Climate models in (palaeo-) Climate models in (palaeo-) climatic researchclimatic research

How can we use climate models as tools for How can we use climate models as tools for hypothesis testing in (palaeo-) climatic research hypothesis testing in (palaeo-) climatic research and how can we apply this to understand climate and how can we apply this to understand climate change from the Cretaceous to the near future? change from the Cretaceous to the near future?

Nanne WeberNanne Weber

Temperature 1900-2100Temperature 1900-2100

Different scenario’s for GreenHouse (GH) Gases

+0.6C

History of climateHistory of climate

last 300 Myr last 3 Myr last 1000yr

last 50 kyr

cold warm

Outline of this talk

1. What is a climate model?2. Middle Holocene (6000 years Before

Present =6 kyr BP): stable climate with warm NH summers

3. The Last Glacial Maximum (LGM, 21 kyr BP): cold, low GH Gas levels and large continental caps

4. The Paleocene-Eocene Thermal Maximum (PETM, 55 Myr BP): warm and high GH Gas levels

A climate model describes the A climate model describes the Earth’s climate systemEarth’s climate system

Computations are done on

a grid with finite size• You have to choose a grid

• Grid size is always limited

• Small systems are not simulated, so have to be parameterised

Small ~ grid distance

• Model can only be validated at large spatial scale

Important processes that have to be parameterised:

• Turbulent transport of heat, impuls, moisture

• Clouds

• Precipitation

• Boundary layers

• Radiation

Climate models tend to be very sensitive to these processes!!

Completeness of model: this is determined by availability, computer

resources and research question

Model= a set of mathematical equations which are solved on a grid by a computer

*Equations describe many different processes (e.g. incoming radiation, cloud formation, heat transport, snow melt) in one or more components (e.g. atmosphere, ocean, vegetation) of the climate system*With a given spatial resolution*With given boundary conditions (e.g. glacial ice sheets, Greenhouse gases for 2100, Cretaceous land-sea mask)

Climate modelsClimate models

The middleHolocene(6 kyr BP)

Northern Africa wet

southern Europe and India also wet, northern

Europe dry

Het midden HoloceenHet midden Holoceen

Middle Holocene

The middleHolocene(6 kyr BP)

Hypothesis: changes in precipitation

(and vegetation) at 6 ka due to orbital

forcing

This holds also for cyclic patterns in

sedimentary records

Orbital Orbital parameters: parameters: main cyclesmain cycles

Orbital parameters at 6kyr BP:Orbital parameters at 6kyr BP:higher NH summer insolationhigher NH summer insolation

Changes in monthly-mean insolation as a function of

latitude

The middle The middle HoloceneHolocene

changes in changes in summer summer

temperaturetemperatureand and

precipitationprecipitation(6k minus 0k)(6k minus 0k)

Monsoon Monsoon precipitation precipitation

over Africa at 6 over Africa at 6 kyr BP: kyr BP:

1) models 1) models underestimate underestimate the signal as the signal as indicated by indicated by

biomesbiomes2) vegetation 2) vegetation

and ocean and ocean feedbacks help!feedbacks help!

Figure from IPCC Third

Assessment Report (2001)

The middleHolocene(6 ka BP)

Hypothesis: changes in precipitation

(and vegetation) at 6 ka due to orbital

forcing

OK???

The Last Glacial MaximumThe Last Glacial Maximum

LGMLGM PIPI

COCO22 ppm ppm 185 185 280 280

CHCH44 ppb ppb 350 350 760 760

NN22O ppbO ppb 200 200 270 270

LGM versus Pre-Industrial (PI)

climate:

lower GH-gas concentrations

LGM

The Last Glacial Maximum (LGM) The Last Glacial Maximum (LGM) period of maximum extent of NH period of maximum extent of NH

continental ice sheetscontinental ice sheets

Hypothesis:Hypothesis:

changes in changes in annual annual

temperaturetemperatureand and

precipitationprecipitation(21k minus 0k)(21k minus 0k)are due to low are due to low GH-gas and ice GH-gas and ice

sheetssheets

Regional coolingRegional cooling

Cooling over the Cooling over the north Atlantic: north Atlantic:

data (diamonds) data (diamonds) and 16 different and 16 different

models models

Figure from

Kageyama et al.

(2006)

Hypothesis OK? Hypothesis OK?

Yes for regional-Yes for regional-mean cooling, but mean cooling, but

we do not (yet) we do not (yet) understand understand

spatial details spatial details

If we do a transient run, with prescribed insolation (upper), CO2 (middle) and ice (lower), do we find a realistic Antarctic temperature?

If we do a number of transient runs, each with separate forcings (insolation, CO2 and ice), where do we find a response to what?

Different aspects of climate forced by different factors!!

SAT – CO2, ice Monsoon – orbital

What forces variations methane?

One hypothesis is:variations in the wetland source

Wetland area for today and LGM (Kaplan, 2002)

Extended Extended hypothesis:hypothesis:

in the cold and in the cold and dry LGM climate dry LGM climate wetland area is wetland area is

reduced and reduced and emissions are emissions are

much lower than much lower than todaytoday

Methane model: compute wetland area and emissions from climatic fields (temperature, bottom moisture, etc)

black: NH high-latitudesgreen: tropics

yellow: SH high-latitudes

PETM: extremely warm, because of high GH-gas concentrations?

Zachos et al. (2001)

Large ‘mismatch’ in temperature between model (red line) and

data (dots: red=PETM, yellow=just

before or after PETM)

Fig. from Sluijs (2006)

PETM simulation with EC-Earth,PETM simulation with EC-Earth,with CO2=1400 ppm and prescribed sea surface with CO2=1400 ppm and prescribed sea surface

temperatures. Results for DJF surface tempstemperatures. Results for DJF surface temps

EC-Earth runs:polar warmth seems to be

due to combination of feedbacks (sea-ice and

snow, clouds,..)

Comparison of simulated temps with data (green dots)

TenTen

Can one step twice into the same river?