Laura at 08:35, 22 May 2026

2026-05-22T08:35:06Z

← Older revision		Revision as of 08:35, 22 May 2026
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			{{ChapterNavigation
			\| cover = Cover-WGI.jpg
			\| reporturl = IPCC:AR6/WGI
			\| reporttitle = WGI
			\| prevurl = IPCC:AR6/WGI/Chapter-4
			\| nexturl = IPCC:AR6/WGI/Chapter-6
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Laura: Laura moved page IPCC:Wg1:Chapter:Chapter-5 to IPCC:AR6/WGI/Chapter-5 without leaving a redirect

2026-05-18T16:17:37Z

Laura moved page IPCC:Wg1:Chapter:Chapter-5 to IPCC:AR6/WGI/Chapter-5 without leaving a redirect

← Older revision	Revision as of 16:17, 18 May 2026
(No difference)

Laura: #119

2026-05-15T11:56:28Z

#119

← Older revision		Revision as of 11:56, 15 May 2026
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	This box presents an assessment of interactions between the carbon and water cycles that influence the dynamics of the biosphere and its interaction with the climate system. It also highlights carbon–water trade-offs arising from the use of land-based climate change mitigation options. Individual aspects of the interactions between the carbon and water cycles are addressed in separate chapters (Sections 5.2.1, 5.4.1, 8.2.3, 8.3.1, 8.4.1 and 11.6). The influence of wetlands and dams on methane emissions is assessed elsewhere (Sections 5.2.2, 5.4.7 and 8.3.1), as well as the consequences of permafrost thawing ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2\|Section 9.5.2]] and Box 5.1) and/or increased flooding (Sections 8.4.1, 11.5 and 12.4) on wetland extent in the northern high latitudes and wet tropics.		This box presents an assessment of interactions between the carbon and water cycles that influence the dynamics of the biosphere and its interaction with the climate system. It also highlights carbon–water trade-offs arising from the use of land-based climate change mitigation options. Individual aspects of the interactions between the carbon and water cycles are addressed in separate chapters (Sections 5.2.1, 5.4.1, 8.2.3, 8.3.1, 8.4.1 and 11.6). The influence of wetlands and dams on methane emissions is assessed elsewhere (Sections 5.2.2, 5.4.7 and 8.3.1), as well as the consequences of permafrost thawing ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2\|Section 9.5.2]] and Box 5.1) and/or increased flooding (Sections 8.4.1, 11.5 and 12.4) on wetland extent in the northern high latitudes and wet tropics.

	~~'''~~'''Does elevated CO~~'''~~ <sub>2</sub> ~~'''~~alleviate the ~~impa''' '''cts~~ of drought?'''~~'''~~		'''Does elevated CO <sub>2</sub> alleviate the impacts of drought?'''

	Increasing atmospheric CO <sub>2</sub> concentration enhances leaf photosynthesis and drives a partial closure of leaf stomata, leading to higher water-use efficiency (WUE) at the leaf canopy and ecosystem scales ( [[#Norby--2011\|Norby and Zak, 2011]] ; [[#De%20Kauwe--2013\|De Kauwe et al., 2013]] ; [[#Fatichi--2016\|Fatichi et al., 2016]] ; [[#Knauer--2017\|Knauer et al., 2017]] ; [[#Mastrotheodoros--2017\|Mastrotheodoros et al., 2017]] ). Since AR5 (Box 6.3), a growing body of evidence from tree-ring and carbon isotopes further confirms an increase of plant water-use efficiency over decadal to centennial time scales, with some evidence for a stronger enhancement of photosynthesis compared to stomatal reductions ( [[#Frank--2015\|Frank et al., 2015]] ; [[#Guerrieri--2019\|Guerrieri et al., 2019]] ; [[#Adams--2020\|Adams et al., 2020]] ).		Increasing atmospheric CO <sub>2</sub> concentration enhances leaf photosynthesis and drives a partial closure of leaf stomata, leading to higher water-use efficiency (WUE) at the leaf canopy and ecosystem scales ( [[#Norby--2011\|Norby and Zak, 2011]] ; [[#De%20Kauwe--2013\|De Kauwe et al., 2013]] ; [[#Fatichi--2016\|Fatichi et al., 2016]] ; [[#Knauer--2017\|Knauer et al., 2017]] ; [[#Mastrotheodoros--2017\|Mastrotheodoros et al., 2017]] ). Since AR5 (Box 6.3), a growing body of evidence from tree-ring and carbon isotopes further confirms an increase of plant water-use efficiency over decadal to centennial time scales, with some evidence for a stronger enhancement of photosynthesis compared to stomatal reductions ( [[#Frank--2015\|Frank et al., 2015]] ; [[#Guerrieri--2019\|Guerrieri et al., 2019]] ; [[#Adams--2020\|Adams et al., 2020]] ).

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	In conclusion, it is ''very likely'' that elevated CO <sub>2</sub> leads to increased WUE at the leaf level, concurrent with enhanced photosynthesis. Increased CO <sub>2</sub> concentrations alleviate the effects of water deficits on plant productivity ( ''medium confidence'' ) but there is ''low confidence'' for its role under extreme drought conditions. There is ''low confidence'' that increased WUE by vegetation will substantially reduce global plant transpiration and diminish the frequency and severity of soil moisture and streamflow deficits associated with the radiative effect of higher CO <sub>2</sub> concentrations.		In conclusion, it is ''very likely'' that elevated CO <sub>2</sub> leads to increased WUE at the leaf level, concurrent with enhanced photosynthesis. Increased CO <sub>2</sub> concentrations alleviate the effects of water deficits on plant productivity ( ''medium confidence'' ) but there is ''low confidence'' for its role under extreme drought conditions. There is ''low confidence'' that increased WUE by vegetation will substantially reduce global plant transpiration and diminish the frequency and severity of soil moisture and streamflow deficits associated with the radiative effect of higher CO <sub>2</sub> concentrations.

	~~'''~~'''How does drought affect the ~~terres''' '''trial~~ CO~~'''~~ <sub>2</sub> ~~'''~~sink?'''~~'''~~		'''How does drought affect the terrestrial CO <sub>2</sub> sink?'''

	Water availability controls the spatial distribution of photosynthesis – gross primary productivity (GPP) – over a larger part of the globe ( [[#Beer--2010\|Beer et al., 2010]] ) and, at local scale, drought decreases GPP more than respiration ( [[#Schwalm--2012\|Schwalm et al., 2012]] ) over most ecosystem types. This makes water availability a major climatic driver of variability in net ecosystem exchange ( [[#Jung--2017\|Jung et al., 2017]] ; [[#Humphrey--2018\|Humphrey et al., 2018]] ). In addition to suppressing photosynthesis, field evidence suggests that droughts reduce the land CO <sub>2</sub> sink, also through increasing forest mortality and promoting wildfire ( [[#Allen--2015\|Allen et al., 2015]] ; [[#Brando--2019\|Brando et al., 2019]] ; [[#Abram--2021\|Abram et al., 2021]] ).		Water availability controls the spatial distribution of photosynthesis – gross primary productivity (GPP) – over a larger part of the globe ( [[#Beer--2010\|Beer et al., 2010]] ) and, at local scale, drought decreases GPP more than respiration ( [[#Schwalm--2012\|Schwalm et al., 2012]] ) over most ecosystem types. This makes water availability a major climatic driver of variability in net ecosystem exchange ( [[#Jung--2017\|Jung et al., 2017]] ; [[#Humphrey--2018\|Humphrey et al., 2018]] ). In addition to suppressing photosynthesis, field evidence suggests that droughts reduce the land CO <sub>2</sub> sink, also through increasing forest mortality and promoting wildfire ( [[#Allen--2015\|Allen et al., 2015]] ; [[#Brando--2019\|Brando et al., 2019]] ; [[#Abram--2021\|Abram et al., 2021]] ).

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	In conclusion, there is ''high confidence'' that the global net land CO <sub>2</sub> sink is reduced on interannual scale when regional-scale reductions in water availability associated with droughts occur, particularly in tropical regions. There is also ''high confidence'' that the global land sink will become less efficient due to soil moisture limitations and associated drought conditions in some regions for high-emissions scenarios, specially under global warming above 4°C. However, there is ''low confidence'' on how these water cycle feedbacks will play out in lower emissions scenarios (at 2°C global warming or lower) due to uncertainties in regional rainfall changes and the balance between the CO <sub>2</sub> fertilization effect, through WUE, and the radiative impacts of greenhouse gases.		In conclusion, there is ''high confidence'' that the global net land CO <sub>2</sub> sink is reduced on interannual scale when regional-scale reductions in water availability associated with droughts occur, particularly in tropical regions. There is also ''high confidence'' that the global land sink will become less efficient due to soil moisture limitations and associated drought conditions in some regions for high-emissions scenarios, specially under global warming above 4°C. However, there is ''low confidence'' on how these water cycle feedbacks will play out in lower emissions scenarios (at 2°C global warming or lower) due to uncertainties in regional rainfall changes and the balance between the CO <sub>2</sub> fertilization effect, through WUE, and the radiative impacts of greenhouse gases.

	~~'''~~'''What are the limits of carbon dioxide removal from a water ~~cyc''' '''le~~ perspective?'''~~'''~~		'''What are the limits of carbon dioxide removal from a water cycle perspective?'''

	Carbon dioxide removal (CDR) options based on terrestrial carbon sinks will require the appropriation of significant amounts of water at the landscape level. Most mitigation pathways that seek to limit global warming to 1.5°C or less than 2°C require the removal of about 30 to 300 GtC from the atmosphere by 2100 ( [[#Rogelj--2018b\|Rogelj et al., 2018b]] ). Bioenergy with carbon capture and storage (BECCS), and afforestation/reforestation are the dominant CDR options used in climate stabilization scenarios, implying large requirements for land and water ( [[#5.6\|Section 5.6]] ; [[#Beringer--2011\|Beringer et al., 2011]] ; [[#Boysen--2017b\|Boysen et al., 2017b]] ; [[#Fajardy--2017\|Fajardy and Mac Dowell, 2017]] ; [[#Jans--2018\|Jans et al., 2018]] ; [[#Séférian--2018b\|Séférian et al., 2018b]] ; [[#Yamagata--2018\|Yamagata et al., 2018]] ; [[#Stenzel--2019\|Stenzel et al., 2019]] ). A review of freshwater requirements for irrigating biomass plantations shows a range between 15 and 1250 km <sup>3</sup> per GtC of biomass harvest. This is equivalent to a water requirement of 99–8250 km <sup>3</sup> for the median BECCS deployment of around 3.3 GtC yr <sup>−1</sup> ( [[#Smith--2016\|Smith et al., 2016]] ) in <2°C-scenarios ( [[#Stenzel--2021\|Stenzel et al., 2021]] ), assuming that biomass is converted to electricity, which is substantially less efficient than converting biomass to heat. These large ranges are the result of different assumptions about the type of biomass and yield improvements, management, and land availability. The use of alternative feedstocks, such as wastes, residues and algae, would lead to smaller water requirements ( [[#Smith--2019\|Smith et al., 2019]] ).		Carbon dioxide removal (CDR) options based on terrestrial carbon sinks will require the appropriation of significant amounts of water at the landscape level. Most mitigation pathways that seek to limit global warming to 1.5°C or less than 2°C require the removal of about 30 to 300 GtC from the atmosphere by 2100 ( [[#Rogelj--2018b\|Rogelj et al., 2018b]] ). Bioenergy with carbon capture and storage (BECCS), and afforestation/reforestation are the dominant CDR options used in climate stabilization scenarios, implying large requirements for land and water ( [[#5.6\|Section 5.6]] ; [[#Beringer--2011\|Beringer et al., 2011]] ; [[#Boysen--2017b\|Boysen et al., 2017b]] ; [[#Fajardy--2017\|Fajardy and Mac Dowell, 2017]] ; [[#Jans--2018\|Jans et al., 2018]] ; [[#Séférian--2018b\|Séférian et al., 2018b]] ; [[#Yamagata--2018\|Yamagata et al., 2018]] ; [[#Stenzel--2019\|Stenzel et al., 2019]] ). A review of freshwater requirements for irrigating biomass plantations shows a range between 15 and 1250 km <sup>3</sup> per GtC of biomass harvest. This is equivalent to a water requirement of 99–8250 km <sup>3</sup> for the median BECCS deployment of around 3.3 GtC yr <sup>−1</sup> ( [[#Smith--2016\|Smith et al., 2016]] ) in <2°C-scenarios ( [[#Stenzel--2021\|Stenzel et al., 2021]] ), assuming that biomass is converted to electricity, which is substantially less efficient than converting biomass to heat. These large ranges are the result of different assumptions about the type of biomass and yield improvements, management, and land availability. The use of alternative feedstocks, such as wastes, residues and algae, would lead to smaller water requirements ( [[#Smith--2019\|Smith et al., 2019]] ).

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	~~'''~~'''What is permafrost carbon and why should we be ~~conc''' '''erned~~ about it?'''~~'''~~		'''What is permafrost carbon and why should we be concerned about it?'''

	Soils in the Arctic and other cold regions contain perennially frozen layers, known as permafrost. Soils in the northern permafrost region store a large amount of organic carbon, estimated at 1460–1600 PgC across surface soils and deeper deposits ( [[#Hugelius--2014\|Hugelius et al., 2014]] ; [[#Strauss--2017\|Strauss et al., 2017]] ; [[#Mishra--2021\|Mishra et al., 2021]] ). Of that carbon, permafrost soils and deposits store 1070–1360 PgC, of which 300–400 PgC are in the first metre, and the rest at depth. The remaining 280–340 PgC are in permafrost-free soils within the permafrost region. These carbon deposits have accumulated over thousands of years due to the slow rates of organic matter decomposition in frozen and/or waterlogged soil layers, but these frozen soils are highly decomposable upon thaw ( [[#Schädel--2014\|Schädel et al., 2014]] ).		Soils in the Arctic and other cold regions contain perennially frozen layers, known as permafrost. Soils in the northern permafrost region store a large amount of organic carbon, estimated at 1460–1600 PgC across surface soils and deeper deposits ( [[#Hugelius--2014\|Hugelius et al., 2014]] ; [[#Strauss--2017\|Strauss et al., 2017]] ; [[#Mishra--2021\|Mishra et al., 2021]] ). Of that carbon, permafrost soils and deposits store 1070–1360 PgC, of which 300–400 PgC are in the first metre, and the rest at depth. The remaining 280–340 PgC are in permafrost-free soils within the permafrost region. These carbon deposits have accumulated over thousands of years due to the slow rates of organic matter decomposition in frozen and/or waterlogged soil layers, but these frozen soils are highly decomposable upon thaw ( [[#Schädel--2014\|Schädel et al., 2014]] ).

	~~'''~~'''Is permafrost carbon already thawing and emitting ~~gr''' '''eenhouse~~ gases?~~'''~~'''		'''Is permafrost carbon already thawing and emitting greenhouse gases?'''
	The permafrost region was a historic carbon sink over centuries to millennia ( ''high confidence'' ) ( [[#Loisel--2014\|Loisel et al., 2014]] ; [[#Lindgren--2018\|Lindgren et al., 2018]] ). Currently though, thawing soils due to anthropogenic warming are losing carbon from the decomposition of old frozen organic matter, as found via carbon 14 ( <sup>14</sup> C) signature of respiration at sites undergoing rapid permafrost thaw ( [[#Hicks%20Pries--2013\|Hicks Pries et al., 2013]] ), of dissolved organic carbon in rivers draining watersheds with permafrost thaw ( [[#Vonk--2015\|Vonk et al., 2015]] ; [[#Wild--2019\|Wild et al., 2019]] ), and of methane (CH <sub>4</sub> ) produced in thawing lakes ( [[#Walter%20Anthony--2016\|Walter Anthony et al., 2016]] ).		The permafrost region was a historic carbon sink over centuries to millennia ( ''high confidence'' ) ( [[#Loisel--2014\|Loisel et al., 2014]] ; [[#Lindgren--2018\|Lindgren et al., 2018]] ). Currently though, thawing soils due to anthropogenic warming are losing carbon from the decomposition of old frozen organic matter, as found via carbon 14 ( <sup>14</sup> C) signature of respiration at sites undergoing rapid permafrost thaw ( [[#Hicks%20Pries--2013\|Hicks Pries et al., 2013]] ), of dissolved organic carbon in rivers draining watersheds with permafrost thaw ( [[#Vonk--2015\|Vonk et al., 2015]] ; [[#Wild--2019\|Wild et al., 2019]] ), and of methane (CH <sub>4</sub> ) produced in thawing lakes ( [[#Walter%20Anthony--2016\|Walter Anthony et al., 2016]] ).

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	Since AR5, there have been new studies showing that permafrost thaw also leads to nitrous oxide (N <sub>2</sub> O) release from soil ( [[#Abbott--2015\|Abbott and Jones, 2015]] ; [[#Karelin--2017\|Karelin et al., 2017]] ; [[#Wilkerson--2019\|Wilkerson et al., 2019]] ), a previously unaccounted source. However, this release is unquantified at the pan-Arctic scale.		Since AR5, there have been new studies showing that permafrost thaw also leads to nitrous oxide (N <sub>2</sub> O) release from soil ( [[#Abbott--2015\|Abbott and Jones, 2015]] ; [[#Karelin--2017\|Karelin et al., 2017]] ; [[#Wilkerson--2019\|Wilkerson et al., 2019]] ), a previously unaccounted source. However, this release is unquantified at the pan-Arctic scale.

	~~'''~~'''What does the paleo~~''' '''~~record tell us?'''~~'''~~		'''What does the paleo record tell us?'''

	Large areas of Alaska and Siberia are underlain by frozen, glacial-age, ice- and carbon-rich deposits, and many of these areas show evidence of thermokarst processes during Holocene warm periods. Rapid warming of high northern latitudes contributed to permafrost thaw, liberating labile organic carbon tothe atmosphere ( [[#Köhler--2014\|Köhler et al., 2014]] ; [[#Crichton--2016\|Crichton et al., 2016]] ; [[#Winterfeld--2018\|Winterfeld et al., 2018]] ; [[#Meyer--2019\|Meyer et al., 2019]] ), supporting the vulnerability of these areas to further warming ( [[#Strauss--2013\|Strauss et al., 2013]] , 2017).		Large areas of Alaska and Siberia are underlain by frozen, glacial-age, ice- and carbon-rich deposits, and many of these areas show evidence of thermokarst processes during Holocene warm periods. Rapid warming of high northern latitudes contributed to permafrost thaw, liberating labile organic carbon tothe atmosphere ( [[#Köhler--2014\|Köhler et al., 2014]] ; [[#Crichton--2016\|Crichton et al., 2016]] ; [[#Winterfeld--2018\|Winterfeld et al., 2018]] ; [[#Meyer--2019\|Meyer et al., 2019]] ), supporting the vulnerability of these areas to further warming ( [[#Strauss--2013\|Strauss et al., 2013]] , 2017).

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	In conclusion, several independent lines of evidence indicate that permafrost thaw did not release vast quantities of fossil CH <sub>4</sub> associated with the transient warming events of the LDT. This suggests that large emissions of CH <sub>4</sub> from old carbon sources will not occur in response to future warming ( ''medi'' ''um confidence'' ).		In conclusion, several independent lines of evidence indicate that permafrost thaw did not release vast quantities of fossil CH <sub>4</sub> associated with the transient warming events of the LDT. This suggests that large emissions of CH <sub>4</sub> from old carbon sources will not occur in response to future warming ( ''medi'' ''um confidence'' ).

	~~'''~~'''What level of emissions do we expect~~''' '''~~in the future?'''~~'''~~		'''What level of emissions do we expect in the future?'''

	Near-surface permafrost is projected to decrease significantly under future global warming scenarios ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2\|Section 9.5.2]] ), thus creating the potential for releasing CO <sub>2</sub> and CH <sub>4</sub> to the atmosphere, and act as a positive carbon–climate feedback.		Near-surface permafrost is projected to decrease significantly under future global warming scenarios ( ''high confidence'' ) ( [[IPCC:Wg1:Chapter:Chapter-9#9.5.2\|Section 9.5.2]] ), thus creating the potential for releasing CO <sub>2</sub> and CH <sub>4</sub> to the atmosphere, and act as a positive carbon–climate feedback.

Laura: /* Chapter 5: Global Carbon and other Biogeochemical Cycles and Feedbacks */

2026-05-13T14:03:39Z

Chapter 5: Global Carbon and other Biogeochemical Cycles and Feedbacks

← Older revision		Revision as of 14:03, 13 May 2026
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	This chapter should be cited as:		'''This chapter should be cited as:'''

	Canadell, J.G., P.M.S. Monteiro, M.H. Costa, L. Cotrim da Cunha, P.M. Cox, A.V. Eliseev, S. Henson, M. Ishii, S. Jaccard, C. Koven, A. Lohila, P.K. Patra, S. Piao, J. Rogelj, S. Syampungani, S. Zaehle, and K. Zickfeld, 2021: Global Carbon and other Biogeochemical Cycles and Feedbacks. In ''Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change'' [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 673–816, doi: [https://doi.org/10.1017/9781009157896.007 10.1017/9781009157896.007] .		Canadell, J.G., P.M.S. Monteiro, M.H. Costa, L. Cotrim da Cunha, P.M. Cox, A.V. Eliseev, S. Henson, M. Ishii, S. Jaccard, C. Koven, A. Lohila, P.K. Patra, S. Piao, J. Rogelj, S. Syampungani, S. Zaehle, and K. Zickfeld, 2021: Global Carbon and other Biogeochemical Cycles and Feedbacks. In ''Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change'' [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 673–816, doi: [https://doi.org/10.1017/9781009157896.007 10.1017/9781009157896.007] .

imported>Design: /* FAQ 5.4 | What Are Carbon Budgets? */

2026-05-11T08:42:54Z

FAQ 5.4 | What Are Carbon Budgets?

Show changes

IPCC:AR6/WGI/Chapter-5 - Revision history

Laura at 08:35, 22 May 2026

Laura: Laura moved page IPCC:Wg1:Chapter:Chapter-5 to IPCC:AR6/WGI/Chapter-5 without leaving a redirect

Laura: #119

Laura: /* Chapter 5: Global Carbon and other Biogeochemical Cycles and Feedbacks */

imported>Design: /* FAQ 5.4 | What Are Carbon Budgets? */