Hot Earth; Hotter Planet
Climate Change Feature Feature 2

Ancient analogs show that a hotter planet is the geological norm

If the past is a key to the present, then we need to embrace a hotter planet. The earth spent about 670 million years out of its 4.5-billion-year history in glacial periods. So, glacial periods only account for only 15% of the earth’s existence, and the planet’s normal state is a hot, mostly ice-free planet. This normal, ice-free earth supports large inland seas, and vast areas of the continental coasts are underwater. Houston, Miami, New Orleans, Charleston New York, Boston, and Washington DC are all sites for scuba diving in an ice-free world.

Our view of what’s typical stems from Homo sapiens having evolved during one of the relatively rare glacial periods in the earth’s history. This particular period began about 34 million years ago. Therefore, global warming pushes us from a normal cool-earth state towards an unwelcome warmer planet from a human standpoint. However, from a geological perspective, global warming moves us back to the earth’s normal warm conditions.

The fact that a warm planet is normal does not provide humans with any comfort because approximately 33% of the world’s population lives within 100 meters of sea level. An ice-free planet submerges the first 66 of those 100 meters under the ocean. While this affects vast areas of land and provides pause for thought, it does not describe the most significant threats that climate change might bring. These will be threats to the oceans.

Hot earth of the past

We can peek back into the planet’s geological history to get a view of a hotter planet. The waning days of the dinosaurs from 100 to 66 million years ago set the stage for a tropical world. Average temperatures during this period were over 6 °C (10 °F) higher than at the end of the 20th century.

Polar ice didn’t exist in the late Cretaceous, and the planet was awash with water. Pangea, the supercontinent, had already fragmented, and many of the continents were punctuated and divided by shallow inland seas. North America was split into two pieces; the midcontinent and the West Coast. An inland seaway ran the continent’s length from the Gulf of Mexico to the northern tip of Canada, separating these landmasses.

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.

 Circulation patterns

Today, at each of the poles, robust polar vortex systems create mid and high latitude air currents like the jet stream. These winds are responsible for much of the weather we experience daily. They also generate a variety of surface currents in the earth’s oceans. The polar vortex systems draw energy from substantial temperature differences between the mid-latitudes and the poles. But warm Cretaceous polar regions would have weakened these weather systems.

A lack of strong temperature gradients between the equator and poles also created a Cretaceous planet where seasonal changes were less pronounced. The smaller temperature differential between the equator and poles then decreased the oceans’ capacity for transporting heat from the equator to the poles. Thermohaline circulation in the earth’s oceans probably declined or disappeared. Thus, currents like the North Atlantic Gulf Stream weakened or ceased to exist.

Weakened ocean currents and reduced atmospheric circulation had a knock-on effect on the ocean’s chemistry. Oceans today enjoy a high rate of “turnover,” where dense water from the ocean’s surface sinks to the ocean floor creating deep-sea currents. These deep-ocean currents eventually rise to the surface where the atmosphere replenishes them with oxygen. But a lack of ocean turnover in the Cretaceous would have set the stage for deep ocean anoxia (oxygen depletion) and black shales.

Black shales

Periods of ocean anoxia appear in the geological record as black shales. Oxygen-dependent life in the anoxic, deep oceans disappeared during specific periods in the Late Cretaceous, and black shales accumulated across the globe.

Black shales are fine-grained sedimentary rocks containing lots of organic matter. The organic matter is mixed in with clay and silt particles, but it is the organic matter that makes black shales distinct from regular shales.

Typically, the remains of dead plants and animals fall to the ocean floor, where bacteria consume them. However, when the deep-ocean waters are anoxic, there are no ocean bottom organisms to eat the organic matter reigning in from above, and it becomes incorporated in the sediments. This high organic content often gives the resulting shales a dark, blackish color.

The Cretaceous atmosphere was also rich in carbon-based greenhouse gases that helped sustain a warm planet. But the black shales provided a carbon store that served to remove some of the CO2 from the Cretaceous atmosphere and fix that carbon in ocean-bottom sediments.

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

In a twist of irony, we are now burning that ancient 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.

However, as a society, we must think beyond the crisis and conjure up a vision of the future where our grandchildren inhabit planet earth and enjoy a rich quality of life. A hotter planet is coming, but will we plan for it?

 Are current climate goals achievable?

Voices on news and social media that deny climate change and global warming are a bit self-delusional. Warming is happening, and it is human-made. Thus far, the earth’s governments have not fully come to grips with either of these facts.

On the other hand, the voices arguing that we can stop global warming at a 1.5°C rise are also deluding themselves. The 2015 Paris Climate Agreement included nearly 200 countries and set ambitious goals. The basic agreement focused on limiting the increase in average global warming to 2°C above pre-industrial levels. The agreement also had a stretch goal of limiting the temperature rise to 1.5°C. These ambitious goals rely on drastically curtailing fossil fuel use before 2050. But the devil is in the details. 

In 2014, IPCC calculations said that limiting Co2-equivalent (CO2-eq) levels in the atmosphere to 450 ppm would give us a good chance of meeting the 2°C goal. However, the problem is that by 2018 NOAA reported that we were already at 496 ppm CO2-eq.

For those not familiar with the term, CO2-eq is a measure of collective greenhouse gas concentrations translated into the warming potential of CO2 only. If actual atmospheric CO2 levels were 410 ppm and the CO2-eq level was 500 ppm, then we interpret that the warming potential of all gases other than CO2 is equal to a 90 ppm increase in CO2 levels.

At a fundamental level, we have already surpassed the necessary conditions to limit warming to 2°C. There are, of course, other remedial actions that we could take, like scaling up carbon sequestration. However, all such efforts require a collective will to succeed.

Government cooperation

The USA is withdrawing from the Paris Agreement. The current administration is also attempting to suppress research and knowledge about climate change. Since the USA is the second-largest carbon emitter in the world, its absence from the Paris Agreement is a blow to any efforts to limit temperature rise. Environmental regulations are being rolled back in countries like the USA and Brazil. The world saw an unprotected Amazon rain forest burn out of control in recent years.

The probabilities are decreasing that coordinated world efforts to tame greenhouse gas levels will succeed primarily because the political will does not currently exist. The responsibility for significant emission reductions rests with the world’s major industrial powers. This collection of quasi-democracies and authoritarian governments is not well suited for the tasks outlined in the Paris Agreement.

Rate of Change

Setting aggressive goals is an excellent way to kickstart a process. However, it is not a guarantee that those goals are achievable. Increasingly it appears that global temperatures will rise above the 2°C threshold. Beyond this level are tipping points where positive environmental feedback loops will take over and drive temperatures higher regardless of the actions taken by humans.

As many climate change skeptics point out, the earth has been warming and cooling for 4.5 billion years. But what makes modern, Anthropocene, climate history different is the rate of change. At the end of the last ice age, average global temperatures experienced a geologically rapid increase of 5°C in 6000 years (a rate of 0.8°C per 1000 years). However, over the past 50 years, the temperature increase rate had been equivalent to 13°C per 1000 years. This rate of increase is unprecedented in geological history, except during mass extinctions.

The rate of change does matter. All mass extinctions share a common theme: Climate change occurs faster than evolutionary change can keep up. Climate change and species extinction today, in the Anthropocene, are faster than at any period of the earth’s long history. A hotter planet is coming. But whether humanity can meet the challenge is unknown.


A dose of realism in environmental research (Source: ArcheanWeb) – Also:


Cretaceous Period (National Geographic) – Also:

Disturbing Animation Shows What Earth Would Look Like if All the Ince Melted (Fiona MacDonald – Science Alert) – Also:

Hothouse Earth: our planet has been here before – here is what it looked like (The Conversation) – Also:

What can the Cretaceous tell us about our climate? (Philip Pika – EGU Blogs) – Also:

Four degrees of separation: lessons from the last Ice Age (Prairie Climate Central) – Also:

Abrupt Climate change During the Last Ice Age (Matthew Schmidt & Jennifer Hertzberg – The Nature Education Knowledge Project) – Also:

What is causing climate change?  (Committee on Climate Change) – Also:

The Ups and Downs of Global Warming (NASA) – Also:

CO2 Currently Rising Faster than the PETM Extinction Event (Rob Painting – Skeptical Science) – Also:

Temperature and atmospheric CO2 concentration estimates through the PETM using triple oxygen isotope analysis of mammalian bioapatite (Gehler, Gingerich, & Pack – PNAS) – Also:

Climate Change 2014 Synthesis Report Summary for Policymakers – Also:

The NOAA Annual Greenhouse Gas Index (AGGI) – Also:

Glossary: Carbon dioxide equivalent (Eurostat) – Also:

When global warming made our world super-hot (Colin Barras – BBC) – Also:

Smithsonian: Ocean – Find Your Blue Also:

The oceans are acidifying at the fastest rate in 300 million years. How bad could it get? Also:

Very large release of mostly volcanic carbon during the Paleocene-Eocene Thermal Maximum Also:

What are the greenhouse gas changes since the Industrial Revolution? Also:

Feature Photo: Upper Terraces of Mammoth Hot Springs (Photographer – Brocken Inaglory) – This file is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license –

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.