HOLIVAR2006 Abstracts
Modelling of the evolution of climate during the Holocene.
Michel Crucifix
Hadley Centre for Climate Prediction and Research, Met Office, Exeter, EX1 3PB, UK
Contact: Michel Crucifix (michel.crucifix@metoffice.gov.uk)
Palaeo-environmental observations covering the Holocene display long-term trends as well as variability at various time scales. These are the expression of the response of the internal dynamics of climate to a series of "forcings", ranging from the well-known astronomical forcing to the more difficult to quantify solar and volcanic forcings. Greenhouse gas records reveal increasing trends since 6000 years ago that also need to be explained. Climate models are then necessary to understand these data and ultimately improve our understanding of climate. However, these models may be very complex and computationally expensive. It is therefore necessary to further idealise the problem. How this is done is the subject of this lecture.
Box models are typically used to represent the dynamics of the ocean carbon cycle. Coupled to dynamical vegetation models, they have been used to assess the carbon intake by vegetation over the last 8000 years, or to study the impact of coral-reef build up during the Holocene. So far, these exercises tend not to support Ruddiman's hypothesis of human-driven greenhouse gas increase over the last 6000 years.
Two-dimensional models (or "2.5-D") of the atmosphere constitute very efficient cores to which models of the slow components of climate, like the ocean, vegetation, and ice sheets may be coupled. A few examples are given, such as the evolution of ice sheets (another facet of Ruddiman's hypothesis), vegetation feedback, and carbon balance. We also comment on the timing of a "Holocene thermal optimum" in the Northern Hemisphere that results from the combination of decreasing ice sheet volume and decreasing summer insolation.
Low-resolution 3-D models can also produce 9000-year integrations, even though they are much more expansive. They are also used to explore internal models of climate variability, resonant response to stochastic events (freshwater pulses, volcanic eruptions), and continuous, idealised forcing.
Higher-resolution 3-D may finally be configured to study the evolution of climate during the Holocene. Two strategies exist: "accelerate" the forcing, such that the actual integration is much smaller than 9000 years; or perform a series of shorter, equilibrium experiments, sometimes with idealised orbital configurations.
The lecture finishes with a brief discussion of tools used to optimise the comparison with palaeo-environmental records, such as glacier and isotope models, with relevant applications.
Michel Crucifix graduated in Physics from Namur University (1998), followed by a Master in Physics (1999) and a doctorate in Sciences (2002) in Louvain-la-Neuve (Belgium). The subject of his thesis was "modelling glacial-interglacial climates over the last glacial-interglacial cycle". The origin and mechanisms of glacial-interglacial cycles that have paced the Earth over the last million years remain his favourite subjects since he was appointed permanent research scientist at the Meteorological Office Hadley Centre (UK) in 2002. He is author or co-author of about 25 peer-reviewed articles on the subject, some of which were produced in the context of broad international and multidisciplinary collaborations, like the Palaeoclimate Modelling Intercomparison Project. His most recent work focuses on vegetation-climate interactions and cloud feedbacks.