Learning from Past Climate Changes

In the last 540 million years, five mass extinction events shaped the history of the Earth. Those events were related to extreme climatic changes and were mainly caused by asteroid impacts, massive volcanic eruption, or the combination of both.  On a global scale the main forces behind climatic change are: solar forcing, atmospheric composition, plate tectonics, Earth’s biota, and of course, us. Human activity is a major driver of the dynamics of Earth system. From hunter-gatherer and agricultural communities to the highly technological societies of the 21st century, humans have driven the climate Earth system towards new, hotter climatic conditions. Until the Industrial Revolution, the average global CO2 levels fluctuated between about 170 ppm and 280 ppm. But with the beginning of the Industrial Era, that number risen above 300 ppm, currently averaging an increase of more than 2 ppm per year. The average monthly level of CO2 in the atmosphere in last April exceeded the 410 ppm for first time in history. Thus we could hit an average of 500 ppm within the next 45 years, a number that has been unprecedented for the past 50–100+ million years according to fossil plant-based CO2 estimates. This current human-driven change far exceed the rates of change driven by geophysical or biosphere forces that have altered the Earth System trajectory in the past, and it poses severe risks for health, economies and political stability. Learning from past climatic changes is critical to our future.

Planktonic foraminifera from the Sargasso Sea in the North Atlantic Ocean. (Photograph courtesy Colomban de Vargas, EPPO/SBRoscoff.)

Microfossils from deep-sea are crucial elements for the understanding of our past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. Over millions of years, these skeletons accumulate in the deep ocean to become a major component of biogenic deep-sea sediments. The importance of microfossils as tool for paleoclimate reconstruction was recognized early in the history of oceanography. John Murray, naturalist of the CHALLENGER Expedition (1872-1876) found that differences in species composition of planktonic foraminifera from ocean sediments contain clues about the temperatures in which they lived. The ratio of heavy and light Oxygen in foraminifera shells can reveal how cold the ocean was and how much ice existed at the time the shell formed. Another tool to reconstruct paleotemperatures is the ratio of magnesium to calcium (Mg/Ca) in foraminiferal shells. Mg2+ incorporation into foraminiferal calcite  is influenced by the temperature of the surrounding seawater, and the Mg/Ca ratios increase with increasing temperature.

Diatoms and radiolarians are susceptible to different set of dissolution parameters than calcareous fossils, resulting in a different distribution pattern at the sea floor and have been used for temperature estimates in the Pacific and in the Antarctic Oceans, especially where calcareous fossils are less abundant. Diatom assemblages are also used in reconstructions of paleoproductivity.

Scanning electron microscope image of different types of pollen grains. Image from Wikipedia.

Pollen and other palynomorphs proved to be an extraordinary tool to paleoenvironmental reconstruction too. Pollen analysis involves the quantitative examination of spores and pollen at successive horizons through a core, specially in lake, marsh or delta sediments, especially in Quaternary sediments where the parent plants are well known. This provide information on regional changes in vegetation through time, and it’s also a valuable tool for archaeologists because it gives clues about man’s early environment and his effect upon it.

Stomatal frequency of land plants, which has been shown in some species to vary inversely with atmospheric pCO2, has been used to estimate paleo-pCO2 for multiple geological time periods. Stomata are the controlled pores through which plants exchange gases with their environments, and play a key role in regulating the balance between photosynthetic productivity and water loss through transpiration.

Temple I on The Great Plaza and North Acropolis seen from Temple II in Tikal, Guatemala. From Wikimedia Commons

Paleoecological records indicate that the transition to agriculture was a fundamental turning point in the environmental history of Mesoamerica. Tropical forests were reduced by agricultural expansion associated with growing human populations. Also soil loss associated with deforestation and erosion was one of the most consequential environmental impacts associated with population expansion in the Maya lowlands. This environmental crisis ended with the collapse of the Classic Maya society.

Human activity has significantly altered the climate in less than a century. Since 1970 the global average temperature has been rising at a rate of 1.7°C per century, and the rise in global CO2 concentration since 2000 is 10 times faster than any sustained rise in CO2 during the past 800,000 years. Today the most politically unstable countries are also places where environmental degradation affected food production and water supply. Other human societies have succumbed to climate change – like the Akkadians – while others have survived by changing their behavior in response to environmental change. We have the opportunity to protect the future of our own society by learning from the mistakes of our ancestors.


David Evans, Navjit Sagoo, Willem Renema, Laura J. Cotton, Wolfgang Müller, Jonathan A. Todd, Pratul Kumar Saraswati, Peter Stassen, Martin Ziegler, Paul N. Pearson, Paul J. Valdes, Hagit P. Affek. Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry. Proceedings of the National Academy of Sciences, 2018; 201714744 DOI: 10.1073/pnas.1714744115

Nicholas P. Evans et al., Quantification of drought during the collapse of the classic Maya civilization, Science (2018); DOI: 10.1126/science.aas9871 

Will Steffen, et al.; Trajectories of the Earth System in the Anthropocene; PNAS (2018) DOI: 10.1073/pnas.1810141115


Our once and future oceans

Earth is the only planet in our Solar System with high concentrations of gaseous diatomic oxygen. Simultaneously, this unique feature of Earth’s atmosphere has allowed the presence of an ozone layer that absorbed UV radiations. The progressive oxygenation of the atmosphere and oceans was sustained by an event of high organic carbon burial, called the Paleoproterozoic Lomagundi Event (ca. 2.3-2.1 billion years ago), which lasted well over 100 million years.

Oxygen is fundamental to life, and influences biogeochemical processes at their most fundamental level. But the oxygen content of Earth has varied greatly through time. In Earth history there have been relatively brief intervals when a very significant expansion of low-oxygen regions occurred throughout the world’s oceans. The discovery of black shales at many drill sites from the Atlantic, Indian, and the Pacific Ocean led to the recognition of widespread anoxic conditions in the global ocean spanning limited stratigraphic horizons. In 1976, S. O. Schlanger and H. C. Jenkyns termed these widespread depositional black shale intervals as “Oceanic Anoxic Events”. This was one of the greatest achievement of the DSDP (Deep Sea Drilling Project).

Corals one of the most vulnerable creatures in the ocean. Photo Credit: Katharina Fabricius/Australian Institute of Marine Science

Human activity is a major driver of the dynamics of Earth system. After the World War II, the impact of human activity on the global environment dramatically increased. Over the past 50 years, open ocean lost an estimated 2%, or 4.8 ±2.1 petamoles (77 billion metric tons), of its oxygen, and ocean oxygen minimum zones (OMZs) have expanded by an area about the size of the European Union. Deoxygenation is linked to other ocean stressors, including warming and acidification.

Ocean warming reduces the solubility of oxygen, and raises metabolic rates accelerating the rate of oxygen consumption. Warming also influence on thermal stratification and indirectly enhances salinity driven stratification through its effects on ice melt and precipitation. The increased stratification alters the mainly wind-driven circulation in the upper few hundred meters of the ocean and slows the deep overturning circulation. Intensified stratification may account for the remaining 85% of global ocean oxygen loss by reducing ventilation nd by affecting the supply of nutrients controlling production of organic matter and its subsequent sinking out of the surface ocean. Warming is predicted to exacerbate oxygen depletion in coastal systems through mechanisms similar to those of the open ocean.

Time scale [Gradstein et al., 2005] illustrating the stratigraphic position and nomenclature of OAEs (From Jenkyns, 2010).

The geological records show that large and rapid global warming events occurred repeatedly during the course of Earth history. The growing concern about modern climate change has accentuated interest in understanding the causes and consequences of these ancient abrupt warming events. The early Toarcian Oceanic Anoxic Event  (T-OAE; ∼183 mya) in the Jurassic Period is associated with a major negative carbon isotope excursion, mass extinction, marine transgression and global warming. Besides, the marked expansion of the oxygen minimum zone over the last decades, is quite similar to the model originally invoked for the genesis of Cretaceous OAEs. The better understanding of the Mesozoic ocean-climate system and the formation of OAEs would help us to predict environmental and biotic changes in a future greenhouse world.



Jenkyns, H. C. (2010), Geochemistry of oceanic anoxic eventsGeochem. Geophys. Geosyst.11, Q03004, doi:10.1029/2009GC002788.

Holz, M., Mesozoic paleogeography and paleoclimates – a discussion of the diverse greenhouse and hothouse conditions of an alien world, Journal of South American Earth Sciences (2015), doi: 10.1016/j.jsames.2015.01.001

Tennant, J. P., Mannion, P. D., Upchurch, P., Sutton, M. D. and Price, G. D. (2016), Biotic and environmental dynamics through the Late Jurassic–Early Cretaceous transition: evidence for protracted faunal and ecological turnover. Biol Rev. doi:10.1111/brv.12255 


The origin of modern phytoplankton.


A single valve of the diatom Thalassiosira pacifica, the coccolithophore Scyphospahaera apsteinii and a pair of phycomas of Pterosperma moebii. (From Falkowski 2004)

The term “phytoplankton” derives from Greek roots: φψτος (phytos) –related to plants – and πλαγκτον (plankton) meaning a “wanderer” or a “drifter”. It was coined by Christian Gottfried Ehrenberg in 1897 and describes a diverse, polyphyletic group of mostly single-celled photosynthetic organisms that drift with the currents in marine and fresh waters.

The evolutionary history of eukaryotic phytoplankton has been studied through morphological fossils and molecular biomarkers such as lipids or nucleic acids. Organic walled fossils  made by eukaryotic phytoplankton occur in rocks as old as 1.6 to 1.8 billion years, but their morphological diversity is low and their phylogenetic relationships obscure.

The acritarchs were dominant forms of eukaryotic phytoplankton during the NeoProterozoic and the Paleozoic. These forms diversified markedly, in parallel with the Cambrian and Ordovician radiations of marine invertebrates. The group began to decline in the Late Devonian. And of course, the End-Permian mass extinction marked a major transition in ocean ecosystem structure.


Geologic range and relative diversity of eukaryotic phytoplankton. From Martin 2012.

Diatoms, dinoflagellates and coccolithophores, appeared in the geologic record during the Mesozoic. The radiation of this modern eukaryotic phytoplankton is paralleled with a long-term increase in sea level with and expansion of flooded continental shelf area.

These taxa have been grouped under the informal heading of “red” algal lineages primarily on the basis of their chlorophyll-c plastids. The shift from green to red phytoplankton lineages may have actually begun during the late Paleozoic.

Trace elements were important in defining the evolutionary trajectory of these groups of phytoplankton. While green algae need higher concentrations of iron, zinc and copper, red forms need higher amounts of manganese, cobalt and cadmium. Geologic evidence indicates that oxygen levels in the Mesozoic were much higher and that helped the micronutrients used by the red phytoplankton to remain dissolved in the oceans and available for uptake.

Diatoms living between crystals of annual sea ice in McMurdo Sound, Antarctica. From Wikimedia Commons.

Diatoms living between crystals of annual sea ice in McMurdo Sound, Antarctica. From Wikimedia Commons.

But, changes in the availability of macronutrients, such as phosphorus, also contributed significantly to the success of these groups. The patterns of diversity in the fossil records of dinoflagellates and coccolithophorids are roughly concordant, but differs with that of diatoms. This reflect different ecological strategies. In contrast to dinoflagellates and coccolithophores, diatoms have evolved a nutrient storage vacuole that can retain sufficiently high concentrations of nitrate and phosphate such that a cell can undergo several divisions without the need for external macronutrients.

Human activities are altering ocean conditions at a speed unsurpassed in our Earth’s history. In modern oceans, the coccolithophores and other calcifying phytoplankton that live in surface waters may be devastated by the acidification, which reduces the availability of minerals needed to make and maintain their shells.
As the oceans warm, they may also become increasingly stratified, impeding upwelling and circulation. In such conditions, dinoflagellates could increase the frequency and surface area covered by toxic blooms in coastal habitats.


Martin, R. E. and Quigg, A. 2012 Evolving Phytoplankton Stoichiometry Fueled Diversification of the Marine Biosphere. Geosciences. Special Issue on Paleontology and Geo/Biological Evolution. 2:130-146.

Falkowski PG, Katz ME, Knoll AH, Quigg A, Raven JA, Schofield O, Taylor FJ., The evolution of modern eukaryotic phytoplankton, Science. 2004 Jul 16; 305(5682):354-60