Friday, December 4, 2020

Polar-ward climate zones shift and consequent tipping points

by Andrew Glikson

The concept of a global climate tipping point/s implies a confluence of climate change processes in several parts of the world where regional climate changes can combine as a runaway shifts to a new climate state. Conversely the shift of climate zones can constitute the underlying factor that triggers extreme weather events which culminate in tipping points. These shifts include the expansion of the tropics, tropical cyclones, mid-latitude storms and weakening of boundaries of the polar vortex, allowing breach of air masses of contrasting temperatures through the jet stream polar boundary, with ensuing snow storms and heatwaves.

Figure 1. Climate tipping points (McSweeney 2020)

The migration of climate zones toward the poles appears to constitute a major factor in triggering tipping points in the Earth system (Figures 1 and 2), including (from north to south):
  1. permafrost loss 
  2. expansion of the Boreal forest at the expense of the tundra
  3. disintegration of the Greenland ice sheet
  4. breakdown of the Atlantic meridional overturning circulation (AMOC) caused by an increased influx of freshwater into the North Atlantic 
  5. Amazon forest dieback 
  6. West African monsoon shift 
  7. Indian monsoon shift 
  8. Coral reef die-off
  9. West Antarctic ice disintegration
Not included in this list are the increased desertification and the extensive fires in parts of the continents, including the Arctic, Siberia, western North America, the Mediterranean, Brazil and Australia.

Figure 2. Monthly anomalies for October 2020 by NOAA (National Centers for Environmental Information)

A conflation of regional climate developments into global climate tipping point/s, namely a shift in state of the Earth climate is likely, although the details of this process are not clear. Alternatively it is the migration of climate zones toward the poles, indicated by climate zone maps, which is triggering regional events.
Figure 3. High anomalies over the Arctic from Nov. 2019 to Oct. 2020 (NASA image)

Here I list some of these likely relationships: 
  • In the Arctic sea ice extent in October 2020 was lower by 36.8% than during 1981-2010 (Figure 2). High anomalies have hit the Artic Ocean and Siberia over the 12-month period from November 2019 to October 2020 (Figure 3). The warming of the Arctic is driven by (1) a decline in albedo due to ice melt and exposure of open water surfaces; (2) the albedo flip generated by formation of thin water surfaces above ice sheets and glaciers, and (3) the penetration of warm air masses through the weakened circum-Arctic jet stream (Figure 4.). 
  • The tropics are expanding at a rate of near-50 km per decade (Jones 2018) and have widened about 0.5° latitude per decade since 1979 (Staten et al. 2018). With warming and desertification effects across North Africa and the Mediterranean Sea this is leading to draughts and fires in southern Europe. The shift of climate zones toward the poles, at a rate approximately 50 to 100 km per decade, as well as sea level rise, is changing the geography of the planet. Once sea level reaches equilibrium temperatures it will attain at least 25 meters above the present, by analogy to Pliocene level (before 2.6 million years ago).
  • As climate zones shift northward an increase of winter precipitation of up to 35% is recorded in mid to northern Europe during the 21st century, with increases of up to 30% in north-eastern Europe. In 2020 Europe had the warmest October on record and North America the heaviest snow precipitation on record (Figure 2). 
  • In Australia a southward migration of the tropical North Australia climate zone and the high pressure ridge separating it from the southern terrain dominated by the Westerlies and the precipitation-bearing spirals of the Antarctic-sourced vortex southward, with consequent droughts in southern and southwestern parts of the continent. 
Figure 4. The Arctic jet stream, summer, 1988, NASA. Extreme melting in 
Greenland’s ice sheet is linked to warm air delivered by the wandering jet 
stream, a fast-moving belt of westerly winds created by the convergence of 
cold air masses descending from the Arctic and rising warm air masses from 
the tropics that flow through the lower layers of the atmosphere.

As evident from the above the shift in climate zones constitutes the underlying factor which triggers extreme weather events and tipping points.

Figure 5. Arctic surface-air temperature anomalies for July 2020.

Since the onset of the industrial age, in particular since about 1960-70, global warming accelerated at by one to two orders of magnitude faster than during the last glacial termination (~16000 – 8000 years ago) and much earlier. Mass extinction events in the Earth history have occurred when environmental changes took place at a rate to which species could not adapt. Plants and animals are currently dying off at a rate 100 to 1000 times faster than the mean rate of extinction over geological timescales.

The Intergovernmental Panel for Climate Change (IPCC AR5) projects linear warming to 2300 and 2500, which however does not take full account of amplifying feedbacks from a range of sources (Trajectories of the Earth system in the Anthropocene). These include reduced CO2 sequestration in the warming oceans, albedo changes due to melting of ice, enrichment of the atmosphere in water vapor, desiccation and burning vegetation, release of methane from permafrost. Nor do these linear trends take account of the stadial effects of the flow of cold ice melt water into the oceans (Glikson, 2019).

According to the National Oceanic and Atmospheric Agency (NOAA) global warming has accelerated significantly during 2015-2020. The danger inherent in temperature rise to about 4 degrees Celsius by 2100 is underpinned by the consequences at lower temperature rise of +1 to +2 degrees Celsius, already in evidence. Thus, whereas the mean land-ocean temperature rise between 1880-2020 is +1.16 degrees Celsius, the average rise in continental temperatures during this period has already reached +1.6 degrees Celsius, beyond the upper limit proposed by the Paris Accord. The rise in temperatures is driving a three-fold to six-fold rise in extreme weather events since 1980 (Figure 6.), including severe storms, tropical storms, flooding, droughts and wildfires (NOAA 2018).

Figure 6. The growth in the frequency of extreme weather events in the US during 1980-2018

Large-scale melting of the Greenland and Antarctica ice sheets, discharging cold ice melt water, is already cooling of parts of the oceans. The clash between cold air masses and tropical fronts would increase storminess, in particular along coastal boundaries and islands. Such storminess, along with intensified tropical cyclones, would render island chains increasingly vulnerable.

To date most suggestions for mitigation and adaptation are woefully inadequate to arrest global warming. Reductions in carbon emissions, which are absolutely essential, may no longer be adequate to arrest accelerating greenhouse gas and temperature levels. At the current level of carbon dioxide (>500 parts per million equivalent CO2+methane+nitrous oxide), reinforced by amplifying feedbacks from land and oceans, the remaining option would be to sequester (down-draw) greenhouse gases from the atmosphere.

A global imperative.

Andrew Glikson

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

The Asteroid Impact Connection of Planetary Evolution
The Archaean: Geological and Geochemical Windows into the Early Earth
Climate, Fire and Human Evolution: The Deep Time Dimensions of the Anthropocene
The Plutocene: Blueprints for a Post-Anthropocene Greenhouse Earth
Evolution of the Atmosphere, Fire and the Anthropocene Climate Event Horizon
From Stars to Brains: Milestones in the Planetary Evolution of Life and Intelligence
Asteroids Impacts, Crustal Evolution and Related Mineral Systems with Special Reference to Australia