PAlaeo-Constraints on Monsoon Evolution and Dynamics

Monsoon systems influence the water supply and livelihoods of over half of the world. Observations are too short to provide estimates of monsoon variability on the multi-year timescale relevant to the future or to identify the causes of change on this timescale. The credibility of future projections of monsoon behavior is limited by the large spread in the simulated magnitude of precipitation changes. Past climates provide an opportunity to overcome these problems. This project will use annually-resolved palaeoenvironmental records of climate variability over the past 6000 years from corals, molluscs, speleothems and tree rings, together with global climate-model simulations and high-resolution simulations of the Indian, African, East Asia and South American monsoons, to provide a better understanding of monsoon dynamics and interannual to multidecadal variability (IM). We will use the millennium before the pre-industrial era (850-1850 CE) as the reference climate and compare this with simulations of the mid- Holocene (MH, 6000 years ago) and transient simulations from 6000 year ago to ca 850 CE. We will provide a quantitative and comprehensive assessment of what aspects of monsoon variability are adequately represented by current models, using environmental modelling to simulate the observations. By linking modelling of past climates and future projections, we will assess the credibility of these projections and the likelihood of extreme events at decadal time scales. The project is organized around four themes: (1) the impact of external forcing and extratropical climates on intertropical convergence and the hydrological cycle in the tropics; (2) characterization of IM variability to determine the extent to which the stochastic component is modulated by external forcing or changes in mean climate; (3) the influence of local (vegetation, dust) and remote factors on the duration, intensity and pattern of the Indian, African and South American monsoons; and (4) the identification of palaeo-constraints that can be used to assess the reliability of future monsoon evolution.

Practical Impact on Climate Services. PACMEDY will deliver an assessment of how well state-of-the-art climate models, the models that are used to make projections of near-term climate changes and form the basis of the projections made by climate services, represent climate conditions outside the modern range. By combining model results and palaeo-observations, it will provide a set of new constraints on the realism of representation of tropical hydrologic budgets. Such assessments, at the very least, provide information that allows climate services to define the level of uncertainty inherent in any projections that are to be used for planning purposes. However, PACMEDY intends to go beyond this by working with the climate services to develop ways in which identified biases and key uncertainties at different timescales can be factored into the provision of regional projections. Fusing the strengths of state-of-the-art modeling and palaeoclimate constraints should provide more realistic projections and a better foundation for planning.

Policy Impact. The development of climate mitigation policies strongly depends on the availability and reliability of climate projections. The development of climate adaptation policies is strongly dependent on these projections and on the climate services they provide. The timeframe for adaptation actions can range from years to decades, and knowledge of the uncertainties inherent in projections on this timeframe is crucial. Nearly half the world is dependent on monsoon precipitation, and many of these countries are particularly vulnerable to climate change. PACMEDY will deliver an improved understanding of monsoon dynamics, measures of the reliability of projections in the tropics, approaches to improve the characterization of future monsoon evolution, and an assessment of the likelihood of extreme droughts and flood. We will ensure that this information is made available to policy-makers in a timely fashion. We will work with policy-related organisations both at a national level (e.g. LWEC in the UK, Brazilian Interministerial Committee on Climate Change) and at a international level (e.g. via the Intergovernmental Panel on Climate Change, IPCC and the Intergovernmental Platform on Biodiversity and Ecosystem Services, IPBES).

PACMEDY will deliver results in time for the next IPCC Assessment Report. Our results will contribute both to Working Group 1 (the Scientific Basis) since we will address the issue of how the changes in mean climate and modes of variability affect tropical climates. They will also contribute to Working Group 2 (Impacts, Adaptation and Vulnerability) since they will address regional monsoon changes and feedbacks and extreme events in the African, Indian, East Asian and South American monsoon regions. The use of detection/attribution methods will also provide new information of the role of different forcing factors at these regional scales. Several participants were engaged in the previous IPCC assessment and expect to make a contribution to the next Assessment.

PACMEDY will also deliver information about ecosystem responses to long-term climate change and IM variability, and this will assist in assessing the likely impact of future changes on natural vegetation, ecosystem services and biodiversity. This work will contribute to the report of the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES).

The work in WP4 of the PACMEDY project and stakeholder conference planned in the final year of the project (see below) are designed to produce and highlight results have direct policy relevance for long-term planning.