Editing IPCC:AR6/SR15/Chapter-3 (section)

=== 3.3.11 Global Synthesis ===

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Table 3.2 features a summary of the assessments of global and regional climate changes and associated hazards described in this chapter, based on the existing literature. For more details about observation and attribution in ocean and cryosphere systems, please refer to the upcoming IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC) due to be released in 2019.

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'''Table 3.2'''

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'''Summary of assessments of global and regional climate changes and associated hazards'''

Confidence and likelihood statements are quoted from the relevant chapter text and are omitted where no assessment was made, in which case the IPCC Fifth Assessment Report (AR5) assessment is given where available. GMST: global mean surface temperature, AMOC: Atlantic Meridional Overturning Circulation, GMSL: global mean sea level.

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{| class="wikitable"
|-
!
! Observed change (recent past versus pre-industrial)
! Attribution of observed change to human-induced forcing (present-day versus pre-industrial)
! Projected change at 1.5°C of global warming compared to pre-industrial (1.5°C versus 0°C)
! Projected change at 2°C of global warming compared to pre-industrial (2°C versus 0°C)
! Differences between 2°C and 1.5°C of global warming
|-
! GMST<br />
anomaly
| GMST anomalies were 0.87°C (±0.10°C ''likely'' range) above pre-industrial (1850–1900) values in the 2006–2015 decade, with a recent warming of about 0.2°C (±0.10°C) per decade ( ''high confidence'' )
[Chapter 1]

| The observed 0.87°C GMST increase in the 2006–2015 decade compared to<br />
pre-industrial (1850–1900) conditions was mostly human-induced ( ''high confidence'' )
<br/><br/>
Human-induced warming reached about 1°C (±0.2°C ''likely'' range) above pre-industrial levels in 2017

[Chapter 1]

| 1.5°C
| 2°C
| 0.5°C
|-
! Temperature extremes
| Overall decrease in the number of cold days and nights and overall increase in the number of warm days and nights at the global scale on land ( ''very likely'' )
Continental-scale increase in intensity and frequency of hot days and nights, and decrease in intensity and frequency of cold days and nights, in North America, Europe and Australia ( ''very likely'' )

Increases in frequency or duration of warm spell lengths in large parts of Europe, Asia and Australia ( ''high confidence'' ( ''likely'' )), as well as at the global scale ( ''medium confidence'' )

[Section 3.3.2]

| Anthropogenic forcing has contributed to the observed changes in frequency and intensity of daily temperature extremes on the global scale since the mid-20th century ( ''very likely'' )
[Section 3.3.2]

| Global-scale increased intensity and frequency of hot days and nights, and decreased intensity and frequency of cold days and nights ( ''very likely'' )
Warming of temperature extremes highest over land, including many inhabited regions ( ''high confidence'' ), with increases of up to 3°C in the mid-latitude warm season and up to 4.5°C in the high-latitude cold season ( ''high confidence'' )

Largest increase in frequency of unusually hot extremes in tropical regions ( ''high confidence'' )

[Section 3.3.2]

| Global-scale increased intensity and frequency of hot days and nights, and decreased intensity and frequency of cold days and nights ( ''very likely'' )
Warming of temperature extremes highest over land, including many inhabited regions ( ''high confidence'' ), with increases of up to 4°C in the mid-latitude warm season and up to 6°C in the high-latitude cold season ( ''high confidence'' )

Largest increase in frequency of unusually hot extremes in tropical regions ( ''high confidence'' )

[Section 3.3.2]

| Global-scale increased intensity and frequency of hot days and nights, and decreased intensity and frequency of cold days and nights ( ''high confidence'' )
Global-scale increase in length of warm spells and decrease in length of cold spells ( ''high confidence'' )

Strongest increase in frequency for the rarest and most extreme events ( ''high confidence'' )

Particularly large increases in hot extremes in inhabited regions ( ''high confidence'' )

[Section 3.3.2]

|-
! Heavy precipitation
| More areas with increases than decreases in the frequency, intensity and/or amount of heavy precipitation ( ''likely'' )
[Section 3.3.3]

| Human influence contributed to the global-scale tendency towards increases in the frequency, intensity and/or amount of heavy precipitation events ( ''medium confidence'' )
[Section 3.3.3; AR5 Chapter 10 (Bindoff et al., 2013a) <sup>[[#fn:r364|364]]</sup> ]

| Increases in frequency, intensity and/or amount heavy precipitation when averaged over global land, with positive trends in several regions ( ''high confidence'' )
[Section 3.3.3]

| Increases in frequency, intensity and/or amount heavy precipitation when averaged over global land, with positive trends in several regions ( ''high confidence'' )
[Section 3.3.3]

| Higher frequency, intensity and/or amount of heavy precipitation when averaged over global land, with positive trends in several regions ( ''medium confidence'' )
Several regions are projected to experience increases in heavy precipitation at 2°C versus 1.5°C ( ''medium confidence'' ), in particular in high-latitude and mountainous regions, as well as in eastern Asia and eastern North America ( ''medium confidence'' )

[Section 3.3.3]

|-
! Drought and dryness
| ''High confidence'' in dryness trends in some regions, especially drying in the Mediterranean region (including southern Europe, northern Africa and the Near East)
''Low confidence'' in drought and dryness trends at the global scale

[Section 3.3.4]

| ''Medium confidence'' in attribution of drying trends in southern Europe (Mediterranean region)
''Low confidence'' elsewhere, in part due to large interannual variability and longer duration (and thus lower frequency) of drought events, as well as to dependency on the dryness index definition applied

[Section 3.3.4]

| ''Medium confidence'' in drying trends in the Mediterranean region
''Low confidence'' elsewhere, in part due to large interannual variability and longer duration (and thus lower frequency) of drought events, as well as to dependency on the dryness index definition applied

Increases in drought, dryness or precipitation deficits projected in some regions compared to the pre-industrial or present-day conditions, but substantial variability in signals depending on considered indices or climate model ( ''medium confidence'' )

[Section 3.3.4]

| ''Medium confidence'' in drying trends in the Mediterranean region and Southern Africa
''Low confidence'' elsewhere, in part due to large interannual variability and longer duration (and thus lower frequency) of drought events, as well as to dependency on the dryness index definition applied

Increases in drought, dryness or precipitation deficits projected in some regions compared to the pre-industrial or present-day conditions, but substantial variability in signals depending on considered indices or climate model ( ''medium confidence'' ).

[Section 3.3.4]

| ''Medium confidence'' in stronger drying trends in the Mediterranean region and Southern Africa
''Low confidence'' elsewhere, in part due to large interannual variability and longer duration (and thus lower frequency) of drought events, as well as to dependency on the dryness index definition applied

[Section 3.3.4]

|-
! Runoff and river flooding
| Streamflow trends mostly not statistically significant ( ''high confidence'' )
Increase in flood frequency and extreme streamflow in some regions ( ''high confidence'' )

[Section 3.3.5]

| Not assessed in this report
| Expansion of the global land area with a significant increase in runoff ( ''medium confidence'' )
Increase in flood hazard in some regions ( ''medium confidence'' )

[Section 3.3.5]

| Expansion of the global land area with a significant increase in runoff ( ''medium confidence'' )
Increase in flood hazard in some regions ( ''medium confidence'' )

[Section 3.3.5]

| Expansion of the global land area with significant increase in runoff ( ''medium confidence'' )
Expansion in the area affected by flood hazard ( ''medium confidence'' )

[Section 3.3.5]

|-
! Tropical and extra-tropical cyclones
| ''Low confidence'' in the robustness of observed changes
[Section 3.3.6]

| Not meaningful to assess given ''low confidence'' in changes, due to large interannual variability, heterogeneity of the observational record and contradictory findings regarding trends in the observational record
| Increases in heavy precipitation associated with tropical cyclones ( ''medium confidence'' )
| Further increases in heavy precipitation associated with tropical cyclones ( ''medium confidence'' )
| Heavy precipitation associated with tropical cyclones is projected to be higher at 2°C compared to 1.5°C global warming ( ''medium confidence'' ). ''Limited evidence'' that the global number of tropical cyclones will be lower under 2°C of global warming compared to under 1.5°C of warming, but an increase in the number of very intense cyclones ( ''low confidence'' )
|-
! Ocean circulation and temperature
| Observed warming of the upper ocean, with slightly lower rates than global warming ( ''virtually certain'' )
Increased occurrence of marine heatwaves ( ''high confidence'' )

AMOC has been weakening over recent decades ( ''more likely than not'' )

[Section 3.3.7]

| ''Limited evidence'' attributing the weakening of AMOC in recent decades to anthropogenic forcing
[Section 3.3.7]

| colspan="3"| Further increases in ocean temperatures, including more frequent marine heatwaves ( ''high confidence'' )
AMOC will weaken over the 21st century and substantially so under high levels (more than 2°C) of global warming ( ''very likely'' )

[Section 3.3.7]

|-
! rowspan="2"| Sea ice
| rowspan="2"| Continuing the trends reported in AR5, the annual Arctic sea ice extent decreased over the period 1979–2012. The rate of this decrease was ''very likely'' between 3.5 and 4.1% per decade (0.45 to 0.51 million km <sup>2</sup> per decade)
[AR5 Chapter 4 (Vaughan et al., 2013) <sup>[[#fn:r365|365]]</sup> ]

| rowspan="2"| Anthropogenic forcings are ''very likely'' to have contributed to Arctic sea ice loss since 1979
[AR5 Chapter 10<br />
(Bindoff et al., 2013a) <sup>[[#fn:r366|366]]</sup> ]

| At least one sea-ice-free Arctic summer after about 100 years<br />
of stabilized warming ( ''medium confidence'' )[Section 3.3.8]
| At least one sea-ice-free<br />
Arctic summer after about<br />
10 years of stabilized warming ( ''medium confidence'' )[Section 3.3.8]
| Probability of sea-ice-free Arctic summer greatly reduced at 1.5°C versus 2°C of global warming ( ''medium confidence'' )
[Section 3.3.8]

|-
| colspan="3"| Intermediate temperature overshoot has no long-term consequences for Arctic sea ice cover<br />
( ''high confidence'' )
[3.3.8]

|-
! Sea level
| It is ''likely'' that the rate of GMSL rise has continued to increase since the early 20th century, with estimates that range from 0.000 [–0.002 to 0.002] mm yr <sup>–2</sup> to 0.013 [0.007 to 0.019] mm yr <sup>–2</sup>
[AR5 Chapter 13<br />
(Church et al., 2013) <sup>[[#fn:r367|367]]</sup> ]

| It is very ''likely'' that there is a substantial contribution from anthropogenic forcings to the global mean sea level rise since the 1970s
[AR5 Chapter 10 (Bindoff et al., 2013a) <sup>[[#fn:r368|368]]</sup> ]

| Not assessed in this report
| GMSL rise will be about 0.1 m (0.00–0.20 m) less at 1.5°C versus 2°C global warming ( ''medium confidence'' )[Section 3.3.9]
|-
! Ocean<br />
chemistry
| Ocean acidification due to increased CO <sub>2</sub> has resulted in a 0.1 pH unit decrease since the pre-industrial period, which is unprecedented in the last 65 Ma ( ''high confidence'' )
[Section 3.3.10]

| The oceanic uptake of anthropogenic CO <sub>2</sub> has resulted in acidification of surface waters ( ''very high confidence'' ).
[Section 3.3.10]

| colspan="3"| Ocean chemistry is changing with global temperature increases, with impacts projected at 1.5°C and, more so, at 2°C of warming ( ''high confidence'' )
[Section 3.3.10]

|}
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