Attribution of Stratospheric Temperature Trends to Forcings

Attribution of Stratospheric Temperature Trends to Forcings

A coupled chemistry-climate model (CCM) study

Richard S. Stolarski

NASA GSFC

In collaboration with Steven Pawson, Anne Douglass, Paul Newman, Mark Schoeberl and Eric Nielsen

SPARC Temperature Trends Meeting, Washington DC, April 12-13, 2007

Difference in Temperature between Past and Future Simulations

Past simulation used observed SSTs (1950-2005). Future simulation used model-generated SSTs (1996-2100). Overlap period was used to adjust future temperatures to make a consistent time series.

Contours indicate the latitude and altitude dependence of the adjustments. All temperatures used in this analysis are annual means.

Model Simulated Temperature Change 1979-1998 (K/decade)

Upper Stratospheric Cooling by Radiation to Space

Ozone HoleCooling

Dynamic Response to Ozone Hole

TroposphericWarming

Can we quantitatively separate the contributions of ozone change and greenhouse gases in our simulations?

Fitting Model Temperature Time Series to EESC, CO2, and CH4 Terms:

Midlatitude Upper Stratosphere

We will fit this function with 4 terms:

Mean + a1•EESC + a2 •CO2 + a3 • CH4

Thin line: model output

Thick line: Fit with All Terms

CO2 Term (+mean)

EESC Term (+mean)

CH4 Term (+mean)

Fitting Model Temperature Time Series to EESC, CO2, and CH4 Terms:

Midlatitude Upper Stratosphere

Fit uses entire 140-year simulation time series

Fitting Model Temperature Time Series to EESC, CO2, and CH4 Terms:

Midlatitude Upper StratosphereThin line:

model output

Thick line: Fit with All Terms

CO2 Term (+mean)

EESC Term (+mean)

CH4 Term (+mean)

Change from 1979-1998

-1.9K EESC

-0.7K CO2

-0.4K CH4

-3.0K Total

Thin line: model output

Thick line: Fit with All Terms

CO2 Term (+mean)

EESC Term (+mean)

CH4 Term (+mean)

Change from 1979-1998

-1.9K EESC

-0.7K CO2

-0.4K CH4

-3.0K Total

Change from 2006-2025

+0.7K EESC

-1.3K CO2

-0.6K CH4

-1.2K Total

Fitting Model Temperature Time Series to EESC, CO2, and CH4 Terms:

Midlatitude Upper Stratosphere

Relative Contribution of EESC, CO2, and CH4 to Temperature

Change at 1 hPa 40oN

1979-1998 2006-2025

Term Sens (K/ppbv) MR (ppbv) T (K) MR (ppbv) T (K)

EESC -9.2±0.3x10-1 +2.1x100 -1.9 -7.6x10-1 +0.7

CO2 -2.4±0.1x10-5 +2.9x104 -0.7 +5.5x104 -1.3

CH4 -2.0±0.2x10-3 +1.9x102 -0.4 +2.9x102 -0.6

Total -3.0 -1.2

How long must the record be to separate effects by

time-series analysis?

Effect of Length of Record on Fitting: Graphical Illustration: 40oN, 1hPa

Effect of Length of Record on Fitting: Graphical Illustration: 40oN, 1hPa

Continued

Sensitivity to EESC and CO2 as a Function of Endpoint of Output

40oN 1hPa

These are trends from 1979 through 1998 calculated from output from 1979 through various end years. Thin lines in left panels are 2.

Some Other Locations in the

Stratosphere

Northern Mid Latitude Lower Stratosphere

1979-1998 2006-2025

Term Sens (K/ppbv) MR (ppbv) T (K) MR (ppbv) T (K)

EESC -1.3±0.4x10-1 +2.1x100 -0.3 -7.6x10-1 +0.1

CO2 -3.0±0.4x10-6 +2.9x104 -0.09 +5.5x104 -0.16

CH4 -6.5±1.5x10-4 +1.9x102 -0.13 +2.9x102 -0.19

Total -0.5 -0.25

Methane term includes effects of increased HOx

60% EESC

Sensitivity to EESC and CO2 as a Function of Endpoint of Output

40oN 50hPa Statistically significant after 2020, but uncertainty never gets less than 50%

Fitting Model Temperature Time Series to EESC, CO2, and CH4 Terms: Antarctic Lower Stratosphere

Thin line: model output

Thick line: Fit with All Terms

CO2 Term (+mean)

EESC Term (+mean)

CH4 Term (+mean)

Antarctic lower stratospheric temperature is dominated by ozone hole from ~1960 through 2100. Greenhouse gas terms are minor.

Fitting Model Temperature Time Series to EESC, CO2, and CH4 Terms: Antarctic Upper Stratosphere

Antarctic upper stratospheric temperature increases due to dynamic response to ozone hole through 2000. Thereafter, temperature decreases as ozone hole recovers and GHGs continue to cause decrease.

Map of EESC Term Using Output from 1979 to:

2010

2025 2040

2015 2020

2060

Color Regions are Statistically Significant

Greenhouse Gas (CO2) Term Using Output from 1979 to:

2010

2025 2040

2015 2020

2060

Some Tentative Conclusions

• Simulations indicate about 2/3 of annual temperature trend of upper stratosphere due to ozone decrease

• Simulations indicate about 60% of annual temperature trend of lower mid latitude stratosphere due to ozone decrease

• Upper stratospheric ozone effect should be able to be separated from greenhouse gas effect with present data: a few more years are needed to reduce uncertainties (of course this assumes a lot about the simulation’s representation of variability and its lack of QBO and solar cycle).