Editing IPCC:AR6/SROCC/Chapter-1 (section)

=== 1.2.1 Ocean and Cryosphere in Earth’s Energy, Water and Biogeochemical Cycles ===

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The ocean and cryosphere play a key role in the Earth system. Powered by the Sun’s energy, large quantities of energy, water and biogeochemical elements (predominantly carbon, nitrogen, oxygen and hydrogen) are exchanged between all components of the Earth system, including between the ocean and cryosphere (Box 1.1, Figure 1).

During an equilibrium (stable) climate state, the amount of incoming solar energy is balanced by an equal amount of outgoing radiation at the top of Earth’s atmosphere (Hansen et al., 2011 <sup>[[#fn:r29|29]]</sup> ). At the Earth’s surface energy from the Sun is transformed into various forms (heat, potential, latent, kinetic, and chemical), that drive weather systems in the atmosphere and currents in the ocean, fuel photosynthesis on land and in the ocean, and fundamentally determine the climate (Trenberth et al., 2014 <sup>[[#fn:r30|30]]</sup> ). The ocean has a large capacity to store and release heat, and the Earth’s energy budget can be effectively monitored through the heat content of the ocean on time scales longer than one year (Palmer and McNeall, 2014 <sup>[[#fn:r31|31]]</sup> ; von Schuckmann et al., 2016 <sup>[[#fn:r32|32]]</sup> ; Cheng et al., 2018 <sup>[[#fn:r33|33]]</sup> ). The large heat capacity of the ocean leads to different characteristics of the ocean response to external forcings compared with the atmosphere (Sections 1.3, 1.4). The reflective properties of snow and ice also play an important role in regulating climate via the albedo effect. Increased amounts of solar energy are absorbed when snow or ice are replaced by less reflective land or ocean surfaces, resulting in a climate change feedback responsible for amplified changes.

Water is exchanged between the ocean, atmosphere, land and cryosphere as part of the hydrological cycle driven by solar heating (Box 1.1, Figure 1; Trenberth et al., 2007 <sup>[[#fn:r33|33]]</sup> ; Lagerloef et al., 2010 <sup>[[#fn:r34|34]]</sup> ; Durack et al., 2016 <sup>[[#fn:r35|35]]</sup> ). Evaporation from the surface ocean is the main source of water in the atmosphere, which is moved back to the Earth’s surface as precipitation (Gimeno et al., 2012 <sup>[[#fn:r36|36]]</sup> ). The hydrological cycle is closed by the eventual return of water to the ocean by rivers, streams, and groundwater flow, and through ice discharge and melting of ice sheets and glaciers (Yu, 2018). Hydrological extremes related to the ocean include floods from extreme rainfall (including tropical cyclones) or ocean circulation-related droughts (Sections 6.3, 6.5), while cryosphere-related flooding can be caused by rapid snow melt and melt water discharge events (Sections 2.3, 3.4).

Ninety-two percent of the carbon on Earth that is not locked up in geological reservoirs (e.g., in sedimentary rocks or coal, oil and gas reservoirs) resides in the ocean (Sarmiento and Gruber, 2002 <sup>[[#fn:r37|37]]</sup> ). Most of this is in the form of dissolved inorganic carbon, some of which readily exchanges with CO 2 in the overlying atmosphere. This represents a major control on atmospheric CO 2 and makes the ocean and its carbon cycle one of the most important climate regulators in the Earth system, especially on time scales of a few hundred years and more (Sigman and Boyle, 2000 <sup>[[#fn:r38|38]]</sup> ; Berner and Kothavala, 2001 <sup>[[#fn:r39|39]]</sup> ). The ocean also contains as much organic carbon (mostly in the form of dissolved organic matter) as the total vegetation on land (Jiao et al., 2010 <sup>[[#fn:r40|40]]</sup> ; Hansell, 2013 <sup>[[#fn:r41|41]]</sup> ). Primary production in the ocean, which is as large as that on land (Field et al., 1998 <sup>[[#fn:r42|42]]</sup> ), fuels complex food-webs that provide essential food for people.

Ocean circulation and mixing redistribute heat and carbon over large distances and depths (Delworth et al., 2017 <sup>[[#fn:r43|43]]</sup> ). The ocean moves heat laterally from the tropics towards polar regions (Rhines et al., 2008 <sup>[[#fn:r44|44]]</sup> ). Vertical redistribution of heat and carbon occurs where warm, low-density surface ocean waters transform into cool high-density waters that sink to deeper layers of the ocean (Talley, 2013 <sup>[[#fn:r45|45]]</sup> ), taking high carbon concentrations with them (Gruber et al., 2019 <sup>[[#fn:r46|46]]</sup> ). Driven by winds, ocean circulation also brings cold water up from deep layers (upwelling) in some regions, allowing heat, oxygen and carbon exchange between the deep ocean and the atmosphere (Oschlies et al., 2018 <sup>[[#fn:r47|47]]</sup> ; Shi et al., 2018 <sup>[[#fn:r48|48]]</sup> ) and fuelling biological production (Sarmiento and Gruber, 2006 <sup>[[#fn:r49|49]]</sup> ).

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