Posted on 10/18/2002 5:34:27 PM PDT by Lessismore
Tropical climate fluctuates on various time scales, from interannual (exemplified by the strong 1997-1998 El Niño) to thousands of years (such as the greening of the Sahara from ~10,000 to ~5500 years ago) (1). At present, 75% of the world's inhabitants live in the tropics. Low-latitude climate fluctuations thus have a considerable societal impact. Because instrumental records are short, paleoclimate records are essential to understanding the full range of tropical climate variability, its effects on Earth's energy budget and water vapor cycle, the connections between high- and low-latitude climate, and the sensitivity of the tropics to future climate change.
On page 589 of this issue, Thompson et al. (2) present the first ice core record from Africa. The record complements ice core evidence for tropical climate change from other continents (3). The new record comes from Kilimanjaro, Africa's highest peak, which lies near the equator in the East African monsoon region. The nearly continuous, high-resolution record spans the entire Holocene from ~11,500 years ago to the present. The authors infer temperature fluctuations from changes in the ice oxygen isotope ratio, d18O. They use high concentrations of insoluble dust and major aerosol species as fingerprints of large-scale drying trends, and peaks in F- and Na+ as possible indicators of more local erosion during strong but brief drying events.
At the millennial time scale, Thompson et al. (2) distinguish two climate phases: conditions warmer and wetter than today from ~11,000 to ~4000 years ago, and a relatively dry and cool climate over the past ~4000 years. This pattern agrees with numerous continental and marine records showing that the monsoon circulation and related rainfall over the northern tropics and equatorial East Africa were considerably stronger during the early to mid-Holocene as a result of changes of Earth's orbit around the Sun (1, 4).
Superimposed on these general trends are abrupt events on time scales of decades or centuries that strongly affect human societies. Understanding these short-term variations requires accurate and well-dated records. Unfortunately, dating tropical ice cores is a difficult exercise. The depth-age model used by Thompson et al. (2) does not provide an absolute chronology. The small amount of organic material does not yield consistent 14C ages. The authors assign ages to three ice layers. The 1952 layer is identified accurately on the basis of a 36Cl peak from a thermonuclear bomb test. The age for the core base derives from comparison with an Eastern Mediterranean speleothem. The third age is based on assumed relationships between d18O minima and solar activity minima in the upper part of the sequence.
These correlations are questionable and the precise timing of climate events proposed by the authors should be regarded with caution. Nonetheless, the reliability of the Kilimanjaro ice core chronology seems to be supported by its coherence with the timing of prominent known events.
Thompson et al. (2) suggest a substantial cooling in equatorial Africa during the Little Ice Age (from ~1270 to 1850), a cold spell in Europe and many other parts of the world (3, 5, 6). This large-scale event led to different hydrological responses over East Africa and the western Indian Ocean. Lake Malawi, in the southern East African tropics, experienced a period of low water level (5). In contrast, the Lake Naivasha basin in Kenya was relatively wet (7), especially during the Maunder Minimum of solar activity, when a minimum in monsoon strength occurred in the Arabian Sea (8). This illustrates the complexity of Little Ice Age climate variability in the region.
The authors add new evidence for three major dry spells at ~4000, ~5200, and ~8300 years before the present, already identified at some sites in the northern monsoon domain and related to large-scale events (1, 4, 9). A drought ~4000 years ago, linked to societal disturbances in Egypt, Mesopotamia, and India, has been identified in the Middle East and North Africa (4) and associated with a cooling in the North Atlantic (10).
The rapid drying and cooling event ~5200 years ago falls within the abrupt termination of the "African humid period" in the northern tropics. This event can be explained by feedback processes associated with changes in sea surface temperature and vegetation cover, which amplified the climate response to gradual changes in solar radiation received at the top of Earth's atmosphere (11).
The tropical drought at 8300 years before the present corresponds to a brief cooling in northern high latitudes and a global reduction in the atmospheric methane concentration (5). An oxygen isotope study of a stalagmite from Oman, where high d18O values are ascribed to reduced monsoon rainfall intensity, suggests that this centennial decrease in tropical precipitation was primarily controlled by changes in solar activity. These changes likely induced changes in atmospheric or ocean circulation that amplified the response to solar input change (9).
Thompson et al. (2) assume a positive relation between d18O and air temperature, and thus they interpret a marked d18O depletion from 6500 to 5200 years ago as a substantial cooling. A mid-Holocene cooling-wetting in equatorial East Africa has been suggested from glacier advance, pollen, and lake sediment records (12).
But important questions arise from the short-term d18O-inferred temperature fluctuations at Kilimanjaro. In the tropics, the d18O values of rainfall exhibit a far stronger correlation with rainfall amount than with air temperature (13). The few detailed lake and stalagmite oxygen isotope records available in the East African-South Asian monsoon domain (9, 14, 15) have thus served as a proxy for variations in monsoon rainfall.
Recently, a postglacial oxygen record from diatom silica in some alpine lakes at Mount Kenya, supported by a well-constrained 14C chronology (15), was interpreted by considering the factors governing the regional isotopic rainfall composition. The authors (15) concluded that centennial- to millennial-scale d18O fluctuations primarily reflect variations in moisture balance and cloud height driven by sea surface temperature anomalies over the tropical South Indian Ocean.
The Mount Kenya and Kilimanjaro isotope profiles show similarities (see the figure), but the marked d18O depletions at ~6500 to 5200 years before the present were interpreted differently: Barker et al. argue that they reflect anomalously heavy snowfall (15), while Thompson et al. interpret them in terms of a substantial cooling (2).
The relative roles of temperature, water vapor trajectory, and precipitation amount on the tropical-montane isotope records thus remain controversial (15). Improving the global network for isotopic composition of precipitation (GNIP) (16) should resolve this question by helping to calibrate the tropical rainfall d18O composition in terms of climate parameters.
The unique ice core record of African climate presented by Thompson et al. (2) is also probably the last. The Kilimanjaro ice fields are shrinking fast in response to global warming, as are other tropical glaciers (3, 13). If the climatic trends of the 20th century continue, the ice on Kilimanjaro will disappear in the next 15 to 20 years (2). In the tropics, human societies suffer more from declining or irregular water resources than from changes in temperature. But global warming may have serious implications for local populations that depend on glacier meltwaters for farming, irrigation, or hydroelectric power. There is also an urgent need to collect high-quality cores from tropical glaciers that will not preserve paleoclimatic archives for much longer.
Then, in his last paragraph, he trots out the great bugaboo of faux science, "global warming". In fact, he has in hand evidence of this phenomenon -- the shrinking tropical glaciers. But, from the context, I don't believe he relates today's "global warming" to the variability he has been writing about for the previous 14 paragraphs.
E.g., he accepts the Little Ice Age (1270-1850) as perfectly natural, but would probably ascribe "global warming" to the automobile...
Not necessarily true. There would likely still be ice in cracks and crevices in the mountain surface even if the glaciers disappeared.
Water arrives from cometary particles and the like constantly
and has done so, presumably, since the planet's formation.
"Constantly" in planetary terms, yes, but not that many have hit the earth in the "few moments" (again in planetary terms) that man has been around.
Please note I said "Water arrives from cometary particles..."
It is an ongoing thing.
Every few seconds a "snowball" the size of a
small house breaks up as it approaches Earth
and deposits a large cloud of water vapor in
Earth's upper atmosphere.
Well, then we better get busy sending some H2O to the moon in order to assist God with keeping the planet in balance.
Nah. Remember how they used to tell us to drink
12 glasses of water a day? And now they say, don't?
For Gaia's sake, DO !
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