Detection and attribution of temperature change in the lower stratosphere

Nathan Gillett, Ben SanterNathan Gillett, Ben Santer

Model data

Will start by using IPCC 20Will start by using IPCC 20thth century simulations century simulations with at least ozone depletion and volcanic aerosol.with at least ozone depletion and volcanic aerosol.

Ben Santer has calculated T4 temperatures from Ben Santer has calculated T4 temperatures from all forced runs of these models, and ensembles of all forced runs of these models, and ensembles of the PCM with multiple forcings.the PCM with multiple forcings.

Model and observational data both available for Model and observational data both available for the period 1980—1999.the period 1980—1999.

Tem

pera

ture

ano

mal

y (K

)

Global mean T4 MSU4

temperatures in 20th century simulations of IPCC AR4 models with at least greenhouse gases, volcanic aerosols and ozone.

80°S - 80°N.

Power spectra of global mean T4

Variability Variability in global in global mean T4 mean T4 realistically realistically simulated in simulated in all-forced all-forced runs.runs.

Variability Variability underestimaunderestimated in ted in control.control.

Most Most variability variability at >1yr is at >1yr is forced in forced in model.model.

T4 trend patterns – 1980-1999RSS UAH

IPCC AR4 models

Trends in K/year

Detection and attribution

Detection and attribution techniques allow us to Detection and attribution techniques allow us to objectively test model-observations consistency, objectively test model-observations consistency, and test for evidence of the response to particular and test for evidence of the response to particular forcings in the observations.forcings in the observations.

We need simulations with individual forcings in We need simulations with individual forcings in order to attribute to separate forcings.order to attribute to separate forcings.

Method applied here using PCM data (model top Method applied here using PCM data (model top ~47km, 7 stratospheric levels), but could be ~47km, 7 stratospheric levels), but could be applied using other models.applied using other models.

Calculation of attributable trends

Data are filtered to retain large-scale, long-Data are filtered to retain large-scale, long-timescale variability. I used 2-yr means of 25 T4 timescale variability. I used 2-yr means of 25 T4 spherical harmonics.spherical harmonics.

Observations are regressed onto model response Observations are regressed onto model response patterns for each forcing in a multiple regression.patterns for each forcing in a multiple regression.

Regression coefficients and uncertainties are used Regression coefficients and uncertainties are used to scale cooling/warming due to each forcing.to scale cooling/warming due to each forcing.

T4 timeseries in PCM

Temperature Anomaly (K)

RSS

GHGs

Ozone

ALL

Detection results

Regression coefficients for UAH T4 temperatures, 1980-1999

0

1

2

3

4

5

6

0.5

Re

gre

ss

ion

co

eff

icie

nts

Ozone Volcanos GHGs+sulphate+solar

Regression coefficients for RSS T4 temperatures, 1980-1999

0

1

2

3

4

5

6

0.5

Re

gre

ss

ion

co

eff

icie

nts

Ozone Volcanos GHGs+sulphate+solar

Attributable temperature trends in T4

Attributable trends in RSS T4 temperatures, 1980-1999

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

-0.5

Att

rib

uta

ble

tre

nd

(K

/de

ca

de

)

Ozone Volcanos GHGs+sulphate+solarRSS

Attributable trends in UAH T4 temperatures, 1980-1999

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

-0.5

Att

rib

uta

ble

tre

nd

(K

/dec

ade)

Ozone Volcanos GHGs+sulphate+solarUAH