Monday, November 16, 2020

Accelerated global warming and stadial cooling events: IPCC oversights regarding future climate trends

 by Andrew Glikson

The linear nature of global warming projections by the IPCC (2014) Assessment Report (AR5) (Figure 1) appears to take little account of stadial cooling events, such as have followed peak temperature rises in previous interglacial stages. The linear trends appear to take only limited account of amplifying positive feedback effects of the warming from land and ocean. A number of factors cast doubt on IPCC climate change projections to 2100 AD and 2300 AD, including:
Figure 1 (a) IPCC average surface temperature change to 2100 relative to 1986-2005 IPCC AR5;
(b) IPCC average surface temperature change to 2300 relative to 1986-2005 IPCC AR5

However, global temperature measurements for 2015-2020 indicate accelerated warming due to both the greenhouse effect reinforced by a solar radiation maximum (Hansen and Sato 2020) (Figure 2).

Figure 2. Accelerated Global Warming reinforced by both greenhouse 
gases and a solar maximum Hansen and Sato, 2020

The weakening of the northern Jet stream, due to polar warming and thus reduced longitudinal temperature contrasts, allows penetration of warm air masses into the polar region and consequent fires (Figure 3). The clash between tropical and polar air and water masses (Figure 3A) leads to regional storminess and contrasting climate change trajectories in different parts of the Earth, in particular along land-ocean boundaries and island chains. 

The weakening of the jet stream and migration of climate zones constitute manifestations of an evolving Earth’s energy imbalance¹, namely a decrease in reflection of solar radiation from Earth to space and thereby global warming. Earth retained 0.6 Watt/m² during 2005-2010 and 0.87 Watt/m² during 2010-2020 (Hansen and Sato 2020), primarily due to a rise in greenhouse gases but also due to a solar radiation peak. During 2015-2020 global warming rates exceeded the 1970-2015 warming rate of 0.18°C/per decade, a deviation greater than climate variability. Hansen and Sato (2020) conclude the accelerated warming is caused by an increasing global climate forcing, specifically by the role of atmospheric aerosols.

Figure 3 A. Undulating and weakening jet stream and the polar vortex and penetration
of warm air, inducing Arctic warming and fires.     B. Satellite images of Wildfires
ravaging parts of the Arctic
, with areas of Siberia, Alaska, Greenland and Canada
engulfed in flames and smoke. While wildfires are common at this time of year, record-
breaking summer temperatures and strong winds have made 2020 fires particularly bad.

Bronselaer et al., 2018 modelled a meltwater-induced cooling of the southern hemisphere toward the end 21st century by as low as -1.5°C (Figure 4A). Hansen et al. 2016 estimated the time frame of 21st century stadial cooling event as dependent on the rates of ice melt (Figure 4B), reaching near global extent toward the end of the century (Figure 4C).

Figure 4 A. 2080–2100 meltwater-induced sea-air temperature anomalies relative to
the standard RCP8.5 ensemble (Bronselaer et al., 2018). Hatching indicates where the
anomalies are not significant at the 95% level;  B. Negative temperature anomalies
through the 21st-22nd centuries signifying stadial cooling intervals (Hansen et al., 2016);
C. A model of Global warming for 2096, where cold ice melt water occupies large parts
of the North Atlantic and circum-Antarctica, raises sea level by about 5 meters and
decreases global temperature by -0.33°C (Hansen et al., 2016).

With the concentration of greenhouse gases rising by approximately 47% during the last century and a half, faster than almost any observed rise in the Cenozoic geological record, the term “climate change” refers to an extreme shift in state of the atmosphere-ocean system. The greenhouse gas rise and temperature rise rates are faster than those of the K-T mass extinction, the Paleocene-Eocene extinction and the last glacial termination.

The consequences for future climate change trends include:
  • Further expansion of the tropical climate zones and a polar-ward shift of intermediate climate zones, leading to encroachment of subtropical deserts over fertile Mediterranean zones. 
  • Spates of regional to continent-scale fires, including in Brazil, Siberia, California, around the Mediterranean, Australia.
  • A weakened undulating jet stream (Figure 3) allowing penetration of and clashes between warm and cold air and water masses, with ensuing storms. 
  • In Australia the prolonged drought, low vegetation moisture, high temperatures and warm winds emanating from the northern Indian Ocean and from the inland, rendering large parts of the continent tinder dry and creating severe fire weather subject to ignition by lightning.
  • The delayed melting of the large ice sheets due to hysteresis², would be followed by sea level rise to Pliocene levels, ~25 meters above pre-industrial levels, once sea level reaches equilibrium with temperature of 2 to 3 degrees Celsius or higher, changing the geography of the continents.
It would follow from these considerations that succeeding periods of peak temperatures, extensive melting of the ice sheets, flow of ice melt into the oceans and thereby stadial cooling would lead to clashes between tropical fronts and cooling masses of air, producing storminess, in particular along continental margins and island chains. The modelled time frame of these developments (Figure 4B) may be cyclical, or may extend further in time and place as long as the ice sheets continue to breakdown.

¹ Earth's energy imbalance is the difference between the amount of solar energy
by Earth and the amount of energy the planet radiates to space as heat.
If the imbalance is 
positive, more energy coming in than going out, we can expect
Earth to become warmer in t
he future — but cooler if the imbalance is negative.
² Hysteresis is the dependence of the state of a system on its history. For example the 
melting of an ice sheet may occur slowly depending on its previous state.

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