labs_title

Global land carbon sink response to temperature and precipitation varies with ENSO phase

Y. Fang, A.M. Michalak, C.R. Schwalm, D.N. Huntzinger, J.A. Berry, P. Ciais, S. Piao, B. Poulter, J.B. Fisher, R.B. Cook, D. Hayes, M. Huang, A. Ito, A. Jain, H. Lei, C. Lu, J. Mao, N.C. Parazoo, S. Peng, D.M. Ricciuto, X. Shi, B. Tao, H. Tian, W. Wang, Y. Wei and J. Yang

The response of terrestrial carbon uptake to climate change remains a major source of uncertainty in projecting future climate. One way to reduce this uncertainty is to assess the sensitivity of terrestrial carbon uptake to key drivers under varying climate conditions. In the case of the El Nino-Southern Oscillation (ENSO), for example, existing studies disagree on whether temperature or precipitation drives the ENSO response of the tropical land biosphere. Here, we find that the dominant driver in fact depends on the phase of ENSO, with temperature dominating post El Nino and precipitation dominating post La Nina. Furthermore, mechanistic models of the terrestrial biosphere do not capture this phase-dependence, implying potential biases in modeling climate impacts on future terrestrial carbon uptake. Our study highlights the need to better understand the joint impacts of heat and water on tropical ecosystem.


Figure: The response of the annual atmospheric growth rate of CO2 (AGR) to tropical mean annual temperature (TMAT) and precipitation (TMAP) differs by ENSO phase, and models do not capture this response under any ENSO conditions. Filled circles represent partial correlations between detrended anomalies of AGR and TMAT, and between detrended anomalies of AGR and TMAP. Whiskers represent uncertainties of correlations. Ellipses represents the one-standard deviation envelope for the distribution specified by the mean and covariance of the same partial correlations estimated using detrended annual tropical NEE as simulated by ten terrestrial biosphere models in place of AGR. Empty diamonds represent the mean correlations across these models, while full diamonds represent partial correlations derived using the ensemble average NEE across all models. In all cases, correlations are calculated over the period 1959–2010, with black representing all years, red indicating post El Niño, and blue indicating post La Niña. The pink, blue and grey shaded areas delineate the range for significant partial correlations (p < 0.05) for post El Niño (|r| > 0.46), post La Niña ((|r| > 0.44) and all year (|r| > 0.28) cases using a two-tailed test.

Abstract

Climate variability associated with the El Niño-Southern Oscillation (ENSO) and its consequent impacts on land carbon sink interannual variability have been used as a basis for investigating carbon cycle responses to climate variability more broadly, and to inform the sensitivity of the tropical carbon budget to climate change. Past studies have presented opposing views about whether temperature or precipitation is the primary factor driving the response of the land carbon sink to ENSO. Here, we show that the dominant driver varies with ENSO phase. Whereas tropical temperature explains sink dynamics following El Niño conditions (r TG,P = 0.59, p < 0.01), the post La Niña sink is driven largely by tropical precipitation (r PG,T = -0.46, p = 0.04). This finding points to an ENSO-phase-dependent interplay between water availability and temperature in controlling the carbon uptake response to climate variations in tropical ecosystems. We further find that none of a suite of ten contemporary terrestrial biosphere models captures these ENSO-phase-dependent responses, highlighting a key uncertainty in modeling climate impacts on the future of the global land carbon sink.

Fang, Y., A.M. Michalak, C.R. Schwalm, D.N. Huntzinger, J.A. Berry, P. Ciais, S. Piao, B. Poulter, J.B. Fisher, R.B. Cook, D. Hayes, M. Huang, A. Ito, A. Jain, H. Lei, C. Lu, J. Mao, N.C. Parazoo, S. Peng, D.M. Ricciuto, X. Shi, B. Tao, H. Tian, W. Wang, Y. Wei, J. Yang (2017) "Global land carbon sink response to temperature and precipitation varies with ENSO phase", Environmental Research Letters, 12:064007, doi:10.1088/1748-9326/aa6e8e.