Editing IPCC:AR6/WGII/TS (section)

=== Reasons for concern (RFC) ===

<div id="h3-19-siblings" class="h2-siblings"></div>

'''TS.C.12 More evidence now supports the five major RFCs about climate change, describing risks associated with unique and threatened systems (RFC1), extreme weather events (RFC2), distribution of impacts (RFC3), global aggregate impacts (RFC4) and large-scale singular events (RFC5) (''' '''''high confidence''''' '''). (''' Figure TS.4, Table TS.1) { 16.6.3, Figure 16.15 }

'''TS.C.12.1 Compared to AR5 and SR15, risks increase to high and very high levels at lower global warming levels for all five RFCs (''' '''''high confidence''''' '''), and transition ranges are assigned with greater confidence.''' Transitions from high to very high risk emerge in all five RFCs, compared to just two RFCs in AR5 ( ''high confidence'' ). As in previous assessments, levels of concern at a given level of warming remain higher for RFC1 than for other RFCs. (Table TS.1, TS.AII) { 16.6.3, Figure 16.15 }

'''TS.C.12.2 Limiting global warming to 1.5°C would ensure risk levels remain moderate for RFC3, RFC4 and RFC5 (''' '''''medium confidence''''' '''), but risk for RFC2 would have transitioned to a high risk at 1.5°C and RFC1 would be well into the transition to very high risk (''' '''''high confidence''''' ''').''' Remaining below 2°C warming (but above 1.5°C) would imply that risk for RFC3 through RFC5 would be transitioning to high, and risk for RFC1 and RFC2 would be transitioning to very high ( ''high confidence'' ). By 2.5°C warming, RFC1 will be at very high risk ( ''high confidence'' ), and all other RFCs will have begun their transitions to very high risk, with ''medium confidence'' for RFC2, RFC3 and RFC4, and ''low confidence'' for RFC5. (Table TS.1) { 16.6.3, Figure 16.15 }

'''TS.C.12.3 While the RFCs represent global risk levels for aggregated concerns about ‘dangerous anthropogenic interference with the climate system’, they represent a great diversity of risks, and in reality, there is not one single dangerous climate threshold across sectors and regions.''' RFC1, RFC2 and RFC5 include risks that are irreversible, such as species extinction, coral reef degradation, loss of cultural heritage or loss of a small island due to sea level rise. Once such risks materialise, the impacts would persist even if global temperatures subsequently declined to levels associated with lower levels of risk in an ‘overshooting’ scenario, for example where temperatures increase over ‘well below 2°C above pre-industrial’ for multi-decadal time spans before decreasing ( ''high confidence'' ). (Figure TS.4, see also TS.C.13) { 16.6.3, Figure 16.15 }

'''Table TS.1 |''' Updated assessment of risk level transitions for the five reasons for concern (RFC) { 16.6.3 }

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

! '''Example of impacts (not comprehensive)'''

! '''Updated risk level based on observed and modelled impacts'''

! '''Warming level'''

|-
| rowspan="2"| '''RFC1 Unique and threatened systems''' : ecological and human systems that have restricted geographic ranges constrained by climate-related conditions and have high endemism or other distinctive properties. Examples include coral reefs, the Arctic and its Indigenous Peoples, mountain glaciers and biodiversity hotspots.

| Coral bleaching, mass tree and animal mortalities, species extinction; decline in sea-ice dependent species, range shifts in multiple ecosystems

| In transition from moderate to high

| 1.1°C ( ''ve'' ''ry high confiden'' ''ce'' )

|-
| Further decline of coral reef (by 70–90% at 1.5°C) and Arctic sea-ice dependent ecosystems; insects projected to lose &gt;50% climatically determined geographic range 2°C; reduced habitability of small islands; increased endemic species extinction in biodiversity hotspots

| Projected to transition from high to very high risk

| 1.2°C–2.0°C ( ''high confidenc'' ''e'' )

|-
| rowspan="2"| '''RFC2 Extreme weather events''' : risks/impacts to human health, livelihoods, assets and ecosystems from extreme weather events such as heatwaves, heavy rain, drought and associated wildfires and coastal flooding.

| Increased heat-related human mortality, wildfires, agricultural and ecological droughts, water scarcity; short-term food shortages; impacts on food security and safety, price spikes; marine heatwaves estimated to double in frequency.

| In transition to high risk at present

| 1.0°C–1.5°C ( ''h'' ''igh confid'' ''ence'' )

|-
| Significant projected increases in fluvial flood frequency and resultant risks associated with higher populations; at least 1 d yr -1 with a heat index above 40.6°C for about 65% of megacities at 2.7°C and close to 80% at 4°C; soil moisture droughts 2–3 times longer; agricultural and ecological droughts more widespread; simultaneous crop failure across worldwide breadbasket regions; malnutrition and increasing risk of disease.

| Projected to transition to very high risk (new in AR6)

| 1.8°C–2.5°C ( ''me'' ''dium confidence'' )

|-
| rowspan="3"| '''RFC3 Distribution of impacts''' : risks/impacts that disproportionately affect particular groups, such as vulnerable societies and socio-ecological systems, including disadvantaged people and communities in countries at all levels of development, due to uneven distribution of physical climate change hazards, exposure or vulnerability.

| Increasing undernutrition, stunting and related childhood mortality, particularly in Africa and Asia and disproportionately affecting children and pregnant women; distributional impacts on crop production and water resources

| Current risk level is moderate

| 1.1°C ( ''h'' ''igh confidence'' )

|-
| Risk of simultaneous crop failure in maize estimated to increase from 6% to 40%; increasing flood risk in Asia, Africa, China, India and Bangladesh; high risks of mortality and morbidity due to heat extremes and infectious disease with regional disparities

| Projected to transition to high risk

| 1.5°C–2.0°C ( ''m'' ''edium confidence'' )

|-
| Much more negative impacts on food security in low to mid-latitudes; substantial regional disparity in risks to food production; food-related health projected to be negatively impacted by 2°C–3°C warming; heat-related morbidity and mortality, ozone-related mortality, malaria, dengue, Lyme disease and West Nile fever projected to increase regionally and globally

| Projected to transition to very high risk

| 2.0°C–3.5°C ( ''m'' ''edium confidence'' )

|-
| rowspan="3"| '''RFC4 Global aggregate impacts''' : impacts to socio-ecological systems that can be aggregated globally into a single metric, such as monetary damages, lives affected, species lost or ecosystem degradation at a global scale.

| Aggregate impacts on biodiversity with damages of global significance (e.g., drought, pine bark beetles, coral reef ecosystems); climate-sensitive livelihoods like agriculture, fisheries and forestry would be severely impacted

| In transition to moderate risk

| 1.1°C ( ''medium confidence'' )

|-
| Estimated 10% relative decrease in effective labour at 2°C; global exposure to multi-sector risks approximately doubles between 1.5°C and 2°C; global population exposed to flooding projected to rise by 24% at 1.5°C and by 30% at 2°C warning; reduced marine food provisioning, fishery distribution and revenue value with projected approximate 13% decline in ocean animal biomass.

| Projected to transition to high risk

| 1.5°C–2.5°C ( ''me'' ''dium confidence'' )

|-
| Widespread death of trees, damage to ecosystems and reduced provision of ecosystem services over temperature range 2.5°C–4.5°C; projected global annual damages associated with sea level rise of USD31,000 billion yr -1 in 2100 for 4°C warming scenario.

| Projected to transition to very high risk (new in AR6)

| 2.5°C–4.5°C ( ''lo'' ''w confidence'' )

|-
| rowspan="3"| '''RFC5 Large-scale singular events''' : relatively large, abrupt and sometimes irreversible changes in systems caused by global warming, such as ice sheet disintegration or thermohaline circulation slowing, sometimes called tipping points or critical thresholds.

| Mass loss from both Antarctic (whether associated with marine ice sheet instability or not) and Greenland ice sheets is more than seven times higher over the period 2010–2016 than over the period 1992–1999 for Greenland and four times higher for the same time intervals for Antarctica; in Amazon forest, increases in tree mortality and a decline in carbon sink are reported

| Current risk level is moderate

| 1.1°C ( ''high confidence'' )

|-
| Implications for 2000-year commitments to sea level rise from sustained mass loss from both ice sheets as projected by various ice sheet models, reaching 2.3–3.1 m at 1.5°C peak warming and 2–6 m at 2°C peak warming; risk of savannisation for Amazon alone was assessed to lie between 1.5°C and 3°C, with a median value at 2°C

| Projected to transition to high risk

| 1.5°C–2.5°C ( ''me'' ''dium confidence'' )

|-
| Uncertainties in projections of sea level rise at higher levels of warming, long-term equilibrium sea level rise of 5–25 m at mid-Pliocene temperatures of 2.5°C; potential for Amazon forest dieback between 4°C and 5°C; risk of ecosystem carbon loss from tipping points in tropical forest and loss of Arctic permafrost.

| Projected to transition to very high risk (new in AR6)

| 2.5°C–4°C ( ''l'' ''ow confidence'' )

|}

<div id="Temporary" class="h2-container"></div>

<span id="temporary-overshoot"></span>