Global positive gross primary productivity extremes and climate contributions during 1982–2016

Highlights

• Summer in the Northern Hemisphere is vital for the global positive GPP extremes.

• Grasslands contributed the most to positive GPP extremes.

• Sufficient precipitation would boost the global terrestrial ecosystem's carbon uptake to form positive GPP extremes.

• Global warming will not be conducive to the carbon uptake of the terrestrial ecosystem.

Abstract

Gross primary production (GPP) quantifies the photosynthetic uptake of carbon by the terrestrial ecosystem. Positive GPP extremes represent the potential capacity of the terrestrial ecosystem to uptake carbon dioxide. Studying the positive GPP extreme is vital for the global carbon cycle and mitigation of global warming. With increasing climate extreme events, many kinds of research focus on studying negative GPP and the negative impact of climatic extremes on GPP. There is still a lack of research on positive GPP extremes and whether climatic extremes could be beneficial to global carbon uptake. In this study, we used daily Boreal Ecosystem Productivity Simulator (BEPS) to simulate GPP of the global terrestrial ecosystem during 1982–2016 and combined TRENDY models to detect positive GPP extremes and investigate the effects of climate extremes on GPP. We found the results of the TRENDY models have large differences in some areas of the globe, and the BEPS model driven by remote sensing data could be more suitable for simulating the long-term time series of global terrestrial GPP. Compared to other plant functional types, grasslands contributed the most to positive GPP extremes, accounting for approximately 41.6% (TRENDY) and 34.8% (BEPS) of the global positive GPP extremes. The probabilities of positive GPP extremes caused by positive precipitation extremes were significantly higher than those caused by temperature and radiation in most areas of the globe, indicating that sufficient precipitation (not a flood) would boost the carbon uptake ability of the global terrestrial ecosystem to form positive GPP extremes. On the contrary, the partial correlation coefficients between temperature and GPP were negative in most areas of globe, suggesting that global warming will not be conducive to carbon uptake of the terrestrial ecosystem. This study may provide new knowledge on the global positive GPP extremes.

Introduction

Climate change is and will continue to impact the natural environment and human well-being (Parry et al., 2007; Frank et al., 2015). Current projections, based upon contrasted emission scenarios, suggest somewhere between 0.3 and 4.8 °C warmings by the end of this century (IPCC, 2013). Moreover, the frequency and intensity of climate extremes are projected to further increase in the mid-to-late 21st century due to the ongoing global warming (Niu et al., 2018; Sui et al., 2018). Climate change, especially climate extreme events, may have a potential impact on the terrestrial carbon cycle (Du et al., 2018; von Buttlar et al., 2018). Terrestrial ecosystems are more sensitive to climate extremes than to gradual climate change because they typically show greater response strengths during shorter response times to these extremes (Frank et al., 2015; Hanson et al., 2006). Global climate extremes could affect the composition, structure, and function of the terrestrial ecosystem (Frank et al., 2015). Therefore, studying the impact of climate extremes on the global carbon cycle is crucially important (Ciais et al., 2005; Schwalm et al., 2012; Seneviratne et al., 2012).

Gross primary production (GPP) quantifies the amount of carbon dioxide fixed into organic compounds through photosynthesis by land plants (Campbell et al., 2017), and it is the basis of the global carbon cycle. GPP is the largest carbon flux in the global terrestrial ecosystem, driving a variety of ecological functions, such as vegetation respiration, growth, etc. Changes in GPP may significantly affect the global carbon cycle (W.Z. Chen et al., 2019).

Extreme events are generally defined as statistically unusual episodes or occurrences, which are beyond the bounds of typical or normal variability (Reichstein et al., 2013). Positive extremes represent a statistically unusual episode in a positive direction, which opposite to negative extremes. The most widely definition of extremes is the anomaly departing one or more standard deviations from the average value (Liu et al., 2013; Linthicum et al., 1999; Xu et al., 2012). In this study, after detrending the GPP time series, positive GPP extremes are defined as GPP at least 1.5 times the standard deviation higher than the mean value.

Positive GPP extremes represent the potential capacity of the terrestrial ecosystem to uptake carbon dioxide. Studying negative GPP extremes and their control mechanisms is an important step for developing adaptation strategies and risk reduction in the context of future climate change (W.Z. Chen et al., 2019). Comparing to negative GPP extremes, studying positive GPP extremes and their control mechanisms are useful for finding optimal climate conditions for vegetation growth, which are beneficial for developing planting strategies. Moreover, studying the positive GPP extreme is vital for the global carbon cycle and mitigation of global warming.

There are massive studies focusing on the negative impact of climate extremes on the global carbon cycle, such as extreme droughts (Lewis et al., 2011; Phillips et al., 2009), heat waves (Ciais et al., 2005; Lesk et al., 2016; Reichstein et al., 2007), wind storms (Sun et al., 2012), ice storm (Sun et al., 2012; Stone, 2008), and other climate extremes (Hong et al., 2011; Poulter et al., 2014). These climate extremes exerted huge impacts on the terrestrial ecosystem. For example, a European heatwave in 2003 caused an amount of CO₂ loss in Western European equivalent to that had been absorbed from the atmosphere during the previous three to five years under normal weather conditions (Ciais et al., 2005; Vetter et al., 2008). During 2000–2004, drought results in carbon sink reduction of 30–298 TgC yr⁻¹ in the western North America (Schwalm et al., 2012). In the 1999 storm, Lothar wind storms reduced 30% of Europe's carbon sink (Lindroth et al., 2009). But not all climate extremes can cause extreme impacts in terrestrial ecosystems (Frank et al., 2015).

There are also some studies that focus on negative GPP extremes and their control mechanisms. For example, W.Z. Chen et al. (2019) used monthly GPP simulated by ecological models to analyze negative GPP extremes in China during 1982–2015, and found that climate extremes decreased China's annual GPP by 2.8%. Furthermore, they reported that GPP negative anomalies can be explainable by drought in northern China and by temperature extremes in southern China. Reichstein et al. (2013) explored the mechanisms and impacts of climate extremes on the terrestrial carbon cycle. They investigated the control mechanisms of the hundred largest negative GPP extreme events during 1982–2011, and indicated that there are 56% of negative events caused by water scarcity and 14% of events by extremely high temperatures. Zscheischler et al. (2014) pointed out that GPP extremes are associated with climate anomalies; and more specifically, negative GPP extremes are mostly associated with droughts.

However, the effects of climate extremes on the global carbon cycle are diverse; not all climate extreme events will adversely affect the terrestrial ecosystem. Wang et al. (2018) used terrestrial biosphere models to simulate China's terrestrial ecosystem GPP during 1982–2015 and found that favorable climate extreme events contributed to positive GPP extremes in the terrestrial ecosystem in China. But there is still a lack of study on positive GPP extremes and their drivers on a global scale. Most previous studies mainly focus on negative climate extremes and their impacts, but do not analyze whether climate extremes are beneficial to global carbon uptake.

In this study, we used the BEPS (Boreal Ecosystem Productivity Simulator) and TRENDY models to detect positive GPP extremes, and investigated the relationship between GPP extremes and climate extreme events on a global scale during the period from 1982 to 2016. The specific objectives are: (i) to explore the spatial patterns of global positive GPP extremes and the contribution of Plant Function Types (PFTs) during 1982–2016; (ii) to investigate the effects of positive extremes of temperature, precipitation, and solar radiation on the carbon uptake of the global terrestrial ecosystem. The outcome of this study may provide new knowledge on the impact of positive climate extremes on the productivity of global terrestrial ecosystems.