Observations on Late Pleistocene cooling and precipitation in the lowland Neotropics

Abstract

Although accurate reconstructions of Amazonian palaeoclimates are central to understanding the distribution and history of Neotropical biodiversity, current reconstructions based on proxy data are discordant and subject to intense debate. We review some current thinking in Amazonian climatology and incorporate some new ideas in an attempt to explain the apparently contradictory records of palaeoprecipitation from across the Amazon Basin. We suggest that palaeoecologists need to recognise that organised convective radiation, the process that induces most wet season rainfall in Amazonia, should be treated as a phenomenon related to, but separate from, the passage of the inter-tropical convergence zone (ITCZ). Decoupling ITCZ migration from larger convective activity provides a mechanism to account for observed palaeoclimatic heterogeneity. Patterns of observed precipitation change are consistent with the long-term persistence of closed forest across much of Amazonia, indicating that the greatest changes in precipitation during the last glacial maximum came during the wet season, which would have little negative impact on forest extent. Neotropical cooling at the last glacial maximum (LGM) is widely accepted, although the estimates of that cooling range between 1 C and > 5 C. In answering the basic question 'What is meant by cooling?' we observe that interhemispheric ice mass asymmetry may have caused cooling to be manifested differently according to location. A terrestrial cooling of ca. 5 C, as well as radiative cooling and event-based cooling combined to induce vegetation change. Probably, both absolute temperature and mean monthly minima were reduced by polar air incursions in the northern Neotropics. However, in the southern Neotropics, mean monthly minima were reduced by more frequent incursions of Patagonian air masses, but absolute minima may have been largely unchanged.

Hydrology and climatology at Laguna La Gaiba, lowland Bolivia: complex responses to climatic forcings over the last 25 000 years

“…It is interesting to note that today, during the austral winter, high winds that accompany cold polar fronts from the southern high latitudes (locally known as 'surazos'), mix the shallow Laguna La Gaiba. We can speculate that these may have been more common during the last glacial, a suggestion also made by Bush and Silman (2004) and Cruz et al (2006).…”

“…Such an explanation might also explain the contrast in full glacial conditions between Laguna la Gaiba and NE and SE Brazil noted by Whitney et al (2011). A similar mechanism has been suggested by Bush and Silman (2004). Again, it is difficult to place the Altiplano sites into this framework as convection over Amazonia is usually identified as the main moisture source for this area, although Vizy and Cook (2007) Some of the complexity of the YD across the tropical Americas is discussed by Bush and Metcalfe (2012).…”

Synergisms among Fire, Land Use, and Climate Change in the Amazon

“…In addition, the draconian projections of Cox et al seem at variance with known historical changes in the Amazon. Despite considerable Pleistocene cooling and drying, for instance, Amazonian forests were evidently more stable in their geographic distribution than was previously thought (77,78). Finally, regional circulation models (RCMs), which better represent local topography, geographic features, and land-cover changes than do GCMs (79), suggest that Amazonian vegetation might be surprisingly resistant to climate change (78,80).…”

Influence of seasonal variation on the hydro-biogeochemical characteristics of two upland lakes in the Southeastern Amazon, Brazil

“…However, the impacts of these land use changes may not yet exceed the magnitude of natural variability of hydrologic cycles, but they do signal that transition to a disturbance-dominated regime must be considered as a climate driver (Davidson et al 2012). Considering the effects of climate seasonality on lacustrine environments, it is widely demonstrated that the rainfall regime in short time scales has a direct influence on lake levels of the Amazon region (Bush et al 2002, Bush and Silman 2004, Bush and Metcalfe 2012. However, information related to how long lake levels respond to daily or monthly rainfall variations is almost unknown.…”