What is a climate model?
A digital representation of the climate system that describes the behaviour of the various components of the system.
Models can treat physical and chemical processes and interactions, and use numerical approximation.
What are key forcings to the typical climate model?
Solar radiation
Concentration of GHGs, volcanic gases and particles.
Why do we model climate?
- Better forecasting and prediction of climate change.
- Climate reconstruction and understanding past processes to better estimate the future
What factors need to be known to simulate a past climate?
- Earth’s orbit
- Solar irradiance
- Volcanic and anthropogenic aerosols
- GHGs
For before the modern instrumental period of observations, 2 or more of these must be estimated from proxy data.
Describe the long term solar variability derived from sunspots
A cycle of 11 - year sunspots have been noted, where higher numbers of sunspots emits more irradiance. We can also derive solar flux from cosmogenic isotopes (14C and 10Be).
How is solar flux derived from cosmogenic isotopes?
- intensity of solar wind and total solar irradiance are positively correlated.
- The magnetic field associated with solar wind deflects cosmic radiation.
- Cosmic radiation generates 14C and 10Be
- We associate a high total solar irradiance with low production rates of 14C and 10Be, and we obtain records of these from ice cores and trees.
What two ways do we reconstruct long term volcanic forcing with?
Via measurements of:
- Acidity, which relates to maxima in sulphate aerosols.
- Sulphate Ion content in seawater.
V. Aerosols cool the surface and warm the upper atmosphere, affecting circulation.
What are energy balance models?
These are the simplest models that consider the balance between incoming shortwave radiation and outgoing longwave radiation., a global EBM treats Earth as a single point, a 1D EBM divides Earth into discrete latitudinal bands.
Benefits of energy balance models?
- Quick to run
- Easy to understand
- used in studies of sensitivity of Earth system to major radiational changes.
- However, they leave out many processes.
What are global climate models (GCMs)?
These are full 3d representations of climate, comprising at least the atmosphere and oceans, and now commonly sea ice.
These are the most comprehensive models used routinely for climate simulations.
What are Earth System Models (ESMs)?
These are developments on GCMs that include more elements, such as land ice, icebergs, biosphere and the carbon-cycle.
These are more comprehensive than GCMs but have not been fully developed as of yet.
How does a GCM work?
Earth surface is divided into grid points, where conditions are specified for each point, at each atmospheric and oceanic layer (can be multiple).
Equations are solved at each grid box to give meteorological values (temp, pressure, flow velocity) along time steps.
What size processes can atmospheric GCMs predict?
They can only really resolve large processes, such as planetary waves, cyclones and cloud clusters.
Their resolution is too coarse to notice thunder storms, boundary layer turbulence or anything small (known as parameterisations).
Whats is the Palaeoclimate Modelling Intercomparison Project?
We cannot run GCMs for a long time due to computation requirements, so to reconstruct palaeo conditions most runs are made to obtain a snapshot of the past world.
The PMIP combined experiments from 18 modelling centres, to reconstruct mid-Holocene mean surface air temp and precipitation.
What are EMICS (Earth system models of intermediate complexity)?
These are models that sacrifice complexity of GCMs to provide long simulations to understand palaeoclimatic conditions, their simplicity allows them to be run up to 100,000 years.
What two uses to EMICs have?
- Allowing long-term simulations over many millennia.
2. Simulating the interaction of as many components of the climate system as possible in an efficient manner.
Examples of EMICS
- The Climber Model, a very coarse 2D atmospheric model with a monthly temporal resolution.
- The LOVECLIM is another coarse 3D model with the aim of studying as many processes as possible: carbon dynamics, geochemistry, climate-coupling.
What can cause changes in atmospheric composition?
- Volcanism can change composition abruptly by introducing volcanic gases.
- Biochemical evolution - oxygen comprised 1% atmosphere 600Mya, nowadays comprises 23%.
- Greenouse gases, can be changed by anthropogenic or volcanic activity.
How do plate tectonics alter earth system flows?
Have they done it during the Quaternary?
They can alter the ocean and atmospheric circulation through continental drift.
- Darien Gap (Between N & S america) closed 3.5Mya.
- Rise of Tibet
During the Quaternary the continents have effectively not moved, so are not a cause of climate change during the Quaternary.
How could human activity impact climate system?
- Emissions since industrial revolution.
- Spread of agriculture for 4000 years.
- Major fires for 1000s of years.
Is there an oscillatory pattern in sunspots?
Sunspots cycle on a timescale of roughly 11 years, variations in length of sunspot cycles correlate with temperature changes.
Orbital Theory Concept
Imbrie et al. 1993: “temporal changes in insolation that are astronomically driven cause significant changes in the global climate…. account for much of the climate variability of Quaternary glacial-interglacial cycles”.
What three cyclic parameters characterize Earth’s orbit?
- Eccentricity (shape) of orbit, changes from circular to elliptical and back every 96 kyr, changes incoming insolation.
- Obliquity (tilt) of Earth’s axis, varies between 21.5-25 degrees on a 42 kyr cycle, changing insolation distribution.
- Precession (wobble) of axis cycles on 21kyr cycle, causing precession of the equinox.
Effects of Eccentricity on global radiation reciept:
The extremes of this cycle cause a 0.3% change in the total radiation receipt. a 1% change would cause a 1 degree change.
Glacial cooling events involve 5-10 degrees cooling, so eccentricity cannot explain this, HOWEVER the timing of eccentricity changes correspond to 100kyr ice age cycles.
