1) Observed climate changes are a combination of long-term human influences and natural variations on interannual to decadal timescales. Natural variations include both natural forcings like volcanoes and internal fluctuations that occur spontaneously.
2) In Europe, natural climate variability has at times obscured or intensified human-caused warming due to stronger year-to-year variability compared to global and tropical regions. Emergence of human-caused changes is delayed in Europe compared to other regions.
3) Modes of climate variability like the North Atlantic Oscillation organize variability in Europe and can amplify or attenuate projected human-caused changes, especially at regional scales and in the near-term through 20
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Climate change and internal variability in Europe
1. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
9 August 2021
#ClimateReport #IPCC
SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
Climate change and internal variability
in EUROPE
Christophe CASSOU
Lead Author WGI Chapter 3
Centre National de Recherche Scientifique (France)
2. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
• Observed climatic changes since pre-industrial era at any spatial scale are a combination of long-term
human-caused changes and natural variations on time scales from days to decades.
Regardless of future levels of global warming, this combination will continue in the future.
• Natural variations consist of both natural radiatively-forced signals (due to volcanic eruptions or solar
variations) and internal fluctuations of the climate system, which occur spontaneously i.e. in the absence
of any radiative forcings.
• Natural climate variability has temporarily obscured/attenuated or
intensified human-caused climate change at decadal time scales
• Year-to-year variability on top of human-caused warming is mostly
controlled by internal variability
Observed global temperature changes
with respect to 1850-1900
Example of intensification
Example of attenuation
Year-to-year fingerprint of
internal variability
• The ratio between the long-term human-induced change
(signal) and the amplitude of year-to-year internal variability
(noise) [ = a metric that is attached to our perception of past
and future climate changes], differs from global to regional
scales and between regions.
Source: Figure SPM.1 AR6
How about Europe?
Background
3. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
The year-to-year internal variability (noise) is
stronger at midlatitudes than in the tropics.
It is especially true over Europe.
1
Source: Fig 1.14 AR6
Regional properties of internal variability 1
4. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
In all European areas, the human-caused
temperature rise (signal) is stronger than
global mean temperature changes and than
the rise over most tropical lands
The year-to-year internal variability (noise) is
stronger at midlatitudes than in the tropics.
It is especially true over Europe.
2
1
Source: Fig 1.14 AR6
2
Observed annual temperature changes in 2020
Regional properties of internal variability and warming
1
5. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
Emergence
Source: Fig.TS24 AR6
1
2
Time of emergence = date from which the new mean climate
corresponds to unfamiliar conditions in preindustrial period
(unfamiliar=less than about 2% of chance of occurrence based on
observed years over preindustrial time i.e. ratio signal/noise =2)
• Emergence “delayed” over Europe with a
latitudinal gradient due to greater weight of
internal variability going north despite stronger
warming
1981-1988 for Mediterranean (MED)
1997-2004 for WCE and EEU
2005-2012 for Northern Europe (NEU)
• Emergence differs between regions, between
climate variables and between seasons
Weight of internal Variability:
Regional >> global
Mid-latitudes >> Tropics
Precipitation >> Temperature
Winter >> Annual >> Summer
6. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
• At oceanic- or continental-basin scales, internal variability is usually organized through so-called modes of
variability, defined as recurrent space-time structures of variability with intrinsic spatial patterns, seasonality
and timescales (see Technical Annex IV, AR6).
• At oceanic- or continental-basin scales, the variability of the climate system on top of human-caused climate
trends can be described to a large extent at seasonal-to-multidecadal timescales by the occurrence and
often combination of several modes of climate variability which lead to local impacts and remote responses
through teleconnection processes .
• 7 modes of interannual variability + 2 modes of decadal variability have been assessed in AR6
NAM-NAO (North Atlantic Oscillation – Northern Annular Modes)
SAM (Southern Annular mode)
ENSO (El Nino Southern Oscillation)
IOB (Indian Ocean Basin) & IOD (Indian Ocean Dipole)
AZM (Atlantic Zonal Mode) & AMM (Atlantic Meridional Mode)
AMV (Atlantic Multidecadal Variability)
PDV (Pacific Decadal Variability)
Modes of variability (MoVs)
NAO
Fraction of surface air temperature (SAT) and
precipitation (pr) variance explained at interannual
timescales for European regions
Europe
Source: Table TS.4 AR6
7. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
• Since preindustrial period, natural climate variability have temporarily obscured and
intensified human-caused climate change on interannual to decadal time scales
Internal variability as a modulator of past but also future
human-caused changes
• Natural drivers and internal variability WILL either amplify or attenuate projected
human-caused changes in mean climate and climatic impact drivers(CIDs),
including extremes, especially at regional scales and in the near-term [2020-2040],
but with little effect on centennial global warming (high confidence)
Near-term cooling at any particular location with respect to present climate could occur
and would be consistent with the global surface temperature increase due to human
influence (high confidence)
• Modulations driven by internal variability are important to consider in planning for the
full ranges of possible changes for risk assessment and regional adaptation
8. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
• Use of larger ensembles of Global Circulation Models to better explore and account for
internal variability at near-term and at regional scale, and in particular the use of a
collection of so-called single model initial condition large ensembles, a novelty in AR6.
Source: Figure 1.21, AR6
Assessment of climate outcomes at near-term
• Accounting better for internal variability strengthens the conclusion on the forced response.
Different phasing of
internal variability
Range of
internal
variability
Ensemble
mean
9. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
Source: Line et al. (2021), PC
Full range of temperature outcome over Europe in winter
@near-term
Human-caused
best estimate response:
Warming [Europe] = +0.89o
Difference of temperature between near-term [2020-2040] and historical [1995-2014] periods
for 30 individual members
One model: CNRM-CM6, one scenario : ssp2-4.5
Warmest
Coldest
Ensemble mean [30 members]
10. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
Source: Line et al. (2021), PC
Internal variability can partially mask human-caused
warming @near-term (ex. North Europe region – NEU)
Distribution of the temperature difference
between near-term [2020-2040]
and historical [1995-2014] periods
averaged over NEU for
ssp2-4.5
Human-caused best estimate
NEU
10%
25%
Mean
75%
90%
NEU
Warmest
Coldest
11. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
Source: Line et al. (2021), PC
Internal variability partially masks discernible
differences between scenarios over NEU @near-term
Human-caused best estimate
NEU
NEU
Distributions for the 4 illustrative
scenarios are statistically
undistinguishable over NEU
@near-term
Distribution of the temperature difference
between near-term [2020-2040]
and historical [1995-2014] periods
averaged over NEU for
ssp2-4.5
ssp1-2.6
ssp3-7.0
ssp5-8.5
12. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
Modulations driven by internal variability
are important to consider in planning for
the full ranges of possible changes for risk
assessment and regional adaptation
strategies, especially @near-term
because internal variability can
significantly amplify or attenuate human
caused changes
Over Europe, accounting for internal
variability is essential as its weight is large
with respect to other regions.
Summary
13. SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
9 August 2021
#ClimateReport #IPCC
SIXTH ASSESSMENT REPORT
Working Group I – The Physical Science Basis
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