Anthropogenic vs natural climate change: which one is faster?

22/01/2021

Globally, weather broadcasters publish one extreme weather event after the other, attributing 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 extremes 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 thinning 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: when did it start and how is it measured?

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 since and the Earth's average temperature is now 1.5 °C above pre-industrial values. Recent models have shown that we may reach a 4° increase by the end of this century if we follow a business-as-usual trajectory (no climate change mitigation). The main, but not the only, effect of higher CO2 levels is global warming.

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

The video from NASA below shows 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 states that average temperatures should not rise above 1.5 °C to stay within safe planetary boundaries, the natural limits that define a safe world for humans.  

CO2 readings have been recorded systematically since 1958, when an American chemist, Dr Charles Keeling, started to take measurements on Mauna Loa in Hawaii (where the air is very clean). The now famous "Keeling Curve" shows the steady increase in CO2 in the atmosphere since. 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 is published every day by the Scripps Institution of Oceanography. 

Stable CO2 levels: Last Ice Age until the industrial Revolution

In pre-industrial times, calculated CO2 levels in the atmosphere were around 260-270 ppm. This concentration has been stable throughout the last 10,000 years, i.e. since the end of the Last Ice Age (see the graph below). By the early sixties of last century, CO2 levels had reached to 320 ppm, and in December 2020, we measured 414 ppm.

Let's learn more about past climate change.

Ice house vs hot house episodes

Through geological time climate has changed continuously. Earth has undergone extreme climates and has transformed, more than once, from an 'ice house' to a 'hot house' (or greenhouse) world. During an ice house, the poles are covered in ice, which often extended to much lower latitudes than now, whereas in a hot house, the poles are ice-free. Such episodes can last tens of thousands, to tens of millions of years.

Today we are in an ice-house world: Antarctica has been covered in ice for over 33 million years, and the Artic for almost 3 million years. Within a timespan of a human life, we will swap from ice to hot house: Models of the University of London predict the Arctic will become ice free in summers from 2040 onward. 

One very extreme ice house happened in the Proterozoic Era (from 2500 to 541 myr ago). Earth was almost completely covered in snow and ice. Scientists refer to it as the Snowball Earth (see image). 

Let's look at the causes of natural climate change.

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 changes in the frequency and amount of volcanic eruptions, photosynthesis, dying and decomposing organisms, and erosion of the land.
  • 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.

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 two past examples of a ice house world and a hot house 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 capture CO2, 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. In turn, 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.

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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/