Predicting the response of the Amazon rainforest to persistent drought conditions under current and future climates: a major challenge for global land surface models

Abstract

Abstract. While a majority of global climate models project drier and longer dry seasons over the Amazon under higher CO 2 levels, large uncertainties surround the response of vegetation to persistent droughts in both present-day and future climates. We propose a detailed evaluation of the ability of the ISBA CC (Interaction Soil-Biosphere-Atmosphere Carbon Cycle) land surface model to capture drought effects on both water and carbon budgets, comparing fluxes and stocks at two recent throughfall exclusion (TFE) experiments performed in the Amazon. We also explore the model sensitivity to different water stress functions (WSFs) and to an idealized increase in CO 2 concentration and/or temperature. In spite of a reasonable soil moisture simulation, ISBA CC struggles to correctly simulate the vegetation response to TFE whose amplitude and timing is highly sensitive to the WSF. Under higher CO 2 concentrations, the increased wateruse efficiency (WUE) mitigates the sensitivity of ISBA CC to drought. While one of the proposed WSF formulations improves the response of most ISBA CC fluxes, except respiration, a parameterization of drought-induced tree mortality is missing for an accurate estimate of the vegetation response. Also, a better mechanistic understanding of the forest responses to drought under a warmer climate and higher CO 2 concentration is clearly needed.

Improving the ISBA<sub>CC</sub> land surface model simulation of water and carbon fluxes and stocks over the Amazon forest

“…4.2, this is due to the increased LE in the PS and PS+R versions. Note that the ISBA CC soil moisture content was also successfully evaluated at K67 and at Caxiuanã (Joetzjer et al, 2014;Fig. 3, top panels).…”

“…It had never been tested on mature trees and tropical species and does not perform well when tested in the Amazon as shown below. Therefore, we propose an alternative parameterization assuming a constant f 0 coherent with in situ observations (Domingues et al, 2007) and validated against the two artificial drought experiments completed in the eastern Amazon (Joetzjer et al, 2014, and references within). Later in this paper, we call version PS ISBA CC with these different values of A m,max , f 0 and the modified WSF.…”

Development and evaluation of CNRM Earth system model – CNRM-ESM1

“…This carbon cycle module evolved from the physically based Interactions between Soil, Biosphere and Atmosphere (ISBA) model (Noilhan and Mahfouf, 1996;Noilhan and Planton, 1989) and is able to simulate the surface carbon fluxes and the terrestrial carbon pools. The carbon fluxes module was extensively tested over France and Europe (Sarrat et al, 2007;Szczypta et al, 2012), and the carbon cycle module was tested for temperate and high-latitude regions (Gibelin et al, 2006 and was used more recently in studies of carbon cycling over the Amazon basin (Joetzjer et al, 2015(Joetzjer et al, , 2014, permafrost regions (Rawlins et al, 2015) and at global scale . In this work, this terrestrial carbon cycle module is coupled to a global climate model for the first time.…”

Modelling Freshwater Resources at the Global Scale: Challenges and Prospects

“…In case of 4°C global warming, the high temperatures lead to decreased vegetation cover, such that runoff as a ratio of precipitation is projected to increase worldwide (Murray et al 2012). Large uncertainties also surround the response of vegetation to persistent droughts in both present-day and future climates, especially over the Amazon rainforest (Joetzjer et al 2014).…”