Metrics for quantification of influence on climate Ayite-Lo Ajovan, Paul Newman, John Pyle, A.R. Ravishankara Co-Chairs, Science Assessment Panel July 14, 2014Climate Metrics, OEWG Paris  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. July 14, 2014Climate Metrics, OEWG Paris Longer  slower recovery Shorter  faster recovery Time stop emission No loss, No recovery Infinitely long lifetime Atmospheric concentration Production determines how much accumulates. The lifetime determines how rapidly the atmosphere is “cleansed.” Atmospheric lifetime is also a metric

What is Greenhouse Effect? Earth Atmosphere  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 CO 2 increases. July 14, 2014Climate Metrics, OEWG Paris 20143

What is Greenhouse Effect? CO 2 O3O3 July 14, 2014Climate Metrics, OEWG Paris E in E out  Clearly see absorption by CO 2 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 Wavelength Energy going out (proper units) Energy distribution governed by fundamental laws of physics

Radiative Forcing Earth Atmosphere Radiative Forcing is a metric July 14, 2014Climate Metrics, OEWG Paris 20145

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… Atmosphere Earth July 14, 2014Climate Metrics, OEWG Paris 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- CO 2 : 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 CO 2 -eq emission = emissions (e.g., in metric tonnes) x GWP (100) July 14, 2014Climate Metrics, OEWG Paris Area CO 2 Years since emission Radiative Forcing Area A

Global Warming Potentials (GWP)  Estimate of “time-integrated” radiative forcing for a species relative to that of the same mass of CO 2  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 CO 2.  GWPs do not lead to equivalence with the temporal evolution of the temperature response or that of other climate variables. July 14, 2014Climate Metrics, OEWG Paris 20148

9 Blue = CO 2 Red = gas w/ ~15 yr lifetime Years after emission Temperature Change t1t1 t2t2  T(CO2)  T(X) Global Temperature Potential (GTP) July 14, 2014Climate Metrics, OEWG Paris 2014 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 CO 2 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. July 14, 2014Climate Metrics, OEWG Paris

GTP vs. GWP o Similar trends o GTP ≈ GWP when lifetime roughly equal to, or greater than, 100 yrs. o GTP< GWP for shorter -lived substances July 14, 2014Climate Metrics, OEWG Paris

July 14, 2014Climate Metrics, OEWG Paris 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.

July 14, 2014Climate Metrics, OEWG Paris THANK YOU FOR YOUR ATTENTION

July 14, 2014Climate Metrics, OEWG Paris 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, 2014Climate Metrics, OEWG Paris