Privileged Planet: fairly constant temperature range (Introduction)

by David Turell , Saturday, January 28, 2023, 17:05 (454 days ago) @ David Turell

Allows liquid water to persist:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/27_january_2023/407...

"For billions of years, Earth’s surface climate has varied in temperature within a relatively narrow range, allowing liquid water to persist and ensuring the habitability of our rocky planet. The breakdown of rocks during weathering is thought to be central in keeping the planet warm enough, and not too cold, by providing a stabilizing feedback in the carbon cycle. Brantley et al. (1) reconcile measurements from the laboratory with those made across landscapes in soils and rivers to quantify the overall temperature dependence of this important feedback mechanism. They find that this process only works to stabilize temperatures when minerals are supplied quickly enough by erosion and if rainfall can quench the thirst of the weathering reactions. Thus, the role of weathering in steering Earth’s climate is likely to have changed as the continents shifted and collided.

"Carbon dioxide (CO2) is a greenhouse gas that plays a major role in controlling the temperature at Earth’s surface. Every year, volcanoes release ∼0.1 petagrams of carbon (PgC) per year as CO2 from the interior of the planet.... this volcanic leak of carbon quickly adds up over Earth’s long history: In just a million years, volcanoes would release enough carbon to almost triple the atmosphere and ocean carbon stores. Left unchecked, this buildup of CO2 would lead to runaway warming.

"Instead, CO2 is removed from the atmosphere when specific minerals in rocks are chemically broken down or weathered. The carbonic acid formed by mixing CO2 with rainwater can dissolve silicate minerals. The weathering reactions remove CO2 from the atmosphere and create bicarbonate, a dissolved form of carbon, which is flushed from soils into rivers and onward to the ocean. There, the weathering products are used to make new carbonate minerals that lock up the carbon. Altogether, the net impact of silicate weathering is the removal of CO2 from the atmosphere.

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"Brantley et al. used a thermodynamic framework to assess the temperature dependence of silicate weathering rates. Using calculations grounded in the Arrhenius equation that describes the relationship between reaction rate and temperature, they combined the numerous laboratory assessments with global soil profiles and river catchments. All of these datasets show an increase of silicate weathering with temperature, although the field rates increase more with temperature than those from experiments in the lab. This can be explained by an array of processes that happen in landscapes, including physical mechanisms such as fracturing, the formation of new minerals during weathering, and the role of microorganisms. Notably, Brantley et al. show that many locations around the world have a weak link between temperature and silicate weathering because either there is not enough mineral supplied to weather or they do not have enough water.

"A convergent theme of the study of Brantley et al. and prior work is that only if minerals are supplied fast enough can the silicate weathering thermostat work. Where erosion is low, the temperature sensitivity is muted or apparently nonexistent. Chemical weathering models and proxies of CO2, climate, weathering, and erosion also provide support for these conclusions. For example, recent work has suggested that global erosion could have doubled over the past 10 million to 15 million years and made Earth’s surface a more sensitive thermostat by supplying minerals and making reactions more sensitive to temperature and water supply, which affects climate and atmospheric CO2.

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"The temperature dependence of silicate mineral weathering provides a persistent thermostat to guide Earth’s long-term climate. However, changes in patterns of erosion, water supply, and organic carbon cycling have likely changed the thermostat’s settings as the continents have drifted and collided over millions of years. Geochemical approaches in the laboratory, in the field, and in silico are crucial to further untangling the dynamics of the carbon cycle on Earth and other rocky planets."

Comment: by luck or by design the Earth's silicate weathering and the carbon cycle have kept the Earth's temperature range very moderate.