Showing posts with label melting. Show all posts
Showing posts with label melting. Show all posts

Thursday, July 9, 2020

Global warming and ice sheet melting: Portents of a Younger dryas-like stadial event

Global warming and ice sheet melting:
Portents of a Younger dryas-like stadial event
by 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).

Figure 1 A. The cold ocean region south of Greenland visible on the NASA's 2015 global mean
temperatures (NASA/NOAA; 20 January 2016), the warmest year on record since 1880;
B. Circum-Antarctic summer surface temperatures, showing the large Weddell Sea and other
cold Sub-Antarctic ocean anomalies related to the flow of ice melt water into the ocean, and a seasonal
warming anomaly in the Ross Sea due to upwelling of warm salty water from the circum-Antarctic current.

Stadial events

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 2A Air temperatures at the Last glacial maximum (20-16 kyr), BÖlling-Allerod warm peak,
Younger dryas (12,900 to 11,600 years ago) and 8.2 kyr Laurentian stadial event. This image
shows temperature changes, determined as proxy temperatures, taken from the central region

of Greenland's ice sheet during the Late Pleistocene and beginning of the Holocene.

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).

Figure 4 A. The ~8.2 kyr Laurentian stadial event in a coupled climate model (Wiersma et al. 2011);
B. Reconstructed CO₂ concentrations for the interval between ~8,700 and ~6,800 BP, based on
CO 2 extracted from air in Antarctic ice of Taylor Dome (Wagner et al. 2002).
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.

Figure 5 (a) Evolution of sea surface temperatures in five glacial-interglacial transitions recorded in
ODP 
1089 at the sub-Antarctic Atlantic Ocean (Cortese et al. 2007). Grey lines – δ 18 O measured on
Cibicidoides plankton; Black lines – sea surface temperature. Marine isotope stage numbers are 
indicated on top of diagrams. Note the stadial following interglacial peak temperatures; (b) the last 
glacial maximum and the last glacial termination. Olds- Oldest dryas; Old – Older dryas; Yd – Younger dryas.

Figure 6 (A) Reconstructed atmospheric CO₂ variations during the Late Cretaceous–early Tertiary
derived from the Stomata indices of fossil leaf cuticles calibrated by using inverse regression
and stomatal ratios (Beerling et al. 2002);
(B) Simulated atmospheric CO₂ at and after the Palaeocene-Eocene boundary (after Zeebe et al. (2009).
Compare the CO₂ ppm/year values with the current rise of 2 to 3 ppm/year;
(C) Global CO₂ and temperature during the last glacial termination (After Shakun et al., 2012)
(LGM - Last Glacial Maximum; OD – Older dryas; BA - Bølling–Allerød; YD - Younger dryas);

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.
Figure 7. Expansion of the tropical African climate zone (vertical red lines) into subtropical and Mediterranean
climate zones to the north and south (Migration, Environment and Climate Change, International Organization
for Migration, Geneva, Switzerland (Regional Maps on Migration, Environment and Climate Change.

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

Figure 9. Greenland and Antarctic ice mass change. GRACE data are extension of Velicogna et al. (2014)
gravity data. MBM (mass budget method) data are from Rignot et al. (2011). Red curves are gravity data
for Greenland and Antarctica only; small Arctic ice caps and ice shelf melt add to freshwater input.
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.

Conclusions


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.


Andrew Glikson
Dr Andrew Glikson
Earth and Paleo-climate scientist
ANU Climate Science Institute
ANU Planetary Science Institute
Canberra, Australia



Books:
The Asteroid Impact Connection of Planetary Evolution
http://www.springer.com/gp/book/9789400763272
The Archaean: Geological and Geochemical Windows into the Early Earth
http://www.springer.com/gp/book/9783319079073
Climate, Fire and Human Evolution: The Deep Time Dimensions of the Anthropocene
http://www.springer.com/gp/book/9783319225111
The Plutocene: Blueprints for a Post-Anthropocene Greenhouse Earth
http://www.springer.com/gp/book/9783319572369
Evolution of the Atmosphere, Fire and the Anthropocene Climate Event Horizon
http://www.springer.com/gp/book/9789400773318
From Stars to Brains: Milestones in the Planetary Evolution of Life and Intelligence
https://www.springer.com/us/book/9783030106027
Asteroids Impacts, Crustal Evolution and Related Mineral Systems with Special Reference to Australia
http://www.springer.com/us/book/9783319745442



Tuesday, May 19, 2020

An uncharted 21-23rd centuries’ climate territory

by Andrew Glikson

Precis

21–23ʳᵈ centuries’ transient ocean cooling events (stadials), triggered by ice melt flow from the Greenland and Antarctic ice sheets into the adjacent oceans, herald conditions analogous in part to those of the Younger Dryas stadial (12.9–11.7 kyr) which succeeded the pre-Holocene Bölling-Allerod thermal maximum. The subsequent Younger Dryas cooling event was associated with penetration of polar air masses and ocean currents, leading to storminess, analogous to recent breaching of the weakened polar jet stream boundary, ensuing in major snow storms in North America and Europe and cooling of parts of the North Atlantic Ocean and parts of the circum-Antarctic ocean triggered by the flow of ice melt water from melting glaciers.

21–23ʳᵈ Centuries’ Stadial freeze events

IPCC climate change projections for 2100-2300 portray linear to curved temperature progressions (SPM-5). By contrast, examination of transient cooling events (stadials) which ensued from the flow of ice melt water into the oceans during peak interglacial warming events portray abrupt temperature variations (Fig. 1). The current flow of ice melt water from Greenland and Antarctica ensuing from Anthropogenic global warming is leading to regional ocean cooling in the North Atlantic near Greenland and around Antarctica (Rahmstorf et al, 2015; Hansen et al. (2016); Bronselaer et al. 2018; Purkey et al. 2018; Vernet et al. 2019) (Fig. 2). The incipient developments of ice melt-derived cold water pools in ocean regions adjacent to the large ice sheets imply portents of future stadial events such as, inexplicably, are not indicated by the predominantly linear IPCC climate projections for the 21–23ʳᵈ centuries (IPCC AR5). By contrast, as modelled by Hansen et al. (2016) and Bronselaer et al. (2018), under high greenhouse gas and temperature rise trajectories (RCP8.5), the ice meltwater flow into the oceans from the Antarctic and Greenland ice sheets would lead to cooling of large regions of the ocean, with major consequences for future climate projections. This would include the build-up of large cool ocean pools in the North Atlantic south of Greenland (Rahmstorf et al, 2015) (Fig. 2A) and around Antarctica (Fig. 2B).

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 amounts to 496 ppm (NOAA, 2019), close to transcending the melting points of large parts of the Greenland and Antarctica ice sheets. Given the extreme rise in temperature since the mid-20th Century, where the oceans heat contents is rising, an incipient cooling of near-surface sub-Greenland and sub-Antarctic ocean regions raises the question whether incipient stadial events, perhaps analogous to the Younger Dryas stadial (Johnsen et al. 1972; Severinghaus et al. 1998), may be developing?

Interglacials, late Pleistocene and early Holocene stadial events

Stadial effects in the late Pleistocene record follow peak interglacial temperatures (Cortese et al., 2007) (Fig. 1). During the last glacial termination (LGT) stadial effects included the Oldest Dryas at ~16 kyr, the Older Dryas at ~14 kyr and the Younger Dryas at 12.9 - 11.7 kyr (Fig. 3), the latter with sharp transitions as short as 1 to 3 years (Steffensen et al., 2008), signifying a return to glacial conditions. A yet younger stadial event is represented at ~8.4 - 8.2 kyr when large-scale melting of the Laurentian ice sheet ensued in the discharge of cold water via Lake Agasiz (Matero et al. 2017; Lewis et al., 2012) into the North Atlantic Ocean. The Laurentian cooling involved temperature and CO₂ decline of ~25 ppm over ~300 years (Fig. 3B and C) and a decline of the North Atlantic Thermohaline circulation.

Figure 1. (a) Evolution of sea surface temperatures in 5 glacial-interglacial transitions recorded in
ODP 1089 at the sub-Antarctic Atlantic Ocean. Grey lines – δ¹⁸O measured on Cibicidoides plankton; 
Black lines – sea surface temperature. Marine isotope stage numbers are indicated on top of diagrams.
Note the stadial following interglacial peak temperatures (Cortese et al. 2007).
 (b) The last glacial maximum and the last glacial termination.
Olds  Oldest Dryas; Old – Older Dryas; Yd – Younger Dryas.

Greenland and Antarctica ice melt events

Oxygen isotopes (¹⁸O/¹⁹O), argon isotopes (⁴⁰Ar/³⁹Ar) and nitrogen isotopes (¹⁵N/¹⁴N) studies of Greenland ice cores (Johnsen et al. 1972; Severinghaus et al. 1998) indicate a rise in temperature to -36°C, followed by a sharp fall to -50°C (Table 1; Fig. 3A). At lower latitudes the mean temperatures drop about -2°C and 6°C (Table 1). In the southern hemisphere temperatures dropped by about -2°C in lower latitudes and about -8°C at high latitudes and (Fig. 4; Table 1) Shakun and Carlson, 2010).

Figure 2. A. The cold ocean region south of Greenland visible on NASA's 2015 global mean temperatures, the warmest year on record since 1880. Colors indicate temperature anomalies (NASA/NOAA; 20 January 2016);
B. Circum-Antarctic summer surface temperatures, showing the large Weddell Sea cold anomaly
and a seasonal warming anomaly in the Ross Sea due to upwelling of warm salty water.


Table 1.

Cooling intervals (stadial events) during late Pleistocene and early Holocene interglacial phases.
Isotopic Stage 
Agemax kyr
Agemin kyr
∆tkyr
TmaxoC SST
TminoC SST
∆ToC 
Stadial MIS 11-12 Ref. A
434 kyr
424 kyr
10 kyr
19.3oC SST
13.4oC SST
-5.9oC
Stadial MIS 9-10  Ref. B
346 kyr
331 kyr
5 kyr
19oC SST
13oC SST
-6oC
Stadial MIS 7-8   Ref. C
243.5 kyr
241.5 kyr
2.0 kyr
18oC SST
15.5oC SST 
-2.5oC
Stadial MIS 5-6 Ref. D
136 kyr
130 kyr
6.0 kyr
19oC SST
15.2oC SST
-3.8oC
Younger Dryas stadial ice core In Greenland
MIS 1-2-3  Ref. E
12.86 kyr
11.64 kyr
1.22 kyr
Greenland ice core
-50oC Greenland ice core
-14±3oC
Greenland ice core
Younger Dryas at lower and mid-latitudes of the NH (Fig. 4) Ref. F
12.86 kyr
11.64 kyr
1.22 kyr


-2 to -6oC
8.3 kyr Stadial  Ref. G
8.45 kyr
8.1 kyr
0.35 kyr
-28oC
CO₂=310 ppm
-30oC
CO₂=275 ppm
-2.0oC
Figure 3. A. Temperature variations during the late Pleistocene to the beginning of the Younger Dryas stadial
and the onset of the Holocene, determined as proxy temperatures from ice cores of the central Greenland ice sheet;
B. The ~8.2 kyr stadial event in a coupled climate model (Wiersma et al. 2011);
C. Reconstructed CO₂ concentrations for the interval between ~8,700 and ~6,800 B.P. 

based on CO₂ extracted from air in Antarctic ice of Taylor Dome (Wagner et al. 2002).
Figure 4. 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)

Antarctic ice melt dynamics


Circum-Antarctic surface air temperatures, precipitation and sea-ice cover (Bronselaer et al. (2018), including testing the effects of ice-shelf melting, identifies penetration of relatively warm circumpolar deep water below 400 m into the grounding line underlying the ice shelf (Figs 5, 6A). The flow of ice melt water into the adjacent ocean forms an upper cold water layer away from ice shelf areas (Figure 6B). These authors indicate the flow of ice-sheet meltwater results in a decrease of global atmospheric warming, shifts rainfall northwards, and increases sea-ice area and offshore subsurface Antarctic Ocean temperatures.

Figure 5. Schematic circulation and water masses in the Antarctic continental shelf (Purkey et al., 2018) displaying layering of the sub-Antarctic into a cold ice melt-derived upper layer (-2.1°C) overlying a warmer water zone (-1.0°C) which acts as a source of modified warm water penetrating the grounding zone of the glacial ice shelf.

Figure 6. A. The grounding zone where the bedrock-grounded ice sheet transits to a freely floating ice shelf over several km. The floating ice shelf changes in elevation in response to tides, atmospheric air pressure and oceanic processes. B. The Helium (∆He% - Temperature proxy) profile in the Amundsen Sea. The black dots indicate the sampling depth, and the grey dotted lines indicate the isopycnal (density) lines. The shelf break is located at about ∼280 km).

In turn warmer salty water from the circum-polar deep water (CDW) from the circum-Antarctic current can penetrate below the cold off-shelf layer, as is the case in the Weddell Sea Gyre (Figures 5, 6 and 7).

Figure 7. Penetration of relatively warm and salty water from the circum-Antarctic current below the cold off-shelf surface layer of the Weddell Sea Gyre.

Global stadial cooling events


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 at times dependent on the rates of ice melt (Fig. 8). 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). These authors suggest stadial cooling of about -2°C lasting for several decades (Fig. 8B), depending on ice melt rates, can affect temperatures in Europe and North America.

Figure 8. A. Model surface air temperature (°C) change in 2055–2060 relative to 1880–1920 for modified forcings.
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. 9), 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. The SAT response shows the effect of ice meltwater becomes weaker as the ocean becomes more stratified as a result of both moderate to deep level warming and cooling/freshening at the surface (Fig. 6B). 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 9. A. 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.
B. Time series of the global-mean sea-air temperature (SAT) anomaly relative to the 1950–1970 mean.
Orange shows the standard ensemble and blue shows the meltwater-included ensemble. Solid lines show ensemble means, the dark shading shows the 95% uncertainty in the mean and the light shading shows the full ensemble spread of 20-year means. The green bar indicates the period when the standard and meltwater ensembles diverge.

Based on the paleoclimate record, global warming and rates of melting and surface cooling around parts of Antarctica and the North Atlantic (Fig. 2) would determine the future climate of large parts of Earth. Transient stadial cooling events, inherently associated with meters-scale sea level rise, would result in increased temperature polarities between subpolar and tropical latitudes, leading to storminess where polar-derived and tropical-derived air masses and ocean currents collide. Regional to global stadial cooing would, in principle, last as long as ice sheets remain. Once the large ice sheets are exhausted a transition takes place toward tropical Miocene-like and even Eocene-like conditions about 4 to 5 degrees Celsius warmer than Holocene climate conditions, which allowed agriculture and thereby civilization to emerge.


Andrew Glikson
Dr Andrew Glikson
Earth and Paleo-climate scientist
ANU Climate Science Institute
ANU Planetary Science Institute
Canberra, Australia

Books:
The Asteroid Impact Connection of Planetary Evolution
http://www.springer.com/gp/book/9789400763272
The Archaean: Geological and Geochemical Windows into the Early Earth
http://www.springer.com/gp/book/9783319079073
Climate, Fire and Human Evolution: The Deep Time Dimensions of the Anthropocene
http://www.springer.com/gp/book/9783319225111
The Plutocene: Blueprints for a Post-Anthropocene Greenhouse Earth
http://www.springer.com/gp/book/9783319572369
Evolution of the Atmosphere, Fire and the Anthropocene Climate Event Horizon
http://www.springer.com/gp/book/9789400773318
From Stars to Brains: Milestones in the Planetary Evolution of Life and Intelligence
https://www.springer.com/us/book/9783030106027
Asteroids Impacts, Crustal Evolution and Related Mineral Systems with Special Reference to Australia
http://www.springer.com/us/book/9783319745442


Monday, October 8, 2018

What Does Runaway Warming Look Like?

The forcing caused by the rapid rise in the levels of greenhouse gases is far out of line with current temperatures. A 10°C higher temperature is more in line with these levels, as illustrated by the image below.


Carbon dioxide levels have been above 400 ppm for years. Methane levels above 1900 ppb were recorded in September 2018. Such high levels are more in line with a 10°C higher temperature, as illustrated by the above graph based on 420,000 years of ice core data from Vostok, Antarctica, research station.

How fast could such a 10°C temperature rise eventuate? The image below gives an idea.


Such runaway warming would first of all and most prominently become manifest in the Arctic. In many ways, such a rise is already underway, as the remainder of this post will show.

High Arctic Temperatures

Why are Arctic temperatures currently so high for the time of year?


As warmer water enters the Arctic Ocean from the Atlantic and Pacific Oceans, there is no thick sea ice left to consume this heat. Some of this heat will escape from the Arctic Ocean to the atmosphere, as illustrated by above dmi.dk  image showing very high temperatures for the time of the year over the Arctic (higher than 80°C latitude).


Above dmi.dk image shows that Arctic temperatures are increasingly getting higher during Winter in the Northern Hemisphere.


Similarly, above NASA image shows that Arctic temperatures are increasingly getting higher during Winter in the Northern Hemisphere.


As the Arctic warms up faster than the rest of the world, the Jet Stream is becoming more wavy, allowing more hot air to move into the Arctic, while at the same time allowing more cold air to move south.

Above image shows that the air over the Beaufort Sea was as warm as 12.8°C or 55°F (circle, at 850 mb) on October 2, 2018. The image also illustrates that a warmer world comes with increasingly stronger cyclonic winds.


The images above and below shows that on October 2 and 7, 2018, the sea surface in the Bering Strait was as much as 6°C or 10.7°F, respectively 6.4°C or 11.6°F warmer than 1981-2011 (at the green circle).


As temperatures on the continent are coming down in line with the change in seasons, the air temperature difference is increasing between - on the one hand - the air over continents on the Northern Hemisphere and - on the one hand - air over oceans on the Northern Hemisphere. This growing difference is speeding up winds accordingly, which in turn can also speed up the influx of water into the Arctic Ocean.

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Start of freezing period

Here's the danger. In October, Arctic sea ice is widening its extent, in line with the change of seasons. This means that less heat can escape from the Arctic Ocean to the atmosphere. Sealed off from the atmosphere by sea ice, greater mixing of heat in the water will occur down to the seafloor of the Arctic Ocean, while there is little or no ice buffer left to consume an influx of heat from the Atlantic and Pacific Oceans, increasing the danger that warm water will reach the seafloor of the Arctic Ocean and destabilize methane hydrates. 

Rising salt content of Arctic Ocean

It's not just the influx of heat that is the problem. There's also the salt. Ice will stay frozen and will not melt in freshwater until the temperature reaches 0°C (or 32°F). Ice in saltwater on the other hand will already have melted away at -2°C (or 28.4°F).

The animation of the right shows salty water rapidly flowing through the Bering Strait.

With the change of seasons, there is less rain over the Arctic Ocean. The sea ice also seals the water of the Arctic Ocean off from precipitation, so no more fresh water will be added to the Arctic Ocean due to rain falling or snow melting on the water.

In October, temperatures on land around the Arctic Ocean will have fallen below freezing point, so less fresh water will flow from glaciers and rivers into the Arctic Ocean. At that time of year, melting of sea ice has also stopped, so fresh water from melting sea ice is no longer added to the Arctic Ocean either.

Pingos and conduits. Hovland et al. (2006)
So, the Arctic Ocean is receiving less freshwater, while the influx of water from the Atlantic and Pacific Oceans is very salty. This higher salt content of the water makes it easier for ice to melt at the seafloor of the Arctic Ocean. Saltier warm water is causing ice in cracks and passages in sediments at the seafloor of the Arctic Ocean to melt, allowing methane contained in the sediment to escape.

[ click on images to enlarge ]
The image on the right, from a study by Hovland et al., shows that hydrates can exist at the end of conduits in the sediment, formed when methane did escape from such hydrates in the past. Heat can travel down such conduits relatively fast, warming up the hydrates and destabilizing them in the process, which can result in huge abrupt releases of methane.

Heat can penetrate cracks and conduits in the seafloor, destabilizing methane held in hydrates and in the form of free gas in the sediments.

Methane

peak methane levels as high as 2859 ppb
On October 2 and 7, 2018, peak methane levels were as high as 2838 ppb, respectively 2859 ppb, as the images on the right shows. Methane levels over the Beaufort Sea have been high for some time, and have remained high at very high altitudes.

The threat is that a number of tipping points are going to be crossed, including the buffer of latent heat, loss of albedo as Arctic sea ice disappears, methane releases from the seafloor and rapid melting of permafrost on land and associated decomposition of soils, resulting in additional greenhouse gases (CO₂, CH₄, N₂O and water vapor) entering the Arctic atmosphere, in a vicious self-reinforcing cycle of runaway warming.

A 10°C rise in temperature by 2026?


Above image shows how a 10°C or 18°F temperature rise from preindustrial could eventuate by 2026 (from earlier post).

The situation is dire and calls for comprehensive and effective action, as described at the Climate Plan.


Links

• Temperature Rise
https://arctic-news.blogspot.com/2017/08/temperature-rise.html

• Mean Methane Levels reach 1800 ppb
https://arctic-news.blogspot.com/2013/06/mean-methane-levels-reach-1800-ppb.html

• Why are methane levels over the Arctic Ocean high from October to March?
https://arctic-news.blogspot.com/2015/01/why-are-methane-levels-over-the-arctic-ocean-high-from-october-to-march.html

• Blue Ocean Event
https://arctic-news.blogspot.com/2018/09/blue-ocean-event.html

• Feedbacks
https://arctic-news.blogspot.com/p/feedbacks.html

• The Threat
https://arctic-news.blogspot.com/p/threat.html

• Extinction
https://arctic-news.blogspot.com/p/extinction.html

• Aerosols
https://arctic-news.blogspot.com/p/aerosols.html

• How extreme will it get?
https://arctic-news.blogspot.com/2012/07/how-extreme-will-it-get.html

• Climate Plan
https://arctic-news.blogspot.com/p/climateplan.html