States.  Large ice sheet growth was also seen in Europe, where the Fennoscandian ice sheet covered approximately 4 million square kilometers (Flint, 1957; Daly, 1973).  The weight of these massive ice sheets was so great that they actually depressed the earth's crust, in some cases by as much as 700-800 meters (Flint, 1971; West, 1977), creating gravity anomalies that are still detectable (Peltier, 1987).

In Antarctica, ice covered the continent and flowed to the sea, where it broke apart to form massive icebergs.  In fact, iceberg discharges in both hemispheres were so numerous during the peak of the last ice age (Bond et al., 1992; Broecker et al., 1992) that they covered half the area of the world's oceans (Erickson, 1990). As a result, sea surface temperatures in oceanic polar front regions dropped by as much as 6 to 10 °C, while the tropical oceans cooled from 2 to 8 °C (CLIMAP, 1981; Hughes, 1996; Beck et al., 1997; Webb et al., 1997).

The presence of the large ice sheets forced major changes in the general circulation of the atmosphere.  In North America, mean wind direction was primarily from the northwest (Wells, 1983), bringing cooler air further south.  Wind speeds also increased, by as much as 50-80% above current values in the North Atlantic and Southern oceans (Crowley and North, 1991), enhancing oceanic heat loss even more (Crowley and Parkinson, 1988).  In the Northern Hemisphere, the polar front moved 1200 km south of its current location to 34 °N (Delcourt, 1979; Delcourt and Delcourt, 1983).  As a result, annual temperatures in the mid-latitudes decreased by as much as 10 °C (Flint, 1971; Barry, 1983; Guiot et al., 1989), and by as much as 15-20 °C during the winter (Delcourt, 1979; Watts, 1980).

With few exceptions, the planet was much drier during the last ice age.  Ice cores from Greenland and Antarctica suggest that precipitation decreased about 50% in earth's polar regions during this period (Beer et al., 1985; Herron and Langway, 1985; Lorius et al., 1985).  Dust concentrations in the cores additionally suggest that most of earth's ice-free land was more arid than it is now (Petit et al., 1981; Hammer et al., 1985; Petit et al., 1990).  Other evidence points to the formation of large sand dunes in Central America and the sub-Sahara region of Africa, and to the contraction and fragmentation of the Amazon rainforest into a few small regions of high precipitation (Haffer, 1969; Prance, 1982).  Concurrently, the salinity of the Mediterranean Sea rose by 1-3%, while that of the Red Sea rose as much as 10% (Thunell et al., 1987; Thunell et al., 1988; Thunell and Williams, 1989).  In fact, the concentration of salt in the Red Sea was so high that it may have approached the upper tolerance limit of most planktonic organisms (Crowley and North, 1991).

Terrestrial plants throughout the world also suffered from lower atmospheric carbon dioxide concentrations during earth's last glacial episode, as the air's CO2 content fell to a level of approximately 180 ppm (Berner et al., 1980; Delmas et al., 1980; Barnola et al., 1987).  This drop in atmospheric CO2 was driven largely by the increased ability of colder water to hold more dissolved CO2 (Adey, 1991; Butler, 1991) and by the much larger growth rates of phytoplanktonic organisms in earth's open oceans (Pedersen, 1983; Muller et al., 1983; Lyle, 1988; Lyle et al., 1988), which removed more CO2 from the air (McElroy, 1983; Knox; 1984).  The enhanced growth rates of these tiny but multitudinous organisms were largely the result of the greater amounts of elemental iron (Martin and Fitzwater, 1988; Martin, 1990; Martin, 1992) contained in the augmented quantities of dust (DeAngelis et al., 1987; Gaudichet et al., 1988; Legrand et al., 1988) that were carried to them by the stronger winds of that period (Parkin and Shackleton, 1973; Sarnthein et al., 1981; Roger and Wilson, 1989).  Numerous studies have revealed that this low level of atmospheric CO2 concentration (180 ppm) greatly reduces vegetative productivity (Polley et al., 1992a, 1992b; Polley et al., 1993); and, had the CO2 concentration of the air dropped much lower, it is likely that several plant extinctions would have occurred, since many plants find it difficult to survive at CO2 concentrations on the order of 50 to 100 ppm (Idso, 1989; Salisbury and Ross, 1978).

Large and rapid shifts in climate have been detected in areas of the North Atlantic, Greenland, and in Antarctica from deep-sea sediment cores, ice cores, lake sediments, and pollen series that cover this period of time.  Most of these records provide climatic descriptions of the last glacial cycle, with some continuing on through the Emian interglacial over 120,000 years ago.

In Greenland, rapid warming - approximately 7°C in a few decades - was observed around 11,500 years ago (Dansgaard et al., 1989; Johnsen et al., 1992; Grootes et al., 1993). Alley et al. (1993) also report evidence of even more rapid shifts in precipitation patterns, and other authors have noted swift changes in atmospheric circulation (Taylor et al., 1993; Mayewski et al., 1993).  Sea surface temperature changes of around 5°C, associated with sudden changes in oceanic circulation, also occurred in a few decades in the Norwegian Sea (Lehman and Keigwin, 1992).  Similar warming following the latest deglaciation occurred in regions of the Southern Hemisphere, though the warming there was less abrupt (Suggate, 1990; Denton and Hendy, 1994; Salinger, 1994; Jouzel et al., 1995).

During the last glacial cycle, large warm-cold oscillations have been detected in central Greenland ice cores (Johnsen et al., 1992).  Rapid warmings of between 5 and 7 °C occurred in a few decades, followed by periods of slower cooling and then a rapid return to glacial conditions.  Around 20 such interstadial events occurred during the last glacial period and lasted between 500 and 2000 years (Dansgaard et al., 1993).  Similar rapid changes have been discovered in North Atlantic deep-sea cores, indicating massive iceberg discharges from the Northern Hemisphere ice sheets (Bond et al., 1993; Mayewski et al., 1994; Bond and Lotti, 1995); and they were followed by abrupt shifts to warmer sea surface temperatures.  Additional records from Western Europe, North America, and China (Grimm et al., 1993; Guiot et al., 1993; Porter and An Zhisheng, 1995) document rapid shifts in climate during the last glacial period; and such records have prompted the IPCC to categorize these interstadials as "at least hemispheric in their extent" (Houghton et al., 1996).

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