GAIA IS WOBBLY
Three kinds of wobble: Earth undergoes cyclical changes in its orbit’s shape, known as eccentricity (top); variations in the direction of the rotational axis, known as precession (middle); and variations in the angle its rotational axis is tilted with respect to the orbital plane, known as obliquity (bottom).

NASA/JPL-Caltech


Faint Young Sun

Magnitude: No net temperature effect

Time frame: Constant

Though the sun’s brightness fluctuates on shorter timescales, it brightens overall by 0.009% per million years, and it has brightened by 48% since the birth of the solar system 4.5 billion years ago.

Scientists reason that the faintness of the young sun should have meant that Earth remained frozen solid for the first half of its existence. But, paradoxically, geologists have found 3.4-billion-year-old rocks that formed in wave-agitated water. Earth’s unexpectedly warm early climate is probably explained by some combination of less land erosion, clearer skies, a shorter day and a peculiar atmospheric composition before Earth had an oxygen-rich atmosphere.

Clement conditions in the second half of Earth’s existence, despite a brightening sun, do not create a paradox: Earth’s weathering thermostat counteracts the effects of the extra sunlight, stabilizing Earth’s temperature (see next section).

Carbon Dioxide and the Weathering Thermostat

Magnitude: Counteracts other changes

Time frame: 100,000 years or longer

The main control knob for Earth’s climate through deep time has been the level of carbon dioxide in the atmosphere, since carbon dioxide is a long-lasting greenhouse gas that blocks heat that tries to rise off the planet.


Volcanoes, metamorphic rocks and the oxidization of carbon in eroded sediments all emit carbon dioxide into the sky, while chemical reactions with silicate minerals remove carbon dioxide and bury it as limestone. The balance between these processes works as a thermostat, because when the climate warms, chemical reactions become more efficient at removing carbon dioxide, putting a brake on the warming. When the climate cools, reactions become less efficient, easing the cooling. Consequently, over the very long term, Earth’s climate has remained relatively stable, providing a habitable environment. In particular, average carbon dioxide levels have declined steadily in response to solar brightening.

However, the weathering thermostat takes hundreds of thousands of years to react to changes in atmospheric carbon dioxide. Earth’s oceans can act somewhat faster to absorb and remove excess carbon, but even that takes millennia and can be overwhelmed, leading to ocean acidification. Each year, the burning of fossil fuels emits about 100 times more carbon dioxide than volcanoes emit — too much too fast for oceans and weathering to neutralize it, which is why our climate is warming and our oceans are acidifying.

Plate Tectonics

Magnitude: Roughly 30 degrees Celsius over the past 500 million years

Time frame: Millions of years

The rearrangement of land masses on Earth’s crust can slowly shift the weathering thermostat to a new setting.

The planet has generally been cooling for the last 50 million years or so, as plate tectonic collisions thrust up chemically reactive rock like basalt and volcanic ash in the warm, wet tropics, increasing the rate of reactions that draw carbon dioxide from the sky. Additionally, over the last 20 million years, the building of the Himalayas, Andes, Alps and other mountains has more than doubled erosion rates, boosting weathering. Another contributor to the cooling trend was the drifting apart of South America and Tasmania from Antarctica 35.7 million years ago, which initiated a new ocean current around Antarctica. This invigorated ocean circulation and carbon dioxide–consuming plankton; Antarctica’s ice sheets subsequently grew substantially.

Earlier, in the Jurassic and Cretaceous periods, dinosaurs roamed Antarctica because enhanced volcanic activity, in the absence of those mountain chains, sustained carbon dioxide levels around 1,000 parts per million, compared to 415 ppm today. The average temperature of this ice-free world was 5 to 9 degrees Celsius warmer than now, and sea levels were around 250 feet higher.

Asteroid Impacts

Magnitude: Approximately 20 degrees Celsius of cooling followed by 5 degrees Celsius of warming (Chicxulub)

Time frame: Centuries of cooling, 100,000 years of warming (Chicxulub)

The Earth Impact Database recognizes 190 craters with confirmed impact on Earth so far. None had any discernable effect on Earth’s climate except for the Chicxulub impact, which vaporized part of Mexico 66 million years ago, killing off the dinosaurs. Computer modeling suggests that Chicxulub blasted enough dust and sulfur into the upper atmosphere to dim sunlight and cool Earth by more than 20 degrees Celsius, while also acidifying the oceans. The planet took centuries to return to its pre-impact temperature, only to warm by a further 5 degrees Celsius, due to carbon dioxide in the atmosphere from vaporized Mexican limestone.

How or whether volcanic activity in India around the same time as the impact exacerbated the climate change and mass extinction remains controversial.


Evolutionary Changes

Magnitude: Depends on event; about 5 degrees Celsius cooling in late Ordovician (445 million years ago)

Time frame: Millions of years

Occasionally, the evolution of new kinds of life has reset Earth’s thermostat. Photosynthetic cyanobacteria that arose some 3 billion years ago, for instance, began terraforming the planet by emitting oxygen. As they proliferated, oxygen eventually rose in the atmosphere 2.4 billion years ago, while methane and carbon dioxide levels plummeted. This plunged Earth into a series of “snowball” climates for 200 million years. The evolution of ocean life larger than microbes initiated another series of snowball climates 717 million years ago — in this case, it was because the organisms began raining detritus into the deep ocean, exporting carbon from the atmosphere into the abyss and ultimately burying it.

When the earliest land plants evolved about 230 million years later in the Ordovician period, they began forming the terrestrial biosphere, burying carbon on continents and extracting land nutrients that washed into the oceans, boosting life there, too. These changes probably triggered the ice age that began about 445 million years ago. Later, in the Devonian period, the evolution of trees further reduced carbon dioxide and temperatures, conspiring with mountain building to usher in the Paleozoic ice age.



The concentration of oxygen and carbon dioxide in Earth’s atmosphere from the end of the Precambrian 542 million years ago to the present is charted along with the appearance of various photosynthesizers and the occurrence of mass extinction events. The evolution of conifer trees in the Devonian period, for instance, initiated a precipitous rise in oxygen and a drop in carbon dioxide that triggered a mass extinction.


Samuel Velasco/Quanta Magazine; source: doi.org/10.3390/biom9080322


Large Igneous Provinces

Magnitude: Around 3 to 9 degrees Celsius of warming

Time frame: Hundreds of thousands of years

Continent-scale floods of lava and underground magma called large igneous provinces have ushered in many of Earth’s mass extinctions. These igneous events unleashed an arsenal of killers (including acid rain, acid fog, mercury poisoning and destruction of the ozone layer), while also warming the planet by dumping huge quantities of methane and carbon dioxide into the atmosphere more quickly than the weathering thermostat could handle.

In the end-Permian event 252 million years ago, which wiped out 81% of marine species, underground magma ignited Siberian coal, drove up atmospheric carbon dioxide to 8,000 parts per million and raised the temperature by between 5 and 9 degrees Celsius. The more minor Paleocene-Eocene Thermal Maximum event 56 million years ago cooked methane in North Atlantic oil deposits and funneled it into the sky, warming the planet by 5 degrees Celsius and acidifying the ocean; alligators and palms subsequently thrived on Arctic shores. Similar releases of fossil carbon deposits happened in the end-Triassic and the early Jurassic; global warming, ocean dead zones and ocean acidification resulted.


If any of that sounds familiar, it’s because human activity is causing the same effects today.

As a team of researchers studying the end-Triassic event wrote in April in Nature Communications, “Our estimates suggest that the amount of CO2 that each … magmatic pulse injected into the end-Triassic atmosphere is comparable to the amount of anthropogenic emissions projected for the 21st century.”