Climate Change in the Past – How did our planet respond?

What can we learn from the Paleocene–Eocene Thermal Maximum (PETM)?

One of the best ways to explore how human-induced global warming will affect Earth in future is to study the way our planet responded to climate change in the past.
The Paleocene–Eocene Thermal Maximum (PETM) is one of the most intense and abrupt period of global warming in the past. It occurred around 56 million years ago, at the boundary between the Paleocene and Eocene. This warming has been linked to a rapid increase in the concentration of greenhouse gases in Earth’s atmosphere, which drove up global temperatures by more than 5 °C in just a few thousand years. The fossil record gives us the means of understanding how life was affected by the PETM, and so provides an excellent opportunity to study the relationships between evolution, extinction, migration and climate change.

climate in the past
Past and future trends in global mean temperatures spanning the last 67 million years. Oxigene isotope values in deep sea benthic foraminifera from sediment cores are a measure of global temperature in ice volume. Temperature is relative to the 1961-1990 global mean. If emissions are constant after 2100 and not stabilized before 2250 global climate by 2300 might enter the Hothouse World of the early Eocene. (Credit Westerholt et al. CENOGRID)

The average global surface temperature reached around 32°C which is 18 degrees above the pre industrial level (14°C).
Climate sensitivity was about 3.1° C per doubling of CO2 . These values are similar to those assessed by the IPCC.
The duration of the carbon input is difficult to constrain, a few thousand years seems to be very likely. The emission rates were indeed 10 times slower than the current annual average.

Combined data and modelling studies offer a potential way forward by suggesting simulacra of the traces left in the geologic record that indicate a short carbon input duration. Each of the age-model independent methods outlined here has caveats in its application; however, a consensus appears to be emerging that the carbon emissions that drove the CIE occurred over just a few thousand years. This still suggests emissions rates about 10× slower than the current annual average, but is similar to predicted rates of additional carbon release from natural carbon cycle feedbacks“
Sandra Kirtland Turner, 2018.

The climate of the late Eocene
At the time of the PETM, the world was already much warmer than it is today. The high latitudes and polar regions were more or less ice-free, and were populated by a diverse assemblage of plants and animals. Alligators, which today are found only in the warm tropics and subtropics, occurred well within the Arctic Circle during the early Eocene. The climate in southern North America (at a latitude of ~30° N) was roughly tropical, with high temperatures and lots of rainfall, and small seasonal differences between summer and winter. This warm phase had begun in the Cretaceous period, peaked in the early Eocene, and continued to the end of the Eocene, when global temperatures dropped and ice sheets formed over the Antarctic. By the early Palaeogene period, the arrangement of the continents was quite similar to that of today although the Atlantic Ocean was not as wide as it is now, and India was only just beginning to collide with the rest of Asia, there was no land bridge between North and South America and Antarctica, on the other hand was connected to South America, the Drake Passage formed much later.

50 – 55 million years ago India began to collide with Asia forming the Tibetan plateau and Himalayas. Australia, which was attached to Antarctica, began to move rapidly northward. North and South America was not yet connected. (Paleomap Project – C.R.Scotese)

The climate was much wetter than today with the increase in evaporation rates peaking in the tropics. It’s very likely that much more of this moisture was transported polewards than today. Warm weather would have predominated as far north as the Polar basin.with subtropic temperatures at the poles and a tropical rainforest environment in Germany. Unlike modern rainforests, its latitude would have made it seasonal combined with equatorial temperatures, a weather system and corresponding environment unmatched anywhere on Earth today.

At the start of the PETM, the ocean circulation patterns changed radically in the course of under 5,000 years. Global-scale current directions reversed due to a shift in overturning from the southern hemisphere to northern hemisphere overturning.This „backwards“ flow persisted for 40,000 years.Such a change would transport warm water to the deep oceans, enhancing further warming.
In parts of the oceans, especially the north Atlantic Ocean,bioturbation was absent. This may be due to bottom-water anoxia, or by changing ocean circulation patterns changing the temperatures of the bottom water. However, many ocean basins remained bioturbated through the PETM.
The amount of freshwater in the Arctic Ocean increased, in part due to northern hemisphere rainfall patterns, fueled by poleward storm track migrations under global warming conditions.

Marine microorganisms
Several groups of single-celled planktonic organisms, all of which are common in the oceans today, were greatly affected by the PETM. Dinoflagellates are primitive microorganisms that have characteristics of both plant and animal cells. They are typically 0.02–0.15 millimetres in diameter, and they propel themselves through the oceans using whip-like protuberances called flagella (dinoflagellate means ‘whirling whip’). Some species form protective capsules called resting cysts, in which the organism remains dormant throughout the winter. It is these cysts that are preserved and form the dinoflagellate fossil record. Within this group, the genus Apectodinium  became globally dominant during the PETM, and expanded its range from the warm subtropics to cover most of the globe. This may have been because the warmer high latitudes during this interval allowed Apectodinium to spread farther away from the equator, or it might have been the result of increased nutrient influxes into the oceans.
Coccolithophores are smaller than dinoflagellates, and range in size from 0.00025 to 0.03 mm. They are covered in tiny plates of calcium carbonate called coccoliths, which fall to the sea floor when the organism dies. Coccoliths can accumulate in such abundance as to form rocks in their own right, and they are the main constituent of chalk, which forms the White Cliffs of Dover. Several species of coccolithophore went extinct during the PETM, and a number of new species appeared.
Foraminifera are widespread and abundant in the modern oceans, and inhabit both the surface waters and the sea floor. Planktonic foraminifera are typically less than 0.1 mm in diameter. These forms diversified during the PETM, with several new species appearing in this interval. However, foraminifera that lived on the sea floor (benthic forms) suffered a major extinction event, with 30–50% of species going extinct during the PETM. This was probably the result of rapid warming of the ocean-bottom waters, and an associated decline in the concentration of dissolved oxygen there.
Mammals underwent profound evolutionary and biogeographic changes at the Paleocene–Eocene boundary. Three groups that incorporate many modern mammal species appeared suddenly at this time: Artiodactyla, which includes deer, camels and cows; Perissodactyla, which includes horses and rhinoceroses; and Primates, which includes monkeys, gorillas and humans. These groups probably originated in Asia and then rapidly dispersed to Europe and North America, all within the space of a few thousand years. It seems likely that movement between continents occurred over high-latitude land bridges (such as Greenland or the currently submerged land bridge under the Bering Strait), which only became warm enough to access during the PETM. A number of more ancient Paleocene mammals also went extinct at this time.
The best-known record of mammalian evolution throughout this interval, and indeed for much of the Cenozoic, comes from the Western Interior of North America. In the Bighorn and Clarks Fork basins of Wyoming , sediments that were deposited on ancient flood plains record in great detail environmental change across the PETM. Mammal fossils recovered from this interval not only show the rapid first appearances of the artiodactyls, perissodactyls and primates in this region, but also demonstrate that some types of mammal became smaller during the PETM. Fossils of the now-extinct ground-dwelling herbivores Ectocion and Copecion from the PETM interval are reconstructed as approximately half the weight of those before and after it, and several other mammal groups that survived the PETM show the same pattern. The earliest members of the artiodactyls, perissodactyls and primates were also much smaller than their immediate descendants. Elevated atmospheric carbon dioxide concentrations have been shown in laboratory experiments to reduce leaf digestibility and nutritional value for herbivores, which results in slower growth rates. The higher concentration of atmospheric greenhouse gases during the PETM therefore seems like a better explanation for mammalian dwarfing than the increase in temperature itself.


The Arctic looked like this 56 million years ago – modern Baldcypress Swamp in Louisiana. Photo by Jan Kronsell CC BY-SA 3.0

Plants are rarely fossilized whole, and most of our information on the floral response to the PETM comes from fossil leaves and pollen. The detailed record from the Bighorn Basin in Wyoming shows that several new species migrated into this region during the PETM, both from Europe and from southern North America. Warming of the higher latitudes during the PETM facilitated these migrations, by allowing plant species adapted to warmer climates to expand their ranges past the hotter low latitudes. High-latitude land bridges between Europe and North America would have become usable, and as with the mammals, some European plants spread into North America.

Pollen records from Mississippi and Alabama on the US Gulf of Mexico show that this region was the source of some of the plant species that migrated north into Wyoming during the PETM. Several of the immigrant species from Europe also reached this far into southern North America. The plant communities of the US Gulf Coast also suffered an extinction event, with approximately 20% of pollen types disappearing at the Paleocene–Eocene boundary .
In the tropics of Colombia, the pollen records show that several new species of plants appeared during the PETM and the early Eocene. Many of the species that were present during the Paleocene persisted through the PETM and into the Eocene, and only a relatively small number of extinctions took place. The equatorial forests, therefore, not only survived the PETM warmth, but seem to have flourished in it, with enhanced speciation and limited extinction increasing the number of plant species present.

It is clear that the PETM affected different groups of organisms and habitats in a range of ways. Many species simply expanded their ranges into higher latitudes, and mammals and plants were able to move into new continents. Microorganisms in the oceans’ surface waters experienced few negative impacts during the PETM, whereas benthic foraminifera underwent severe extinctions. Tropical plants did well during the warming, perhaps because they were already adapted to warm conditions. The higher concentrations of atmospheric carbon dioxide throughout the PETM decreased the nutritional value of plant material, however, leading to a temporary decrease in the size of some herbivorous insects and mammals. Feeding intensity among herbivorous insects also increased.
Jardine, Phil. 2011. The Paleocene-Eocene Thermal Maximum. Palaeontology Online, Volume 1, Article 5, 1-7.

Climate proxies, such as ocean sediments (depositional rates) indicate a duration of ∼83 ka, with ∼33 ka in the early rapid phase and ∼50 ka in a subsequent gradual phase.
The most likely method of recovery involves an increase in biological productivity, transporting carbon to the deep ocean. This would be assisted by higher global temperatures and CO2 levels, as well as an increased nutrient supply (which would result from higher continental weathering due to higher temperatures and rainfall; volcanoes may have provided further nutrients).

By the time our great-grandchildren have children of their own, we humans will likely have broken a climate record that has stood unchallenged for 56 million years.
New research has found that humans are pumping nearly 10 times more carbon dioxide into the atmosphere than what was emitted during Earth’s last major warming event, called the Palaeocene-Eocene Thermal Maximum (PETM).
If carbon emissions continue to rise in the future, mathematical models predict that within the next few hundred years, we could be facing another PETM-like event.

If the PETM global warming event is similar to today’s, we know it’s not a perfect comparison. The world was a very different place 55 million years ago. For instance, even before the PETM, the planet was already so warm that there were no ice caps. Nor should we take too much comfort from the fact that the PETM didn’t cause a major mass extinction,  because while current global warming doesn’t threaten the existence of our species, it does threaten our way of life. What we’re talking about is massive changes that could cause a rather unbelievable amount of human suffering and loss of the things we all hold dear. Scott Wing.Smithonian Institution

I can only underline Scott Wing’s statements, but I’m afraid our current climate change scenario will have impacts on our planet which not only threaten our way of life but will also threaten people’s very existence in general. Looking at the latest IPCC report „Climate Change 2022: Impacts, Adaptation and Vulnerability“ and here at the Mid to Long-term Risks (2041–2100) with max 5°C warming there seems to be little hope of less than catastrophic conditions, when it comes to PETM levels of more than 10°C.

IPCC report „Climate Change 2022: Impacts, Adaptation and Vulnerability
Mid to Long-term Risks (2041–2100)

Biodiversity loss and degradation, damages to and transformation of ecosystems are already key risks for every region due to past global warming and will continue to escalate with every increment of global warming (very high confidence). In terrestrial ecosystems, up to 48% of species assessed will likely face very high risk of extinction at global warming levels of 5°C.

In ocean and coastal ecosystems, risk of biodiversity loss increases to high to very high across most ocean and coastal ecosystems by 3°C (medium to high confidence, depending on ecosystem). Very high extinction risk for endemic species in biodiversity hotspots is projected to at least double from 2% between 1.5°C and 2°C global warming levels and to increase at least tenfold if warming rises from 1.5°C to 3°C (medium confidence).

Risks in physical water availability and water-related hazards will continue to increase by the mid- to long-term in all assessed regions, with greater risk at higher global warming levels (high confidence). Projected increases in direct flood damages are higher by 1.4 to 2 times at 2°C and 2.5 to 3.9 times at 3°C compared to 1.5°C global warming without adaptation (medium confidence). At global warming of 4°C, approximately 10% of the global land area is projected to face increases in both extreme high and low river flows in the same location, with implications for planning for all water use sectors (medium confidence).

Climate change will increasingly put pressure on food production and access, especially in vulnerable regions, undermining food security and nutrition (high confidence).  Increases in frequency, intensity and severity of droughts, floods and heatwaves, and continued sea level rise will increase risks to food security (high confidence) in vulnerable regions from moderate to high between 1.5°C and 2°C global warming level, with no or low levels of adaptation (medium confidence). At 2°C or higher global warming level in the mid-term, food security risks due to climate change will be more severe, leading to malnutrition and micro-nutrient deficiencies, concentrated in Sub-Saharan Africa, South Asia, Central and South America and Small Islands (high confidence). Global warming will progressively weaken soil health and ecosystem services such as pollination, increase pressure from pests and diseases, and reduce marine animal biomass, undermining food productivity in many regions on land and in the ocean (medium confidence). At 3°C or higher global warming level in the long term, areas exposed to climate-related hazards will expand substantially compared with 2°C or lower global warming level (high confidence), exacerbating regional disparity in food security risks (high confidence).

Climate change and related extreme events will significantly increase ill health and premature deaths from the near- to long-term (high confidence). Globally, population exposure to heatwaves will continue to increase with additional warming, with strong geographical differences in heat-related mortality without additional adaptation (very high confidence). Climate-sensitive food-borne, water-borne, and vector-borne disease risks are projected to increase under all levels of warming without additional adaptation (high confidence). In particular, dengue risk will increase with longer seasons and a wider geographic distribution in Asia, Europe, Central and South America and sub-Saharan Africa, potentially putting additional billions of people at risk by the end of the century (high confidence). Mental health challenges, including anxiety and stress, are expected to increase under further global warming in all assessed regions, particularly for children, adolescents, elderly, and those with underlying health conditions (very high confidence).

Climate change risks to cities, settlements and key infrastructure will rise rapidly in the mid- and long-term with further global warming, especially in places already exposed to high temperatures, along coastlines, or with high vulnerabilities (high confidence). Globally, population change in low-lying cities and settlements will lead to approximately a billion people projected to be at risk from coastal-specific climate hazards in the mid-term under all scenarios, including in Small Islands (high confidence). The population potentially exposed to a 100-year coastal flood is projected to increase by about 20% if global mean sea level rises by 0.15 m relative to 2020 levels; this exposed population doubles at a 0.75 m rise in mean sea level and triples at 1.4 m without population change and additional adaptation (medium confidence). Sea level rise poses an existential threat for some Small Islands and some low-lying coasts (medium confidence).

In the mid- to long-term, displacement will increase with intensification of heavy precipitation and associated flooding, tropical cyclones, drought and, increasingly, sea level rise (high confidence). At progressive levels of warming, involuntary migration from regions with high exposure and low adaptive capacity would occur (medium confidence). At higher global warming levels, impacts of weather and climate extremes, particularly drought, by increasing vulnerability will increasingly affect violent intrastate conflict (medium confidence)
IPCC report „Climate Change 2022: Impacts, Adaptation and Vulnerability

Having read this scenario of impacts and vulnerabilty based on about 3.5 degrees above our present level – can anyone imaging what life would be like at 10+C levels?

ACT NOW – SPEAK UP – Appeal to world leaders, urge your city, your bank, your employer to take urgent action toward net-zero emissions – UN Environment Program

To preserve a livable climate, greenhouse-gas emissions must be reduced by half by 2030 and to net zero by 2050. Bold, fast, and wide-ranging action needs to be taken by governments and businesses. But the transition to a low-carbon world also requires the participation of citizens – especially in advanced economies.

ActNow is the United Nations campaign for individual action on climate change and sustainability.

Every one of us can help limit global warming and take care of our planet. By making choices that have less harmful effects on the environment, we can be part of the solution and influence change.

Use the app to log your actions and contribute to the global count.

Bernd Riebe SEP 2022

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