Hot Earth
Climate Change Daily Earth Science

A hot earth analog

We can peek back into the planet’s geological history to get a view of a hot earth analog. The waning days of the dinosaurs and the early emergence of mammals as earth’s masters set the stage for a truly warm world. For 66 million years, from about 100 million years ago to 34 million years ago, the earth’s average temperature was over 6 °C (10 °F) higher than at the end of the 20th century. At one point, about 50 to 55 million years ago, the average global temperatures spiked about 15 °C (25 °F) higher than today. Greenhouse gas concentrations like CO2 and methane were also much higher than today.

Polar ice did not exist during this hot earth period, and glaciers in the high mountains disappeared. So, the planet earth was awash with water.  Pangea, the supercontinent, had already fragmented, and most of the continents had similar shapes to today. Also, many of these continents were punctuated and divided by shallow inland seas. 

North America was two separate pieces; the mid-continent and the West Coast, because an inland seaway separated these isolated landmasses.  Open ocean ran the length of the continent from the Gulf of Mexico to the northern tip of Canada.

The climate

Climatic conditions in the late Cretaceous allowed tropical plants to flourish in the polar regions. Also, the temperature difference between the tropics and the poles was low compared to today. This lower temperature differential had a profound effect on atmospheric and oceanic circulation. 

Today, strong polar vortex systems, at each of the poles, create mid and high latitude air currents like the jet stream. These atmospheric currents are responsible for much of the weather we experience today. They also generate a variety of surface currents in the earth’s oceans.

Our hot earth analog had a weaker temperature gradient between the equator and poles, so seasonal changes were less pronounced. Additionally, the earth’s capacity for transporting heat from the equator to the poles declined significantly. Robust thermohaline circulation in the earth’s oceans disappeared, so currents like the North Atlantic Gulf Stream weakened or ceased to exist. 

Weakened ocean currents and reduced atmospheric circulation also had a knock-on effect on ocean chemistry. Oceans today enjoy a high rate of “turnover” where water from the surface sinks and deep ocean water upwells to the surface. This process keeps the deep ocean waters oxygenated.  

Disruption of vigorous ocean circulation during the late Cretaceous led to the development of anoxic conditions in the deep oceans. This period of anoxia appears in the geological record as black shales. Deposits of these shales exist around the globe. So, during the deposition of these shales, oxygen-dependent life forms in the deep oceans disappeared.

Carbon recycling

The Cretaceous atmosphere was rich in carbon-based greenhouse gasses that helped sustain a warm planet. The black shales provided a carbon store that served to remove CO2 from the Cretaceous atmosphere and fix that carbon in deep-sea sediments.

Today, black shales from the Cretaceous are a prolific source of oil in our modern society. These ancient shales were buried deep in the earth, where heat and time transform their organic components into liquid oil. 

In a twist of irony, we are now burning that carbon and releasing the Cretaceous greenhouse gases back into the atmosphere. So, an ancient hot world is helping humans propel themselves into another period of warm climatic conditions.  

(For more see – Embrace a Hot Earth)


Sources

Cretaceous Period (National Geographic) – https://www.nationalgeographic.com/science/prehistoric-world/cretaceous/ Also:

Disturbing Animation Shows What Earth Would Look Like If All the Ice Melted (Fiona MacDonald – Science Alert) – https://www.sciencealert.com/disturbing-animation-shows-what-earth-would-look-like-if-all-the-ice-melted Also:

Hothouse Earth: our planet has been here before – here is what it looked like (The Conversation) – http://theconversation.com/hothouse-earth-our-planet-has-been-here-before-heres-what-it-looked-like-101413 Also:

What can the Cretaceous tell us about our climate? (Philip Pika – EGU Blogs) – https://blogs.egu.eu/divisions/cl/2018/08/20/what-can-the-cretaceous-tell-us-about-our-climate/ Also:

Feature Image: Cretaceous fauna, lower Cretaceous Wellcome M0008775.jpg – Wellcome Trust, a global charitable foundation – This file is licensed under the Creative Commons Attribution 4.0 International license. – https://creativecommons.org/licenses/by/4.0/deed.en

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.