Editing IPCC:AR6/WGII/Chapter-2 (section)

==== 2.4.4.1 Observed Browning of Rivers and Lakes ====

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In boreal coniferous areas, there has been an increase in the transporting of terrestrial-derived dissolved organic carbon (DOC) into rivers and lakes, which has caused increased opacity and a shift toward a brown colour (browning). There was little assessment of this in AR5. This process is driven by climate change, and stems from hydrological intensification, greening of the Northern Hemisphere and degradation of carbon sinks in peatlands ''(robust evidence, high agreement'' ) ( [[#Solomon--2015|Solomon et al., 2015]] ; [[#Catalán--2016|Catalán et al., 2016]] ; [[#de%20Wit--2016|de Wit et al., 2016]] ; [[#Finstad--2016|Finstad et al., 2016]] ; [[#Creed--2018|Creed et al., 2018]] ; [[#Hayden--2019|Hayden et al., 2019]] ). These factors enhance terrestrial productivity, alter vegetation communities and affect the hydrological control of the production and transport of DOC ( [[#Weyhenmeyer--2016|Weyhenmeyer et al., 2016]] ). Non-climate-related drivers of browning are: declining atmospheric sulphur deposition, forestry practices and LULCCs (see Table SM2.1 for detail).

Browning creates a positive feedback to climate by absorbing photosynthetically active radiation, which accelerates upper water (epilimnetic) warming ( [[#Solomon--2015|Solomon et al., 2015]] ). Browning of lakes leads to shallower and more stable thermoclines, and thus overall deep water cooling ( [[#Solomon--2015|Solomon et al., 2015]] ; [[#Williamson--2015|Williamson et al., 2015]] ), and can provoke a transition of the seasonal mixing regime from a mixed lake (polymictic) to one that is seasonally stratified ( [[#Kirillin--2016|Kirillin and Shatwell, 2016]] ).

The ecological responses of browning are a concomitant effect of climate change and nutrient status. Results from long-term, large-scale lake experiments have been variable, showing both strong synergistic effects ( [[#Urrutia-Cordero--2016|Urrutia-Cordero et al., 2016]] ) and no significant effects of browning on plankton community food webs ( [[#Rasconi--2015|Rasconi et al., 2015]] ). Browning has driven a shift from auto- to heterotrophic/mixotrophic-based production ( [[#Urrutia-Cordero--2017|Urrutia-Cordero et al., 2017]] ) and supports heterotrophic metabolism of the bacterial community ( [[#Zwart--2016|Zwart et al., 2016]] ). Browning may also accelerate primary production through the input of nutrients associated with dissolved organic matter (DOM) in nutrient-poor lakes and increase cyanobacteria, which cope better with low light intensities ( [[#Huisman--2018|Huisman et al., 2018]] ) and toxin levels ( [[#Urrutia-Cordero--2016|Urrutia-Cordero et al., 2016]] ). However, the synergistic impacts of browning and climate change on aquatic communities depends on regional precipitation patterns ( [[#Weyhenmeyer--2016|Weyhenmeyer et al., 2016]] ), watershed type ( [[#de%20Wit--2016|de Wit et al., 2016]] ) and the length of the food chain ( [[#Hansson--2013|Hansson et al., 2013]] ). Quantitative attribution of browning to climate change remains difficult ( ''medium evidence'' , ''medium agreement'' ).

In summary, new studies since AR5 have explicitly estimated the effects of warming and browning on freshwaters in boreal areas, with complex positive and negative repercussions on water temperature profiles (lower vs. upper water) ( ''high confidence'' ) and primary production ( ''medium confidence'' ).

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[[File:91f7befa6456321ae09212fa9420be41 IPCC_AR6_WGII_Figure_2_005.png]]

'''Figure 2.5 | Large-scale observed changes in freshwater ecosystems attributed to climate change over more than four decades.''' For description and references, see Sections 2.3.3, 2.4.2 and 2.5.3.6.2.

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