Transcript SOES 3015: Lessons from the Past: The IPCC view of

SOES 3015
Palaeoclimate Models I:
IPCC Context, Model Design & Use
Bob Marsh
Lecture Overview
 Palaeoclimate modeling & the IPCC - being increasingly
incorporated into the IPCC process - why?
 What sort of models are used?
 Range of numerical models used in palaeoclimate studies:
• “Simple” / “Box” models of reduced dimensionality
• Earth System Models of Intermediate Complexity (EMICs)
• General Circulation Models (GCMs)
 Which model for which problem?
 GCM/EMIC Practical Matters: Components; Computing &
Numerical Issues; Parameters; Boundary Conditions
Into the “Anthropocene” …
 Coined in 2000 by atmospheric chemist Paul Crutzen,
famous for work on stratospheric ozone depletion
 Influence of human behavior on the Earth in recent
centuries so significant as to constitute new geological era
 Since industrial revolution (c. 1850) or advent of agriculture
& civilization c. 6000 BC?
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Analogues for the Anthropocene?
 The last millennium - puts recent global warming in a
longer (recent-past) context - now modelled with EMICs
 Other past climates of relevance to IPCC, because they
tell us about climate sensitivity: Last Glacial Maximum;
Previous Interglacial (125 ka BP)
 Pliocene warmth - the last “recent” time that Earth was
substantially warmer, perhaps as warm as we expect in the
late 21st century? (+2°C)
 The Paleocene-Eocene Thermal Maximum - a deeper-time
warning of carbon cycle & other feedbacks/surprises?
IPCC Assessment Report 4 (A4), 2007
 3 Working Groups (WGs):
WG1: The Physical Science Basis of Climate Change
WG2: Climate Change Impacts, Adaptation and Vulnerability
WG3: Mitigation of Climate Change
 WG1 Report in 11 Chapters:
1. Historical Overview of Climate Change Science
2. Changes in Atmospheric Constituents and in Radiative
Forcing
3. Observations: Surface and Atmospheric Climate Change
4. Observations: Changes in Snow, Ice and Frozen Ground
5. Observations: Oceanic Climate Change and Sea Level
6. Palaeoclimate
7. Couplings Between Changes in the Climate System and
Biogeochemistry
8. Climate Models and their Evaluation
9. Understanding and Attributing Climate Change
10. Global Climate Projections
11. Regional Climate Projections
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Some history - The “Hockey Stick”: An Iconic Graph
 central feature of the Third Assessment Report, “TAR” (IPCC, 2001)
Courtesy: Third Assessment Report, “TAR” (IPCC, 2001)
Hockey Stick Controversy 1: Uncertainty in
method(s) used for temperature reconstruction …
 different data/methods yield different millennial temperature change
 not clear whether curves are regionally-biased (to European climate)
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Hockey Stick Controversy 2: The longer context …
 How anomalously warm was the Earth 1000 years ago?
 But how globally representative was the European MWP?
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Some reconciliation in AR4 (IPCC, 2007)
 More observations, a probabilistic approach, support the Hockey
Stick in broad terms, although uncertainty varies through time
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Further evidence provided by models
These are
Boundary
Conditions
 Models confirm: pre-industrial climate change largely explained by
“natural” forcing; recent warming mostly due to anthropogenic CO2 forcing
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Models: How much Complexity ?
 High?
Preliminary results of Imperial College Ocean Model (ICOM)
Horizontal mesh in idealized basin
3-D visualization of convective event
Courtesy: Applied Modelling and Computation Group, Imperial College London
Models: How much Complexity ?
 … or Low?
2-box model of the thermohaline
circulation (Stommel 1961)
Redrawn based on model by: Stommel, H., (1961) Thermohaline
convection with wo stable regimes of flow. Tellus, v. 13: p. 224-230.
Wiley.
Predicts THC hysteresis
(two stable states)
Reproduced by permission of American Geophysical Union: Rahmstorf, S., Crucifix, M.,
Ganopolski, A., Goosse, H., Kamenkovich, I., Knutti, R., Lohmann, G., Marsh, R., Mysak, L.A.,
Wang, Z., Weaver, A.J., Thermohaline circulation hysteresis: A model intercomparison.
Geophys. Res. Lett., v. 32, L23605, 6 December 2005, Copyright [2005] American Geophysical
Union.
Climate modeling: (very) basic principles
 The essence of climate, hence all climate models, is the
radiation balance of the Earth (remember SOES1008?)
 In simplest form, there is a balance between Incoming
Short-Wave Radiation (ISWR) and Outgoing Long-Wave
Radiation (OLWR):
(1- ) S =   Ts4
• is albedo (clouds, ice, snow, vegetation)
• S is incident solar radiation
• is atmospheric emissivity (associated with GHGs)
• is the Stefan-Boltzmann constant
• Ts is global-mean surface temperature
Heat Balance Equation - general form
 Evolution of temperature field (1D - in latitude, )
C
Ts  
t
Net heating
(= 0 for
steady
state)
Q
 A  (1  )S  FTs 

ISWR
OLWR
C = heat capacity;
Meridional
divergence
of heat
transport
A  = area of zone centred on latitude
;
Q = advective- diffusiveheat transport
One-dimensional (meridional)
Energy Balance Model schematic
(sketch by John Shepherd)
N.B. mixing between boxes (double-headed arrows) plus
uni-directional transport (single-headed arrows) …
Courtesy John Shepherd, University of Southampton
Increasing Realism: Earth System Models
of Intermediate Complexity (EMICs)
 Millennial transient simulations presently restricted to somewhat
simplified Earth System Models of Intermediate Complexity (EMICs):
Many components/processes - e.g., “GENIE”
framework for Earth system modeling
Low resolution / reduced dimensionality e.g., CLIMBER-2 (PIK, Germany)
Schematic only, for accurate model refer to: Petoukhov, V., Ganopolski, A., Brovkin, V.,
Claussen, M., Eliseev, A., Kubatzki C., Rahmstorf, S., (2000) CLIMBER-2: a climate system
model of intermediate complexity. Part I: model description and performance for
present climate. Climate Dynamics, v. 16, no. 1, p. 1-17.
 Around a dozen such models developed in last 10-15 years
 Sufficiently fast & complex to simulate past/future millennia
 Combine simulations & statistics provide best way forwards?
Simulating Different Climates with GENIE:
Present; Recent Past; Deep Past
 Easily configured to simulate wide range of climates, changing
boundary conditions (orbital parameters, ice sheets, GHG
concentrations, even land/ocean configuration):
 Modern climate
 Last Glacial
 55 Million years ago
 Also used to explicitly simulate proxies of climate, sea level, ocean
circulation, carbon cycle (ongoing research)
 Additional coupling with models of biogeochemistry, sediments, to
produce synthetic ocean core data (see next lecture)
GENIE climate simualtions (www.genie.ac.uk)
Paleo-Modeling: GCMs
 IPCC focus on equilibrium climate change at key time-slices in the past
 More complex modeling is appropriate, using Ocean-Atmosphere
General Circulation Models (GCMs or OAGCMs)
 GCM simulations coordinated by PMIP, now in second phase:
(a.k.a. PMIP-1)
(see http://pmip2.lsce.ipsl.fr/)
 PMIP-2 focus on state-of-art Atmosphere-only &
Coupled Ocean-Atmosphere General Circulation Models:
• 17 AGCMs in study of European LGM climate (Ramstein et al. 2007)
• 6 full OAGCMs in study of Atlantic MOC at LGM (Weber et al. 2007)
Courtesy: Laboratoire des Sciences du Climat et de l'Environment
Climate Change @ Last Glacial Maximum (1)
Multi-model mean SST change:
Regional T change:
proxy
range
AR4 (2007, Fig. 6.5)
 Multi-model mean conceals large global & regional differences
 Partly due to differences in ocean circulation
 LGM & Pre-industrial “slab-ocean” GCM movies (if time), courtesy
of Bristol Research Initiative for the Dynamic Global Environment
(BRIDGE), Univ. of Bristol (led by Prof. Paul Valdes) …
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Climate Change @ Last Glacial Maximum (2)
 How relevant to present-day climate change?
 Similar (but opposite) GHG change as in year 2000, relative to preindustrial (e.g., CO2 ± 100 ppmv)
 Corresponding change in radiative forcing of -2.8 W m-2
 Further changes in land surface albedo (ice sheets, lowered sea
level) changed radiative forcing by -3.2 W m-2
 Reduced vegetation may have contributed -1 W m-2
 Increased atmospheric aerosols (ditto) -1 W m-2
 Total change in radiative forcing @ LGM approx. -8 W m-2
 Based on proxies, LGM global cooling in the range 4-7°C (relative to
pre-industrial climate) - indication of climate sensitivity (°C per W m-2)
 Models simulate radiative perturbation of -4.6 to -7.2 W m-2, and
global cooling of 3.3 to 5.1°C
 Conclude that PMIP-2 models broadly pass the “LGM test”
Climate change @ Previous Interglacial (1)
 The Eemian, 130±1 to 116±1 ka BP
 Multi-model mean summer surface air temperature change (left)
 Three-model mean annual minimum Greenland ice sheet
thickness & extent (right)
AR4 (2007, Fig. 6.10)
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Climate Change @ Last Interglacial (2)
 How relevant to present-day climate change?
 Warmer than present (pre-industrial) interglacial, due to different
orbital forcing:
solar insolation anomalies (in time vs. latitude)
Northern
winter
Northern
summer
Box 6.1, Figure 1
LIG
Holocene
 Crucial relevance for fate of the Greenland Ice Sheet & hence future
rates of sea-level rise
 No standardized PMIP-type inter-comparison yet (coming soon)
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Climate “Surprises”? - e.g., PETM (~55 Ma BP)
 Observations: carbon cycle
excursion, warming, acidification
 CO2 source:
• Volcanism?
• Clathrates?
• Terrestrial Organic Carbon?
 Trigger: Change in ocean
circulation/mixing?
 Warming much amplified at
high latitudes (+20°C in Arctic)
 Are we heading for a similar
perturbation?
AR4 (2007, Fig. 6.10)
Courtesy: Climate change 2007, The physical scientific basis IPCC Assessment Report 4 (A4), 2007
Post-AR4 Development
 Hansen et al. (2007) “Climate change and trace gases”, in
Phil. Trans. R. Soc., criticizes AR4 for conservatism:
(Fig. 2 from Hansen et al., 2007)
• Select a reconstructed sea
level record:
• Infer changes in radiative
forcing due to ice sheets
(inverse sea level, blue) &
GHGs (from ice cores, red):
• Simply “model” global temp.
(blue) & reproduce most of
proxy record (red):
Courtesy of the Royal Society: Hansen, J., Sato, M., Kharecha, P., Russell, G., Lea, D.W.,
Siddall, M., (2007) Climate change and trace gases. Phil. Trans. R. Soc. A , v. 365, p. 19251954.
 Draws attention to GHGs other than CO2 - higher climate sensitivity?
 Claims ice sheets highly dynamic - may melt faster than expected?
GCM/EMIC Practical Matters (1)
 Components/Schemes:
For basic climate simulation: Atmosphere; Ocean; Sea Ice
For “fast” carbon cycle: Land Scheme (including hydrology);
Biogeochemistry (marine & terrestrial)
For “slow” carbon cycle: Ocean Sediments; Rock Weathering (lithosphere);
Volcanoes; Other exotica? (e.g., Methane Hydrates)
For glacial phenomena: Ice Sheets; Icebergs (including sediment transport,
to reproduce Heinrich Events); Atmospheric Chemistry (to model glacialinterglacial variations of methane)
 Computing/numerical issues:
Horizontal structure (“grid-point” or “spectral”) & horizontal resolution (latlong increments, or wavenumber of truncation, respectively)
Vertical scheme (layers, levels) and vertical resolution
Timestep (atmosphere << ocean, usually)
GCM/EMIC Practical Matters (2)
 Parameters controlling:
Tracer mixing (horizontal, vertical, oriented along preferred surfaces, e.g.,
constant density in the ocean)
Boundary layers (Lower atmosphere; Ocean surface and bottom)
Convection (ocean deep sinking; atmospheric moist convection)
Albedo (clouds, vegetation type, sea ice)
Biogeochemical processes (marine & terrestrial)
 Boundary Conditions:
In all cases: Land-Ocean configuration (geometry, bathymetry, orography,
permitted throughflows); ice-covered gridpoints; catchment areas (for runoff
of precipitation over land); Insolation (Solar Constant)
If no carbon cycle: CO2 (and other radiatively-active gas) concentrations
If glacial simulation: orbital parameters (eccentricity, obliquity, precession);
evolving ice sheets (if no ice sheet module)
Summary
 At present, and for the last 20 years, leading policy advice
on climate change is provided by the IPCC
 Latest Assessment Report (AR4) features Palaeoclimate
chapter, for the first time
 Previous (TAR) paleo focus (& controversy) on
reconstructing the last millennium, now modelled in AR4
 Model simulations now more widely featured, more focus in
AR4 on lessons from past: Climate sensitivity at LGM; GrIS
sensitivity at LIG; pre-Quaternary extreme warmth & carbon
cycle perturbations - analogues for future “hot-house” climate
 Post-AR4 publications suggest AR4 use of palaeo record
too conservative, even wrong: ice sheets to change more
rapidly than expected; overturning may strengthen rather than
weaken; high latitudes to warm even more than expected?
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