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Projected trends in
global temperature
Effects of Climate Change
on U.S. Air Quality
Loretta J. Mickley, Harvard University
Shiliang Wu, Eric Liebensperger, Moeko Yoshitomi,
Dominick Spracklen, Brendan Field
Daniel Jacob, David Rind, Cynthia Lin, David Streets
Day-to-day weather plays a large role in surface air quality (1).
Northeast
Average num. of days
Number of summer days with ozone
exceedances (> 84ppb) over northeast U.S.
Probability
of ozone exceedance
vs. daily max. temperature
1988, hottest on
record
1992, coldest on
record, due to
Pinatubo eruption
Ozone is strongly correlated with temperature in observations; this is due to
(1) chemistry, (2) biogenic VOC emissions, (3) joint association with stagnation.
Lin et al., 2001
Day-to-day weather plays a large role in surface air quality (2).
cyclone
cold front
EPA ozone levels
L
Stalled high pressure
system increases
ozone due to:
• increased biogenic
emissions
• clear skies
• weak winds
• high temperatures.
cold front
85-104 ppb
105-124 ppb
L
3 days later
Cold front pushes
smog poleward and
aloft on a warm
conveyor belt.
O2
hn
STRATOSPHERE
8-18 km
O3
Chemistry of tropospheric ozone
What processes will climate change affect?
TROPOSPHERE
transport
hn
O3
NO2
NO
OH
HO2
hn, H2O
Deposition
CO, VOC
Nitrogen oxide radicals; NOx = NO + NO2
generally limiting
combustion, soils, lightning
Tropospheric
ozone
precursors
H2O2
Methane
wetlands, livestock, natural gas
Nonmethane volatile organic compounds (NMVOCs)
vegetation, combustion, industry
CO (carbon monoxide)
combustion, VOC oxidation
A decrease in cyclone frequency across mid-latitudes has been
observed (and calculated) for recent decades.
Observed trend in winter cyclone
frequency over Northern Hemisphere.
Calculated trend in summer
cyclone frequency, from
GISS 2x2.5 GCM, 23 layers
McCabe et al., 2001
Climate models tend to predict decrease in mid-latitude cyclogenesis in future due to:
• decrease in meridional temperature gradient
• more efficient poleward transport of latent heat
Leibensperger et al., 2007
Ozone exceedances over eastern United States anti-correlate with
cyclone frequency over Southern Canada/ Great Lakes region.
sample summertime storm tracks, 1979-81
weak
correlation
Strong
anti-correlation
NCEP/NCAR reanalysis
Correlation of 1980-2006
summertime ozone exceedances
with cyclone number in the red and
green boxes for each summer.
Cyclone trend from NCEP/NCAR reanalysis
would imply large effect on ozone trends.
Observed 1980-2006 trend in
summertime cyclone number,
40-50 N
Interannual variability of
cyclone frequency and
ozone exceedance is highly
anticorrelated (r = - 0.64).
Cyclone trend works
against improvements in
emissions.
Number of summertime
ozone exceedance days in
Northeast dropped from 38
in 1980 to ~8 in 2006, but
would have dropped to 0 in
absence of cyclone trend.
Number of O3 exceedances in Northeast
Leibensperger et al., in prep.
Global change and air pollution (GCAP):
Five models working together to provide information on climate
change impacts
GISS general circulation model
1950 Spin-up
2000
changing greenhouse gases
2025
2050
2100
MM5
Mesoscale
model
archive met fields
Model for
precursor
emissions
2075
GEOS-CHEM
Global chemistry model
archive
chemistry
archive
met fields
CMAQ
Regional
chemistry model
Pollution episodes double in duration in 2050s due to decreasing
frequency of cyclones ventilating the eastern U.S.
July - August
2045-2052
GISS GCM simulations for 2050s
vs. present-day climate.
CO tracer: constant, present-day
anthropogenic emissions
+ sink of present-day OH fields.
1995-2002
Changes at high end of
distribution due to 10-20%
decrease in summertime
cyclogenesis.
Fewer cold front passages allow pollutants to build up.
Mickley et al., 2004
2000-2050 A1 climate change increases summertime ozone.
Changes in summer 8-h avg. daily maximum ozone
from 2000-2050 climate change
Higher
temperatures,
stagnation
ppb
Increased water
vapor sink
• Models agree that 2000-2050 climate change will decrease background ozone but
increase surface ozone. We find an increase of 3-5 ppb over large regions.
• Most but not all models find maximum effect during pollution episodes (up to 10 ppb in
ours) due to increased stagnation and higher temperatures.
• Most models find significant effects in Northeast/ Midwest.
• Many models disagree in Southeast partly due to different mechanisms for oxidation of
biogenic isoprene (the dominant VOC precursor)
Wu et al., 2008
Maximum JJA temperatures
Maximum JJA 8-h avg ozone
Increased maximum temperatures during heat waves leads to increased ozone.
99th
percentile
2050s
2000s
Midwest
Cumulative probability (%)
Wu et al., 2008
Max. 8-hr-avg ozone
Daily maximum temperature (K)
Effect of 2000-2050 climate change on pollution episodes.
2050s climate
2000s climate
40% decrease in NOx
2050s climate + DNOx
Cumulative probability (%)
Amplification of temperatures at extreme due to
• soil moisture feedbacks
• 17% decreased cyclone frequency.
We can use ozone-temperature correlations as a test of model
sensitivity to climate change, esp in Southeast US
Correlations of daily max-8h-avg ozone with daily max temperature in Jun-Aug
GCAP model
present-day climate (3 years)
Regions of strongest
correlation in model and
observations.
Observations (1980-1998)
correlation
coefficient R
-1
-.5
0
.5
1
Yoshitomi et al., in progress
EPA wildfire project: How will changing forest fire
frequency affect future air quality over the United States?
Area burned and temperature in
Canada over the last century
Most of the variability in
wildfire frequency is due to
year-to-year changes in
surface temperatures and
precipitation.
Forest fires in western US
have big impact on particle
concentrations and on
surface ozone.
Gillet et al., 2004
What will happen next? Dominick Spracklen has developed
a fire prediction tool to calculate future area burned using
GISS GCM meteorological variables.
. . . 2050 and beyond
Results show far:
Future 2040-2050 biomass consumption by wildfires over the
western United States is ~50% greater than for present-day.
Annual total biomass consumed by forest fires, 1996-2055
Plot shows standardized departures from the 1996-2055 mean.
Rynda Hudman at Harvard is investigating
effect of future forest fires on surface ozone
concentrations.
Spracklen et al., in progress.
Increased 2050s wildfire activity leads to 3-6 ppb increase in
afternoon mean ozone in July over western U.S.
JJA surface ozone
Biomass burning NOx emissions
2000
Mean of 5 ppbv
enhancement due
to fires a > 2 SD
2051
[Gg NO]
Hudman et al., in progress
[ppbv]
Conclusions
In the 1980-2006 NCEP/NCAR record, we find a decline in cyclone
frequency of -0.15 cyclones/y, which may have limited the efficacy of air
pollution regulations.
For 2050-2000 A1B climate change, we calculate a10-20% decline in cyclone
frequency, lengthening pollution episodes over Midwest and Northeast.
Climate change is expected to degrade U.S. ozone air quality. The summer
average daily max-8h ozone increases by 2-5 ppb over large areas due to
2000-2050 climate change with the IPCC A1B scenario. Largest effect is
during pollution episodes, when ozone increases by as much as 10 ppb.
Summertime ozone concentrations are predicted to increase by 3-6 ppb over
parts of western US due to increasing wildfire frequency in the 2050s.
Projected trends in
global NOx emissions
for a range of scenarios.
Extra slides
Why is ozone in the Southeast insensitive to climate change?
Isoprene has competing effects on ozone
Isoprene + OH
RO2
(OH sink)
RO + NO2 (O3 formation)
O3
RO2 + NO
RONO2
Isoprene + O3
M
(sink for NOx)
(O3 sink)
Isoprene emissions
2000
Wu et al., 2008
2050 / 2000
O3
Sensitivity test: Isoprene
emissions +30%  Δ(O3)