Paleo-Earth System Modelling

Transcription

1 Paleo-Earth System Modelling Paul Valdes School of Geographical Sciences, University of Bristol

2 Structure of Talk Introduction: Why do we need a paleoperspective to Earth System Models? Example 1: Palaeoclimate Model Intercomparison Project (PMIP) Example 2: Paleo-atmospheric composition. Example 3: Rapid paleoclimate change. Example 4: Palaeoclimate studies of the distant past

3 Why Study Paleoclimates Test our understanding of Earth system dynamics Previous paleoclimate research has highlighted missing features of Earth System Models Test Earth System Models Especially important now that we are including slow components of Earth System Fundamental science ( cultural science ) E.g. Evolution of life. Direct Commercial and Policy Applications for longterm changes Oil formation strongly linked to climate Many other mineral reserves sensitive to climate Long term climate change relevant for nuclear waste disposal and may also show thresholds for CO2 increases.

4 What time periods to study? (From Frakes, 1979)

5 What time periods to study? No direct climate analogues for the future Test climate model processes Choose periods which have a large signal to uncertainty ratio Last 2000 years Signal/Uncertainty = (~ o C/~0.2 oc) > 1 Last Glacial Cycle (0 125,000 years ago) Signal/Uncertainty = (0-5 oc / 1-2 oc) > 1 Cretaceous period (100 million years ago) Signal/Uncertainty = (10-15 oc/ 5-10 oc) >1

6 What Climate Model to use? The dynamic hierarchy of models State-of-the-art model Rigorous test of these models comparison to paleo periods Lower resolution version of above Uncertainty in boundary conditions Intermediate complexity models Development of conceptual models, systems approach, emergent properties, but NOT the best predictive model Energy balance/ box models Educational, illustrative etc. No model is complete. Definitions of each type of model will change with time

7 Glacial Interglacial cycles Fundamental changes of the Earth System which are still poorly understood, especially in a quantitative sense Amplitude of variability shows us that Earth system feedbacks are essential is we are to explain these observations Physical and biological feedbacks both important

8 Palaoeclimate Model Intercomparison Project: PMIP understanding the mechanisms of climate change by examining such changes in the past, when the external forcings were large and relatively well known and when various kinds of geological evidence provide evidence of what actually happened providing a framework for the evaluation of climate models in order to determine how far they are able to reproduce climate states radically different from that of the present day

9 PMIP I ( ) Last Glacial Maximum (21,000 years ago). Change orbit, CO2, land ice sheet. Two simulations: Prescribed Sea Surface Temperature (CLIMAP) Slab ocean model Mid-Holocene (6,000 years ago). Change orbit only. Atmosphere only simulation.

10 Failure of the GCM s in the mid-holocene Data from Sandy Harrison

11 Simulated changes in African monsoon: Land and ocean Feedbacks Picture by Sandy Harrison

12 Last Glacial Maximum: Tropics Ferrara et al., 1999 Rosell-Melé et al., 1998 Harrison, 2000 Large scale patterns not too bad from slab ocean model, but some gradients wrong and N.Atlantic. CLIMAP SST wrong

13 LGM Coupled-Atmosphere Ocean models Annual mean surface air temperature 21ka-0ka MRI ECBILT Court esy of C D Hewit t

14 PMIP II (2002-?) Same goals as PMIP I but emphasis the use of the same models being used for future climate change predictions Hence using atmosphere-ocean-vegetation models Problems about spinning-up models for LGM Simulations 1000 years long LGM and mid-holocene time periods + Glacial Inception and Early Holocene + Fresh water hosing simulations

15 Example 2: Modelling Methane at LGM: Main natural sources are from wetlands (~160 Tg CH4 per year) and from termites (~27 Tg CH4 per year) Wetland emissions depend on extent of wetlands and the amount of decaying material (i.e. net primary productivity of vegetation) Main sink of methane is the reaction with the hydroxyl radical (e.g. CH4 + OH CH3 + H2O) OH concentrations is influenced by reactions with many other compounds in the atmosphere (e.g. CO + OH CO 2 + H) These include emissions of NO x (from soils and lightning), and organic volatile compounds such as isoprene (C 5H8) and terpenes (C 10H16) (from vegetation)

16 Models of the Methane Cycle Climate (atmosphere/ocean) Model Wetland area, methane emissions, Biomass burning Land Surface Hydrology Terrestrial Vegetation Cover Terrestrial Carbon Cycle Terrestrial Nitrogen Cycle Atmospheric Chemistry Isoprene/Terpene emission Soil and Lightning NO x Co-workers David Beerling and Colin Johnson.

17 Results

18 Example 3: Rapid Climate Changes

19 Slab ocean simulations forced by: Examine how important are circulation changes in the ocean

20 Temperature Change

21 Greenland Climate Change

22 Fresh water hosing and predictability Sensitivity to time scale of fresh water pulse AND Strength of Atlantic THC when fresh water pulse imposed (size and duration of pulse uncertain) Initial conditions From Renssen et al. 2002

23 Cretaceous Earth System Models Move continents High CO2 (4 x pre-ind) Reduced solar constant. NO permanent ice Spin-up for 100 years, followed by 5000 oceanonly, followed by another 100 years

24 Warm Cretaceous coupled oceanatmosphere simulation:

25 Why is model so warm? Consider simple global mean, annual mean energy balance. The radiative forcing due to: Increased CO2 ~ +8 Wm-2 Albedo reduction Cloud cover changes ~ +8 Wm-2 ~ +10 Wm-2 In contrast, UGAMP model under similar conditions suggested cloud forcing of -8Wm-2

26 Runaway Greenhouse? Temperature does not stabilise if CO2 = 4 x pre-ind. and no solar constant change Temperature does stabilise (at similar to that shown) if: CO2 = 3 x pre-ind. and solar constant reduced by 0.6%

27 Future Work Coupled atmos.-ocean (and carbon cycle) models will require long spin-ups (a few 1000 years) Chemistry-climate models require comprehensive chemistry and multi-decadal simulations. Non equilibrium models will require long ( years) simulations, and multi-member ensembles New Earth System components will require multi-physics (multicomponent) ensembles because in many cases we are still arguing over the basic equations. New components will allow for much more rigorous model-data comparisons (e.g. isotopes, dust) and will require much better collaboration between modellers and data

28 Summary Paleo climate studies have shown the need to incorporate most components of the Earth system. Previous paleo modelling studies showed that models were missing key processes, but model-data comparisons hampered by limited knowledge of input boundary conditions New generation of Earth system models will not require as many input boundary conditions, hence testing of the models will be easier, but computationally very expensive. Many new questions can be addressed, and many more time periods now possible. Some are good tests of models. ALL are good for testing our understanding. A truly exciting time to come!