# Presentation: 920058

## Constraining uncertainties in observed methane inter-annual variability: 

**Constraining uncertainties in observed methane inter-annual variability: **
**A proposal for multi-model **
**decadal hindcast simulations**

- TF HTAP Workshop, Washington, DC, June 11, 2008

**Arlene Fiore/Frank Dentener/**
**Peter Hess/Isabelle Bey**
**And**
**???**

## Estimates of current CH4 emissions 

***c/o Michael Raupach, CSIRO, Australia; studies cited in IPCC TAR, AR-4***

**Estimates of current CH****4**** emissions**

**Methane sink (OH) also ~30% uncertain [*****Stevenson et al*****., 2006]**

**Total CH****4**** source ~600 Tg yr****-1****, ~60% anthropogenic [*****IPCC AR-4*****]****>25% uncertainty in present-day CH****4**** sources**

**anthropogenic**

**Plants?**
**Keppler et al., 2006**
**Ferreti et al., 2006**
**Houweling et al., 2006**
**Kirshbaum et al., 2005**

## Why have methane concentrations leveled off?How well do we understand inter-annual variability?

- Major driver for 97-98 anomaly?
- -- Biomass burning (CH4 emission,
- OH suppression via CO)
- [e.g.Butler et al., 2005; Duncan et al., 2003;
- Bousquet et al., 2006;Langenfelds et al., 2002]
- -- Wetlands
- [e.g. Dlugokencky et al. 2001;
- Cunnold et al., 2002; Wang et al., 2004,
- Mikaloff Fletcher 2004, Chen and Prinn, 2006]

- CH4 (ppb)

- CH4 growth rate (ppb yr-1)

## Model studies indicate different drivers for observed decadal trends

| Study | Period | Approach | Major driver of trends / IAV* |
| --- | --- | --- | --- |
| Law and Nisbet, 1996 | 1980-1994 | 2D CTM | 1991 on FSU emis decline |
| Bekki & Law, 1997 | 1980-1992 | 2D CTM | wetlands & OH  |
| Dlugokencky et al, 2003 | 1984-2002 | Obs. analysis | approach to steady state? |
| Karlsdottir and Isaksen, 2000 | 1980-1996 | 3D CTM (met fixed) | OH (+anthrop SE Asian emis, -strat. o3) |
| Johnson et al, 2002 | 20 years | 3D CTM (emis fixed) | OH (water vapor) |
| Warwick et al, 2002 | 1980-1998  | 3D CTM (OH fixed) | transport  |
| Dentener et al, 2003ab | 1979-1993 | semi-inverse 3D CTM | OH (mainly water vapor) |
| Wang et al, 2004 | 1988-1997 | 3D CTM  | anthrop emis & OH (-strat O3) |
| Dalsoren and Isaksen, 2006 | 1990-2001 | 3D CTM (only emis. vary) | OH (+anthrop. CO, NOx, NMVOC emissions) |
| Fiore et al, 2006 | 1990-2004 | 3D CTM (only met. varies) | OH (lightning NOx) + T |
| Bousquet et al, 2006 | 1984-2003 | inverse 3D CTM 
(OH from CH3Cl inversion) | anthrop emis; post-1998 +anthrop - wetlands |
| Khalil et al, 2007 | 23 years | Obs. analysis | constant emis. and lifetime |
| Drevet et al., 2008 | 1990-2005 | 3D CTM | Anthrop. emis + OH |

- * *Many of these studies also identify a large role for wetlands and BB on IAV*

**Notes:**
Bold = “most complete” in terms of considering sources of OH/CH4 emission variability; Add work from Isabelle’s group (also bold)?  Italics = studies focused on IAV rather than trends
Note that many forward modeling studies suggest that OH may have increased (slightly) whereas CH3Cl inversion suggest constant or slight decrease in recent years.

## Atmospheric Chemistry and Climate (AC&C): Effort focused on representation of chemistry-climate interactions in earth system models

***Objective***: Use observations of trends and variability in atmospheric composition over
- recent decades to test models used to project future atmospheric chemistry and climate
- and to derive and quantify objective measures of uncertainty.
***Each hindcast experiment defined by***:
- -- a multi-year series (post-1980) of measurements of atmospheric trace species.
- -- a clear objective grading criteria for evaluating model success.
- -- a set of required diagnostics to facilitate model comparison and evaluation.
- -- multi-year external forcings (e.g., emissions) needed to drive the simulations.
- -- guidelines on the types of chemical models and meteorological fields that can usefully participate.

**Activity #1: CHEMICAL HINDCASTS**

##  Simple tracers (CFCs and N2O)
 Aerosols
 Ozone Variability (including simulations of OH)
 Methane Variability.

** **Aerosols
*** ******Ozone Variability (including simulations of OH)***
*** ******Methane Variability.***

***Chemical Hindcasts Proposed*****:**

## Drevet, Bey, 2008

## Joint AC&C and TRANSCOM Activity to examine methane variability?

**Can we begin to reconcile conclusions **
**from top-down and bottom-up approaches?**

**Forward model hindcast, including sources of OH variability: 1980-present**
- -- ozone (and aerosol) precursor emissions (anth., biomass burning, biogenic, lightning)
- -- methane specified to observed values (lower boundary or fixed abundances)
- -- stratospheric ozone columns
- -- meteorology (reanalysis and/or GCM driven by observed SSTs)

**Forward model output available for input to methane inversions:**
- -- Interannually varying monthly mean OH fields (ensemble mean; uncertainty)
- -- Archived methane loss by tropospheric OH (also to stratosphere / soils)
- = implied global emissions, may help with a priori error estimates

**Multi-year inversion modeling for methane:**
- -- Interannually varying monthly mean OH fields (forward model ensemble mean)
- -- Constant OH (multi-year forward model ensemble mean?)
- -- Coordinate with HYMN (Hydrogen, Methane and Nitrous oxide: Trend variability, budgets and interactions with the biosphere)

## Possible Time Line

- 2008: define activity as a community, gauge participant interest
- 	       Initial feedback from Transcom community (last week)
- 		- working document available (frank.dentener@jrc.it)
- 2009: prepare input datasets for forward model hindcast simulations; begin forward model simulations; assemble a priori methane emissions, observations for inversions
- 2010: forward model simulations complete, prepare archived OH fields + methane loss rates for inversions; begin inversion studies; analysis of forward model results for OH/CH4
- 2011: complete inverse studies + analysis