Metrics for quantification of influence on climate

Ayite-Lo Ajovan, Paul Newman,

John Pyle, A.R. Ravishankara

Co-Chairs, Science Assessment Panel

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Described in detail in the 2014 SAP report

Will be available at the end of this year

Simple parameters (a form of common currency) for

comparing atmospheric gases for their climate effects

Atmospheric lifetime: Concept

Assume that L is proportional to n.

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Longer slower recovery

Shorter faster recovery

Time

stopemission

No loss,No recovery

Infinitely long lifetime

Atm

osph

eric

con

cent

rati

on

• Production determines how much accumulates.

• The lifetime determines how rapidly the atmosphere is “cleansed.”

Atmospheric lifetime is also a metric

What is Greenhouse Effect?

EarthAtmosphere

Greenhouse Gas (GHG) absorbs Infra Red (IR) radiation; heats surface (& lower atm)

Changes in GHG abundance change the energy balance - hence climate. - How much? When?

Arrhenius predicted (over 100 years ago) that temperature would increase if CO2 increases.

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What is Greenhouse Effect?

CO2

O3

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EinEout

Clearly see absorption by CO2 and other greenhouse gases Greenhouse effect is REAL! You can see it in the spectrum

(Otherwise, earth would be a frozen planet!). Adding gases traps more energy, but it takes time for the entire

Earth Land-Atmosphere-Ocean system to warm. Eventually the system comes to equilibrium

IR WavelengthE

nerg

y go

ing

out

(pro

per

unit

s)

Energy distribution governed by fundamental laws of physics

Radiative Forcing

Earth

Atmosphere

Radiative Forcing is a metric

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Radiative Forcing

Definition: Change in net irradiance at the tropopause (top of the radiative atm.) subsequent to a perturbation after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with surface and tropospheric temperatures and state held fixed at the unperturbed values

RF (wm-2) is a direct measure of the extent to which the Earth’s energy budget is out of balance to stray away from its “normal” state. (Balance between incoming solar radiation and outgoing infrared radiation)

RF is not measured for GHGs - it is calculated using input from basic lab measurements of the gases.

Why use this?: T = x RF ; Positive RF – heating; Negative RF– cooling; a good measure of relative GHG contributions. Now extended to aerosols…

AtmosphereEarth

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• Instantaneous change in the outgoing flux• A measure of what has happened… a look

back• Does not account for time dependence of

emission

Global warming potential (GWP)Red: 15 yr. Blue- CO2: 5-200 yr. “lifetime” (from C- cycle modeling)… CO2 “lifetime” is comlex

o 100 year GWP has become the coin for policy makers

o The time horizon is a policy choice – not a scientific choice

CO2-eq emission = emissions (e.g., in metric tonnes) x GWP (100)

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Area CO2

Years since emission

Rad

iati

ve F

orci

ng

10020

Area A

Global Warming Potentials (GWP)

Estimate of “time-integrated” radiative forcing for a species

relative to that of the same mass of CO2

Developed for the first IPCC assessment (Derwent, Rodhe, and

Wuebbles, 1990)--- very similar to ODP!

Extensively used in national and international policy

GWPs are a relative measure of the total energy added to the

climate system by a component in question relative to that added

by CO2.

GWPs do not lead to equivalence with the temporal evolution of

the temperature response or that of other climate variables.

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9

Blue = CO2

Red = gas w/ ~15 yr lifetime

Years after emission

Tem

pera

ture

Cha

nge

t1 t2

T(CO2)

T(X)

Global Temperature Potential (GTP)

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Rate of decrease determined by property of Earth systmem

Global Temperature change Potentials

A “temperature outcome” metric and does not give other climate responses.

An estimate of the relative temperature increase on a per unit mass of emissions basis due to emissions of a greenhouse gas relative to that due to CO2 emissions for the chosen time horizon (Shine et al., 2005)

First included in IPCC AR5. Accounts for climate sensitivity and the exchange of heat

between the atmosphere and the ocean, GTPs include physical processes that GWPs do not.

GTPs also incorporate extra uncertainties (compared to GWPs) by including the climate response in the analysis.

GTPs are somewhat sensitive to the specific climate model used to calculate them and also to the background scenario.

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GTP vs. GWP

o Similar trendso GTP ≈ GWP when lifetime roughly equal to, or greater than, 100 yrs.o GTP< GWP for shorter -lived substances

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Summary

1. There are a number of metrics for measuring climate change due to emissions of a substance

2. Most common to date is Global Warming Potential (GWP)

3. The GWP time horizon is NOT determined by science; 100 year is the most commonly used time horizon.

4. There is rough correspondence between GWP and GTP. Higher GWP implies a higher GTP.

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THANK YOU FOR

YOUR ATTENTION

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Extra slide

Factors that influence energy & hence climate

Ocean (heat storage/circulation)

Earth’s orbit around the sun

solar intensity

Land processes and Ice-cover

Atmospheric Processes

Timescales of changes that are of current interest….

Decadal to century timescales…July 14, 2014 Climate Metrics, OEWG Paris 2014 15