Anthropogenic and Natural Contributions to the Lengthening of the Southern Hemisphere Summer Season

Abstract

This study provides the first quantitative assessment of observed long-term changes in summer season timing and length in the Southern Hemisphere (SH) and its sub-regions over the past 60 years, enabling a global completeness by complimenting such characteristics previously reported for the Northern Hemisphere (NH). Using an objective algorithm based on temperature indices, relative measures of summer onset, withdrawal, and duration are determined at each land location over the period 1953–2012. Significant widespread summer season lengthening, due to earlier onset and delayed withdrawal, has occurred across the SH, a longer period for extreme heatwave events and wildfires to potentially occur. The asymmetric magnitude (onset versus withdrawal) in summer season lengthening is slightly less over the SH compared to the NH. Contributions of anthropogenic and natural factors to the observed trends in summer season characteristics were investigated using Coupled Model Intercomparison Project phase 5 (CMIP5) multimodel simulations integrated with observed external forcings (anthropogenic plus natural, ALL), greenhouse-gas forcing only (GHG), and natural forcing only (solar and volcanic activities, NAT). Overall, consistent with the NH, increased greenhouse-gases were the main cause of observed changes in the SH with negligible contribution from other external forcings. ALL and GHG simulations also reproduced a slight tendency for earlier summer onset to contribute more to summer lengthening. Proportions of observed regional trends in summer season indices attributable to trends in long-term internal variability in the SH, namely the Interdecadal Pacific Oscillation (IPO) and Southern Annular Mode (SAM), suggests such variability can only explain up to ∼12%, supporting the dominant role of greenhouse-gas forcing.

Weakened amplitude and delayed phase of the future temperature seasonal cycle over China during the twenty‐first century

“…(2020) indicated that the surface warming induced by greenhouse gases and anthropogenic aerosols is stronger in winter than in summer in the Northern Hemisphere. Temperature seasonality changes caused by anthropogenic forcing can also be clearly detected in other regions, such as in the Southern Hemisphere (Park et al ., 2018; Santer et al ., 2018; Weller et al ., 2020), China (Qian and Zhang, 2019), and the Tibetan Plateau (Duan et al ., 2017). Besides anthropogenic forcing, natural variability also affects temperature seasonality (Tesouro et al ., 2005; Stine et al ., 2009).…”

“…For example, a trend toward decreased amplitude and advanced phase in the Northern Hemisphere average temperature during the twentieth century has been found (e.g., Mann and Park, 1996; Wallace and Osborn, 2002; Stine et al ., 2009). Statistically significant lengthening of the summer season was presented across the majority of land regions in the Southern Hemisphere during 1953–2012 (Weller et al ., 2020). Cassou and Cattiaux (2016) showed an earlier onset of summer by approximately 10 days over Europe between the 1960s and 2000s.…”

Changes in lengths of the four seasons in China and the relationship with changing climate during 1961–2020

“…The extension of summer and the shortening of winter have been detected around the globe in the past half‐century (e.g., Schwartz et al ., 2006; Dong et al ., 2010; Peña‐Ortiz et al ., 2015; Allen and Sheridan, 2016; Park et al ., 2018; Weller et al ., 2020; Wang et al ., 2021a; 2021b) and will continue and even strengthen in the future according to model projections (Hochman et al ., 2018; Wang et al ., 2021a). The changes in season lengths and onsets in China have also received some attention (e.g., Dong et al ., 2010; Yu et al ., 2010; Yan et al ., 2011) and have been found to be generally similar to hemisphere‐scale results.…”

Inter‐Model Differences in Future Summer Onset Over the Northern High Latitudes

“…In particular, the summer season timing and duration is important in line with the recent increases in heat wave events that caused tremendous damages across the globe (Pfleiderer et al., 2019; Zscheischler & Seneviratne, 2017). The observed summer season has lengthened over both hemispheres during the recent decades, which has been attributed to anthropogenic greenhouse warming (Allen & Sheridan, 2016; Park et al., 2018; Peña‐Ortiz et al., 2015; Weller et al., 2020). As the upcoming twenty‐first century faces unprecedented warming conditions (Diffenbaugh & Scherer, 2011; Masson‐Delmotte et al., 2021), summer season will be further expanded (Park et al., 2022; Ruosteenoja et al., 2019; J. Wang et al., 2021).…”