212 Imprint of Climatic Zonation on Marine Sediments
A number of temperature proxies are available for extracting climate fluctuations
from valVed sediments, including microfossil assemblages (warm-water and coldwater indiciators; Sect. 6.1.5), oxygen isotopes (Sect. 7.3.2) and alkenone content of
organic matter. The alkenone method (see S. C. Brassell et aI., 1986, Nature 320:
129) relies on the observation that double-bond abundance within certain long-chain
lipid molecules (unsaturated methyl ketones, C37) decreases with temperature. The
alkenones are contributed to the sediment by the remains of coccolithophores, in
coastal regions predominantly by Emiliania huxleyi.
When reading the record of corals or varves to assess climate variability, we come
up against a major difficulty. As we go back in time within such high-resolution
records, we soon enter the "Little Ice Age", which is a climatic period known to be
highly "untypical" when compared with the last several thousand years. The Little
Ice Age lasted from roughly 1450 to 1850 AD. It was characterized by advances of
mountain glaciers in most parts of the world, and by occasional spells of unusually
cold winters in North America, Europe, and Asia. In Europe, we commonly see
paintings from the 18th century which show ice-covered rivers and lakes that have
not been frozen for a life time! Thus, a portion of the recent warming we have
experienced (Fig. 7.16) may simply be due to a return to more "normal" conditions,
such as existed in the early Middle Ages before the Little Ice Age. How, then, are we
going to tell which part of the recent warming is "natural" and which (if any) is due
to human influence? Clearly, this problem can only be attacked by looking at even
longer records, to extract the long-period variability of climate.
It is usually assumed that the Sun's radiation is constant - in fact, the average
irradiation of a surface facing the Sun at the mean Earth-Sun distance is referred to as
the "solar constant" (ca. l.95 cal/cm 2 /min). Unfortunately, the convenient assumption contained in this label is unfounded: The Sun's output is variable. On the scale
of centuries this is documented, for example, for changes in abundance of sun spots.
Using spectral analysis, we can see the effects of sun-spot cycles (22 yr and 11 yr) in
many tree-ring data, and even in some marine valVes (although less distinctly). So,
perhaps the Little Ice Age is part of a long-term solar cycle. Alternatively (or in
addition), it may owe its excess of severe winters to unusually intense volcanic
activity between the 15th and 19th centuries.
The methods of marine geology - or paleoceanography - can add to the store of
knowledge which we need to be able to say, eventually, "yes, we see the global
warming". The records recovered from corals and varves establish not only the amplitudes and frequencies of large-scale climatic variations (such as the "El Nifto"
phenomenon in the Pacific). They also tell us about the likelihood of abrupt climatic
change. Such changes, which may be defmed as unusually fast transitions from one set
of climatic conditions to another, occur on various time scales, with various amplitudes. In Section 9.2.4, a dramatic change taking but decades at the beginning of the
Holocene will be discussed. The last such change took place in the 1830s, perhaps as
a result of unusal volcanic activity. This change is seen both in tree rings and in marine
varves (in Santa Barbara Basin, off California). There is a possibility that such climate
changes are not completely reversible: a new condition may arise and stabilize itself,
although the factor which produced the change has ceased to operate.
A number of temperature proxies are available for extracting climate fluctuations
from valVed sediments, including microfossil assemblages (warm-water and coldwater indiciators; Sect. 6.1.5), oxygen isotopes (Sect. 7.3.2) and alkenone content of
organic matter. The alkenone method (see S. C. Brassell et aI., 1986, Nature 320:
129) relies on the observation that double-bond abundance within certain long-chain
lipid molecules (unsaturated methyl ketones, C37) decreases with temperature. The
alkenones are contributed to the sediment by the remains of coccolithophores, in
coastal regions predominantly by Emiliania huxleyi.
When reading the record of corals or varves to assess climate variability, we come
up against a major difficulty. As we go back in time within such high-resolution
records, we soon enter the "Little Ice Age", which is a climatic period known to be
highly "untypical" when compared with the last several thousand years. The Little
Ice Age lasted from roughly 1450 to 1850 AD. It was characterized by advances of
mountain glaciers in most parts of the world, and by occasional spells of unusually
cold winters in North America, Europe, and Asia. In Europe, we commonly see
paintings from the 18th century which show ice-covered rivers and lakes that have
not been frozen for a life time! Thus, a portion of the recent warming we have
experienced (Fig. 7.16) may simply be due to a return to more "normal" conditions,
such as existed in the early Middle Ages before the Little Ice Age. How, then, are we
going to tell which part of the recent warming is "natural" and which (if any) is due
to human influence? Clearly, this problem can only be attacked by looking at even
longer records, to extract the long-period variability of climate.
It is usually assumed that the Sun's radiation is constant - in fact, the average
irradiation of a surface facing the Sun at the mean Earth-Sun distance is referred to as
the "solar constant" (ca. l.95 cal/cm 2 /min). Unfortunately, the convenient assumption contained in this label is unfounded: The Sun's output is variable. On the scale
of centuries this is documented, for example, for changes in abundance of sun spots.
Using spectral analysis, we can see the effects of sun-spot cycles (22 yr and 11 yr) in
many tree-ring data, and even in some marine valVes (although less distinctly). So,
perhaps the Little Ice Age is part of a long-term solar cycle. Alternatively (or in
addition), it may owe its excess of severe winters to unusually intense volcanic
activity between the 15th and 19th centuries.
The methods of marine geology - or paleoceanography - can add to the store of
knowledge which we need to be able to say, eventually, "yes, we see the global
warming". The records recovered from corals and varves establish not only the amplitudes and frequencies of large-scale climatic variations (such as the "El Nifto"
phenomenon in the Pacific). They also tell us about the likelihood of abrupt climatic
change. Such changes, which may be defmed as unusually fast transitions from one set
of climatic conditions to another, occur on various time scales, with various amplitudes. In Section 9.2.4, a dramatic change taking but decades at the beginning of the
Holocene will be discussed. The last such change took place in the 1830s, perhaps as
a result of unusal volcanic activity. This change is seen both in tree rings and in marine
varves (in Santa Barbara Basin, off California). There is a possibility that such climate
changes are not completely reversible: a new condition may arise and stabilize itself,
although the factor which produced the change has ceased to operate.