Global warming and ice sheet melting:Portents of a Younger dryas-like stadial eventby Andrew Glikson
Linear climate projections by the IPCC are difficult to reconcile with the paleoclimate evidence of stadial cooling events which closely succeeded warming peaks, including the Younger dryas (12.9–11.6 kyr ago), Laurentian melt (~8.3 kyr) and earlier interglacial stadials. Each of these events followed peak interglacial temperatures, leading to extensive melting of the ice sheets and transient stadial cooling events. Current global temperature rises in the range of ~ +1.19 ± 0.13 °C (Northern Hemisphere) and higher in the Arctic are consistent with this pattern, leading to the build-up of ice melt pools south of Greenland and around Antarctica. The growth of these pools is likely to progress toward large-scale to a global stadial, inducing differential warming and cooling effects leading to major weather disruptions and storminess, possibly analogous to the Younger dryas and Laurentian melt events.
Linear temperature rise projections by the IPCC are unlikely in view of (1) amplifying feedbacks of greenhouse gases and global warming on land and ocean, and (2) stadial cooling effects due to the flow of ice melt water from the large ice sheets into the North Atlantic Ocean and the circum-Antarctic ocean (Figure 1). Apart from the absolute GHG level (~500 ppm CO₂-equivalent), the high rise rate of ~2-3 ppm CO₂/year and thereby temperature is driving dangerous weather events. The extreme rise in greenhouse gases in the atmosphere is evident from a comparison with past climate events (Figure 6). Linear temperature projections and thereby environment change are complicated by storminess due to collisions between air masses of contrasted temperatures. As the Arctic jet stream weakens, warm air currents from the south and freezing air masses from the north cross the boundary, a pattern already manifested by Arctic heat waves and fires and by penetration of freezing air masses into mid-latitudes, i.e. the “Beast from the East” snow storms. The increasing extent of cold ice melt pools around Greenland and Antarctica (Figure 1) suggest such a process is already in progress, signifying an onset of an interglacial stadial, as modelled by Hansen et al. 2016 and Bronselaer et al. (2016).
Late Pleistocene climate cycles were controlled by orbital parameters of the Milankovitch cycles including eccentricity (~100,000 years), obliquity to the ecliptic plane (~41,000 years) and precession/wobble of the Earth’s axis (~19,000 and ~23,000 years). The Younger dryas of 12,900 to 11,600 years ago following the Allerod BÖlling warm peak and marked by cooling of near -20°C in Greenland and (Figure 2A, B), has major implications for climate change projections for the 21-23rd centuries.
The Younger dryas is the longest of three late Pleistocene stadials (Figure 2A) associated with abrupt climatic changes that took place over the last 16,000 years. According to Steffensen et al. 2008 based on deuterium isotopes in ice cores the abrupt onset of the Younger dryas in Greenland occurred over less than 1 year and ended over less than 3 years (Figure 2B), or about 50 years based on stable water isotopes representing the air temperature record. Evidence for the effects of the Younger dryas stadial has also been identified in tropical and subtropical regions (Shakun and Carlson, 2010) (Figure 3). The underlying factors for the Younger dryas and Laurentian (Figure 4) stadial events are the deglaciation of Northernmost America, flow of cold ice melt water into the North Atlantic Ocean and into North American lakes (Lake Agassiz), and the retreat southward of the North Atlantic Thermohaline Current.
Suggestions of a comet impact origin of the Younger dryas are inconsistent with (1) the recurrence of stadial events following peak interglacial temperatures over the last 420,000 years (Figure 5) and (2) the paucity of clear evidence for a large extraterrestrial impact contemporaneous with the Younger dryas, including the little known age of the radar-detected crater below the Hiawatha Glacier In northwest Greenland.
|Figure 2B. deuterium evidence for onset cooling temperature and terminal
warming of the Younger dryas stadial event (14,740-11,660) (Steffensen et al. 2008).
|Figure 3. Magnitude of late Holocene glacial-interglacial temperature changes
in relation to latitude. Black squares are the Northern Hemisphere (NH),
gray circles the Southern Hemisphere (SH) (Shakun and Carlson, 2010).
The youngest recorded stadial, the Laurentian melt, between ~8,500 and ~8.000 years ago (Figure 4), is indicated by distinctive temperature–CO₂ correlation with global CO₂ decline of ≈25 ppm by volume over ≈300 years, consistent with the lowering of North Atlantic sea-surface temperatures and weakening of the AMOC (Atlantic Meridional Ocean Circulation).
The Younger dryas and the Laurentian stadials are not unique, as peak temperatures in every interglacial event over the last 420,000 years were followed by sharp cooling events (Figure 5). Apart from the absolute level of greenhouse gases (GHG) in the atmosphere the high rate at which GHG concentrations are rising, as shown by comparisons with previous extreme warming events (Figure 6), enhances extreme weather events, as well as retards the ability of fauna and flora to adapt to the new conditions.
The average global land and ocean surface temperature for March 2020 was 1.16°C above the 20th century average global level of 12.7°C. Current CO₂ rise and warming rates exceed that of the Last Glacial Termination (LGT) (21–8 kyr) (Figure 6C), the Paleocene-Eocene Thermal Maximum (PETM) (55.9 Ma) (Figure 6B) and the Cretaceous-Tertiary boundary (K-T) (64.98 Ma) impact event (Figure 6A). The relations between warming rates and the migration of climate zones toward the poles (Figure 7), including changes in the atmosphere and ocean current systems, are in the root of the major environmental changes in these zones.
Future Stadial events
IPCC climate change projections for 2100-2300 portray linear to curved temperature progressions (SPM-5). However, amplifying feedbacks and transient cooling events (Stadials) ensuing from the flow of ice melt water into the oceans during peak interglacial warming events, impose abrupt temperature variations (Figure 5). The current flow of ice melt water from Greenland and Antarctica (Figures 8, 9) is leading to regional ocean cooling in the North Atlantic and around Antarctica (Rahmstorf et al, 2015; Hansen et al. (2016); Bronselaer et al. 2018; Purkey et al. 2018; Vernet et al. 2019) (Figures 1, 8). Under high greenhouse gas and temperature rise trajectories (RCP8.5) this implies future stadial events as modelled by Hansen et al. (2016) (Figure 10) and Bronselaer et al. (2018) (Figure 11).
Depending on different greenhouse emission scenarios (IPCC 2019; van Vuren et. al. (2011), including the CO₂ forcing-equivalents of methane (CH4) and nitrous oxide (N2O), the total CO₂–equivalent rise has reached 496 ppm (NOAA, 2019). As the oceans heat contents is rising, upwelling of warm sublayers is melting the leading edges of continental glaciers (Figure 8). This factor and the flow of ice meltwater from leading glacier fronts and grounding lines lead to stratification of the sub-Antarctic ocean and an incipient onset of a southern ocean stadial (Figure 8).
|Figure 8. The transition from grounded ice sheet to floating ice shelf and icebergs|
Satellite and mass balance measurements of the large ice sheets indicate their rapid reduction (Figure 9). Variations in ice thickness, ice drainage and ice velocity data in 176 Antarctic basins between 1979 and 2017 indicate a total mass loss rise from 40 ± 9 Gt/year in 1979–1990 to 50 ± 14 Gt/year in 1989–2000, 166 ± 18 Gt/year in 1999–2009, and 252 ± 26 Gt/year in 2009–2017 (Figure 9). This amounts to an increased melting by more than 6-fold in about 40 years, contributing an average sea level rise of 3.6 ± 0.5 mm per decade, with a cumulative 14.0 ± 2.0 mm since 1979 (Rignot et al. 2019). The mass loss concentrated in areas closest to warm, salty, subsurface, circumpolar deep water (CDW), consistent with enhanced polar westerlies pushing CDW toward Antarctica.
The Greenland ice sheet contains approximately 2,900,000 GtI of ice. During the exceptionally warm Arctic summer of 2019, Greenland lost 600 GtI of ice. Under global GHG and temperature rise this rate is likely to be exceeded. The Greenland ice sheet may not last much longer than a Century. The Antarctic ice sheet weighs approximately 26,500 Gigaton. For a loss greater than ~250 GtI/year it could last for 105 years or less. For accelerated ice melt rates under rising GHG concentrations it could last for significantly shorter time, except for possible negative feedbacks associated with stadial cooling?
Hansen et al. (2016) suggest that, depending on ice melt rates of the polar ice sheets, transient cooling events (stadials) can be expected to develop over periods dependent on the rates of ice melt (Fig. 10). Stadial cooling of about -2°C lasting for several decades (Figure 10) may affect temperatures in Europe and North America. The model is consistent with a slowdown of the Atlantic Meridional Ocean Circulation (AMOC) (Weaver et al. 2012) and the exceptional growth of a cold water region southeast of Greenland, (Rahmstorf et al, 2015).
|Figure 10. A. Model surface air temperature (◦C) change in 2096;
B. Surface air temperature (◦C) relative to 1880–1920 for several ice melt scenarios.
According to Bronselaer et al. (2018) temporal evolution of the global-mean surface-air temperature (SAT) shows meltwater-induced cooling translates to a reduced rate of global warming (Fig. 11), with a maximum divergence between standard models and models which include the effects of meltwater-induced cooling of 0.38 ± 0.02°C in 2055. As stated by the authors “We demonstrate that the inclusion in the model of ice-sheet meltwater reduces global atmospheric warming, shifts rainfall northwards, and increases sea-ice area”, and “Antarctic meltwater is therefore an important agent of climate change with global impact, and should be taken into account in future climate simulations and climate policy.”
|Figure 11. The 2080–2100 meltwater-induced sea-air temperature anomaly relative to the standard
RCP8.5 ensemble. Hatching indicates where the anomalies are not significant at the 95% level.
Based on the paleoclimate record, global warming, penetration of cold and warm air masses across weakened polar boundaries, increased ice melting rates, sea level rise and near-surface cooling of large ocean tracts (Figures 10, 11), collisions between warm and cold air and water masses and thereby storminess are likely to determine the future climate of large parts of Earth. With rising greenhouse gas levels and their amplifying feedbacks from land and oceans these developments are likely to persist in the long term. The continuing migration of climate zones toward the poles is likely to be disrupted by developing stadial effects and differential warming and cooling effects, leading to major weather disruptions and storminess. Continuing release of greenhouse gases and their amplifying feedbacks could lead to tropical Miocene-like conditions about 4 to 5 degrees Celsius warmer than late Holocene climate conditions which allowed agriculture and thereby civilization to emerge.
Dr Andrew Glikson
Earth and Paleo-climate scientist
ANU Climate Science Institute
ANU Planetary Science Institute
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