by Apostolos VoulgarakisPG Lectures, 9-10th of December 2012

Atmospheric Composition and ClimateAn Introduction

The (complex) composition-climate system:

Source: US Climate Change Science Program.

-> CLIMATE

GLOBAL & REGIONAL

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.”

IPCC 2001Also see: http://www.esrl.noaa.gov/gmd/aggi/

(more than a century later…)

Still in use!!

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 Aura satelliteNASA Discover supercomputer

Composition-climate models

http://www.iac.ethz.ch/groups/knutti/research/index

• 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:

Change in number density Production

LossFlux divergence

The Stratosphere

The Ozone (O3) Hole

Predicted it (early ’70s)

Observed it (mid ’80s)

Perfected (almost!) the theory (late ’80s)

P. Crutzen, S. Rowland, M. Molina

J. Farman

S. Solomon

Cl + O3 → ClO + O2

ClO + O3 → Cl + 2 O

More info: http://www.atm.ch.cam.ac.uk/tour/

CFCs

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

See: toms.gsfc.nasa.go

Cionni et al. (2011), ACP (for IPCC AR5)

http://toms.gsfc.nasa.gov

• Large depletion after the 1980s.

• Stabilization later.

• Ozone loss causes negative forcing.

• Particularly large over the Antarctic.

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

Solomon et al. (2011), Science

• Large amounts injected during major volcanic eruptions, such as Pinatubo.

• Substantial effect of background strat. aerosols as well (diff between two green lines).

Implications for Geoengineering

Pope et al. (2012), Nature CC

McCusker et al. (2011), J. Climate

Robock et al. (2009), GRL

The Troposphere (more complex!)

Gases: Long-lived (CO2 & N2O)

IPCC (2007)

CO2 N2O

• Both increasing steadily in recent decades.

• Note: N2O increases are also anthropogenic (fertilizers).

• Note 2: CFCs.

Gases: Methane (CH4) – the 2nd most importantBreakdown of its budget:

Recent growth:

Van Weele (2010)

IPCC (2007)

• Anthropogenic and wetland sources equally important.

• OH loss crucial.

• Growth has slowed down. (though recovered recently).

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

Transport Transport

A secondary pollutant and a greenhouse gas. What determines its budget:

Stevenson et al. (2006), JGR

Tropospheric ozone budget (in numbers)

Tropospheric ozone forcing

Shindell et al. (2013), ACP (for IPCC AR5)

Stevenson et al. (2012), ACPD (for IPCC AR5)

Future (two scenarios)

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.

Gases: Hydroxyl Radical (OH): The detergent of the atmosphere

O3 + hν

Strat.Trop.

O1D + H2O OH

Stratospheric O3

TAerosols,

Clouds

NOx CO, NMVOCs

V. Naik

Surface reflections

• OH is a major tropospheric oxidant.

• It removes CO/VOCs, is involved in tropospheric ozone (O3) production, and in aerosol formation.

• It is the major sink of CH4 in the atmosphere: OH determines CH4 lifetime.

Tropospheric OH abundances and future changes

Voulgarakis et al. (2013), ACP (for IPCC AR5)

• 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).

Future OH and and stratospheric ozone(in a less drastic scenario; RCP6.0)

Voulgarakis et al. (2013), ACP (for IPCC AR5)

• Strat. O3 recovery less radiation in the troposphere slower photolysis (JO1D) less OH

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.

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 (Beer-Lambert law):

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

Shindell et al. (2012), ACPD (for IPCC AR5)

• 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. (2013), ACP (for IPCC AR5)

Regional temperature sensitivity parameter (β)

Shindell et al. (2009), Nature Geosci.

Voulgarakis and Shindell (2010), J. Climate

Regional temperature sensitivity parameter (β):Results

Voulgarakis and Shindell (2010), J. Climate

• β in 50°S-25°N is better constrained than global β.

(AR4)

Precipitation response to regional forcings

Shindell, Voulgarakis et al. (2012), ACP

• Northern midlatitude black carbon (BC) forcing is more effective in driving precipitation changes in India/Bangladesh than tropical BC forcing.

Action on the policy side