Continental-scale impacts of intra-seasonal rainfall variability on simulated ecosystem responses in Africa

K Guan,H Sato,S P Good,H Li,K K Caylor,E F Wood

doi:10.5194/bg-11-6939-2014

Abstract

Abstract. Climate change is expected to modify intra-seasonal rainfall variability, arising from shifts in rainfall frequency, intensity and seasonality. These intra-seasonal changes are likely to have important ecological impacts on terrestrial ecosystems. Yet, quantifying these impacts across biomes and large climate gradients is largely missing. This gap hinders our ability to better predict ecosystem services and their responses to climate change, especially for arid and semi-arid ecosystems. Here we use a synthetic weather generator and an independently validated vegetation dynamic model (SEIB-Dynamic Global Vegetation Model, DGVM) to virtually conduct a series of "rainfall manipulation experiments" to study how changes in the intra-seasonal rainfall variability affect continent-scale ecosystem responses across Africa. We generate different rainfall scenarios with fixed total annual rainfall but shifts in (i) frequency vs. intensity, (ii) rainy season length vs. frequency, (iii) intensity vs. rainy season length. These scenarios are fed into SEIB-DGVM to investigate changes in biome distributions and ecosystem productivity. We find a loss of ecosystem productivity with increased rainfall frequency and decreased intensity at very low rainfall regimes (<400 mm year−1) and low frequency (<0.3 event day−1); beyond these very dry regimes, most ecosystems benefit from increased frequency and decreased intensity, except in the wet tropics (>1800 mm year−1) where radiation limitation prevents further productivity gains. This result reconciles seemingly contradictory findings in previous field studies on the impact of rainfall frequency/intensity on ecosystem productivity. We also find that changes in rainy season length can yield more dramatic ecosystem responses compared with similar percentage changes in rainfall frequency or intensity, with the largest impacts in semi-arid woodlands. This study demonstrates that intra-seasonal rainfall characteristics play a significant role in influencing ecosystem function and structure through controls on ecohydrological processes. Our results suggest that shifts in rainfall seasonality have potentially large impacts on terrestrial ecosystems, and these understudied impacts should be explicitly examined in future studies of climate impacts.

Highlights

Due to increased water holding capacity in the atmosphere as a consequence of global warming (O’Gorman and Schneider, 2009), rainfall is projected to change in intensity and frequency across much of the world (Easterling et al, 2000; Trenberth et al, 2003; Chou et al, 2013), in conjunction with complex shifts in rainfall seasonality (Feng et al, 2013; Seth et al, 2013)
We further explore the GPP sensitivity space (Fig. 5e and g), and find the following robust patterns: Pattern 2.1: The negative GPP sensitivity tends to happen where mean annual precipitation (MAP) is below 1000 mm year−1 with long rainy season length (Tw> 150 days) and low rainfall frequency (λ < 0.35 event day−1)
We provide a new modeling approach to systematically study the ecological impacts from changes in intra-seasonal rainfall characteristics across various biomes and large climate gradients in the African continent

Summary

Introduction

Due to increased water holding capacity in the atmosphere as a consequence of global warming (O’Gorman and Schneider, 2009), rainfall is projected to change in intensity and frequency across much of the world (Easterling et al, 2000; Trenberth et al, 2003; Chou et al, 2013), in conjunction with complex shifts in rainfall seasonality (Feng et al, 2013; Seth et al, 2013) These changes possibly indicate a large increase in the frequency of extreme events and variability in rainfall (Easterling et al, 2000; Allan and Soden, 2008), and many of these changes may be accompanied with little change in total annual rainfall (Knapp et al, 2002; Franz et al, 2010). Understanding these impacts and their possible future changes on terrestrial ecosystems is critical for maintaining ecosystem services and planning adaptation and mitigation strategies for ecological and social benefits under climate change (Anderegg et al, 2013), especially for arid and semi-arid regions, which covers one third of the land surface

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