Editing Causes of climate change (section)

== Ripple effects ==
===Carbon sinks ===
[[File:Carbon Sources and Sinks.svg|thumb|right|{{CO2}} sources and sinks since 1880. While there is little debate that excess carbon dioxide in the industrial era has mostly come from burning fossil fuels, the future strength of land and ocean carbon sinks is an area of study.<ref>{{cite web |title=CO2 is making Earth greener—for now |url=https://climate.nasa.gov/news/2436/co2-is-making-earth-greenerfor-now/ |url-status=live |archive-url=https://web.archive.org/web/20200227173022/https://climate.nasa.gov/news/2436/co2-is-making-earth-greenerfor-now/ |archive-date=27 February 2020 |access-date=28 February 2020 |publisher=NASA}}</ref>]]
The Earth's surface absorbs {{CO2}} as part of the [[carbon cycle]]. Despite the contribution of deforestation to greenhouse gas emissions, the Earth's land surface, particularly its forests, remain a significant [[carbon sink]] for {{CO2}}. Land-surface sink processes, such as [[carbon fixation]] in the soil and photosynthesis, remove about 29% of annual global {{CO2}} emissions.<ref>{{Harvnb|IPCC SRCCL Summary for Policymakers|2019|p=10}}</ref> The ocean also serves as a significant carbon sink via a two-step process. First, {{CO2}} dissolves in the surface water. Afterwards, the ocean's [[Thermohaline circulation|overturning circulation]] distributes it deep into the ocean's interior, where it accumulates over time as part of the [[carbon cycle]]. Over the last two decades, the world's oceans have absorbed 20 to 30% of emitted {{CO2}}.<ref name=":0" />{{Rp|450|date=November 2012}} Thus, around half of human-caused {{CO2}} emissions have been absorbed by land plants and by the oceans.<ref>{{harvnb|Climate.gov, 23 June|2022|ps=:"Carbon cycle experts estimate that natural "sinks"—processes that remove carbon from the atmosphere—on land and in the ocean absorbed the equivalent of about half of the carbon dioxide we emitted each year in the 2011–2020 decade."}}</ref>

This fraction of absorbed emissions is not static. If future {{CO2}} emissions decrease, the Earth will be able to absorb up to around 70%. If they increase substantially, it'll still absorb more carbon than now, but the overall fraction will decrease to below 40%.<ref>{{harvnb|IPCC AR6 WG1 Technical Summary|2021|p=TS-122|loc=Box TS.5, Figure 1}}</ref> This is because climate change increases droughts and heat waves that eventually inhibit plant growth on land, and soils will release more carbon from dead plants [[Soil carbon feedback|when they are warmer]].<ref>{{harvnb|Melillo|Frey|DeAngelis|Werner|2017}}: Our first-order estimate of a warming-induced loss of 190 Pg of soil carbon over the 21st century is equivalent to the past two decades of carbon emissions from fossil fuel burning.</ref><ref>{{harvnb|IPCC SRCCL Ch2|2019|pp=133, 144}}.</ref> The rate at which oceans absorb atmospheric carbon will be lowered as they become more acidic and experience changes in [[thermohaline circulation]] and [[phytoplankton]] distribution.{{sfn|USGCRP Chapter 2|2017|pp=93–95}}<ref name="Liu2022">{{cite journal |last1=Liu |first1=Y. |last2=Moore |first2=J. K. |last3=Primeau |first3=F. |last4=Wang |first4=W. L. |date=22 December 2022 |title=Reduced CO2 uptake and growing nutrient sequestration from slowing overturning circulation |journal=Nature Climate Change |volume=13 |pages=83–90 |doi=10.1038/s41558-022-01555-7 |osti=2242376 |s2cid=255028552 }}</ref><ref name="PearceYale3602023">{{cite web |last=Pearce |first=Fred |date=18 April 2023 |title=New Research Sparks Concerns That Ocean Circulation Will Collapse |url=https://e360.yale.edu/features/climate-change-ocean-circulation-collapse-antarctica |language=en |access-date=3 February 2024 }}</ref>

===Climate change feedbacks ===
{{Main|Climate change feedback|Climate sensitivity}}[[File:NORTH POLE Ice (19626661335).jpg|thumb|Sea ice reflects 50% to 70% of incoming sunlight, while the ocean, being darker, reflects only 6%. As an area of sea ice melts and exposes more ocean, more heat is absorbed by the ocean, raising temperatures that melt still more ice. This is a positive feedback [[Ice–albedo feedback|process]].<ref>{{cite web |url=https://nsidc.org/cryosphere/seaice/processes/albedo.html |title=Thermodynamics: Albedo |work=NSIDC |access-date=10 October 2017|archive-url=https://web.archive.org/web/20171011021602/https://nsidc.org/cryosphere/seaice/processes/albedo.html |archive-date=11 October 2017 |url-status=live }}</ref>]]

The response of the climate system to an initial forcing is modified by feedbacks: increased by [[positive feedback|"self-reinforcing" or "positive" feedbacks]] and reduced by [[negative feedback|"balancing" or "negative" feedbacks]].<ref>{{cite web |title=The study of Earth as an integrated system |publisher=Earth Science Communications Team at NASA's Jet Propulsion Laboratory / California Institute of Technology |year=2013 |series=Vitals Signs of the Planet |archive-url=https://web.archive.org/web/20190226190002/https://climate.nasa.gov/nasa_science/science/ |archive-date=26 February 2019 |url=https://climate.nasa.gov/nasa_science/science/ |url-status=live}}</ref> The main reinforcing feedbacks are the [[Water vapour feedback|water-vapour feedback]], the [[ice–albedo feedback]], and the net effect of clouds.{{sfn|USGCRP Chapter 2|2017|pp=89–91}}<ref>{{harvnb|IPCC AR6 WG1 Technical Summary|2021|p=58|ps=: The net effect of changes in clouds in response to global warming is to amplify human-induced warming, that is, the net cloud feedback is positive (high confidence)}}</ref> The primary balancing mechanism is [[radiative cooling]], as Earth's surface gives off more [[Infrared|heat]] to space in response to rising temperature.{{sfn|USGCRP Chapter 2|2017|pp=89–90}} In addition to temperature feedbacks, there are feedbacks in the carbon cycle, such as the fertilizing effect of {{CO2}} on plant growth.<ref>{{harvnb|IPCC AR5 WG1|2013|p=14}}</ref>

Uncertainty over feedbacks, particularly cloud cover,<ref>{{harvnb|IPCC AR6 WG1 Technical Summary|2021|pp=58, 59|ps=: clouds remain the largest contribution to overall uncertainty in climate feedbacks}}</ref> is the major reason why different climate models project different magnitudes of warming for a given amount of emissions.<ref>{{harvnb|Wolff|Shepherd|Shuckburgh|Watson|2015|ps=: "the nature and magnitude of these feedbacks are the principal cause of uncertainty in the response of Earth's climate (over multi-decadal and longer periods) to a particular emissions scenario or greenhouse gas concentration pathway."}}</ref> As air warms, [[Relative humidity|it can hold more moisture]]. Water vapour, as a potent greenhouse gas, holds heat in the atmosphere.{{sfn|USGCRP Chapter 2|2017|pp=89–91}} If cloud cover increases, more sunlight will be reflected back into space, cooling the planet. If clouds become higher and thinner, they act as an insulator, reflecting heat from below back downwards and warming the planet.<ref>{{Cite journal |last1=Williams |first1=Richard G |last2=Ceppi |first2=Paulo |last3=Katavouta |first3=Anna |date=2020 |title=Controls of the transient climate response to emissions by physical feedbacks, heat uptake and carbon cycling |url=https://iopscience.iop.org/article/10.1088/1748-9326/ab97c9 |journal=Environmental Research Letters |volume=15 |issue=9 |pages=0940c1 |doi=10.1088/1748-9326/ab97c9 |bibcode=2020ERL....15i40c1W |issn=1748-9326|hdl=10044/1/80154 |hdl-access=free }}</ref>

Another major feedback is the reduction of snow cover and sea ice in the Arctic, which reduces the reflectivity of the Earth's surface.<ref>{{harvnb|NASA, 28 May|2013}}.</ref>
More of the Sun's energy is now absorbed in these regions, contributing to [[polar amplification|amplification of Arctic temperature changes]].<ref>{{Cite journal |last1=Cohen |first1=Judah |last2=Screen |first2=James A. |last3=Furtado |first3=Jason C. |last4=Barlow |first4=Mathew |last5=Whittleston |first5=David |last6=Coumou |first6=Dim |last7=Francis |first7=Jennifer |last8=Dethloff |first8=Klaus |last9=Entekhabi |first9=Dara |last10=Overland |first10=James |last11=Jones |first11=Justin |date=2014 |title=Recent Arctic amplification and extreme mid-latitude weather |url=https://www.nature.com/articles/ngeo2234 |journal=Nature Geoscience |language=en |volume=7 |issue=9 |pages=627–637 |doi=10.1038/ngeo2234 |bibcode=2014NatGe...7..627C |issn=1752-0894|hdl=10871/20621 |hdl-access=free }}</ref> Arctic amplification is also thawing [[permafrost]], which releases methane and {{CO2}} into the atmosphere.<ref name="Turetsky 2019">{{harvnb|Turetsky|Abbott|Jones|Anthony|2019}}</ref> Climate change can also cause methane releases from [[wetland]]s, marine systems, and freshwater systems.{{sfn|Dean|Middelburg|Röckmann|Aerts|2018}} Overall, climate feedbacks are expected to become increasingly positive.<ref>{{harvnb|IPCC AR6 WG1 Technical Summary|2021|p=58|ps=: Feedback processes are expected to become more positive overall (more amplifying of global surface temperature changes) on multi-decadal time scales as the spatial pattern of surface warming evolves and global surface temperature increases.}}</ref>