Anthropogenic vs natural climate change: which one is faster?

Spain was blanketed by half a meter of snow in January of this year. Then the peninsula was lashed with unusually heavy rains and wind. Globally, weather broadcasters publish one extreme after the other and attribute them to climate change. But how fast does climate change really proceed? What are the numbers? How do the extreme weather phenomena of the last decades compare to the climate spikes of the geological past?

Authors: Dorothea Eue / Kathelijne Bonne, 2021. GondwanaTalks.

2020 was the warmest year on record. The northern tip of the Antarctic Peninsula reached record summer temperatures of 18.3 °C in February 2020. In Patagonia glaciers are retreating at a rate of 30-44 m per year, faster than anywhere else. The Great Lakes in the US no longer freeze. And permafrost globally disintegrates, releasing 1,7 billion tons of carbon dioxide into the atmosphere each winter since 2003.

Are we really facing accelerated anthropogenic global warming?

We'll look at the human impact on climate and compare it to climate change in the geological past of our planet. Then we'll look at some causes of global climate change and give a few examples.

Melting ice cap on Greenland (Ian Joughin on imaggeo.egu.eu / CC BY-NC-SA 3.0)
Melting ice cap on Greenland (Ian Joughin on imaggeo.egu.eu / CC BY-NC-SA 3.0)

Human impact on climate change: a few statistics.

The anthropogenic global warming started with the burning of fossil fuels, which release CO2, during the Industrial Revolution that started around 1750. CO2 levels have continued to rise and the Earth's average temperature is now 1.5 °C above the pre-industrial value. Recent models have shown that we may reach a 4° increase by the end of the century if we follow a business-as-usual trajectory (no climate change mitigation). 

Based on these numbers, from the Industrial Revolution until 2100 the global average temperature would increase more than 1° each 100 years.

The video from NASA below show the Earth's average temperature anomalies since the late 19th century. Red is above the average and blue below. 

Keeling curve showing CO2 increase since 1958.

The 2015 Paris climate agreement has states that average temperatures should not rise above 1.5 °C to stay within safe planetary boundaries, these are nine thresholds defined by the Stockholm Resilience Centre.  

The increase in CO2 in the atmosphere is illustrated by the famous Keeling Curve, showing continuous measurements taken on Mauna Loa (Hawaii) since 1958, initiated by the chemist Charles Keeling. The Keeling Curve brought the CO2 rise to the world's attention, but a link between rising temperatures and CO2 levels had already been pointed out much earlier, by the chemist Svante Arrhenius (1859-1927). 

CO2 concentration in the atmosphere is expressed in parts per million (ppm) and daily readings are published by the Scripps Intitution of Oceanography, for which Dr. Keeling worked.

Now, a few numbers to illustrate the anthropogene impact. In pre-industrial times, calculated CO2 levels in the atmosphere were around 228 ppm. This concentration has been relatively stable throughout the last 10,000 years, i.e. since the end of the Last Ice Age (see the graph with Holocene concentrations below). By the early sixties of last century CO2 levels had reached to 320 ppm, and in December 2020, we measured 414 ppm; almost twice the pre-industrial value within less than 300 years.

The main (but not the only) effect of the CO2 increase is global warming. An immediate, visible result is the melting of polar ice. According to models from the University of London, the Arctic will be ice free in summer by 2040. 

Arctic warming is already disrupting the Jetstream and the polar vortex, causing erratic weather patterns in the northern hemisphere, as, for example, sorely felt by the inhabitants of snow-struck Spain in January 2021.

Random spikes or a steady trend?

And what if these extreme data are just random spikes? Could the high temperatures of the last decades just represent a random spike? Let's find out how the fast modern global warming compares to other climate events of the Earth's history.

Climate change in the past

Through geological time climate has changed continuously. Earth has undergone extreme climates and has transformed, more than once, from an 'ice house' to a 'greenhouse' world. During an ice house, the poles are fully covered in ice, which often extended to much lower latitudes than now, whereas in a greenhouse, the poles are ice-free. Today we are in an ice-house world: both the Artic and Antarctica are covered in ice ... for now. Throughout these natural climatic changes, not all species could adapt to the changing world and many species got extinct.

Some of the main ice ages of the past lasted for thousands to millions of years. During the Proterozoic Era for example (from 2500 to 541 myr ago), Earth underwent a few severe ice ages during which it was almost completely covered in snow and ice. Scientists refer to it as the hypothetical Snowball Earth. It took millions of years before enough greenhouse gasas (e.g., from volcanoes) accumulated in the air to make the ice melt. 

And during other times, it could get very hot, for example during Paleocene to Eocene times, as we will see.

Triggers of natural climate change

The causes for initiating natural climate change are complex and intertwined. There are 3 main causes:

  • Natural changes in CO2 levels. CO2 and other greenhouse gases trap heat from the sun. A change in CO2 can result from volcanic eruptions, photosynthesis, dying and decomposing organisms, and changes in erosion of the land. (The anthropogenic CO2 release is due to the burning of organisms that did not decompose and release their CO2 in the past.)
  • Plate tectonics: as large expanses of land change location, solar radiation is distributed differently across the land and sea. Therefore, ocean currents change as well, distributing heat and humidity differently. Climate change caused by plate tectonics takes many millions of years to get established.
  • Astronomical factors: the regular changes in the position of the Earth's rotation axis, which wobbles, also act upon the climate. These oscillation of the Earth cause climate change at the scale of a few thousands to one million years. Examples are the Pleistocene Ice Ages, and Green and Desert Sahara episodes (read more about this phenomenon in our article on the palaeo-Niger River).

Fast, cataclysmic changes in climate (needing minutes to weeks or months) can be caused by asteroid impacts or massive volcanic eruptions (read our article on the Campi Flegrei supervolcano and the assumed end of the Neanderthals in Italy). The aerosols that are released in such events block out sunlight, causing a short, cold period. Normally, these changes do not cause a permanent change in climate, but many species die or get extinct in the process. 

Let's look at a past ice house world and a past greenhouse world.

Extreme cold: The Carboniferous Ice Age.

During the Carboniferous, plants and life in general had conquered the land and flourished. Earth was finally green. Therefore, an incredible amount of photosynthesis was taking place. Photosynthesis is the process by which green plants and microorganisms convert CO2 to oxygen, as part of their metabolism. They extract CO2 from the atmosphere, causing the CO2 levels to drop, with lower temperatures as a result. Due to the lower temperatures, ice caps at the poles got bigger, and these great white expanses reflected more sunlight back into space. The result: it got even colder. This Great Ice Age lasted for about 40 million years. 

Extreme heat: The Paleocene-Eocene Thermal Maximum

Earth experienced its hottest temperature 55 million years ago, at the transition from the Paleocene to the Eocene Epoch. There was a great release of CO2 from the oceans that lasted 20,000 to 50,000 years, causing average temperatures to rise to 5 to 8 degrees higher than today. This episode is known as the Paleocene-Eocene Thermal Maximum (PETM). CO2 levels were around 2000 ppm (five times more than today). The hot episode lasted in total about 200,000 years.

Based on the above numbers, the temperatures during the PETM would have risen 1° each 2500 to 10,000 years. 

And this takes us back to the initial question of this article: Which one is faster, anthropogenic or natural climate change?

Accelerated anthropogenic warming

While the PETM extreme global warming episode was catastrophic enough for many species, according to a recent study (Gingerich, 2019, see sources below), the rate at which humans produce CO2 today is ten times higher than during the peak 55 million years ago. 

And based on our quick calculations in this article, with a rise of 1° each 100 years for the post-industrial warming, as opposed to the 1° per 2500 to 10,000 years for the PETM, the global warming today appears to take place at an even faster rate than that stated by Gingerich. 

Many erratic weather events observed today can therefore to be attributed to CO2 release by human activities. We face unprecedented climatic change and its repercussions if we continue on a business-as-usual and not a build-back-better trajectory.

Did Filomena bother you? I hope it didn't because worse is coming.

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Desintegrating glacier.
Desintegrating glacier.

Go to articles on Climate Change.

Sources:

NASA Earth Observatory. 2020, Great Lakes, not so great ice., https://earthobservatory.nasa.gov/images/146317/great-lakes-not-so-great-ice

NASA Earth Observatory. 2020. Antarctica Melts Under Its Hottest Days on Record. https://earthobservatory.nasa.gov/images/146322/antarctica-melts-under-its-hottest-days-on-record.

NASA Earth Observatory. 2020. Permafrost Becoming a Carbon Source instead of a  Sink. https://earthobservatory.nasa.gov/images/145880/permafrost-becoming-a-carbon-source-instead-of-a-sink

NASA Earth Observatory. 2019. Is HPS-12 the Fastest Thinning Glacier? 

Julienne Stroeve and Dirk Notz 2018 Environ. Res. Lett. 13 103001 Changing state of Arctic sea ice across all seasons.

Gingerich, P. D. (2019). Temporal scaling of carbon emission and accumulation rates: Modern anthropogenic emissions compared to estimates of PETM onset accumulation. Paleoceanography and Paleoclimatology, 34, 329- 335. https://doi.org/10.1029/2018PA003379.

The Royal Society: Basics of climate change. https://royalsociety.org/topics-policy/projects/climate-change-evidence-causes/basics-of-climate-change/