Mean growing season soil PCO, data were obtained for 19 regions of the woi.1 in nine countries. Bivariate anL multiple linear regression analysis with soil log(PC0,) as the dependent variable and TEMP, PRECIP, log (AET), and log(PET) as the four climatic independent variables demonstrated that AET was the best independent predictor of soil PCO, .An improved soil PC0,-AET model was developed by assuming (1) that as AET approaches zero, soil PCO, approaches the atmospheric value and (2) that there is an upper limit to soil PCO, at very high AET. This model has the form log(PC0,) = -3.47 + 2.09 (1 -e-0'00172 AET) where AET is in mm. It explains 67 per cent of the initial variation in the soil PCO, data, predicts a soil log(PC0,) of -3.47 at AET = 0, and an upper limit of 3.5 per cent (log(PC0,) = -1.45) for mean growing season soil PCO, at AET values of 2000mm and above. The results of this study suggest that soil PCO, levels in tropical areas are, on average, higher than those in temperate, alpine, and arctic regions.

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Stalagmite ANJ94-5 from Anjohibe Cave in northwest Madagascar suggest six distinct climate periods from 9.1 to 0.94 ka. Periods I and II (9.1-4.9 ka) were wetter and punctuated by a series of prominent droughts. Periods IV-VI (4-0.94 ka) were much drier and less variable. Period III (4.9-4 ka) marks the transition between wetter and drier conditions and consists of two significant droughts: the first (4.8-4.6 ka) coincides approximately with the end of the African Humid Period and the second (4.3-4.0 ka) may be the expression of the Northern Hemisphere 4.2 ka dry event in northwest Madagascar. Strong positive correlations between δ 13 C and δ 18 O values in Periods I-IV (r = 0.63-0.91) suggest that both isotopes were influenced by natural climate changes indicating that humans may not have been present in the area. In contrast, during Periods V (r = 0.07) and VI (r = −0.12) the "decoupling" of δ 13 C and δ 18 O might signal an impact from human activities starting around 2.5 ka. Rapid changes in climate during the early and middle Holocene, with prominent droughts lasting up to 800 years, did not kill off Madagascar's megafauna, and neither did a human population, present since the early Holocene if evidence from south Madagascar is reliable. However, many extinctions occurred under the more stable climatic conditions of the late Holocene, despite an antiphase climate relationship between northern and southcentral Madagascar. This suggests that initial human colonization, or significant increase in human population, triggered the megafaunal extinctions by hunting and destruction of megafaunal habitats.

Two stalagmites from Anjohibe Cave have annual layers made up of inclusion-rich calcite over inclusion-free calcite or of darker aragonite over clear aragonite. Geochemical evidence indicates that the basal units are deposited slowly in the wet season and the upper units more rapidly in the dry season. For the period with rainfall and temperature data (ad 1951–1992), layer thickness correlates well with the Southern Oscillation Index (SOI), as well as rainfall, water surplus, and actual evapotranspiration (AET) at nearby Majunga. Com parison of the layer record for one stalagmite with 1866–1994 SOI data indicates that layer thickness correlates best with the frequency and intensity of warm, low-phase SO (El Niño) events, not with average SOI conditions. In addition, the 415-year layer thickness time-series from that speleothem agrees remarkably well with historical records of El Niño frequency, with Galápagos (Ecuador) coral records of sea-surface temperature in the eastern Pacific, and with accumulation rates on the Quelccaya Ice Cap of Peru, which are lower at times of high El Niño frequency.