Mid-holocene Experiment

Each model distributed is unique and differs from all the others in three ways: the initial conditions it is started from, the attributes which force it to be in one particular climate state and the parameters which make up the actual model.

Every climate model has to make a number of approximations, called parameterisations. To read more about these, click here. This implies that there are numbers in the model which are given a certain, fixed value. This value is not certain, however, and a range of values could be equally realistic. This experiment will investigate the effect on the modelled climate of varying the value of 20 of the most poorly understood parameters in the model - such as the relationship between the number of raindrops in a cloud and how much it actually rains (to see which parameters are perturbed, click here). It is possible that some combinations of parameters may replicate the past climate equally well, but produce widely different forecasts for what might happen in the future. Some combinations of parameters will not work at all, produce a completely unrealistic climate ( for example an Earth that boils or freezes, or oscillates between very hot and very cold every couple of years) and probably crash the model. It is not possible to tell beforehand what these combinations might be.

Some things which you do not think of as part of the climate nevertheless have a huge effect on the climate - such as volcanoes (after Pinatubo erupted in 1991 the ash it spewed out affected the climate for several years), solar activity, and, of course, the composition of the atmosphere. The orbital configuation of the Earth also act as a forcing on the climate, as it affects the amount of incoming radiation at the top of the atmosphere and the seasonality of the climate. When these forcing mechanisms change, they force the climate to change.

The flap of a butterfly's wings in Brazil can set off a tornado in Texas'. This famous quote sums up the fact that very small differences in what is going on in the world now can have huge effects on what happens in the future. As we cannot have perfect knowledge about what is going on now (down to the scale of individual butterflies) this means that, to produce a complete forecast of everything that might happen in the future, we need to take into account everything that might be happening now. To do this, we need to use a range of starting, or initial, conditions for our models when we start running them to make a climate forecast.

The aim of Experiment 1 in this particular study is to place a paleo-constraint on climate sensitivity. Sensitivity is here defined as the equilibrium temperature response to doubling of pre-industrial carbon dioxide concentrations. Some of the climateprediction.net models have shown a large sensitivity and by applying paleo-forcings to the models, we are testing if the models and their corresponding sensitivity are realistic. The general circulation model results are compared to paleo-observations. All the models are run with a range of initial conditions, parameters and forcings. By providing a framework for the evaluation of the climate models, we are testing if they are able to simulate climates that were different from today. This will improve our confidence in the models projections for future climates.

The period of focus is the mid-Holocene, i.e. ~6000 years before present (6kyBP). Why use the mid-Holocene climate to benchmark our models? The current climate is not in equilibrium. It is changing. The previous period in time with a relatively stable climate was the mid-Holocene, when the climate was stable for a period of about 2000 years and the forcing on the climate is well known. The 6kyBP climate is reasonably well known through paleo- observations.

The model used is HadSM3, a state-of-the-art climate model from the Hadley Centre for Climate Prediction and Research Atmospheric General Circulation Model coupled to a slab ocean. Mid-Holocene boundary conditions are applied to the model, i.e. the orbital configuration is altered to represent that of 6000 years ago. This redistributes the latitudinal and seasonal distribution of incoming solar radiation at the top of the atmosphere in the model. The methane concentrations are also lowered. Experiments 1 and 2 have somewhat different boundary conditions in the mid-Holocene phase, as described below. The model you download consists of 4 different phases, each of 15 model years and with a unique set of initial conditions.
The four phases are as follows:

Phase 1 is the calibration phase of the experiment. In this phase, the temperature of the surface of the ocean is artificially held constant. The movement, or flux, of heat, in or out of the ocean that is required to keep the ocean at a constant temperature is calculated. This is an easy solution to having a very simple ocean in the model, which cannot actually store heat in the way that a real, deep, complex ocean can. The dates given to this phase are 1810-1825.

This is the control phase. This involves running the model for 15 years with the levels of CO2 in the model atmosphere kept constant at pre-industrial levels, 282ppm. Unlike phase 1, here the temperature of the ocean surface is allowed to vary, according to how much energy the ocean receives and emits. However, it is safe to assume that the amount of heat flowing into the oceans is the same as in phase 1, so the heat fluxes calculated in phase 1 are applied. Unless the atmosphere starts doing something very different, and the energy balance at the top of the atmosphere is changed, the temperature of the whole atmosphere should therefore stay the same. If this is the case, the globally averaged surface temperature should also be approximately constant and not change substantially from year to year or drift off to a very different temperature, and we say that the model is stable. The dates given to this phase are 1825-1840.

In this phase the model is forced with pre-historic conditions. The orbital configuration is altered o that of 6000 years before present. I.e. the tilt of the Earth is increased by 0.5 degree and the precession of the equinoxes (the position of the Earth in its orbit around the Sun at the equinoxes) and eccentricity (the shape of the Earth?s orbit around the Sun) of the Earth is changed in accordance with the so-called Milancovitch cycles. Additionally, the methane concentrations are lowered with about 15% compared to the pre-industrial. The model is again run for 15 years and is given the same model dates as in the control phase of 1825-1840.

In this phase the levels of greenhouse gases (you can read more about the Greenhouse Effect here) are doubled and the model is run for a further 15 years. In a good model, the atmosphere should adjust to this change in forcing and eventually settle in a new stable, equilibrium state (which may be the same, warmer or cooler). The dates given to this phase are 2050-2065.

By comparing the single and doubled CO2 steps, we can calculate the climate sensitivity of the models - this is the difference between the globally averaged surface temperature in the model with pre-industrial CO2 and in that with doubled CO2. This is a useful indicator of how a climate model behaves, although it is slightly artificial, as of course carbon dioxide values in the atmosphere do not remain constant for 15 years, but change continuously.

The difference in the model results between the mid-Holocene and control models are compared to the paleo-observations available. A range of paleo-observations are used to test the models; pollen data, lake level data from lake sediment cores, macro fossil data and temperature data deduced from foraminifera, molluscs, diatoms, driftwood, whale fossils and whale bones. The observations have shown that the global monsoon systems were stronger during the mid-Holocene. The most notable feature of the mid-Holocene climate is the increase in the moisture budget across northern Africa. There were extensive wetlands and lakes across the current hyper-arid Sahara. In addition to look for these spatial features in the model results, the resulting change in seasonality from the altered orbital configuration can be seen in the global mean timeseries of temperature and precipitation rate.

There are two parts to this particular paleo-experiment; Firstly the models will be distributed with the four phases as described above. The second part of the Experient these models will be distributed again, only there is a change to the mid-Holocene phase. Phase 3 has altered orbital configuration and reduced methane concentrations as before, now with the additional change of the inclusion of an ice sheet in Eastern North America and the Hudson Bay is expanded. These local changes are hopefully going to improve the simulated climate in Eastern North America, as climate models to date have had difficulties capturing the locally cooler climate of the mid-Holocene. With this second experiment the motivation is how well can we simulate past climates and the models are benchmarked against a set of robust features of the mid-Holocene climate as seen the geological evidence.