Atmospheric Composition and Climate
An Introduction
by Apostolos Voulgarakis
PG Lectures, 9-10th of December 2012
The (complex) composition-climate system:
-> CLIMATE
GLOBAL & REGIONAL
Source: US Climate Change Science Program.
3μ
Greenhouse gases and climate
J. Fourier
J. Tyndall
S. Arrhenius
“If the quantity of carbonic acid (CO2) increases in geometric progression, the
augmentation of the temperature will increase nearly in arithmetic progression.”
(more than a century later…)
Still in use!!
IPCC 2001
Also see: http://www.esrl.noaa.gov/gmd/aggi/
Radiative forcing (RF)
 IPCC (2007): “The change in net irradiance (solar plus
longwave; in W m–2) at the tropopause after allowing for
stratospheric temperatures to readjust to radiative equilibrium,
but with surface and tropospheric temperatures and state held
fixed at the unperturbed values”.
 ΔT = λ*RF (ΔT=global temperature change, λ=climate
sensitivity parameter).
 RF is preferred, as more straightforward than ΔΤ.
IPCC 2013
However..
Global radiative forcing is not always useful, as:
 …temperature response depends on a variety of uncertain feedbacks,
and is highly region-dependent.
 …many forcing agents, such as aerosols and tropospheric ozone
(short-lived) are very inhomogeneous, leading to complex patterns of
forcing and response.
 …a global view of composition and radiation from satellites and from
composition-climate models (both recent developments!) can facilitate
the study of such problems.
NASA Discover supercomputer
NASA Aura satellite
Composition-climate models
• 3-dimensional gridded
atmosphere, often coupled with 3-d
ocean.
• Atmospheric chemistry and
aerosols “sitting on top” of a climate
model.
• Everything as interactive as
possible.
• For each constituent and for each
gridpoint, a continuity equatuion is
solved:
http://www.iac.ethz.ch/groups/knutti/research/index
Change in
number density
Flux divergence Production
Loss
The Stratosphere
The Ozone (O3) Hole
Predicted it (early ’70s)
P. Crutzen, S. Rowland, M. Molina
CFCs
Observed it (mid ’80s)
Cl + O3 → ClO + O2
ClO + O3 → Cl + 2 O
J. Farman
Perfected (almost!) the
theory (late ’80s)
More info:
http://www.atm.ch.cam.ac.uk/tour/
S. Solomon
The Ozone (O3) Hole
P. Crutzen, S. Rowland, M. Molina
“…we have left the Holocene and had entered a
new Epoch—the Anthropocene—because of the
global environmental effects of increased human
population and economic development…”
Stratospheric ozone changes/forcing
• Large depletion after the
1980s.
• Stabilization later.
See: toms.gsfc.nasa.go
http://toms.gsfc.nasa.gov
• Ozone loss causes negative
forcing.
• Particularly large over the
Antarctic.
Cionni et al. (2011), ACP (for IPCC AR5)
Stratospheric ozone effects on trop. circulation
Kang et al. (2011), Science
• Drastic change of future zonal precipitation, due to poleward shift
of extratropical westerly jet.
Stratospheric aerosols
• Large amounts
injected during major
volcanic eruptions,
such as Pinatubo.
• Substantial effect
of background
strat. aerosols as
well (diff between
two green lines).
Solomon et al. (2011), Science
Implications for Geoengineering
McCusker et al. (2011),
J. Climate
Robock et al. (2009), GRL
Pope et al. (2012), Nature CC
The Troposphere
(more complex!)
Gases: Long-lived (CO2 & N2O)
CO2
N 2O
• Both increasing steadily in
recent decades.
• Note: N2O increases are
also anthropogenic
(fertilizers).
• Note 2: CFCs.
IPCC (2007)
Gases: Methane (CH4) – the 2nd most important
Breakdown of its budget:
Recent growth:
Van Weele (2010)
• Anthropogenic and wetland sources
equally important.
• OH loss crucial.
• Growth has slowed down.
(though recovered recently).
IPCC (2007)
Global present-day methane distribution
• Subtle differences between different regions.
• However, still suggestive of where the large emissions are (industrial
areas – especially East Asia – and tropical/extratropical wetlands).
Gases: Tropospheric ozone
A secondary pollutant and a greenhouse gas.
What determines its budget:
Transport
Transport
Tropospheric ozone budget (in numbers)
Stevenson et al. (2006), JGR
Tropospheric ozone forcing
Past
• 1850-2000 forcing is mostly positive,
except for the Antarctic.
• It peaks in the northern subtropics.
• 2000-2100 forcing is large in the
scenario with large methane changes.
Stevenson et al. (2012), ACPD (for IPCC
AR5)
Future (two scenarios)
Shindell et al. (2013), ACP (for IPCC AR5)
Gases: Hydroxyl Radical (OH): The detergent of the
atmosphere
• OH is a major
tropospheric oxidant.
Stratospheric O3
O3 + hν
Surface
reflections
V. Naik
NOx
Strat.
Trop.
• It removes CO/VOCs, is
Aerosols,
Clouds
T
involved in tropospheric
ozone (O3) production,
and in aerosol formation.
O1D + H2O
OH
• It is the major sink of
CO, NMVOCs
CH4 in the atmosphere:
OH determines CH4
lifetime.
Tropospheric OH abundances and future
changes
• Multi-model OH highest
in low latitudes, especially
over polluted regions.
• Changes in the future
mostly negative, due to large
methane increases (sink) in
this drastic scenario
(RCP8.5).
Voulgarakis et al. (2013), ACP (for IPCC AR5)
Future OH and and stratospheric ozone
(in a less drastic scenario; RCP6.0)
• Strat. O3 recovery  less radiation in the troposphere 
slower photolysis (JO1D)  less OH
Voulgarakis et al. (2013), ACP (for IPCC AR5)
Aerosols: major components
• Sulphate (SO4) (both anthropogenic and natural; natural
comes mainly from oceans and volcanoes).
• Black carbon (BC) (mostly anthropogenic; also from natural
fires).
• Organic carbon (both anthropogenic and natural; natural
comes from secondary aerosol formation above forests).
• Mineral dust (mainly natural)
• Sea-salt (natural)
• Nitrate (both anthropogenic and natural)
Optical depth
Optical depth (τ) gives a measure of how opaque a medium is
to radiation passing through it. E.g. aerosol optical depth is
the τ due to aerosol in the medium.
¥
t a = ò r k dz
z
where ρ is the mass density (kg m-3), k is the absorption
coefficient (m2 kg-1), and dz is the vertical path (m). If I0 is the
radiation at the top of the atmosphere, and θ is the zenith
angle, the radiation following aerosol attenuation (I) is (BeerLambert law):
I = I 0 exp(-
ta
cosq
)
More τ terms can be added for gases, or multiple aerosol types.
Aerosols: Present-day models vs satellites (τ)
Shindell et al. (2013), ACP (for IPCC AR5)
Aerosols: Sulphate
• Sulphate particles are produced from gases (through OH
oxidation) in the atmosphere.
• Their main precursors are:
a) anthropogenic or volcanic sulphur dioxide (SO2),
b) dimethyl sulfide (DMS) from biogenic sources, especially
marine plankton.
• Sulphate is mostly scattering (cooling).
Present-day surface sulphate concentration (NASA GISS model)
Aerosols: Black carbon
• Black carbon is emitted in aerosol form (no gas precursors).
• It mainly comes from fossil fuel combustion and biomass
burning.
• BC is mostly absorbing (warming).
Present-day surface anthropogenic (left) and biomass burning (right) BC concentration (NASA GISS model)
Aerosols: Modelled past and future forcing
• Sulphate has caused
significant negative
forcing in the historical
period.
• Black carbon forcing has
been positive.
• Both show a large spread,
and both become smaller in
the future.
Shindell et al. (2012), ACPD (for IPCC AR5)
Shindell et al. (2013), ACP (for IPCC AR5)
Regional temperature sensitivity parameter (β)
Voulgarakis and Shindell (2010), J. Climate
Shindell et al. (2009), Nature Geosci.
Regional temperature sensitivity parameter (β):
Results
(AR4)
• β in 50°S-25°N is better
constrained than global β.
Voulgarakis and Shindell (2010), J. Climate
Precipitation response to regional forcings
• Northern midlatitude black carbon (BC) forcing is more
effective in driving precipitation changes in India/Bangladesh
than tropical BC forcing.
Shindell, Voulgarakis et al. (2012), ACP
Action on the policy side