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Impacts of Aerosols on
Climate Extremes in the USA
Nora Mascioli
Objectives
• To determine the effect of aerosols on the
magnitude, frequency, and duration of
extreme weather events relevant to air quality
concerns in the historical record.
• To assess how these extreme events may
evolve in the future as a result of changing
aerosol and greenhouse gas levels.
Aerosols and Climate Change
• The most abundant
anthropogenic
aerosols are sulfate
and black carbon.
Sulfate aerosols
reflect solar radiation
back into space,
cooling the planet.
• Black carbon absorbs
solar radiation,
heating the planet.
• Aerosol are the
dominant source of
uncertainty in the net
anthropogenic forcing.
Source: IPCC AR4 (FAQ 2.1)
Aerosol Indirect Effects
Source: IPCC AR4, Ch. 2
•It also theoretically increases the lifetime
of the cloud by suppressing precipitation
(cloud lifetime effect).
• Aerosols also
affect climate
indirectly via their
interactions with
clouds.
• Aerosols can act
as cloud
condensation
nuclei (CCN).
• Clouds form with
smaller droplets,
making them more
reflective (cloud
albedo effect)
Changing Aerosol Emissions
• Primary anthropogenic
sources of aerosols are
fossil fuel and biomass
burning. Due to air
quality regulations,
anthropogenic aerosol
emissions peaked in
the US around 1970.
• Globally, aerosol
emissions have not
increased significantly
since 1970.
Extreme Weather
• Extreme weather
events such as heat
waves and dry spells
are important for air
quality management.
• How will aerosols effect
these events?
• We will use indices
provided by the Expert
Team on Climate
Change Detection and
Indices (ETCCDI)
Source: accuweather.com
Extreme Climate Indices
• For this project, we will use:
– Maximum of daily
maximum
temperature(TXx)
– % of days above the 90th
percentile daily maximum
temperature (TX90)
– Warm spell duration
(WSDi)
– Days with rain above a
fixed threshold (R1mm,
R10mm, and R20mm)
– Consecutive dry days
(CDD).
• Results shown are based on
the GFDL-CM3 historical
simulations for eastern
North America.
Source: Giorgi et al, 2000.
Maximum daily maximum temperature (TXx)
Difference between 1980-2006
and 1860-1890 means
• No clear change between 1860 and
the present looking over entire region.
• However, smaller subregions show
changes between the 1860-1890 and
1980-2006 means that may be
statistically significant.
• TXx from 1950 - 1980 appears cooler
than 1920-1950 and 1980-2006
Maximum daily maximum temperature (TXx)
• Left hand plot shows the difference
between the 1980-2006 and 19501980 means at each grid cell.
• 1950-1980 is the period of peak
aerosol emissions in the US.
• 1980-2006 shows uniformly higher
TXx values, compared with 19501980.
Warm Days (TX90)
Difference between 1980-2006
and 1860-1890 means
• PDFs do not show any clear
differences between time
periods.
• Can see opposite signed
changes in Canada and
Eastern U.S.
Warm Spell Duration (WSDi)
•
•
•
Difference between 1980-2006
and 1860-1890 means
Similarly, no discernible changes in
the pdf between time periods
Small changes in WSDi outside of
Florida.
Although mid-Atlantic and midwest
regions saw decreases in TX90
similar to Florida, they do not
translate to equal decreases in
WSDi.
Number of wet days (R1mm)
Difference between 1980-2006
and 1860-1890 means
• Large variance and significant
deviations on decadal time
scales.
• Over the eastern US, there are
general decreases in the
number of wet days.
• Eastern Canada shows an
increase in the number of wet
days.
Very heavy precipitation days
(R20mm)
Difference between 1980-2006
and 1860-1890 means
• The northeast US shows
decreases in the number of
extreme precipitation events.
• Eastern Canada shows a slight
increase in the number of extreme
precipitation events, although the
magnitude of the changes are
generally small.
Next Steps
• Generally, we do not see any significant changes in
the distributions of the climate indices examined
here. However, results thus far suggest that we may
have better luck studying smaller sub-regions. It is
also possible that there may be cancellations
between the effects of aerosols and greenhouse
gases.
• In order to understand the role of aerosols in affecting
these climate indices, we will examine GFDL-CM3’s
aerosol only runs, aerosol and ozone only runs,
greenhouse gas only runs, and control simulations for
the preindustrial and 1990.
• Finally, we will expand our analysis to include other
CMIP5 models that have performed aerosol only
simulations.
Back up Material
Heavy precipitation days (R10mm)
Difference between 1980-2006
and 1860-1890 means
• Similar to R1mm and
R20mm, we see large
decreases in the heavy rain
dains over the Northeast,
mid-Atlantic, and Midwest
US.
• Heavy rain days increase
over Eastern Canada.
Consecutive Dry Days (CDD)
•
•
Difference between 1980-2006
and 1860-1890 means
Slight decreases in CDD over
Canada and southeast US.
The magnitude of the anomalies
are generally less than a day, so
it is unlikely that they are
statistically significant.
Global trends in climate extremes
Source: Sillman et al, 2013
Global trends in climate extremes
•
Source: Sillman et al, 2013
Time series of the (a, b)
global spatial mean and (c, d)
spatial median over all land
grid points of consecutive dry
days (CDD) from 1948 to
2005 of the ensemble mean
(solid) and median (dashed)
of 31 CMIP5 (black) and 18
CMIP3 (green) models as
well as the reanalysis ERA40
(blue) from 1958 to 2001,
ERA interim (cyan) and
NCEP2 (orange) from 1979
to 2010, and NCEP1 (red)
from 1948 to 2005.