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

==== 2.3.3.1 Observed Change in Thermal Habitat and Oxygen Availability ====

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Since AR5, evidence of changes in the temperature of lakes and rivers has continued to increase. Global warming rates for lake surface waters were estimated as 0.21°C–0.45°C per decade between 1970 and 2010, exceeding sea-surface temperature (SST) trends of 0.09°C per decade between 1980 and 2017 ( ''robust evidence'' , ''high agreement'' ) (Figure 2.2; ( [[#Schneider--2010|Schneider and Hook, 2010]] ; [[#Kraemer--2015|Kraemer et al., 2015]] ; [[#O’Reilly--2015|O’Reilly et al., 2015]] ; [[#Woolway--2020b|Woolway et al., 2020b]] ). Warming of lake surface water temperatures was variable within regions ( [[#O’Reilly--2015|O’Reilly et al., 2015]] ) but more homogeneous than deep-water temperature changes ( [[#Pilla--2020|Pilla et al., 2020]] ). Because temperature trends in lakes can vary vertically, horizontally and seasonally, complex changes have occurred in the amount of habitat available to aquatic organisms at particular depths and temperatures ( [[#Kraemer--2021|Kraemer et al., 2021]] ).

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[[File:cca4001e49eec573dada90dcc9bfbf0a IPCC_AR6_WGII_Figure_2_002.png]]

'''Figure 2.2 | Observed global trends in lake and river surface water temperature.'''

'''(a)''' Left panel: map of temperatures of lakes (1970–2010).

'''(b)''' Left panel: map of temperatures of rivers (1901–2010). Note that the trends of river water temperatures are not directly comparable within rivers or to lakes, since time periods are not consistent across river studies. Right panels (a) and (b) depict water temperature trends along a latitudinal gradient highlighting the above average warming rates in northern Polar Regions (polar amplification). Data sources for lakes: ( [[#O’Reilly--2015|O’Reilly et al., 2015]] ; [[#Carrea--2019|Carrea and Merchant, 2019]] ; [[#Woolway--2020a|Woolway et al., 2020a]] ; [[#Woolway--2020b|Woolway et al., 2020b]] ). Data sources for rivers: ( [[#Webb--1992|Webb and Walling, 1992]] ; [[#Langan--2001|Langan et al., 2001]] ; [[#Daufresne--2004|Daufresne et al., 2004]] ; [[#Moatar--2006|Moatar and Gailhard, 2006]] ; [[#Lammers--2007|Lammers et al., 2007]] ; [[#Patterson--2007|Patterson et al., 2007]] ; [[#Webb--2007|Webb and Nobilis, 2007]] ; [[#Durance--2009|Durance and Ormerod, 2009]] ; [[#Kaushal--2010|Kaushal et al., 2010]] ; [[#Pekárová--2011|Pekárová et al., 2011]] ; [[#Jurgelėnaitė--2012|Jurgelėnaitė et al., 2012]] ; [[#Markovic--2013|Markovic et al., 2013]] ; [[#Arora--2016|Arora et al., 2016]] ; [[#Latkovska--2016|Latkovska and Apsīte, 2016]] ; [[#Marszelewski--2016|Marszelewski and Pius, 2016]] ; [[#Jurgelėnaitė--2017|Jurgelėnaitė et al., 2017]] ).

Changes in river water temperatures ranged from −1.21°C to +1.076°C per decade between 1901 and 2010 ( ''medium evidence, medium agreement'' ) ( [[#Hari--2006|Hari et al., 2006]] ; [[#Kaushal--2010|Kaushal et al., 2010]] ; [[#Jurgelėnaitė--2012|Jurgelėnaitė et al., 2012]] ; [[#Li--2012|Li et al., 2012]] ; [[#Latkovska--2016|Latkovska and Apsīte, 2016]] ; [[#Marszelewski--2016|Marszelewski and Pius, 2016]] ). The more rapid increase in surface water temperature in lakes and rivers in regions with cold winters ( [[#O’Reilly--2015|O’Reilly et al., 2015]] ) can, in part, be attributed to the amplified warming in polar and high-latitude regions ( ''robust evidence'' , ''high agreement'' ) ( [[#Screen--2010|Screen and Simmonds, 2010]] ; [[#Stuecker--2018|Stuecker et al., 2018]] ).

Shifts in thermal regime: Since AR5, the trend that lake waters mix less frequently continues ( [[#Butcher--2015|Butcher et al., 2015]] ; [[#Adrian--2016|Adrian et al., 2016]] ; [[#Richardson--2017|Richardson et al., 2017]] ; [[#Woolway--2017|Woolway et al., 2017]] ). This results from greater warming of surface temperatures relative to deep-water temperatures, and the loss of ice during winter which prevents inverse thermal stratification in north temperate lakes ( ''robust evidence, high agreement'' ) ( [[#Adrian--2009|Adrian et al., 2009]] ; [[#Winslow--2015|Winslow et al., 2015]] ; [[#Adrian--2016|Adrian et al., 2016]] ; [[#Schwefel--2016|Schwefel et al., 2016]] ; [[#Richardson--2017|Richardson et al., 2017]] ) ''.''

Oxygen availability: increased water temperature and reduced mixing cause a decrease in dissolved oxygen. In 400 lakes, dissolved oxygen in surface and deep waters declined by 4.1 and 16.8%, respectively, between 1980 and 2017 ( [[#Jane--2021|Jane et al., 2021]] ). The deepest water layers are expected to experience an increase in hypoxic conditions by &gt;25% due to fewer complete mixing events, with strong repercussions for nutrient dynamics and the loss of thermal habitat ( ''robust evidence, high agreement'' ) ( [[#Straile--2010|Straile et al., 2010]] ; [[#Zhang--2015|Zhang et al., 2015]] ; [[#Schwefel--2016|Schwefel et al., 2016]] ).

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