Task 3.1 Perturbations of climate change agents
 

Objectives

The objective of this task is

• to define sets of perturbations of climate change agents

Methodology and scientific achievements related to Task including contribution from partners
This task requires the definition of General Circulation Model (GCM) experiments that are performed by the three METRIC groups running GCMs (i.e. DLR, LMD and UREAD). All model calculations use mixed-layer oceans run to equilibrium with the forcing.

The purpose of these experiments is to test the validity of radiative forcing as a concept by comparing results for identical calculations in the three models. The work was envisaged to progress in two phases:
A. Use highly idealised forcings that test the dependence of the relationship between radiative forcing and climate response, by for example, varying the latitudinal distribution of forcings or the relative importance of the solar or thermal infrared radiation streams in contributing to that forcing. The philosophy broadly follows that adopted by Forster et al. (2000)1.
B. Using results from Work Packages 4 and 5, compare the forcing-response relationship for more realistic changes in constituents.

This task is progressing to schedule. Three sets of idealised experiments were defined (which, including a control run, amount to 13 individual GCM integrations per group) and circulated by email between February 2000 and March 2001. All GCM experiments were run with a global and annual averaged radiative forcing of 1 Wm-2, as determined using the radiation scheme in each GCM, and using the fixed dynamic heating parameterisation to determine the stratospheric temperature changes. Common output was requested to allow easier intercomparison.
1. Changes in carbon dioxide. 4 experiments were requested from each group: (a) CO2 changed at all latitudes, (b) CO2 changed in the tropics (latitudes less than 30 degrees), (c) CO2 changed in the extra-tropics (latitudes greater than 30 degrees), and (d) CO2 changed in northern extra-tropics only.
2. Changes in solar constant. 3 experiments, following 1(a) to 1(c) above.
3. Changes in ozone. The experiments follow those reported by Stuber et al. (2001a, 2001b) which indicated significant departures from a constant forcing-response relationship. Five experiments were requested: (a) changes in lower stratospheric ozone, globally on fixed pressure levels, (b) changes in upper tropospheric ozone globally, following local tropopause, (c) as (b) but tropical ozone change only, (d) as (b) but extra-tropial ozone change only, (e) as (b) but northern extra-tropical ozone change only.
 

Socio-economic relevance and policy implication

No direct socio-economic or policy implications are related to this work package.
 

Discussion and conclusion

The definition was the idealised experiments was quite straight forward. The well defined simulations will allow for a better intercomparison.
 

Plan and objectives for next period

The definition of experiments for the more realistic calculations will be done early in year 2 as anticipated.
 
 
 

Task 3.2 Simulations for idealised forcing perturbations
 

Objectives

The objective of Task 3.2 is

• to perform GCM simulations for idealised perturbations designed in Task 3.1.

The experiments designed under Task 3.1 are being performed by the three groups running GCMs: DLR, LMD (CNRS) and UREAD. The inter-model comparison is performed mainly at UREAD.

The current status of this task is that it is on schedule. The carbon dioxide experiments have been completed and the initial analysis completed; the solar constant experiments are largely completed and analysis has started; and the ozone perturbation calculations have now been initiated and should be complete by the summer of 2001.

The basic scientific point being addressed by this task is the extent to which the nature of the radiative forcing impacts on the climate response. Importantly, we wish to establish whether conclusions drawn from one model are supported by the other GCMs. The basic conceptual framework is as follows: The global-mean radiative forcing RF, can be related to the global-mean surface temperature response dT, by dT= lambda·RF, where 'lambda' is a climate sensitivity parameter. It is well known that there is much intermodel difference between the absolute value of 'lambda' amongst climate models, ranging from around 0.4 to 1.1 K/W/m². Most of this uncertainty is due to problems in modelling cloud feedbacks, and as reported by IPCC (2001) there has been little progress in reducing this uncertainty in recent decades; indeed, this is one reason for using radiative forcing, rather than surface temperature response, as a metric. For a wide range of forcings, GCMs indicate that the value of 'lambda' is approximately independent of the nature of the forcing, but there is concern over the generality of this result. The particular question we address is whether the departure of the value of 'lambda' from its value for a globally homogeneous forcing is similar for all three GCMs.
 

Table 1: Overview of the CO2 experiment results from the tree GCMs (DLR, LMD and UREAD). The colums indicate the particular experiment (i.e. where the CO2 is perturbed). For each GCM, the first row indicates the  CO2 (in ppmv) required to achieve a 1 W/m² forcing for the relevant region. The second row gives the global mean surface temperature change (in K)  and the third row gives the ration of the global-mean surface temperature change with respect to the global CO2 change (i.e. row 2)
 

Table 1 indicates our initial results. Concentrating first on the column labelled "CO2 global", this shows the climate sensitivity for a global change in carbon dioxide. By coincidence, the range of climate sensitivity parameters ? amongst the three different GCMs spans the range from 0.38 to 1.1 K/Wm-2, which is found in the wider community. This is of importance; we will be able to establish whether the dependence of ? on the nature of the forcing depends on the model's own absolute value of 'lambda'.

All three models show a broadly similar response. The row labelled dT/dT* shows the ratio of the global mean surface temperature response for the particular case relative to the global case. All three models show that a 1 W/m² forcing concentrated in the tropics causes a smaller response than the global mean forcing. The LMD model is rather more sensitive to this than the DLR and UREAD results which are in good agreement.

As anticipated, the extra-tropical runs show enhanced sensitivity, again with good agreement between DLR and UREAD, although this agreement is diminished for the perturbation solely in the northern hemisphere.

Figure 3.1 shows the zonal mean surface temperature response. This shows the degree to which the location of the forcing influences the response. This is most marked in the extra-tropical NH case where most of the response is also in the northern hemisphere. It can be seen in all cases that the response of the LMD model at around 60 S is distinct from DLR and UREAD and is believed to be related to the form of the sea-ice parameterisation in the LMD model.

Figure 3.1: Zonal mean surface temperature change (in K) for the four CO2 experiments using the three GCMs (DLR, LMD, and UREAD).
Run CG = Global CO2 (Run 2), Run CT = CO2 where latitude < 30° (Run 3),
Run CE = CO2 where latitude > 30° (Run 4), Run CN = CO2 where latitude > 30°N (Run 5)
 

Socio-economic relevance and policy implication

No direct socio-economic or policy implications are related to this work package.
 

Discussion and conclusion

The overall conclusions of the work so far under this task is that there is encouraging agreement between the models on the degree to which the global (and hemispheric) response depends on the geographical distribution of the forcing. If this result were confirmed by the other experiments, it would indicate that it may be possible to improve the concept of radiative forcing and global warming potential  (GWP). It might be possible to improve the precision with which forcings and GWPs can be compared by taking into account the nature of the individual forcing, by, for example, applying a simple weighting which accounts for this weighting.

Another issue, less significant for the purposes of this task, which is identified here, is the differences amongst the radiation codes. Our experimental design was such that each model applied a 1 Wm-2 global mean forcing, with each group establishing the change in constituent necessary to achieve this in their model. It can be seen that, particularly for the extratropical NH case, significantly different CO2 perturbations are required.
 

Plan and objectives for next period

Further perturbations as designed in Task 3.1 will be studied.
 
 
 

Task 3.3 Simulations for realistic forcings
 

This Task has not yet been started.
 

Plan and objectives for next period

We plan to perform GCM simulations for realistic perturbations of radiative forcing agents.
 
 
 
 
 
 

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