Evolution of the Global Carbon Cycle and Climate Regulation on Earth

Abstract

The existence of stabilizing feedbacks within Earth's climate system is generally thought to be necessary for the persistence of liquid water and life. Over the course of Earth's history, Earth's atmospheric composition appears to have adjusted to the gradual increase in solar luminosity, resulting in persistently habitable surface temperatures. With limited exceptions, the Earth system has been observed to recover rapidly from pulsed climatic perturbations. Carbon dioxide (CO2) regulation via negative feedbacks within the coupled global carbon‐silica cycles are classically viewed as the main processes giving rise to climate stability on Earth. Here we review the long‐term global carbon cycle budget, and how the processes modulating Earth's climate system have evolved over time. Specifically, we focus on the relative roles that shifts in carbon sources and sinks have played in driving long‐term changes in atmospheric pCO2. We make the case that marine processes are an important component of the canonical silicate weathering feedback, and have played a much more important role in pCO2 regulation than traditionally imagined. Notably, geochemical evidence indicate that the weathering of marine sediments and off‐axis basalt alteration act as major carbon sinks. However, this sink was potentially dampened during Earth's early history when oceans had higher levels of dissolved silicon (Si), iron (Fe), and magnesium (Mg), and instead likely fostered more extensive carbon recycling within the ocean‐atmosphere system via reverse weathering—that in turn acted to elevate ocean‐atmosphere CO2 levels.

Mountains, erosion and the carbon cycle

“…The textbook juxtaposition of C sources from solid Earth degassing versus C sinks from silicate weathering has prevailed for decades 17,24 and persists today 53,58 (Fig. 1A).…”

Chance played a role in determining whether Earth stayed habitable

“…Or, conversely, given that the early Earth was not frozen, if the rest of the climate system had remained unaltered then the oceans would now be boiling under today's brighter Sun. Evidently neither occurred, for reasons that are still debated [8][9][10][11][12] ; although plausible solutions exist, in particular when combining multiple processes 13 , as yet there is no firm consensus as to how in reality the 'Faint Young Sun Paradox' was overcome 14 .…”

“…However, the Gaia hypothesis lacks a clear mechanistic basis and is not consistent with the most recent scientific evidence 23 . Another proposal in this class suggests that silicate weathering contributes to a thermostat by removing carbon dioxide more rapidly under warmer climates 12,[24][25][26][27][28][29][30] . Controls on continental silicate weathering rate remain somewhat uncertain, however, with field studies suggesting a strong influence from the rate of supply of newly-exposed (easily weathered) rock 31,32 , as well as from climate 28 .…”

“…Proxy evidence also casts some doubt on whether silicate weathering effectively countered climate changes during Earth's history 33,34 ; for instance, its rate appears not to have decreased as climate cooled gradually through the Cenozoic 33,35 and climate recovery following the intense warming of the Paleocene Eocene Thermal Maximum has been attributed to organic carbon burial rather than silicate weathering 36,37 . As questions have arisen about whether silicate weathering on the continents provides Earth's thermostat, new proposals have suggested that silicate weathering in other locations is key: either islands 27 or the sea floor 12,29 .…”

“…The most widely held view at this time is that "Earth has been a habitable planet for [billions] of years because of the existence of stabilising feedbacks in the global carbon and silicon cycles." 12 . Although there are a few exceptions 23,39,40 , the possible role of chance is typically not even mentioned 12,30,[43][44][45] .…”