What causes climate change?
Image: Olivier Mesnage
Causes
- A brief history of climate change: who knew what, when
- What causes climate change?
- Would the climate be warming without humans?
- Is it just a cycle? (Earth’s wobbly orbit)
- Sunspots & solar activity
- Land use: agriculture & cities
- Volcanoes
- Ocean currents
- Black carbon & ash
- Blue hydrogen
- Greenhouse gases & how they work
- – Carbon dioxide & the carbon cycle
- – Methane (biogenic & ‘natural’ gas)
- – Nitrous oxide
- – Clouds & water vapour
- – Ozone
- – Man-made industrial chemicals
- – Aerosol pollution
- How to start an Ice Age!
- What’s in a name?
Other sections
Home > Climate wiki > What causes climate change?
What causes climate change?
Summary
- 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 many climate forcings. Until humans appeared, all were natural. Now, the most powerful ones are anthropogenic (man-made).
- Different climate forcings each determine how much solar energy arrives from the sun, and how much escape.
- Natural Forcings happen through natural changes; these were very slowly cooling the planet (Fig. 1)
- Anthropogenic Forcings due to human activities are far more powerful. They are causing temperatures to increase much more than natural cooling, and the pace is accelerating (Fig. 1).
The main climate forcings
- Greenhouse gases
- (natural) changed chemistry of the atmosphere: life and volcanoes
- (anthropogenic) changed ichemistry of the atmosphere: human activities, mostly by burning fossil fuels.
- Land use changes destroying biodiversity (anthropogenic; the worst comes from livestock and also growing food for livestock)
- Black carbon (soot) and ash (anthropogenic: forest fires & industrial pollution) and (natural: volcanoes)
- The Milankovitch 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!
Causes
- A brief history of climate change: who knew what, when
- What causes climate change?
- Would the climate be warming without humans?
- Is it just a cycle? (Earth’s wobbly orbit)
- Sunspots & solar activity
- Land use: agriculture & cities
- Volcanoes
- Ocean currents
- Black carbon & ash
- Blue hydrogen
- Greenhouse gases & how they work
- – Carbon dioxide & the carbon cycle
- – Methane (biogenic & ‘natural’ gas)
- – Nitrous oxide
- – Clouds & water vapour
- – Ozone
- – Man-made industrial chemicals
- – Aerosol pollution
- How to start an Ice Age!
- What’s in a name?
Other sections
Home > Climate wiki > What causes climate change?
Summary
- 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 many climate forcings. Until humans appeared, all were natural. Now, the most powerful ones are anthropogenic (man-made).
- Different climate forcings each determine how much solar energy arrives from the sun, and how much escape.
- Natural Forcings happen through natural changes; these were very slowly cooling the planet (Fig. 1)
- Anthropogenic Forcings due to human activities are far more powerful. They are causing temperatures to increase, and the pace is accelerating (Fig. 1).
The main climate forcings
- Greenhouse gases
- (natural) changes in the chemistry of the atmosphere caused mostly by life and volcanoes
- (anthropogenic) changes in the chemistry of the atmosphere caused by human activities, mostly by burning fossil fuels.
- Land use changes destroying biodiversity (anthropogenic; the worst comes from livestock and also growing food for livestock)
- Black carbon (soot) and ash (anthropogenic: forest fires & industrial pollution) and (natural: volcanoes)
- The Milankovitch 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 do forcings work?
If the strength of cooling = warming, the forcings balance one another so the climate stays the same. 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. Humans are pushing so hard that we can see the climate tipping, overwhelming natural cooling forces (Fig. 1). But we can’t be certain when the climate will crash and break, so we just keep pushing. Once certain tipping points are reached, the geological record shows that the climate will become unstable for several thousands of years until a new stable state is reached.
More information
-
- 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 gases 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. (see the next Explainer for details)
- 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
- ~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.
-
Around 2.4 – 2 billion years ago during the Paleoproterozoic era, the earliest life forms were anaerobic cyanobacteria, which produced oxygen as a waste product. There was so many of them producing so much oxygen that that they changed the chemistry of the atmosphere…and poisoned themselves in the process.This is known as the Great Oxidation Event, also called the Great Oxygenation Event, the Oxygen Crisis and the Oxygen Catastrophe. 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 still produce the oxygen we need to survive, and now play an essential role in the carbon cycle.
-
Fossil fuels are literally the carbon in plants and animals that died millions of years ago. As the conditions that made them no longer exist, they cannot be replaced (at least in anything remotely relevant to human lifespans), so they are non-renewable.
Take coal for example. This comes from trees that grew in vast lowland swamp forests during the (appropriately named) Carboniferous Period, 358.9 million years ago (Mya), to 298.9 Mya. The high levels of carbon dioxide and even higher levels of oxygen in the atmosphere during that period plus the collision of continents that created low lying land and a 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 became the ‘fossil’ fuel coal. (See the carbon cycle on this website for more details and how oil and gas was made).
Today, burning this coal is releasing the carbon back into the atmosphere as carbon dioxide (CO2).
-
Where two plates are separating at a region where the rocks contain a lot of carbon, it can belch out large quantities of carbon dioxide (CO2). For example, the East Africa rift releases about 20 megatonnes of CO2 every year from magma below the crust. See this University of Auckland paper, and these research papers:
- 2022: Muller et al; Evolution of Earth’s tectonic carbon conveyor belt, Nature volume 605, pages 629–63
- 2020: Muirhead et al; Displaced cratonic mantle concentrates deep carbon during continental rifting, Nature 582, pp 67–72
-
Carbon already in the atmosphere
(ppm = parts per million; Gt = one gigatonne or one billion tonnes)
- 2.13 Gt of carbon = 1 ppm currently in the atmosphere
- To convert carbon (C) to carbon dioxide (CO2), first divide the atomic mass of carbon (12) by the atomic mass of CO2 (44) = 3.67.
- Then multiply this by 2.13 Gt carbon: 3.67 x 2.13 = 7.8 Gt carbon dioxide
- So, 7.8Gt carbon dioxide = 1ppm of CO2 currently in the atmosphere
- As there is currently around 415ppm* of CO2 in the atmosphere, that’s 415 x 7.8 Gt = 3,373Gt CO2.
* The amount of CO2 in the atmosphere varies seasonally because plants accumulate carbon in the spring and summer and release some back to the air in autumn and winter. As the northern hemisphere has more land and plants, carbon dioxide levels go up in winter because plants become less productive. Annual measurements of carbon dioxide are an average of these ups and downs. On April 11, 2021, CO2 in the atmosphere peaked at 420ppm
Calculations for adding carbon to the atmosphere from emissionsEmissions are NOT the same as concentrations! This is because the ocean and biosphere absorb around 55% of emissions. Currently, about 45% stays in the atmosphere.- To calculate each additional ppm, divide 7.8 Gt / 0.45 = 17.3Gt
- So, it takes about 17.3Gt of CO2 emissions to add 1ppm to the atmosphere
But that number is changing because in the future, the oceans will not be able to absorb as much of this extra CO2 future, which means that fewer emissions will add more CO2 and thus more warming to the atmosphere.Moreover, “Additional ecosystem responses to warming not yet fully included in climate models, such as CO2 and CH4 [methane] fluxes from wetlands, permafrost thaw and wildfires, would further increase concentrations of these gases in the atmosphere (high confidence).” – IPCC 2021 p41.
-
Earth has periodically been 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, that is, they were full of carbon. This, along with global-scale wildfires that also released huge quantities of CO2 into the atmosphere from burning trees, led to a rapid global warming of ~5°C soon after the relatively brief ‘impact winter’ cooling period.
- 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 an ‘impact winter’ that led to an Ice Age.
- The Eocene–Oligocene extinction event 35 million years ago may have been triggered in part by up to five impact events including Popigai, Siberia and Chesapeake Bay, US.
-
- 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
- 2022: Muller et al; Evolution of Earth’s tectonic carbon conveyor belt, Nature volume 605, pages 629–63
- 2021: Kramer et al; Observational evidence of increasing global radiative forcing, Geophysical Research Letters, 25 March
- 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; PNAS 116 (36) 17900-17905
- 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
-
- 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
- 2022: Muller et al; Evolution of Earth’s tectonic carbon conveyor belt, Nature volume 605, pages 629–63
- 2021: Kramer et al; Observational evidence of increasing global radiative forcing, Geophysical Research Letters, 25 March
- 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; PNAS 116 (36) 17900-17905
- 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