by Andrew GliksonThe 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:
- The flow of large volumes of cold ice melt water into the oceans, leading to stadial cooling effects, such as in the North Atlantic (Rahmstorf et al 2015; Glikson, 2019) and around Antarctica (Bonselaer et al., 2018).
- Paleoclimate observations indicate that during the Pleistocene glacial-interglacial cycles, at least for the 800,000 years, every time temperatures reached a peak a sharp cooling followed (Cortese et al. 2007).
- Amplifying feedbacks from land and ocean drive non-linear climate trajectories, due to a lowered capacity of the warming oceans to absorb CO₂, the release of CO₂ from desiccated vegetation and extensive bushfires, decrease in reflection due to melting of ice sheets, increase in infrared absorption by open water and exposed rock surfaces, discharge of methane from melting permafrost and from methane clathrates.
|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.
|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.
|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.
absorbed 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 the future — but cooler if the imbalance is negative.
melting of an ice sheet may occur slowly depending on its previous state.
Dr Andrew Glikson
Earth and Paleo-climate scientist
ANU Climate Science Institute
ANU Planetary Science Institute
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