26 January 2012

Dr. Herron-Thorpe will arrive at 10:00 to have you do the greenhouse gas survey

team assignments for CE 401

any questions from last Tuesday [orbital variations, radiative forcings]

aerosols

HW 4 is posted on the web – due Tuesday 1/31/2012

read MacKay pgs 241-245

HW 3:1a: 0.05°C change in temperature with 1 w/m^2 change in solar irradiance1b: albedo change from 31 to 30% ~ 1°C change in temp1c: eccentric earth orbit ~4°C change in temp

(2) in the figure below, approximately estimate the change in Earth equilibrium temperature (currently 288K) that would occur if CO2 were completely removed from the atmosphere. In the graph below, the large drop in intensity of Earth radiance (an absorbance) near 680 wavenumbers is caused by 392 ppm CO2.

Now I can relate area to temperature. T[K] area[sq]220 310240 423260 583280 783Earth 859300 1062

relationship of area on my piece of graph paper to temperature

Now measure the area under the CO2 absorption: just dot the Earth curve in above the absorbed area and count squares in the absorbed area 76 sq. Subract 76 from 859 = 783 and compute the temperature from the best fit above, or just read it off the curve 783 sq corresponds to a temperature of 280K the change in temperature is about 8°C (288 – 280K).

finish radiative forcing from last time

factors that influence the radiative equilibrium of the Earth system

average solar input: 342 w/m2source IPCC 2007

global warming potential (GWP) of a gas [GWPg]:

a weighting factor to compare the GHG efficiency of a gas relative to CO2. Comparespotency of GHG to that of CO2:

GWPg = Fg x Rg(t) dt / FCO2 x RCO2 dt

where the integral is from time 0 to time TFg = radiative forcing efficiency of the gas in question [w m-2 kg-1]FCO2 = radiative forcing efficiency of CO2 [w m-2 kg-1]

Rg = fraction of the 1 kg of gas remaining in the atmosphere at time tRCO2 = fraction of the 1 kg of CO2 remaining in the atmosphere at time t

radiative forcing efficiency is usually an exponential decay function, or ~ constantwith time, depending on the gas. For CO2 the decay is rapid the first fewdecades as the biosphere absorbs the carbon, then it decays at a muchslower rate corresponding to the slow CO2 uptake of the oceans

Choice of time horizon for GWP depends on what a policy maker is interested in

e.g. CH4 GWP is 62 for 20 yr horizon, 23 for 100 yr, and 7 for 500 yr

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compare CO2 to CH4 and N2O emissions for warming potential:

emissions:CO2 = 27,000 MMt CO2/yr US emissionsCH4 = 370 MMtCH4/yrN2O = 6 MMt N2O/yr

[MMt = million metric tons]

compare impacts to CO2:

CH4: GWP100 = 23*370 = 8510 MMtCO2 equivalentN2O: GWP100 = 296*6 = 1776 MMtCO2 equivalent

aerosols

aerosols and their effects on climate

• big driver – HUGE driver, both human induced and natural RF• not well understood – hardly on the data sheets in 1990• not well measured – satellites and ground networks – short record of data• parameters that affect aerosols and their RF not understood• global effects not well understood – clouds (height and distribution)

increasing complexity of the modeling world

aerosols:• solid particles or liquid particles suspended in the air• size: few nanometers to microns in size: x100 - 1000 variation in size• have major impacts on climate• physical properties: shape, size, chemical composition

• EPA regulates particles in the US PM10 and PM2.5 – CEE 341, 415• key aerosol groups:

• sulfates• organic carbon• black carbon• nitrates• mineral dust• sea salt

• aerosols of clump together to form complex mixtures• 90% of aerosols (by mass) are natural in origin• about 10% of global aerosols are generated by human activity• problems in dealing with aerosol effect: diversity in size, composition and origin;spatial and temporal variability; source; injection height• aerosols removed primarily through cloud processing and wet and dry deposition

aerosol RF effects are categorized into direct and indirect effects:

• direct: mechanism by which aerosols scatter and absorb radiation change in theradiative balance of the Earth system

• organic carbon, sulphate, nitrate, black carbon, dust, biomass burning

• indirect: mechanism by which aerosols modify the microphysical and hence theradiative properties, amount, and lifetimes of clouds

• size, shape, chemical composition, etc.

volcanic pollen sea salt soot

fossil fuel combustion SO2 which reacts with H2O and gases to sulfate aerosols

biomass burning organic carbon and black carbon

transportation sector prolific producer of aerosols

aerosols are usually modeled as spherical in shape – do they look spherical??????

properties: shape, size, composition, chemistry, polarization, index of refraction, mass,

- aerosol optical depth is the fundamental measure of quantity and distribution of aerosols- absorbance is proportional to

exp{-}

where is the optical depth. AOD is a measure of incident light scattered or absorbed.

is prop to path lengthand extinction cross section

2003-2006average AOD

global aerosol distribution. Yellow = coarse particles like dust, red = fine particles like smokeor air pollution.

MODIS data

MODIS 9 Oct 2010

different aerosols scatter or absorb sunlight differently depending on physical prop

black carbon effects - modeled

indirect effects of aerosols: cloud formation and cooling• aerosols play a critical role in cloud formation• natural aerosols are most important• but human produced aerosols have a significant impact

ship tracks – whiteclouds and map ofcloud droplet size where ship exhaustis mixed with cloudlayer, droplets are smaller

measurements of aerosolsfrom satellites and networksof instruments

AERONET

NASA Global Hawk at Edwards AFB, CA

you guys ought to get involved in atmospheric studies – it is one heck of a lot of fun!

outstanding issues in aerosol effects on climate change:

• composition

• optical absorption

• impacts on surface radiation and heating

• long term trends

• total RF