Amplifying feedbacks leading to accelerated planetary temperatures
“The paleoclimate record shouts to us that, far from being self-stabilizing, the Earth's climate
system is an ornery beast which overreacts even to small nudges” (Wally Broecker)
These feedbacks drive a chain reaction of events, accelerating the warming, as follows:
- Melting snow and ice expose dark rock surfaces, reducing the albedo of the polar terrains and sea ice in surrounding oceans, enhancing infrared absorption and heating.
- Fires create charred low-albedo land surfaces.
- An increase in evaporation raises atmospheric vapor levels, enhancing the greenhouse gas effect.
- Whereas an increase in plant leaf area enhances photosynthesis and evapotranspiration, creating a cooling effect, the reduction in vegetation in darkened burnt areas works in the opposite direction, warming land surfaces.
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| Figure 1. The 2021 global climate trends (Hansen, 2021, by permission) |
The current acceleration of global warming is reflected by the anomalous rise of temperatures, in particular during 2010-2020 (
Hansen 2021,
Figure 1 above). Consequently, extensive regions are burning, with 4 to 5 million fires per year counted between about
2004 and 2019. In 2021, global April temperatures are much less than in 2020, due to a moderately strong
La Nina effects.
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| Figure 2. The Palaeocene-Eocene Thermal Maximum recorded by benthic plankton isotopic data from sites in the Antarctic, south Atlantic and Pacific (Zachos et al., 2003). The rapid decrease in oxygen isotope ratios is indicative of a large increase in atmospheric temperatures associated with a rise in greenhouse gases CO₂ and CH₄ signifies approximately +5°C warming. |
Analogies between Anthropocene climate change and major geological climate events reveal the rate of current rise in greenhouse gas levels and temperatures as compared to major geological warming events is alarming. A commonly cited global warming event is the Paleocene-Eocene boundary thermal maximum (PETM) at 55 Ma-ago, reaching +5 degrees Celsius and over 800 ppm CO₂ within a few thousand years
(Figures 2 above and 3A below).
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Figure 3. (A) Simulated atmospheric CO₂ at and following the Palaeocene-Eocene boundary (after Zeebe et al., 2009);
(B) Global CO₂ and temperature during the last glacial termination (After Shakun et al., 2012) (LGM - Last Glacial Maximum; OD – Older dryas; BA - Bølling–Alerød; YD - Younger dryas). Glikson (2020). |
The triggering of a mass extinction event by the activity of organisms is not unique to the Anthropocene. The end Permian mass extinction, the greatest calamity for life in geologic history, is marked in marine carbonates by a negative δ¹³C shift attributed to oceanic anoxia and the emission of methane (CH₄) and hydrogen sulphide (H₂S) related to the activity of methanogenic algae (“purple” and “green” bacteria) (
Ward, 2006;
Kump, 2011). As a corollary anthropogenic climate change constitutes a geological/biological process where the originating species (Homo sapiens) has not to date discovered an effective method of controlling the calamitous processes it has triggered.
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| Andrew Glikson |
A/Prof. Andrew Glikson
Earth and Paleo-climate scientist
The University of New South Wales,
Kensington NSW 2052 Australia
Links image top
• Seasonal origin of the thermal maxima at the Holocene and the last interglacial - by Samantha Bova et al. (2021)
https://www.nature.com/articles/s41586-020-03155-x
• The Last Great Global Warming - by Lee Kump (2011)
https://www.scientificamerican.com/article/the-last-great-global-warming
