Impacts of Aerosols on Climate Extremes in the USA

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Impacts of Aerosols on Climate Extremes in the USA

• Nora Mascioli

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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.

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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)
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Aerosol Indirect Effects

• 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)

Source IPCC AR4, Ch. 2

• It also theoretically increases the lifetime of
  the cloud by suppressing precipitation (cloud
  lifetime effect).

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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.

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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
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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.
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Maximum daily maximum temperature (TXx)

• 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

Difference between 1980-2006 and 1860-1890 means
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Maximum daily maximum temperature (TXx)

• Left hand plot shows the difference between the
  1980-2006 and 1950-1980 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 1950-1980.

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Warm Days (TX90)

• PDFs do not show any clear differences between
  time periods.
• Can see opposite signed changes in Canada and
  Eastern U.S.

Difference between 1980-2006 and 1860-1890 means
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Warm Spell Duration (WSDi)

• 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.

Difference between 1980-2006 and 1860-1890 means
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Number of wet days (R1mm)

• 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.

Difference between 1980-2006 and 1860-1890 means
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Very heavy precipitation days (R20mm)

• 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.

Difference between 1980-2006 and 1860-1890 means
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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-CM3s 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.

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Back up Material
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Heavy precipitation days (R10mm)

• 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.

Difference between 1980-2006 and 1860-1890 means
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Consecutive Dry Days (CDD)

• 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.

Difference between 1980-2006 and 1860-1890 means
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Global trends in climate extremes
Source Sillman et al, 2013
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Global trends in climate extremes

• 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.

Source Sillman et al, 2013