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IPCC WG1 AR5: Key Findings
Relevant to Future Air Quality
Fiona M. O’Connor, Atmospheric Composition & Climate Team,
Met Office Hadley Centre
IPCC AR5 WG1 Report
• WG1 Summary for Policy Makers released 27 Sept 2013
• WG1 Full Report published 30 Sept 2013
• WG1 provides the latest assessment of the physical science with
respect to climate change
• Observed changes are unprecedented and have been seen throughout
the climate system
• It is now possible to link observed changes in many climate components
to human influence
• Climate models have been improved, and can reproduce main features
of observed multi-decadal warming
• WG2 and WG3 on impacts and mitigation to be released in 2014
Outline of Presentation
• Observed Changes in the Climate
• What’s Causing these Changes?
• Future Projections of Climate Change
• Future Projections of Air Quality
• Main Conclusions and Key Uncertainties
Part 1: Observed changes to the
climate
Atmospheric CO2 levels are
unprecedented for 800,000 yrs
Atmospheric CO2 concentrations have
• increased by about 40% since 1750, due to human activity
• exceed values recorded in ice cores for the last 800,000 years
2000s: warmest decade on
record
• Last 3 decades
warmest in
instrumental record.
• From palaeoclimate
records, in the NH,
last 30 years likely
(66-100%) the
warmest period of the
past 1400 years.
• Very likely (90-100%)
that number of warm
days has increased
and cold nights
decreased globally
Almost the whole globe is warming
Annual mean temperature (1901-2012)
Most global land areas analysed have experienced significant warming
including of both maximum and minimum temperature extremes since
1950
Despite robust multi-decadal warming since 1901, there is substantial
decadal variability in the rate of warming with several periods exhibiting
almost no linear trend.
Part 2: What are the causes of the
changing climate?
Changes to Earth’s Energy
Budget
The AR5 estimate for 2011 is 44% greater than the
AR4 estimate for 2005 because of:
• Increased forcing from greenhouse gases
• Revised estimate of aerosol forcing
Attribution of Radiative
Forcing to Emissions
Radiative Forcing from
Aerosols
Time Evolution and Spatial
Heterogeneity in RF
Part 3: Future projections
Scenarios for future change
• AR4: Special Report on
Emission Scenarios (SRES)
are four major families of GHG
emission scenarios each making
different assumptions for future
greenhouse gas pollution, landuse and other driving forces
• AR5: Representative
Concentration Pathways
(RCPs) are four GHG
concentration trajectories, each
describing a different future
value of radiative forcing in
2100. RCPs aim to provide a
range of climate model
responses, rather than being
derived from socio-economic
storylines.
Projections of global average
warming
• By the end of the century, the increase of global mean surface temperature
above 1986-2005 levels is projected to be:
• 0.3-1.7˚C for RCP2.6
• 2.6-4.8˚C for RCP8.5
• Global warming >2˚C is likely (66-100%) for RCP6.0 and RCP8.5
• Global warming >4˚C is unlikely (0-33%) except for RCP8.5
Warming will not be the same
everywhere
• There is very high confidence that long-term warming will be larger
over land than over the ocean, and that the Arctic region will warm
most rapidly.
• Ocean warming will continue for centuries, even if greenhouse gas
emissions are decreased.
There will be large geographical
variations in precipitation change
• For the next few decades, changes in regional-scale precipitation
will be strongly influenced by natural variability.
• Contrast between wet and dry regions and seasons will increase
over most of the globe, though there are regional exceptions.
• Monsoon precipitation is likely to intensify, along with a
lengthening of the monsoon season.
Projections of temperature
over Europe
2016–2035 vs 1986–2005
ensemble mean results from
the ENSEMBLES project
Figure 11.18
Rajczak et al. (2013)
Part 4: Future Projections of Air
Quality (O3 and PM2.5)
AQ Statements in Summary for
Policy Makers
• Range in projections of near-term AQ (surface O3 and PM2.5)
is driven primarily by emissions rather than by climate change
• Globally, warming decreases background surface O3 but
high CH4 (RCP8.5) can offset this decrease
• Higher surface temperatures in polluted regions will trigger
feedbacks in local chemistry and emissions, increasing peak
levels of O3 and PM2.5
• For PM2.5,climate change may alter natural aerosol sources
and wet removal, but no confidence level is attached to the
overall impact
Future Anthropogenic Emissions
SRES
vs
RCPs
Figure 8.2 AR5
Changes in Trop. O3 Burden
Atmospheric
Composition and
Climate Model
Intercomparison
Project (ACCMIP)
Young et al., ACP,
2013
Changes in Near-Term Surface O3
Figure 11.21 (adapted from Fiore et al., 2012)
Global and Regional Changes in
Surface O3
Time series
of O3
Figure 11.23a
Changes in OH/Reactivity
Results from
ACCMIP time
slice experiments
Voulgarakis et al.,
ACP, 2013.
Changes in Aerosol Optical
Depth relative to Yr- 2000
Table AII.5.3
2030
2100
RCP2.6
-14%
-26%
RCP4.5
-4%
-18%
RCP6.0
-12%
-23%
RCP8.5
-4%
-14%
Changes in Surface PM2.5
Time series
of PM2.5
Figure 11.23b
Extreme Weather and AQ
• Extreme AP
episodes are associated with changing weather
patterns, such as heat waves and stagnation episodes
• Climate change has increased the near-term risk of heat waves
• Projected changes in the frequency of regional air stagnation
events remain difficult to assess -> regional AP extremes
• Projections show increases in extreme O3 pollution events over
US/Europe but lack of agreement at regional level
Longer-Term Feedbacks on
Natural CH4 Emissions
Figure 6.36
O’Connor et al., 2010
Summary on AQ
• Lower background AP levels are projected following
the RCPs compared to SRES (high confidence)
• Range in near-term projections of AQ is driven
primarily by emissions rather than by climate change
(medium confidence)
• The total range in emissions — including the CLE and
MFR scenarios—is larger than spanned by the RCPs
• Peak O3 and PM2.5 will increase with climate change
Key Gaps and Uncertainties
• Uncertainties in Future Anthropogenic Emissions
• Uncertainties in BVOC chemistry
• Impact of climate change on BVOC emissions
• Impact of climate change on natural CH4 emissions
• Impact of climate change on transport pathways
• Future Projections of Stagnation Events
• Future Projections of Regional AQ
• Future Projections of Regional OH
• Recent and Future Trends in surface O3
Questions and answers