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

==== 2.5.2.10 Committed Impacts of Climate Change on Terrestrial Ecosystems and Implications of Overshoot ====

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Projections point to potentially large changes of canopy structure and composition within and across the terrestrial biomes in response to climate change and changes in atmospheric CO 2 . These changes will contribute to altered ecosystem carbon uptake and losses, biophysical climate feedbacks (Sections 2.3.2; 2.4.4; 2.5.3.2; 2.5.3.3. 2.5.3.4, 2.5.3.5, Figure 2.10, Table 2.4) and multiple other ecosystem services (Sections 2.5.3, 2.5.4) as well impacts on biodiversity (Sections 2.4.2, 2.4.3, 2.4.4, 2.4.5, 2.5.1.3, 2.5.1.4, 2.5.2, Figure Box 2.1.1, Table Box 2.1.1, Table SM2.4). Until now, most studies project changes over next decades until the end of this century.

However, there is an increasing body of literature that has found continued, longer-term responses of ecosystems to climate change, so-called ‘committed changes’, that arise from lags that exist in many systems. Many processes in ecosystems take more than a few decades to quasi-equilibrate to environmental changes. Therefore, the trends of changing vegetation cover identified in simulations of transient warming continue to show up in simulations that hold climate change at low levels of warming ( ''medium confidence'' ) ( [[#Boulton--2017|Boulton et al., 2017]] ; [[#Pugh--2018|Pugh et al., 2018]] ; [[#Scheiter--2020|Scheiter et al., 2020]] ). Such changes, which could tip ecosystems into an alternative state, could also be triggered by a ‘warming overshoot’ if global warming were to exceed a certain threshold, even if mean temperatures afterwards decline again ( [[#Albrich--2020a|Albrich et al., 2020a]] ).

For instance, even if warming achieved by 2100 remained constant after 2100, such committed responses continue to occur. These include: (1) continued Amazon forest loss ( [[#Boulton--2017|Boulton et al., 2017]] ), consistent with results in [[#Pugh--2018|Pugh et al. (2018)]] that found continued tropical forest cover loss across a range of models and simulation setups, and (2) across Africa, an increased shift towards woody C3 vegetation was found in equilibrium state, the overall response depending on the atmospheric CO 2 concentration ( [[#Scheiter--2020|Scheiter et al., 2020]] ). In [[#Pugh--2018|Pugh et al. (2018)]] , the opposite was found for boreal forest cover, which showed a strong committed increase. The committed changes in vegetation composition correspond to large committed changes in terrestrial carbon uptake and losses ( [[#Boulton--2017|Boulton et al., 2017]] ; [[#Pugh--2018|Pugh et al., 2018]] ; [[#Scheiter--2020|Scheiter et al., 2020]] ), and would plausibly also appear in other ecosystem functioning and services. These studies point to the importance of having not only a multi-decadal but also a multi-century perspective when exploring the impacts of political decisions on climate change mitigation taken now. Even if climate-warming targets are met, published evidence so far suggests that fundamental changes in some ecosystems are ''likely'' as these correspond to well-understood ecosystem physiological responses that trigger long-term changes in composition.

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'''Box 2.1 | Assessing Past Projections of Ecosystem Change against Observations'''
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To assess future climate change impacts on ecosystems, we use models to project their future distribution. Comparing the trends in the observed changes against the projections can help assess the strength of the model projections. In this box, we compare observed trends of changes in ecosystem structure to projections highlighted in previous IPCC reports, specifically AR3 ( [[#IPCC--2001|IPCC, 2001]] ), AR4 ( [[#Fischlin--2007|Fischlin et al., 2007]] ) and AR5 ( [[#Settele--2014|Settele et al., 2014]] ). We use this to assess how well the projections are matching up with observed changes. The map represents studies documenting observed changes in common plant functional groups (e.g., trees, grasses and shrubs). Studies documenting changes in plant functional groups were collated from published papers in natural and semi-natural areas. Studies were included if climate change or interactions between climate change and land use showed a causal link to the observed change (Table SM2.4). Studies were excluded if the changes were only from landscape/land use transformation (e.g., deforestation). In each paper, we recorded the geographical location and type of functional change, and noted the causes. Observed changes are plotted onto a biome map derived from the WWF ecoregions database ( [[#Olson--2001|Olson et al., 2001]] ). Trends in changing plant functional types are good indicators of potential biome shifts and are used to assess how observations match up with projections.

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'''Figure Box 2.1.1 |''' '''Observed changes in the distribution of plant functional types that are caused by climate change or a combination of land use and climate change.''' Shifts in plant functional types are indicative of shift in biome function and structure. Based upon studies listed in Table SM2.4 and section 2.4.

'''Table Box 2.1.1 |''' Comparison of projections on biome change from AR3, AR4 and AR5 ( [[#IPCC--2001|IPCC, 2001]] ; [[#Fischlin--2007|Fischlin et al., 2007]] ; [[#Settele--2014|Settele et al., 2014]] ), with observed changes in ecosystems as assessed in this report (see [[#2.4|Section 2.4]] , Figure Box 2.1.1, Table SM2.4). Observed changes marked in bold show good agreement with past projections; those in red show mismatch with observations and projections.

{| class="wikitable"
|-
! '''Biome'''

! '''AR3'''

! '''AR4'''

! '''AR5'''

! '''Observed trends 1990–2021'''

|-
| ''MTEs''

| Increased disturbance by fire and warming will cause a loss of unique habitats

| Loss of 65% of area due to warming. Increased fire frequencies will favour resprouting plants. An increase in grass dominance. Forest expansion within MTEs due to elevated CO 2 .

| Range contractions of all species

| Increase in water deficit '''and fire activity''' (Sections 2.4.3.6, 2.4.4.2) '''causing a decline in diversity;''' tree mortality (Fig. Box 2.1.1) with resprouting trees worst affected.

'''Increasing dominance of grasses (often alien).''' Increasing dominance of deciduous over evergreen species (Fig. Box 2.1.1).

|-
| ''Tundra''

| Tree and shrub encroachment into tundra

| Increased woody plant growth due to longer and warmer growing seasons and shrub tundra replacing dwarf tundra

Poleward expansion of tundra into polar desert and encroachment of coniferous trees into tundra

| Continued woody expansion in tundra regions with reduced surface albedo due to less snow and more woody cover

| '''Increase in woody shrub cover in tundra and expansion of boreal forest into tundra''' (Fig. Box 2.1.1, 2.4.3.4).

|-
| ''Boreal forest''

| Reduced productivity due to weather-related disturbances (e.g., increased fire risk).

Deciduous broadleaf tree encroachment into boreal forest.

| Extensive boreal tree spread into tundra.

Boreal forest dieback within boreal zone and contraction of boreal forest at southern ecotone with continental grasslands

|
| '''Expansion into tundra and upslope treeline advance''' ( [[#2.4.3.8|Section 2.4.3.8]] and Fig Box 2.1.1).

'''Increased mortality due to drought, fire, beetle infestations''' (Sections 2.4.3.8, 2.4.42.1, 2.4.4.3.1).

|-
| ''Tropical forest''

| Increasing CO 2 concentration would increase NPP

| Increases in forest productivity and biomass through increased CO 2 with localised decreases in the Amazon. Shift in forest species composition.

Expansion of forest area into mesic savanna.

| Shift in the climate envelope of moist tropical forests but forests are less likely to undergo major retractions or expansions than suggested in AR4

| '''Expansion of tropical forest into savannas in Africa, Asia, South America''' ( [[#2.4.3.7|Section 2.4.3.7]] , Fig. Box 2.1.1).

'''Forest biomass increases (though slowing)''' ( [[#2.4.4.4|Section 2.4.4.4]] ).

Forest degradation from drought, warming, fire and shorter residence time of trees ( [[#2.4.3.7|Section 2.4.3.7]] )

'''Shift in species composition''' towards species with more aridity-adapted traits ( [[#2.4.3.7|Section 2.4.3.7]] ).

|-
| ''Temperate forest''

| Forest decline and increased mortality

| Increase in tree mortality from drought-related declines.

A general increase of deciduous vegetation at the expense of evergreen vegetation is predicted at all latitudes.

|
| Map indicates a shift towards deciduous species in western North America (Fig. Box 2.1.1).

'''Tree death due to interactions of drought, pest outbreaks and fire''' (2.4.3.8, 2.4.4.2.1., 2.4.4.3.1)

|-
| ''Grasslands and savannas''

| Increasing CO 2 concentration will increase NPP

| Increased tree dominance in savannas and grasslands (from elevated CO 2 ), with C3 plants benefitting more than C4 plants

| Rising CO 2 will increase the likelihood of woodier states (but the transition will vary in different environments)

| '''Greening and encroachment across tropical and temperate savannas in Africa, Asia, Australia and America''' ( [[#2.4.3.5|Section 2.4.3.5]] ).

'''Expansion of trees into grasslands and advancement of tree lines.'''

Signs of increased C4 grass productivity in drought conditions. Increased C3 grass productivity ( [[#2.4.3.5|Section 2.4.3.5]] ).

|-
| ''Desert/arid shrublands''

|
| An increase in desert vegetation productivity was projected in southern Africa, the Sahel, central Australia, the Arabian Peninsula and parts of central Asia due to a positive impact of rising atmospheric CO 2

|
| '''Greening''' (increased leaf area index [LAI] and woody cover) and increased herbaceous production are occurring at desert–grassland interfaces (Cross-Chapter Paper 3)

|}

Assessment: There is high agreement between observations and projections of tree death in temperate and boreal forests, with current projections (AR6) indicating this trend will continue (Sections 2.4.4.3, 2.5.3.3, 2.5.4). Forest death is most widely recorded in central Europe and western North America (Fig. Box 2.1.1). There is also very high agreement between observations and projections of woody encroachment in savannas, grasslands and tundra, with projections also indicating that this trend is likely to continue (Sections 2.4.3.5, 2.4.3.9, 2.5.2.5, 2.5.2.9, 2.5.4). Observations of desert-greening show good agreement with earlier projections. Patterns of desertification are also occurring, although the geographical match between projections and observations shows moderate agreement, likely due to the strong role of land use in this process. Projections of tropical forest expansion into mesic savannas and boreal forest expansion into tundra also show agreement with the observations.

Projections of the future of Mediterranean shrublands, deserts, xeric shrublands and temperate grassy systems are limited, making assessment of this relationship less clear. It is also unclear, due to limited observations, how widespread a shift there is from deciduous forest species to evergreen forest species. Some observations suggest this is occurring, but it is not clear how widespread this change is and if the geographical pattern is as projected.

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