SIXTH ASSESSMENT REPORT

Working Group I – The Physical Science BasisNew
UnderstandingsinthePhysical
9 August 2021
Climate Change Sciences
Panmao ZHAIIPCC WGI Co-chair

22 May, 2023, Japan

https://www.ipcc.ch/report/ar6/wg1/

2007"Warming of theclimate system isunequivocal…"

1990
Gave a broad overview ofclimate change science,
discussion of uncertainties
and evidence of warming

2013“Human influence on the climate
system is clear.”

1995“The balance of evidence
suggests a discernible
human influence on
global climate”
2001“There is new and stronger
evidence that most of thewarming observed over thelast 50 years is attributableto human activities”

Improvements:

2021
Climate Change 2021:
The Physical Science Basis“It is unequivocal that human
influence has warmed theatmosphere, ocean and land”.

SIXTH ASSESSMENT REPORT

Working Group I – The Physical Science Basis

9 August 2021

Observed Climate change#ClimateReport #IPCC

https://www.ipcc.ch/report/ar6/wg1/

Process
understanding

Past
climatearchives

Clearest picture ofpast, present andfuture climate

Global andregional
climate models

Methodsofanalysesevidence from over
14,000 publications
assessed in the reportSince the 20th century, the global surface temperature has increased rapidly.
Global surface temperature was 1.09°C higher in the last decade than 1850–1900.

2001-2020 warming 0.99°C；2011-2020 warming 1.09°C

• Global warming, but with
regional differences.

• Warming enhanced since 1980s,
especially in the Arctic andmany continental regions.

Changes in temperatures over the land and oceans from 1850-2020

The Earth has warmed by 1.1℃
Warming is unprecedented in more than 2000 years
Changes in global surface temperature relative to 1850-1900

1.1°C

warm
er

Adapted from Figure SPM.1

SIXTH ASSESSMENTREPORT

Working Group I – The Physical Science Basis

目前的气候可能是10万年来最暖的；过去2百万年地球气候在冰期与间冰期冷暖变化的波动；
末次冰期向间冰期过渡期间最大的增暖速率为1.5℃/千年；19世纪后期以来全球地表温升已
经超过了1.1℃，逆转了6千多年以来长期缓慢的变冷趋势，表现为快速的温升趋势！

近期的气候可能是10万年来最暖的M

Changes are widespread, rapid,
and intensifying

FigureHuman influence on climate is unequivocal
Atmospheric concentrations

CO
2

CH4

Observed increases in well-mixed
greenhouse gas (GHG) concentrations
since ~1750 are unequivocally caused byhuman activities
Emissions in the last decade reached thehighest levels in human history

N2
O
thousands of yearsthousandsin the pastof years in the past

Figure 5.4

Climate change is already affecting every region on Earth
Hot extremes
Type of observed change
Colour = Increase/decrease
Low agreement in thetype of change
Limited data and/or
literature

Figure SPM.3

Climate change is already affecting every region on Earth
Heavy precipitation
Type of observed change
Colour = Increase/decrease
Low agreement in thetype of change
Limited data and/or
literature

Figure SPM.3

Changes are widespread, rapid,
and intensifying

FigureThese changes are unprecedented
Fastest sea level rates
in at least 3000 years

Lowest Arctic sea
ice level in at least
1000 years

Fastest sea surface
temperature warming
in at least
11,000 years

Ocean acidification unusual
in at least 2 million years

Ocean heat content
unprecedented
in at least
18,000 years

Unprecedented
glacier melt in at
least
2000 years
Figure FAQ2.2

SIXTH ASSESSMENT REPORT

Working Group I – The Physical Science Basis

9 August 2021

Human influecnes on climate change#ClimateReport #IPCC

https://www.ipcc.ch/report/ar6/wg1/

Human-caused radiative forcing is increasing↑ greenhouse gas
concentrations↓ global cooling
effect of aerosols

Change in effective
radiative forcings(W/m2)

Rate(W/m2 per decade)Figure 2.10

Human influence causes heating of the climate system

Figure FAQ7.1

Optimal Fingerprinting
The scaling factor β gives a measureof how well X simulates themagnitude of climate change signals
in Y:

if β is < 1, X overestimates the signal;
if β is > 1, X underestimates thesignalβ can be used to ‘constrain’ model
projection

Hasselmann 1993,1997; Allen and Tett 1999; Hegerl and Zwiers 2011;

Observed warming is driven by emissions from human activities, with greenhouse gas
warming partly masked by aerosol cooling
Evidence from
attribution studies

human
influence
greenhouse gas
warmingaerosol coolingobservations

climate variability
natural factors
Adapted from Figure SPM.2

Improved understanding of the role of each individual component of human influence
Evidence from radiative forcing andclimate sensitivity studies

CO2
MethaneMainly contribute tochanges in non-CO₂ greenhouse gases

Mainly contribute tochanges in anthropogenic aerosols

Adapted from Figure SPM.2

Human-induced changes are increasingly apparent at regional andlocal scales
Change in surface air temperature
human-caused & natural
natural only

Figure TS.7

Widespread changes are
attributed to human
influence

Main driver of …
Contributes to …
Adapted from Figure FAQ2.2

Confidence has increased in human influenceon many aspects of climate system
Variables

AR5（2013）

AR6（2021）Confidences

GST

Extremely highUnequivical

Large sclae
precipitation

Midum

LikelyExtreme temperature

Extremely highVirtually certain

Extreme Precipitation

MidiumLikelyDrought

Low

Midium/likelylikely（since 1970）

Very likely (sicne 1950）

Reduction in glacials

LikelyVery likely

Mass loss in Green
land ice sheet

LikelyVery likely

Ocean acidification

Very likely

Virtually certain

Heat content increase
in upper ocean

LikelyExtremely likely

Occurance of compound
extremes
Spring snow coverdecline over NH

SIXTH ASSESSMENT REPORT

Working Group I – The Physical Science Basis

9 August 2021

Possible climate futures#ClimateReport #IPCC

https://www.ipcc.ch/report/ar6/wg1/

New shared socio-economic pathways for CMIP6

SSP-RCP Senario
Framework●

CMIP5 used RCPs●

CIMP6 used SSPs

*SSPX-Y: 5 set of illustraive senarios:
SSP1-1.9 Very low GHG emission
SSP1-2.6 Low
SSP2-4.5 Medium
SSP3-7.0 High
SSP5-8.5 Very high GHG emission

Figure CCB1.4, Fig.1

The illustrative set of five SSP scenarios span a broader range of greenhouse gas andair pollutant futures than assessed in earlier WGI reports
Shared Socioeconomic Pathway (SSP) Scenarios
Carbon dioxide (GtCO2/yr)

Very highCO2 emission pathways without
any climate change mitigation

High

Intermediate
Net zero CO2

Low
Very low

CO2 emission pathway
CO2 emission pathways in which CO2 emissions
decline to net zero around or after 2050

Box SPM.1, Figure SPM.4

Broad agreement across multiple lines of evidence, supporting a best estimate ofequilibrium climate sensitivity of 3°C, with a likely range of 2.5°C to 4°C

AR6Equilibrium Climate Sensitivity in IPCC reports

Figure TS.16

Broad agreement across multiple lines of evidence, supporting a best estimate ofequilibrium climate sensitivity of 3°C, with a likely range of 2.5°C to 4°C
Multiple lines of evidence are used to better constrain climate sensitivity

The AR6 best estimate of ECS is
3°C, the likely range is 2.5 to 4°C
and the very likely range is 2 to 5°C.
Figure TS.16

AR6 Constrant projections for GSTchanges

Tokarska et al., 2020

Global warming of 1.5°C and 2°C will be exceeded unless deep reductions in
CO2 and other greenhouse gas emissions occur in the coming decades
Global surface temperature change relative to1850–1900

CO2 emissions

Very
high
High
Intermediate
Low
Very low

1.5°C

Near-term

Figure SPM.8

Global warming of 1.5°C and 2°C will be exceeded unless deep reductions in
CO2 and other greenhouse gas emissions occur in the coming decades
Global surface temperature change relative to1850–1900

2°C

CO2 emissions

Intermediate
In 2100: 2.7°C(2.1 to 3.5°C)

Figure SPM.8

Projections in the context of the Earth’s climate history
Global surface temperature change (relative to 1850-1900)+2.5ºC

Figure TS.1

SIXTH ASSESSMENTREPORT

Working Group I – The Physical Science Basis

Projected change in Global meansea level rise

SSP1-2.6

over 15m

Ref: 1995-2014（~0.2m）

2100

SSP1-1.9

0.38 m

SSP1-2.6

0.44m

SSP2-4.5

0.56m

SSP3-7.0

0.68m

SSP5-8.5

0.77m

SSP5-8.5

Extreme sea level events will increase as GSL rise

Atmosphere

Continued global warming is
projected to further intensify
the global water cycle, including its
variability, global monsoon
precipitation and the severity of
wet and dry events

Land

Ocean

Frozen regions

Many changes in the climate system become larger in direct relation toincreasing global warming↑ frequency and intensity● hot extremes and marine heatwaves● heavy precipitation (+7% per oC)● drought in some regions↑ proportion of intense tropical cyclones

Figure FAQ11.2

SIXTH ASSESSMENTREPORT

Working Group I – The Physical Science Basis

全球变暖背景下温升的未来预估

SSP1-1.9(极低排放情景) :2041-2060为1.6°C
2081-2100为1.4°C

Table SPM.1

SIXTH ASSESSMENTREPORT

Working Group I – The Physical Science Basis

全球变暖背景下温升的未来预估

Table SPM.1. See more in Table
4.5 and Fig 4.11

SIXTH ASSESSMENT REPORT

Working Group I – The Physical Science Basis

9 August 2021

Climate information foradaptation and mitigation#ClimateReport #IPCC

https://www.ipcc.ch/report/ar6/wg1/

Multiple lines of evidence are
distilled for the co-production ofuser-relevant regional climate
information framed by context andvalues

Chapter 10, Figure 10.1

Natural variability can enhance or reduce near term climate changes, particularly
on regional scales

time

Figure 1.13

Regional climate assessment

C
Multiple climatic impact-drivers will change in all regions of the world

Heat

Rain

Snow

cold

drought

ice

&

&

&

Wind

Coastal&Other

oceanic

A climatic impact-driver is a physical climate system conditions(e.g., means, events, extremes) that affect an element of society or
ecosystems

Open
ocean

Worldwide changes in heat, cold, snow and ice, coastal, oceanic climatic
impact-drivers will continue over the 21st century
Changes of CIDs related to the water cycle have a more region specific distribution● All regions are projected to experience changes in at least 5 CIDs.
● 96% of regions are projected to experience changes in at least 10 CIDs.
● 50% of regions are projected to experience changes in at least 15 CIDs.

Regional sea level rise contributes to increases in the frequency and severity of coastal
flooding in low-lying areas and to coastal erosion along most sandy coasts

Extreme sea levels that occurred once per century in the recent past willoccur
• by 2050: 20 to 30 times more frequently
annually or more frequently at about 19–31% of tide gauges• by 2100 : at least 160 times more frequently,
annually or more frequently at 60% (<2°C) to 80% (4°C) of tidegaugesRegional sea level rise contributes to increases in the frequency and severity of coastal
flooding in low-lying areas and to coastal erosion along most sandy coasts

NUMBER OF REGIONS
55

45

35

25

15

5

5 15 25 35 45 55

COASTAL
CLIMATIC
IMPACT-DRIVERS

by 2050 compared to 1960-2014 (2°C global warming)

Figure SPM.9

At 2°C global warming and above, the magnitude of changes increases for droughts,
heavy precipitation and associated flooding events, and for mean precipitationcompared to those at 1.5°C
NUMBER OF REGIONS
55

45

35

25

15

5

5 15 25 35 45 55

WET & DRY
CLIMATIC
IMPACT-DRIVERS

by 2050 compared to 1960-2014 (2°C global warming)
Figure SPM.9

Region-specific changes include intensification of tropical cyclones and/or
extratropical storms

55

45

35

25

15

5

5 15 25 35 45 55

NUMBER OF REGIONS

WIND
CLIMATIC
IMPACT-DRIVERS

by 2050 compared to 1960-2014 (2°C global warming)
Figure SPM.9

The open ocean regions are projected to experience widespread warming,
increased marine heatwaves, loss of oxygen and increased surface salinity
contrasts due to the intensified water cycle
NUMBER OF REGIONS
55

45

35

25

15

5

5 15 25 35 45 55

OPEN OCEAN
CLIMATIC
IMPACT-DRIVERS

by 2050 compared to 1960-2014 (2°C global warming)

Figure SPM.9

Low Likelihood High Impact events
The probability of occurrence is low but potential impacts on society andecosystems could be highTheir chance of occurrence increases with higher global warming
They can occur at the global and regional scale

See Table 4.10

SIXTH ASSESSMENT
二氧化碳累计排放增加会加剧全球变暖（~每年排放40GT）REPORT
Every
tonne
COScienceadds to global warming
Working GroupI – Theof
PhysicalBasis
2 emissions

Figure SPM.10

2390 GtCO2

SIXTH ASSESSMENTREPORT

Working Group I – The Physical Science Basis剩余碳收支Remaining carbon budgets

Table SPM.2

SIXTH ASSESSMENTREPORT

Working Group I – The Physical Science Basis

CO2累积排放量高的情景下，陆地和海洋碳汇占比迅速减少！

Figure SPM.7

Summery:
Recent climate changes are widespread, rapid, and intensifying, and unprecedented inthousands of years.
● Human activities are causing climate change, making extreme climate events, including
heat waves, heavy rainfall, and droughts, more frequent and severe.
● Climate change is already affecting every region on Earth, in multiple ways. The changes
we experience will increase with further warming.
● There’s no going back from some changes in the climate system. However, somechanges could be slowed and others could be stopped by limiting warming.
● Unless there are immediate, rapid, and large-scale reductions in greenhouse gas
emissions, limiting warming to 1.5°C and even 2°C will be beyond reach.
●SIXTH ASSESSMENT REPORT

Working Group I – The Physical Science BasisThank you.
Information:
9 MoreAugust2021
IPCC: www.ipcc.chInteractive Atlas: interactive-atlas.ipcc.ch
IPCC Working Group I TSU:
IPCC Press Office: ipcc-media@wmo.int#ClimateReport #IPCC

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