Climate models in (palaeo-) climatic research How can we use climate models as tools for hypothesis testing in (palaeo-) climatic research and how can we apply this to understand climate change from the Cretaceous to the near future? Nanne Weber
Temperature 1900-2100 Different scenario’s for GreenHouse (GH) Gases +0.6C
History of climate last 300 Myr last 3 Myr last 1000yr last 50 kyr cold warm
Outline of this talk <ul><li>What is a climate model? </li></ul><ul><li>Middle Holocene (6000 years Before Present =6 kyr BP): stable climate with warm NH summers </li></ul><ul><li>The Last Glacial Maximum (LGM, 21 kyr BP): cold, low GH Gas levels and large continental caps </li></ul><ul><li>The Paleocene-Eocene Thermal Maximum (PETM, 55 Myr BP): warm and high GH Gas levels </li></ul>
A climate model describes the Earth’s climate system
<ul><li>Computations are done on </li></ul><ul><li>a grid with finite size </li></ul><ul><li>You have to choose a grid </li></ul><ul><li>Grid size is always limited </li></ul><ul><li>Small systems are not simulated, so have to be parameterised </li></ul><ul><li>Small ~ grid distance </li></ul><ul><li>Model can only be validated at large spatial scale </li></ul>
<ul><li>Important processes that have to be parameterised: </li></ul><ul><li>Turbulent transport of heat, impuls, moisture </li></ul><ul><li>Clouds </li></ul><ul><li>Precipitation </li></ul><ul><li>Boundary layers </li></ul><ul><li>Radiation </li></ul>Climate models tend to be very sensitive to these processes!!
Completeness of model: this is determined by availability, computer resources and research question
Climate models 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)
The middle Holocene (6 kyr BP) Northern Africa wet southern Europe and India also wet , northern Europe dry
Cooling over the north Atlantic: data (diamonds) and 16 different models Figure from Kageyama et al. (2006)
Hypothesis OK? Yes for regional-mean cooling, but we do not (yet) understand 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 hypothesis: in the cold and dry LGM climate wetland area is reduced and emissions are much lower than today
Methane model: compute wetland area and emissions from climatic fields (temperature, bottom moisture, etc) black: NH high-latitudes green: 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, with CO2=1400 ppm and prescribed sea surface temperatures. 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)