Carboniferous Cooling
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Carboniferous Cooling

Cold Weather Returns

Published in the EarthSphere Blog: Cover Image: The Amniote Egg — A Game Changer (©2022 Archean Enterprises, LLC; ArcheanArt)


The Carboniferous Period was a time of tremendous evolutionary change in Earth’s plant and animal kingdoms. Last time in the Forgotten Origins series, we discussed lush, Carboniferous forests, vast swamps, massive coal deposits, and predatory dragonfly-like insects with over two-foot wingspans. But life learned some other tricks during this period.

Last time: Carboniferous Period — The Age of Coal

The Amniote Egg

Once life crawled out of the ocean, it sought to explore a strange new world and go where no life had gone before. But before such a journey was possible, life had to solve some basic problems about living on dry land. One such pressing matter was how to reproduce. Sexual reproduction was nothing new, but that left the problem of the egg.

The Carboniferous Period produced one of life’s great evolutionary innovations, the amniote egg. The movement of animal life onto land created a reproductive dilemma. Prior to the first land settlers, animals lived beneath the waves of ancient oceans. There, they reproduced in a wet, aqueous environment where fertilized eggs developed into new life. But this strategy had its limitations on land. Perhaps these water-bound eggs might survive in damp, low-lying terrains, but they were susceptible to desiccation as climate conditions changed and periods of drought arrived. These eggs stood little chance of survival away from the water’s edge.

Amniote eggs changed this dissatisfactory state of affairs by providing a tough membrane seal around the egg, preventing it from drying out. These were probably rubbery textured membranes at first, but eventually, they led to hard-shell eggs. You will notice there were eggs but no chickens in the Carboniferous, so here we find the answer to the age-old riddle of which came first, the chicken or the egg? This innovation allowed the ancestors of reptiles, birds, and mammals to exploit ecological niches across the continents and far from the oceans.

Amniote eggs encase their embryonic life in a sealed capsule with all the fluids and food needed for a new life to develop. The liquid ingredients for life stay safely inside, yet air, hence oxygen necessary for life, can still permeate the shell. All in all, a very clever evolutionary development.

Self-Regulating Cycles

The biosphere and our environment are in a constant dance of self-regulation. Feedback loops of all types adjust and modulate environmental conditions. The end of the Carboniferous is a classic example of a self-regulating cycle.

The period was a remarkable time for both animal and plant evolution. Conditions were ripe for the growth of thick forests across extensive areas of low, swampy terrain. Ample sunlight, water, and nutrients allowed for explosive growth. The most productive areas were in the equatorial Laurasia landmass, where grand Carboniferous rainforests developed. Geological conditions conspired to create environments where the rapid burial of dead trees and plants allowed for thick, rich coal deposits to develop. Over 300 million years later, these same coal deposits drove our planet’s Industrial revolution and kicked off Anthropocene climate warming.

But coal is just condensed carbon. Trees and other plants extract carbon dioxide from the atmosphere and transform it into organic carbon. During the Carboniferous, this organic carbon was sequestered in biomass and in the forest soils. When plants died, carbon locked in nascent coal deposits was permanently removed from the atmosphere.

The late Carboniferous, when many of the coal beds were deposited, spanned a period of 24 million years. Slowly the amount of carbon dioxide in the atmosphere decreased as the forests sequestered it, leading to global cooling. Cooler and dryer conditions eventually caused the collapse of the Carboniferous rainforests, followed by increased high latitude glaciation.

Success always comes at a price because it changes the environmental balance. The tremendous success of the Carboniferous rainforests depended on a warm climate and lots of carbon dioxide. Eventually, the rainforests significantly depleted the atmospheric carbon dioxide. They became the masters of their own demise.

A Long Time Coming

The environmental saga of the Carboniferous stretched over tens of millions of years. Despite the severe environmental changes, the period ended with only a minor extinction event, centered mainly around rainforest collapse in the tropical latitudes. Environmental shifts over long periods of time provide a buffer for the biosphere, giving evolution a chance to work out solutions as ecosystems slowly change. Average global temperatures shifted from about 20 degrees C in the Early Carboniferous to about 10 degrees C by the end of the period.

The average rate of temperature change during the Carboniferous was about 0.167 degrees C per million years or 6 million years for a temperature drop of 1 degree. We have seen an Anthropocene temperature rise of one degree in 100 years by comparison. The comparative rate of change between now and then is staggering, with Anthropocene change occurring thousands of times faster than in the Carboniferous.

The rate of change matters; the faster the rate of environmental change, the less likely evolution can keep up. The end result of this imbalance is species extinction, with increasing species loss as the rate of change accelerates. Homo Sapiens are clever, but they cannot escape the fallout of rapid climate change. This fallout does not guarantee extinction for a species as adaptable as humans, but life would look very different if our critical marine species suffered a massive population decline. Three billion people currently depend on seafood as their primary source of protein. 

The industrial and technological revolutions of the Anthropocene mean human activities have profound effects on the ecosystems sustaining us. It is entirely possible for us to destroy the ecosystems we ultimately depend on for survival, and become the masters of our own demise.

(Next: Life in the Permian)



Sustainable Seafood (Source: WWF) 

The Carboniferous (Source: Berkeley) 

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.