What causes climate change?
(Image: Olivier Mesnage)
What causes climate change?
- The causes of climate change are often called ‘climate forcings’. This term comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy (heat) reaching Earth’s atmosphere and the amount that escapes.
- If more solar energy escapes than arrives (negative RFS), the planet cools. Conversely, if less energy escapes than gets in (positive RFs), the planet warms (Fig. 1) This is because of the Law of Conservation of Energy, a basic law of thermodynamics, which states that:
“Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.”
- There are several climate forcings; until humans appeared, all of these forcings were natural. Now, some are anthropogenic (man-made).
- Right now, natural forcings are very slowly cooling the climate. However, much more powerful anthropogenic forcings are causing temperatures to increase, and the rate is accelerating.
The main climate forcings
- Greenhouse gasses and aerosols
- Land use (anthropogenic)
- The Milankovich Cycle (natural): how Earth orbits the Sun
- Sunspots and solar activity (natural): variations in solar energy
- Plate tectonics (natural): the position of continents
- Ocean currents (natural): distributing heat and nutrients
- Iron flux (natural): fertilising life in the oceans
- Rocks from space (natural): not often, but dramatic!
How climate forcings work
Different climate forcings each determine how much solar energy arrives from the sun, and how much escape.
- Natural Forcings happen through natural changes
- Anthropogenic Forcings are due to human activities
If the strength of cooling forcings = warming forcings, they balance one another so the climate remains unchanged. But when several cooling forcings happen at the same time, they can push Earth into an ‘ice house’ cold state. Conversely, if several warming forcings compound one another, Earth is forced into a hot ‘greenhouse’ state.
One way to think of it is what happens when two people from opposite directions push a stool. You might both be pushing really hard, but if you’re both applying the same exact force, the stool won’t move.
That’s what’s happening to our climate. We’re pushing too hard, we can see the climate is tipping, overwhelming cooling forces, but as we aren’t sure when it will crash, we just keep pushing. Once certain tipping points are reached, the climate can be very unstable for thousands of years until it reaches a new stable state.
Timeline of events: Earth’s changing climate
- When Earth formed ~4.6 billion years ago, the sun was only 70% as bright as it is today. Earth would have frozen, but the atmosphere was composed of hydrogen sulphide and the greenhouse gasses methane and carbon dioxide, but no oxygen.
- ~4 billion years ago life appeared as blue-green algae called cyanobacteria. Using sunlight, they took carbon dioxide from the air & used water to convert it into energy, just as plants do today.
- Over the next 1.7 billion years, the blue-green algae spread across the entire planet. They took so much carbon dioxide from the atmosphere and released so much oxygen (as a waste product) that by 2.4 billion years ago they had changed the chemistry of the atmosphere and caused a mass-extinction event.
- The methane haze cleared, the skies turned blue, and although the sun was getting brighter (about 7% every billion years) the temperature slowly declined, until Earth became so cold that it may have been covered in ice (Video 1).
- Volcanoes erupted, the planet warmed, but more ‘Snowball Earth’ events followed (although some may have been more ‘slushball’ than ‘snowball’) as the climate see-sawed between warm and cold.
- The last ‘Snowball Earth’ event ended around ~635 million years ago
Video 1: How volcanoes ended ‘Snowball Earth’ events.
- ~600 – 100 million years ago, complex life evolved, continents collided, and natural climate forcings shifted Earth’s climate several times between cool ‘icehouse’ and warm ‘hothouse’ or ‘greenhouse’ climates.
- The warmest ‘greenhouse’ event during this time was the Carboniferous 358.9 – 298.9 million years ago, when atmospheric carbon dioxide (CO2) was ~800ppm (twice as much as today), and sea levels were 80-120m higher.
- Around 100 million years ago multiple natural forcings outlined here, set events in motion that led to the current ice age.
Life caused the first mass extinction event:
Known as the ‘oxygen crisis’ or ‘oxygen catastrophe‘ because cyanobacteria are anaerobic; oxygen is poisonous to them. While almost all of them died out, some were engulfed by eukaryotes to become endosymbiotic cyanobacteria. Over hundreds of millions of years, they evolved into chloroplasts: the green parts inside of plants that we see today, responsible for photosynthesis. They now play an essential role in the carbon cycle.
Named because of the large amounts of coal from trees that grew in vast lowland swamp forests. The high levels of carbon dioxide and even higher levels of oxygen, and the collision of continents that created low lying land and hot wet climate, were the perfect conditions over millions of years for dead trees to fall into swampy ground. Here, they couldn’t be decomposed through normal processes. Instead, they turned into peat and eventually the ‘fossil’ fuel: coal. Today, burning this coal is releasing the carbon back into the atmosphere as carbon dioxide (CO2) (see the carbon cycle).
Where two plates are separating at a region that is high in carbon-bearing rocks, it can belch out large quantities of carbon dioxide (CO2). The East Africa rift releases about 20 megatonnes of CO2 every year from magma below the crust. See this University of Auckland paper.
Rocks from Space:
Earth has been periodically hit by asteroids, comets, and other space debris large enough to instantly change the climate. The best known event was the Cretaceous–Paleogene (K–Pg) extinction event ~65 million years ago when an asteroid impact blew dust, soil, and rocks not only into the atmosphere but also out into space, where it fell back into the upper atmosphere, creating a dust shroud for weeks to months. This stopped sunlight from reaching the surface, which led to a cool ‘impact winter’ for some years.
Worse, the asteroid slammed into rocks rich in carbonates. This along with global-scale wildfires released huge quantities of CO2 into the atmosphere, so rapid global warming of ~5°C soon followed the brief period of cooling.
Evidence of the impact event can be seen in a layer of iridium (dust from the comet) in the Waipara River, North Canterbury.
There is also evidence that large scale volcanic eruptions in what is now Northern India also contributed to the warming.
Dust from an extraterrestrial impact event near (but not into) Earth ~466 million years has been implicated in an Ice Age.
The Eocene–Oligocene extinction event ~35 million year ago may have been triggered in part by up to five impact events including at Popigai, Siberia, and Chesapeake Bay, US.
References and further reading
- Carbon Brief: Why scientists think 100% of global warming is due to humans
- BBC: The event that transformed Earth
- BBC: Earth was a frozen snowball when animals first evolved
- NASA: Snowball Earth may have been slushy
- The Conversation: Hothouse Earth: our planet has been here before—and here’s what it looks like
- Wikipedia: Evolution of chloroplasts
- National Geographic: The Carboniferous
- 2020: Muirhead et al; Displaced cratonic mantle concentrates deep carbon during continental rifting, Nature 582, pp 67–72
- 2019: Ayuso-Fernández et al; Peroxidase evolution in white-rot fungi follows wood lignin evolution in plants;
- 2019: Schmitz et al; An extraterrestrial trigger for the mid-Ordovician ice age: Dust from the breakup of the L-chondrite parent body. Science Advances 5/9
- 2016: Gernon et al; Snowball Earth ocean chemistry driven by extensive ridge volcanism during Rodinia breakup, Nature Geoscience 9, 242-248
- 2015: Rene et al; State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact: Science 350 (6256) 76-78
- 2013 IPPC: Chapter 8: Anthropogenic and Natural Radiative Forcing in: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
- 2009: Koeberl; Late Eocene impact craters and impactoclastic layers—an overview in The Late Eocene Earth—Hothouse, Icehouse, and Impacts: The Geological Society of America Vol 452
- 2004: Pierrehumbert; High levels of atmospheric carbon dioxide necessary for the termination of global glaciation Nature 429, 646–649