WP2 - THE PREDICTABLE PART OF CLIMATE FOR THE NEXT DECADES

Responsable : Eric Guilyardi, LOCEAN

Objectifs et stratégie

A large component of the recent global warming is now attributed to human activities. Global warming will continue during the next decades at a rate depending primarily on the anthropogenic emissions discussed in the previous section. However, the mechanisms and the respective role of internal variability, of natural or anthropogenic forcings on most aspects of recent climate changes (such as sea-ice decrease in the Arctic or precipitation changes in the Sahel) are currently not established. This lack of understanding limits our ability to predict climate evolution over the next few decades.

For the future, the predictability of regional climate for the next decades to century will primarily depend on: (1) the response to changes in long-lived greenhouse gases; (2) the response to regional changes in aerosols and other short-lived species; (3) the low-frequency modes of natural variability.

The goal of WP2 is to coordinate and develop related research at IPSL, both for observations and for modelling, with a focus on three related key objectives:

• Quantify and understand the internal and natural variability of climate. Understand climate fluctuations (e.g. AMOC, ENSO, …), their dependencies on the mean climate state and their response to external forcings (GHG, sun, aerosols,…). A specific effort will be devoted to the study of the last millennium, a period for which decadal to sub-decadal climate variations may be reconstructed from various natural archives, and for which some estimates of forcings associated with solar variations and volcanic eruptions are available.
• Quantify and understand climate changes due to anthropogenic forcing. By contrast to that of GHG, aerosol radiative forcing is subject to a larger uncertainty and the L-IPSL will seek to understand aerosols radiative forcing change in the future. The primary mechanisms involved in the spread of climate feedbacks, in particular those due to radiative feedbacks will also be explored. In this context, the fate of other climate parameters and phenomena will also be addressed (e.g. tropical precipitation change, AMOC, ice sheets dynamics,…).
• Predict and assess climate changes at decadal time scales. Unravelling the respective contributions of external forcing and internal variability in the recent and future decadal change requires to (i) to identify and assess mechanisms that drive climate variability and trend and (ii) to increase our confidence in climate change projections. A joint development and use of global earth system models and observations will help both the detection and attribution of these decadal signals. The possibility to forecast the predictable part of the next decades will be explored using a combination of historical and initialised simulations. For such timescales, the slow components of climate models (ocean, sea-ice,…) need to be initialized near observations and the L-IPSL will seek to develop new methodologies and select appropriate observations.