CMIP forcing - GHG concentrations

Summary

Greenhouse gas concentrations are a key input to climate models. To date, these inputs have been made without using earth observations from satellites. This project aims to address this gap by devising a method that allows us to create climate model inputs from our key observation sources: ice cores, ground-based observations (e.g. flasks) and satellite-based measurements. These inputs will be used in future rounds of the world’s most comprehensive climate model comparison project, the Climate Model Intercomparison Project (CMIP). In addition to the new method, the project also aims to identify the steps required to provide regular (e.g. annual) updates of these climate model inputs.

Background

Climate models are a, if not the, key source of information about our climate’s future. Before relying on future simulations, models perform simulations of our past climate. These past climate simulations are a key part of evaluating the extent to which climate models can reproduce our observed climate, which then informs our confidence in climate models’ future projections.

In order to run climate models, we require inputs. These are generally referred to as climate forcings. This project focuses on the atmospheric greenhouse gas concentration inputs/forcings, particularly those for simulation of our past climate. To date, greenhouse gas forcings have been produced without considering earth observations from satellites. This project will address this gap by devising a new method to include earth observations in climate forcings.

Satellite observations are only available for approximately the last thirty years. However, we generally want to simulate past climate over a period of one hundred years or more. Hence, this project will also have to consider how to combine satellite observations with observations from other methods, specifically ground-based observations and ice core samples. The composite product needs to combine these various sources to create our best-estimate of past greenhouse gases, while avoiding processing artefacts such as spurious discontinuities. This is a challenge. This project will learn from other forcings research projects (such as the volcanic forcing for CMIP project) that have developed solutions to the same challenge.

The output product will improve the inputs we use with climate model simulations. We will translate the learning from creating the past climate forcings into our creation of the future climate forcings too, for example insights how the seasonal cycle of different greenhouse gases is changing with climate change.

The project ultimately feeds into the Coupled Model Intercomparison Project (CMIP). CMIP is a key source of information for future climate projections, underpinning multiple IPCC reports and the policy-relevant insights they generate.

Project plan

Work Package 1 evaluates the greenhouse gas forcings which have already been provided to CMIP in work prior to this project. This work ensures that we have a clear understanding of the strengths and weaknesses of our existing approach before we develop the new approach.

Work Package 2 will develop the new methodology that incorporates earth observations from satellites into greenhouse gas concentration forcings. This will aim to maximise the insights from satellite-observations’ unique spatial and temporal characteristics while also avoiding discontinuities between the forcings provided for the pre-satellite and satellite eras.

Work Package 3 - key output is the greenhouse gas forcings for Phase 7 of CMIP’s (CMIP7’s) future forcing work. This work takes place as part of ScenarioMIP (https://doi.org/10.5194/egusphere-2024-3765). These future-focussed simulations are a key policy-relevant of CMIP7 as a result of their specific focus on the interaction between society’s mitigation actions and the climate.

Work Package 4 combines the results of the previous work packages, translating their insights into updated projections of future greenhouse gas forcings. These insights will result from the insights derived from satellite-observations, something which have been missing from greenhouse gas forcing research to date. This could lead to an updated projection of future greenhouse gas concentrations, with accompanying changes in projections of our future climate.

Work Package 5 will ensure that these insights are documented and comprehensible for users of the forcings. Multiple user guides and manuscripts will be produced to provide both user-focussed documentation as well as research papers that pass the rigours of scientific peer-review.

Work Package 6 and 7 are overarching work packages. At present, the forcings are only produced for CMIP phases, which means every 6-7 years. Work Package 6 will develop a roadmap towards regular delivery of these forcings, where regular is currently envisaged to mean every 6-12 months. Work Package 7 is project management, ensuring that the project delivers on time and that communication with ESA is regular, open and clear.