How stable has past climate been

The major climate changes considered so far in this chapter have taken place relatively slowly. The growth and recession of the large polar ice-sheets between the ice ages and the intervening warmer interglacial periods have taken on average many thousands of years. However, the ice core records such as those in Figures 4.6 and 4.8 show evidence of large and relatively rapid fluctuations. Ice cores from Greenland provide more detailed evidence of these than those from Antarctica. This is because at the summit of the Greenland ice cap, the rate of accumulation of snow has been higher than that at the Antarctica drilling locations. For a given period in the past, the relevant part of the Greenland ice core is longer and more detail of variations over relatively short periods is therefore available.

The data show that the last 8000 years have been unusually stable compared with earlier epochs. In fact, as judged from the Vostok (Figure 4.6) and the Greenland records (Figure 4.8) this long stable period in the Holocene is a

Figure 4.8 Variations in Arctic temperature over the past 100 000 years as deduced from oxygen isotope measurements (in terms of S18O) from the 'Summit' ice core in Greenland. The quantity S18O plotted in Figures 4.6 and 4.7 is the difference (in parts per thousand) between the 18O/16O ratio in the sample and the same ratio in a laboratory standard. The overall shape of the record is similar to that from the Vostok ice core shown in Figure 4.6 but much more detail is apparent in the 'Summit' record's stable period over the last 8000 years. A change of 5 parts per 1000 in S18O in the ice core corresponds to about a 7 °C change in temperature.

unique feature of climate during the past 420 000 years. It has been suggested that this had profound implications for the development of civilisations.8 Model simulations (see Chapter 5) indicate that the detail of long-term changes during the Holocene is consistent with the influence of orbital forcing (Figure 4.7). For instance, some northern hemisphere glaciers retreated between 11 000 and 5000 years ago and were smaller during the later part of this period than they are today. The present-day glacier retreat cannot be attributed to the same natural causes as the decrease in summer insolation due to orbital forcing during the past few millennia has tended to glacier increase.

It is also interesting to inspect the rate of temperature change during the recovery period from the last glacial maximum about 20 000 years ago and compare it with recent temperature changes. The data indicate an average warming rate of about 0.2 °C per century between 20 000 and 10 000 years before present (BP) over Greenland, with lower rates for other regions. Compare this with a temperature rise during the twentieth century of about 0.6 °C and the rates of change of a few degrees Celsius per century projected to occur during the twenty-first century because of human activities (see Chapter 6).

The ice core data (Figure 4.8) demonstrate that a series of rapid warm and cold oscillations called Dansgaard-Oeschger events punctuated the last glaciation. Comparison between the results from ice cores drilled at different locations within the Greenland ice cap confirm the details up to about 100 000 years ago. Comparison with data from Antarctica suggests that the fluctuations of temperature over Greenland (perhaps up to 16 °C) have been larger than those over Antarctica. Similar large and relatively rapid variations are evident from North Atlantic deep sea sediment cores.

Another particularly interesting period of climatic history, more recently, is the Younger Dryas event (so called because it was marked by the spread of an arctic flower, Dryas octopetala), which occurred over a period of about 1500 years between about 12 000 and 10 700 years ago. For 6000 years before the start of this event the Earth had been warming up after the end of the last ice age. But

Dryas event and its rapid end about 10 700 years ago. Dating of the ice core was by counting the annual layers down from the surface; dating of the lake sediment was by the 14C method. A change of 5 parts per 1000 in S18O in the ice core corresponds to about a 7 °C change in temperature.

then during the Younger Dryas period, as demonstrated from many different sources of palaeoclimatic data, the climate swung back again into much colder conditions similar to those at the end of the last ice age (Figure 4.9). The ice core record shows that at the end of the event, 10 700 years ago, the warming in the Arctic of about 7 °C occurred over only about 50 years and was associated with decreased storminess (shown by a dramatic fall in the amount of dust in the ice core) and an increase in precipitation of about 50%.

Two main reasons for these rapid variations in the past have been suggested. One reason particularly applicable to ice age conditions is that, as the icesheets over Greenland and eastern Canada have built up, major break-ups have occurred from time to time, releasing massive numbers of icebergs into the North Atlantic in what are called Heinrich events. The second possibility is that the ocean circulation in the North Atlantic region has been strongly affected by injections of fresh water from the melting of ice. At present the ocean circulation in this region is strongly influenced by cold salty water sinking to deep ocean levels because its saltiness makes it dense; this sinking process is part of the 'conveyor belt' which is the major feature of the circulation of deep ocean water around the world (see Figure 5.18). Large quantities of fresh water from the melting of ice would make the water less salty, preventing it from sinking and thereby altering the whole Atlantic circulation.

This link between the melting of ice and the ocean circulation is a key feature of the explanation put forward by Professor Wallace Broecker for the Younger Dryas event.9 As the great ice-sheet over North America began to melt at the end of the last ice age, the melt water at first drained through the Mississippi into the Gulf of Mexico. Eventually, however, the retreat of the ice opened up a channel for the water in the region of the St Lawrence River. This influx of fresh water into the North Atlantic reduced its saltiness, thus, Broecker postulates, cutting off the formation of deep water and that part of the ocean 'conveyor belt'.10 Warm water was therefore prevented from flowing northward, resulting in a reversal to much colder conditions. The suggestion is also that a reversal of this process with the starting up of the Atlantic 'conveyor belt' could lead to a sudden onset of warmer conditions.

Although debate continues regarding the details of the Younger Dryas event, there is considerable evidence from palaeodata, especially those from ocean sediments, for the main elements of the Broecker explanation which involve the deep ocean circulation. It is also clear from palaeodata that large changes have occurred at different times in the past in the formation of deep water and in the deep ocean circulation. Chapter 3 mentioned the possibility of such changes being induced by global warming through the growth of greenhouse gas concentrations. Our perspective regarding the possibilities of future climate change needs to take into account the rapid climate changes that have occurred in the past.

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