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The aim of this paper is to investigate the longevity and diversity of the Amazonian rainforest and to assess its likely future. Palaeoclimate and palaeoecological records suggest that the Amazon rainforest originated in the late Cretaceous and has been a permanent feature of South America for at least the last 55 million years. The Amazon rainforest has survived the high temperatures of the Early Eocene climate optimum, the gradual Cenozoic cooling, and the drier and lower carbon dioxide levels of the Quaternary glacial periods. Two new theories for the great diversity of the Amazon rainforest are discussed – the canopy density hypothesis and the precessional-forced seasonality hypothesis. We suggest the Amazon rainforest should not be viewed as a geologically ephemeral feature of South America, but rather as a constant feature of the global Cenozoic biosphere. The forest is now, however, entering a set of climatic conditions with no past analogue. The predicted future hotter and more arid tropical climates may have a disastrous effect on the Amazon rainforest. [ABSTRACT FROM AUTHOR]

Abstract Human impacts prior to the Industrial Revolution are not well constrained. We investigate whether the decline in global atmospheric CO 2 concentration by 7–10 ppm in the late 1500s and early 1600s which globally lowered surface air temperatures by 0.15∘C, were generated by natural forcing or were a result of the large-scale depopulation of the Americas after European arrival, subsequent land use change and secondary succession. We quantitatively review the evidence for (i) the pre-Columbian population size, (ii) their per capita land use, (iii) the post-1492 population loss, (iv) the resulting carbon uptake of the abandoned anthropogenic landscapes, and then compare these to potential natural drivers of global carbon declines of 7–10 ppm. From 119 published regional population estimates we calculate a pre-1492 CE population of 60.5 million (interquartile range, IQR 44.8–78.2 million), utilizing 1.04 ha land per capita (IQR 0.98–1.11). European epidemics removed 90% (IQR 87–92%) of the indigenous population over the next century. This resulted in secondary succession of 55.8 Mha (IQR 39.0–78.4 Mha) of abandoned land, sequestering 7.4 Pg C (IQR 4.9–10.8 Pg C), equivalent to a decline in atmospheric CO 2 of 3.5 ppm (IQR 2.3–5.1 ppm CO 2). Accounting for carbon cycle feedbacks plus LUC outside the Americas gives a total 5 ppm CO 2 additional uptake into the land surface in the 1500s compared to the 1400s, 47–67% of the atmospheric CO 2 decline. Furthermore, we show that the global carbon budget of the 1500s cannot be balanced until large-scale vegetation regeneration in the Americas is included. The Great Dying of the Indigenous Peoples of the Americas resulted in a human-driven global impact on the Earth System in the two centuries prior to the Industrial Revolution. Highlights • Combines multiple methods estimating pre-Columbian population numbers. • Estimates European arrival in 1492 lead to 56 million deaths by 1600. • Large population reduction led to reforestation of 55.8 Mha and 7.4 Pg C uptake. • 1610 atmospheric CO2 drop partly caused by indigenous depopulation of the Americas. • Humans contributed to Earth System changes before the Industrial Revolution. [ABSTRACT FROM AUTHOR]

Modelling simulations of palaeoclimate and past vegetation form and function can contribute to global change research by constraining predictions of potential earth system responses to future warming, and by providing useful insights into the ecophysiological tolerances and threshold responses of plants to varying degrees of atmospheric change. We contrasted HadCM3LC simulations of Amazonian forest at the last glacial maximum (LGM; 21 kyr ago) and a Younger Dryas-like period (13-12 kyr ago) with predicted responses of future warming to provide estimates of the climatic limits under which the Amazon forest remains relatively stable. Our simulations indicate that despite lower atmospheric CO2 concentrations and increased aridity during the LGM, Amazonia remains mostly forested, and that the cooling climate of the Younger Dryas-like period in fact causes a trend toward increased above-ground carbon balance relative to today. The vegetation feedbacks responsible for maintaining forest integrity in past climates (i.e. decreased evapotranspiration and reduced plant respiration) cannot be maintained into the future. Although elevated atmospheric CO2 contributes to a positive enhancement of plant carbon and water balance, decreased stomatal conductance and increased plant and soil respiration cause a positive feedback that amplifies localized drying and climate warming. We speculate that the Amazonian forest is currently near its critical resiliency threshold, and that even minor climate warming may be sufficient to promote deleterious feedbacks on forest integrity.