Editing IPCC:AR6/WGII/Chapter-3 (section)

===== 3.4.3.3.4 Time of emergence: species exposure to altered environments =====

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Since SROCC, more studies have assessed the time of emergence for climate-induced drivers ( [[#3.2.3|Section 3.2.3]] ) and the ecosystem attributes through which the impacts manifest. However, as in previous assessments (Table 3.22), the time of emergence for a given driver or ecosystem attribute depends on the reference period, the definition of the signal emergence threshold and the spatio-temporal scales considered (see Box 5.1 in SROCC; [[#Kirtman--2013|Kirtman et al., 2013]] ; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

'''Table 3.22 |''' Conclusions from previous IPCC assessments about projected time of emergence on coastal, ocean and deep-sea systems

{| class="wikitable"
|-
! Oceanic systems and chapter subsection

! Projections

|-
| Coastal ( [[#3.4.2|Section 3.4.2]] )

| ‘Multiple [climate-impact drivers] will emerge [...] in the 21st century under RCP8.5, while the time of emergence will be later and with less [climatic hazards] under RCP2.6. [Non-climate] impacts such as eutrophication add to, and in some cases, exacerbate these large-scale slow climate drivers beyond biological thresholds at local scale (e.g., deoxygenation)’ ( [[IPCC:Wg2:Chapter:Chapter-5#5.3|Section 5.3.7]] in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

|-
| Epipelagic ( [[#3.4.3|Section 3.4.3]] )

| ‘Observed range shifts in response to climate change in some regions such as the north Atlantic are strongly influenced by warming due to both multi-decadal [climate change and] variability, suggesting that there is a longer time of emergence of range shifts from natural variability and a need for longer biological time series for robust attribution’ ( [[IPCC:Wg2:Chapter:Chapter-5#5.2|Section 5.2.3.1.1]] in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

|-
| Open ocean ( [[#3.4.3|Section 3.4.3]] )

| ‘[The timing] for five primary drivers of marine ecosystem change (surface warming and acidification, oxygen loss, nitrate concentration and net primary production change) are all prior to 2100 for over 60% of the ocean area under RCP8.5 and over 30% under RCP2.6 ( ''very likely'' )’ (Figure 1 in Box 5.1 in SROCC, Box 5.1 in SROCC, Executive Summary in SROCC Chapter 5; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

‘Anthropogenic signals are expected to remain detectable over large parts of the ocean, even for the RCP2.6 scenario for pH and SST, but are ''likely'' [to be less conspicuous] for nutrients and NPP [net primary production] in the 21st century. For example, for the open ocean, the anthropogenic pH signal in Earth System Models’ (ESM) historical simulations is ''very likely'' to have emerged for three-quarters of the ocean prior to 1950, and it is ''very likely'' over 95% of the ocean has already been affected, with little discernible difference between scenarios’ (Executive Summary in SROCC Chapter 5, Box 5.1 in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ). ‘The climate signal of oxygen loss will ''very likely'' emerge by 2050 with a ''very likely'' range of 59–80% by 2031–2050 and rising with a ''very likely'' range of 79–91% of the ocean area by 2081–2100 (RCP8.5 emissions scenario). The emergence of oxygen loss is smaller in area for the RCP2.6 scenario in the 21st century and by 2090 the [area where emergence is evident is declining].’ It has also been shown that signatures of altered oxygen solubility or utilisation may emerge earlier than for oxygen levels (Executive Summary in SROCC Chapter 5, Box 5.1 in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ).

|-
| Deep sea (Box 3.3)

| ‘Emergence of risk is expected to occur later at around the mid-21st century under RCP8.5 for abyssal plain and chemosynthetic ecosystems (vents and seeps)’ ( [[IPCC:Wg2:Chapter:Chapter-5#5.2|Section 5.2.5]] in SROCC; [[#Bindoff--2019a|Bindoff et al., 2019a]] ). ‘All deep seafloor ecosystems are expected to be subject to at least moderate risk under RCP8.5 by the end of the 21st century, with cold water corals undergoing a transition from moderate to high risk below 3°C’ (SM5.2 in SROCC; [[#Bindoff--2019b|Bindoff et al., 2019b]] ).

|}

Anthropogenically driven changes in chlorophyll- ''a'' concentrations across an ensemble of 30 ESMs are expected to exceed natural variability under RCP8.5 by 2100 in ~65–80% of the global oceans, when the natural variability is calculated using the ensemble’s standard deviation ( [[#Schlunegger--2020|Schlunegger et al., 2020]] ); however, if two standard deviations are used, then significant trends in chlorophyll- ''a'' concentration are expected under RCP8.5 across ~31% of the global oceans by 2100 ( [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ). In contrast, the anthropogenic signal in phytoplankton community structure, which has a lower natural variability, will emerge under RCP8.5 across 63% of the ocean by 2100 when two standard deviations are used ( ''limited evidence'' ) ( [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ).

The time of emergence of climate impacts on ecosystems will be modulated jointly by species-specific adaptation potential ( [[#3.3.4|Section 3.3.4]] ; [[#Jones--2018|Jones and Cheung, 2018]] ; [[#Collins--2020|Collins et al., 2020]] ; [[#Gamliel--2020|Gamliel et al., 2020]] ; [[#Miller--2020a|Miller et al., 2020a]] ), speed of range shifts and spatial reorganisation ( ''high confidence'' ) (Sections 3.3, 3.4.2, 3.4.3). These ecosystem responses complicate projections of the time of emergence of environmental properties that impact biogeochemical cycling ( [[#Schlunegger--2019|Schlunegger et al., 2019]] ; [[#Schlunegger--2020|Schlunegger et al., 2020]] ; [[#Wrightson--2020|Wrightson and Tagliabue, 2020]] ), ecosystem structure and biodiversity (Figure 3.20a,c; [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ; [[#Trisos--2020|Trisos et al., 2020]] ), and higher trophic levels, including fisheries targets ( [[#Cheung--2020|Cheung and Frölicher, 2020]] ). Better accounting for multiple interacting factors in ESMs (see Box 3.1) will provide insight into how marine ecosystems will respond to future climate ( ''high confidence'' ).

The time of emergence of ecosystem responses supports planning for specific time-bound actions to reduce risks to ecosystems ( [[#3.6.3.2|Section 3.6.3.2.1]] ; [[#Bruno--2018|Bruno et al., 2018]] ; [[#Trisos--2020|Trisos et al., 2020]] ). Although under RCP8.5, climate refugia from SST after 2050 are primarily in the Southern Ocean in tropical waters, these refugia are mainly from deoxygenation ( [[#Bruno--2018|Bruno et al., 2018]] ). Marine assemblages in these places will be exposed to unprecedented temperatures after 2050, peaking in 2075 (Figure 3.20a,b; [[#Trisos--2020|Trisos et al., 2020]] ). In contrast, changes in phytoplankton community structure will emerge earlier, primarily in the Pacific Ocean subtropics and through much of the North Atlantic Ocean (Figure 3.20c,d; [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ). Under RCP8.5, changes in phytoplankton community structure and, to a lesser extent, exposure of marine species to unprecedented temperatures, will emerge earlier in marine protected areas (MPAs), covering ~7.7% of the global oceans ( [[#3.6.2.3|Section 3.6.2.3.2.1]] ; UNEP-WCMC and [[#IUCN--2020|IUCN, 2020]] ; [[#UNEP-WCMC%20and%20IUCN--2021|UNEP-WCMC and IUCN, 2021]] ) as compared with non-MPAs (Figure 3.20b,d). Such assessment can support planning for future MPA placement and extent. Because MPAs can serve as refugia from non-climate drivers (Sections 3.6.2.3, 3.6.3.2.1), they facilitate opportunities for adaptation among marine species and communities in coastal oceans ( [[#3.4.2|Section 3.4.2]] ).

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[[File:8f6c3fd05dd924e5d2f20a2442625694 IPCC_AR6_WGII_Figure_3_020.png]]

'''Figure 3.20 |''' '''Time of exposure to altered environments.'''

'''(a)''' Simulated spatial variation in the time of exposure of marine species to unprecedented temperatures under RCP8.5. Time of exposure is quantified as the median year after which local species are projected to encounter temperatures warmer than the historical maximum within their full geographic range for a period of at least 5 years. This estimate is based on 22 Coupled Model Intercomparison Project 5 (CMIP5) models, and is drawn from data presented by Trisos et al. (2020). Only regions that have times of emergence by 2100 are shown.

'''(b)''' The distribution in the time of exposure to unprecedented temperatures within marine assemblages ( [[#Trisos--2020|Trisos et al., 2020]] ) under RCP8.5 in marine protected areas (in turquoise) and in non-marine protected areas (in purple). Values were calculated after regridding to equal-area 0.5° hexagons.

'''(c)''' Time of emergence for phytoplankton community-structure changes (based on a proxy–ecosystem-model reflectance at 500 nm) under RCP8.5. Only regions with statistically significant ( ''p'' &lt; 0.05) trends that are presently largely ice free and have times of emergence by 2100 are shown. (Based on the results of one numerical model from [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ).

'''(d)''' The distribution in the time of emergence for changes in phytoplankton community structure (same proxy as in panel c) ( [[#Dutkiewicz--2019|Dutkiewicz et al., 2019]] ) under RCP8.5 in marine protected areas (in turquoise) and in non-marine protected areas (in purple). Values were calculated after regridding to equal-area 0.5° hexagons.

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