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

== 2.1 Introduction ==

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=== 2.1.1 Overview ===

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We provide assessments of observed and projected impacts of climate change across species, biomes (vegetation types), ecosystems and ecosystem services, highlighting the processes that are emerging on a global scale. Where sufficient evidence exists, differences in biological responses across regions, taxonomic groups or types of ecosystems are presented, particularly when such differences provide meaningful insights into current or potential future autonomous or human-mediated adaptations. Human interventions that might build the resilience of ecosystems and minimise the negative impacts of climate change on biodiversity and ecosystem functioning are assessed. Such interventions include adaptation strategies and programmes to support biodiversity conservation and Ecosystem-based Adaptation (EbA). The assessments were done in the context of the Convention on Biological Diversity (CBD) and sustainable development goals (SDGs), whose contributions to climate resilient development (CRD) pathways are assessed. This chapter highlights both the successes and failures of adaptation attempts and considers potential synergies and conflicts with land-based climate change mitigation. Knowledge gaps and sources of uncertainty are included to encourage additional research.

The Working Group II Summary for Policymakers of the AR5 stated that ‘many terrestrial and freshwater species have shifted their geographic ranges, seasonal activities, migration patterns, abundances, and species interactions in response to ongoing climate change’ ( [[#IPCC--2014d|IPCC, 2014d]] ). Based on long-term observed changes across the regions, it was estimated that approximately 20–30% of plant and animal species are at risk of extinction when global mean temperatures rise 2–3°C above pre-industrial levels ( [[#Fischlin--2007|Fischlin et al., 2007]] ). In addition, the WGII AR5 Synthesis Report ( [[#IPCC--2014e|IPCC, 2014e]] ) broadly suggested that autonomous adaptation by ecosystems and wild species might occur, and proposed human-assisted adaptations to minimise negative climate change impacts.

Risk assessments for species, communities, key ecosystems and their services were based on the risk assessment framework introduced in the IPCC AR5 ( [[#IPCC--2014b|IPCC, 2014b]] ). Assessments of observed changes in biological systems emphasise detecting and attributing the impacts of climate change on ecological and evolutionary processes, particularly freshwater ecosystems, and ecosystem processes such as wildfires, that were superficially assessed in previous reports. Where appropriate, assessment of interactions between climate change and other human activities is provided.

Land use and land cover change (LULCC) as well as the unsustainable exploitation of resources in terrestrial and freshwater systems continue to be major factors contributing to the loss of natural ecosystems and biodiversity ( ''high confidence'' ). Fertiliser input, pollution of waterways, dam construction and the extraction of freshwater for irrigation put additional pressure on biodiversity and alter ecosystem function ( [[#Shin--2019|Shin et al., 2019]] ). Likewise, for biodiversity, invasive alien species have been identified as a major threat, especially in freshwater systems, on islands and in coastal regions ( ''high confidence'' ) ( [[#IPBES--2018b|IPBES, 2018b]] ; [[#IPBES--2018e|IPBES, 2018e]] ; [[#IPBES--2018c|IPBES, 2018c]] ; [[#IPBES--2018d|IPBES, 2018d]] ; [[#IPBES--2019|IPBES, 2019]] ). Climate change and CO 2 are expected to become increasingly important as drivers of change over the coming decades ( [[#Ciais--2013|Ciais et al., 2013]] ; [[#Settele--2014|Settele et al., 2014]] ; [[#IPBES--2019|IPBES, 2019]] ; [[#IPCC--2019c|IPCC, 2019c]] ).

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=== 2.1.2 Points of Departure ===

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Species diversity and ecosystem function influence each other reciprocally, while the latter forms the necessary basis for ecosystem services ( [[#Hooper--2012|Hooper et al., 2012]] ; [[#Mokany--2016|Mokany et al., 2016]] ). Drivers of impacts on biodiversity, ecosystem function and ecosystem services have been assessed in reports by the IPCC, the Food and Agriculture Organization (FAO), the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES) and the Global Environmental Outlook ( [[#Settele--2014|Settele et al., 2014]] ; [[#FAO--2018|FAO, 2018]] ; [[#IPBES--2018b|IPBES, 2018b]] ; [[#IPBES--2018e|IPBES, 2018e]] ; [[#IPBES--2018c|IPBES, 2018c]] ; [[#IPBES--2018d|IPBES, 2018d]] ; [[#IPBES--2019|IPBES, 2019]] ; [[#UNEP--2019|UNEP, 2019]] ; [[#Secretariat%20of%20the%20Convention%20on%20Biological%20Diversity--2020|Secretariat of the Convention on Biological Diversity, 2020]] ). Most recently, the IPCC Special Report on Climate Change and Land (SRCCL) provided an assessment on land degradation and desertification, GHG emissions and food security in the context of global warming ( [[#IPCC--2019c|IPCC, 2019c]] ), and the IPBES–IPCC joint report on biodiversity and climate change provided a synthesis of the current understanding of the interactions, synergies and feedbacks between biodiversity and climate change ( [[#Pörtner--2021|Pörtner et al., 2021]] ). This chapter builds on and expands the results of these assessments.

Assessment of the impacts of climate change on freshwater systems has been limited in previous assessments, and inter-linkages between terrestrial and freshwater processes have not been fully explored ( [[#Settele--2014|Settele et al., 2014]] ; [[#IPBES--2019|IPBES, 2019]] ). Improved treatment of impacts on terrestrial and freshwater systems is critical, considering the revisions of international sustainability goals and targets, especially the conclusion that many of the proposed post-2020 targets of the CBD cannot be met due to climate change impacts ( [[#Arneth--2020|Arneth et al., 2020]] ).

Previous reports highlighted the possibility of new ecosystem states stemming from shifts in thermal regimes, species composition, and energy and matter flows ( [[#Settele--2014|Settele et al., 2014]] ; [[#Shin--2019|Shin et al., 2019]] ). Projecting such “tipping points” (see Glossary Appendix II) has been identified in previous reports as a challenge since monitoring programmes, field studies, and ecosystem and biodiversity modelling tools do not capture the underlying species–species and species–climate interactions sufficiently well to identify how biological interactions within and across trophic levels may amplify or dampen shifts in ecosystem states ( [[#Settele--2014|Settele et al., 2014]] ; [[#Shin--2019|Shin et al., 2019]] ). Building on these previous analyses and the recent literature, [https://www.ipcc.ch/report/ar6/wg2/chapter/chapter-2 Chapter 2] of AR6 provides new insights compared to those of previous assessments by (i) emphasising freshwater aspects and the interlinkages between freshwater and terrestrial systems, (ii) assessing more clearly the link between biodiversity and ecosystem functioning, (iii) assessing the impacts associated with climate change mitigation scenarios versus those of climate change including interactions with adaptation, and (iv) where possible, places findings in the context of the United Nations (UN) SDGs 2030 and services for human societies.

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=== 2.1.3 Guide to Attribution and Traceability of Uncertainty Assessments ===

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For biological systems, we use the framework for detection and attribution outlined in AR5, in which biological changes observed are not attributed to global but rather to local or regional climate changes ( [[#Parmesan--2013|Parmesan et al., 2013]] ; [[#Cramer--2014|Cramer et al., 2014]] ). However, global distribution of regional responses is desirable to achieve generality, and data in prior reports were concentrated from the Northern Hemisphere. The critique of ‘global’ studies by ( [[#Feeley--2017|Feeley et al., 2017]] ) argues that their naming is misleading, that most of them are far from global, and that a considerable geographic and taxonomic bias remains. This bias is diminishing, as regional data from the Southern Hemisphere is added and there is now representation from every continent.

Overall confidence in attributing biological changes to climate change can be increased in multiple ways ( [[#Parmesan--2013|Parmesan et al., 2013]] ), four of which we list here. First, confidence rises when the time span of biological records is long, such that decadal trends in climate can be compared with decadal trends in biological response, and long-term trends can be statistically distinguished from natural variability. Second, confidence can be increased by examining a large geographic area, which tends to diminish the effects of local confounding factors ( [[#Parmesan--2013|Parmesan et al., 2013]] ; [[#Daskalova--2021|Daskalova et al., 2021]] ). Third, confidence is increased when there is experimental or empirical evidence of a mechanistic link between particular climate metrics and a biological response. Fourth, confidence is increased when particular fingerprints of climate change are documented that uniquely implicate climate change as the causal driver of the biological change ( [[#Parmesan--2003|Parmesan and Yohe, 2003]] ). These conditions constitute multiple lines of evidence, which, when they converge, can provide ''very high confidence'' that climate change is the causal driver of an observed change in a particular biological species or system ( [[#Parmesan--2013|Parmesan et al., 2013]] ).

Important factors that may confound or obscure effects of climate change are the presence of invasive species, changes in land use (LULCC) and, in freshwater systems, eutrophication ( [[#IPCC--2019a|IPCC, 2019a]] ). The temporal and spatial scale of studies also affects estimates of impacts. The most extreme published estimates of biological change tend to be derived from smaller areas and/or shorter time frames ( [[#Daskalova--2021|Daskalova et al., 2021]] ); a recent large global analysis of data for 12,415 species found that differences in study methodology accounted for most of the explained variance in reported range shifts ( [[#Lenoir--2020|Lenoir et al., 2020]] ). The importance of LULCC is frequently stressed, but there is a paucity of studies actually quantifying the relative effects of climate change and LULCC on species and communities. ( [[#Sirami--2017|Sirami et al., 2017]] ) found only 13 such studies: four concluded that effects of LULCC overrode those of climate change, four found that the two drivers independently affected different species and five found that they acted in synergy.

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