sem seminar
TRANSCRIPT
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 1000yrlast 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
270 200 N2O ppb
760 350 CH4 ppb
280 185 CO2 ppm
PILGM
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?