24 January 2012WSU classes were cancelled on the 19th, so will have to speed up a bit

any questions from last Tuesday [WSU cancelled classes on Thursday]?

Earth radiation budget, GHG spectra, and aerosols

HWs 2 & 3 are due today

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

solutions to HW 2 and HW 3 will be on the web today – no late papers accepted

quick guess at annual change per year:

[390 – 310] / [2011-1958] = 1.5 ppm/yr which is approximately [1.5ppm/350ppm] *100 = 0.43%/yr

but the question asks you to graph annual change for each year

HW 2 on Mauna Loa data

notice there is an increase in emissionswith time, not subtle – have nearly tripledand the calculations does not have to feretout a subtle feature in the data. It is veryclear and without dispute!

in ppm

• astronomical forces drive global climate change – natural change

• seasons are driven by astronomical causes, as is the 24 h day/night cycle

• not until the second half of 19th century was it accepted that there were indeed ice ages• three main types of evidence that climate has radically changed in the past

• geological• rock scouring• moraines from glaciers• valley cutting• glacial erratics

• chemical• isotope ratios in fossils and ice cores

• paleontological• fossil geographic distributions

• have been at least 5 major ice ages• Earth has been in an interglacial period for

about 11000 yrs – very stable climate• causes of the ice ages are not understood in detail

• atmospheric composition• changes in Earth orbit that are cyclic• motion of tectonic plates changed winds, ocean currents, etc.• changes in solar irradiance

Earth orbital changes that vary the solar input and cause the ice ages: theMilankovich cycles – these cycles change the solar input to the Earth system

shape of earth’s orbit changes during a cycle of about 100,000 years

eccentricity changes

varies from nearly circularto high eccentricity 0.058with mean 0.028.

Caused by perturbationsfrom the other planets

e = 0.017 currently

axis of rotation changes from about 21.5° to 24.5° --> seasonal variations over a period of 41,000 years. Tilt is the most significant cause of seasonal temp change. Modulates theseasons

axial tilt (obliquity) – increased obliquity increased seasonal amplitude change

the earth’s rotation axis precesses (wobbles) with a period of about 26,000 years

due to tidal forces exerted by sun and moon on solid Earth since Earth is not sphericalaffects climate extremes

axial precession – trend in direction of axis of rotation in inertial space – gyroscopic motion

problems with the Milankovitch theory for ice ages:

• 100,000 yr problem: eccentricity variations should have a smaller impact than the other mechanisms, but this is the strongest climate signal in the data record

• 400,000 yr problem: eccentricity variations also show a 400,000 yr cycle but that cycle is only visible in climate records > 1My ago

• observations of climate changes show behavior much more intense than calculated

• the 23,000 yr cycle dominates, the opposite of what is observed

• the “reinforcement of causes” does not seem strong enough to initiate an ice age

• in the past 400k yrs, Milankovitch cycles match too well to ignore

• so the explanation is not 100% - there are still issues with the explanation

radiative forcing

radiative forcing: IPCC

“Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism. In this report radiative forcing values are for changes relative to preindustrial conditions defined at 1750 and are expressed in watts per square meter (W/m2)”

RF can be used to estimate a subsequent change in equilibrium temperature (T) since temperature must be related to change in radiation (linear or ??):

T = F

where F is the radiative forcing (W m-2), and

is called the mean climate sensitivity factor [°C/(W m-2)] and allows computation of impactson temperature of different GHG

different atmospheric models different values of

RF for CO2, = 0.71 °C/W m-2

for a 1°C atmospheric temperature change, F = 1.41 W m-2

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

sources and sinks, top right 1990 and 2000 levels

GHG sources and sinks

details of greenhouse gases

thermal radiation curves (T)

measured thermal Earth radiation over the Mediterranean sea

H2OCH4N2O

O3CO2

H2O

GHG absorbers are indicated with atmospheric transmission “windows”

radiation of the Earth at equilibrium effective temperature of 288K = +15°C

peak of theradiation curveat about 15 µm

this curve is the Planck curvefor a black body at 288K

CO2 as a molecular absorber

H2O as a molecular absorber

O3 as a molecular absorber

CH4 as a molecular absorber