S04.03 New observations suggest vulnerability of the carbon sink in tropical rainforests

to be a net sink for carbon (C) in the future is an important and open question. The current expectation that rainforests will increasingly absorb anthropogenic C emissions in this century is largely based on coupled global modelling with some support from field studies. However, by considering long-term demographic processes such as tree mortality, we report that the capacity of rainforests to store C is more likely to decrease with global warming by approximately 20 Pg C per degree of warming, implying more rapid growth of global atmospheric CO2 and consequent climate change than is now anticipated. Specifically, we identify a broad, pantropical pattern of decreased above-ground biomass of rainforests growing in warmer climates that is most likely to be due to increased rates of tree mortality and that is explainable by constant mortality in thermal time. Considering 116 pantropical rainforest plots with various long-term mean annual temperatures (T, oC), basal area (BA), a measure that closely relates to total biomass and C stocks (BA, m2 of tree stems ha-1), declines on average by 2.08 m2 ha-1 oC-1. This decline has not been documented previously or factored into global climate models. This pantropical pattern of lower biomass in warmer climates implies a reduced capacity of rainforests to sequester C with global warming. To understand the causes of this broad pattern, we analysed data from North East Australia where 8210 trees in sixteen rainforest plots were measured over a period of 35 years, with T from 18.3 to 25.8 oC and mean annua