New Multi-Model Study: 1.5C Hotter Globally By 2027 (Best Case); By 2023 (Worst Case)
In this study, the period that corresponds to the threshold of a 1.5 °C rise (relative to 1861–1880) in surface temperature is validated using a multi-model ensemble mean from 17 global climate models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). On this basis, the changes in permafrost and snow cover in the Northern Hemisphere are investigated under a scenario in which the global surface temperature has risen by 1.5 °C, and the uncertainties of the results are further discussed. The results show that the threshold of 1.5 °C warming will be reached in 2027, 2026, and 2023 under RCP2.6, RCP4.5, RCP8.5, respectively. When the global average surface temperature rises by 1.5 °C, the southern boundary of the permafrost will move 1–3.5° northward (relative to 1986‒2005), particularly in the southern Central Siberian Plateau. The permafrost area will be reduced by 3.43×106 km2 (21.12%), 3.91×106 km2 (24.1%) and 4.15×106 km2 (25.55%) relative to 1986–2005 in RCP2.6, RCP4.5 and RCP8.5, respectively. The snow water equivalent will decrease in over half of the regions in the Northern Hemisphere but increase only slightly in the Central Siberian Plateau. The snow water equivalent will decrease significantly (more than 40% relative to 1986–2005) in central North America, western Europe, and northwestern Russia. The permafrost area in the Qinghai-Tibetan Plateau will decrease by 0.15×106 km2 (7.28%), 0.18×106 km2 (8.74%), and 0.17×106 km2 (8.25%), respectively, in RCP2.6, RCP4.5, RCP8.5. The snow water equivalent in winter (DJF) and spring (MAM) over the Qinghai-Tibetan Plateau will decrease by 14.9% and 13.8%, respectively.
Keywords
Permafrost; Snow water equivalent; Northern Hemisphere; 1.5 °C global warming
1. Introduction
The cryosphere is a layer in which the Earth’s surface water exists in solid form; it includes glaciers (e.g., mountain glaciers, ice caps, polar ice caps, and ice shelves), frozen ground (seasonal frozen ground and permafrost), snow cover, solid-state precipitation, sea ice, river ice, lake ice, etc. Various components of the cryosphere are mainly distributed in the high latitudes and high-elevation areas, which are quite sensitive to changes in air temperature, which is known as the indicator of climate change. Historically, the cryosphere has often played a key role in triggering changes in climate and gradually forcing changes in the atmosphere. Global warming is the main characteristic of the Earth’s climate over the past century. The results of the global average linear trends in the land and sea surface temperatures indicate that the global average temperature rose by 0.85 °C between 1880 and 2012 (IPCC, 2013a). Over the past 30 years, the rise in temperature over every 10 year interval is higher than at any point in time since 1850. In the Northern Hemisphere (NH), 1983–2012 was likely the warmest 30 year period of the past 1400 years, and has an obvious warming trend in comparison with the previous decade or two (Qin, 2014). Under global warming, the cryosphere has responded to global warming very rapidly, and local glaciers, permafrost, and snow cover have changed markedly (Burke et al., 2013; Park et al., 2015; Rafiq and Mishra, 2016).
Permafrost, one of the main components of the cryosphere, covers approximately one quarter of the Earth's land area and is mainly distributed in the NH. Permafrost is highly sensitive to a warming climate. Its degradation is mainly represented based on the following three variables: the increase in permafrost temperature, the decrease in the extent of permafrost, and the deepening of the active layer thickness. Observations have shown that the permafrost in the Qinghai-Tibetan Plateau (TP), Canada, Alaska, Sweden, and other regions has degraded to various extents (Chen and Li, 2008; Slater and Lawrence, 2013). The temperature increases in permafrost have pronounced differences in different regions. The most significant increases in permafrost temperature were found in northern Alaska, Russia European North, which increased by 2–3 °C between 1971 and 2010 (Romanovsky et al., 2010; Oberman, 2012). The permafrost temperature at a depth of 6 m increased at the rate of 0.02 °C per year between 2006 and 2010 along the Qinghai-Tibet railway (Wu et al., 2012). The active layer thickness tended to deepen in the NH since the mid-1990s, with the most pronounced deepening found in Russia European North, eastern Siberia, and Chukotka. The active layer thickness declined slightly in Canada and western Siberia (IPCC, 2013b). The active layer thickness has increased at a rate of 0.71 cm per year in the eastern TP since 1980 (Zhang and Wu, 2012). The permafrost area in the Arctic decreased at a rate of 0.55×106–0.81×106 km2 per year between 1967 and 2000 (Burke et al., 2013). Warmer permafrost in Vorkuta, which is 10–15 m thick, completely melted between 1975 and 2005, and the southern boundary of the permafrost migrated 80 km northward (IPCC, 2013b).
Snow cover is the most widely distributed component of the cryosphere, and it experiences the most significant seasonal and interannual changes. Snow cover is sensitive to changes in temperature. Climate change, at any time or spatial scale, will trigger a response in snow cover. Satellite passive microwave data have shown that snow extent and snow water equivalent (SWE) in winter experienced a downward trend in the NH between 1978 and 2010, and snow extent increased slightly only in parts of Eurasia (Li et al., 2012; Liu and Li, 2013). Furthermore, snow extent decreased at a rate of (–0.55±0.21) ×106 km2 per decade in spring over the entire NH between the 1980s and the 21st century (Chen et al., 2015; Thackeray et al., 2016), but showed an apparent increase in autumn. The TP had regions with high values of seasonal snow cover (Wang et al., 2009a; Wang et al., 2009b) and presented the annual variation characteristic of alternating between higher and lower snow cover values over the past 40 years. In other words, snow cover in the TP experienced a negative anomaly prior to the 1970s, and then transitioned to a positive anomaly, before finally returning to a negative anomaly in the late 1990s. The snow cover in the TP showed a slight decreasing trend for the entire period of 1970–2010 (Bo et al., 2014). Additionally, glaciers and sea ice also experienced pronounced changes. The most dramatic changes in glaciers in the NH occurred in the European Alps and the Rocky Mountains in western North America, which decreased by approximately 40% and 30%, respectively, over the past few decades; the second most dramatic change occurred along the boundary of the TP, which decreased by approximately 23% (Wang et al., 2015a). The sea ice extent in the Arctic decreased at a rate of 5.91×104 km2 per year in the last two or three decades (Kong et al., 2016), and retreated most rapidly in summer (Liu et al., 2016).
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