The only way to predict the day-to-day weather and changes to the climate over longer timescales is to use computer models.
These models solve complex mathematical equations that are based on well established physical laws that define the behaviour of the weather and climate.
However, it is not possible to represent all the detail in the real world in a computer model, so approximations have to be made. The models are tried and tested in a number of ways:
They are used to reproduce the climate of the recent past, both in terms of the average and variations in space and time.
They are used to reproduce what we know about ancient climates (which are more limited).
The Met Office Hadley Centre model is unique among climate models in that it is used with more regional detail to produce the weather forecasts every day.
Two critical factors have helped us to improve these models over the years. First, our knowledge of the real world has improved, which allows us to improve the models.
Second, the speed and power of computers has increased dramatically, allowing us to represent more detail in the models.
Improvements in the science
The climate system is highly complex, with many potential interactions and feedbacks. Over the years, more of this complexity has been included in models.
In the 1970s, models included a simple representation of the atmosphere. Rain was included but not clouds; carbon dioxide (CO2) concentrations were considered and the radiation (heating) that determines the effect of CO2 on temperature was also included.
Now, current state-of-the-art climate models include fully interactive clouds, oceans, land surfaces and aerosols, etc. The latest models include detailed chemistry and the carbon cycle.
Improvements in computer power
Climate modelling has always made use of the best computers, but has been limited by the available computer power.
A typical model divided the world up in to boxes 600km across with five levels to represent all the vertical structure. They were used to predict changes on timescales of months up to a year or so.
They were mainly used to understand climate processes rather than to predict the future.
The latest Met Office Hadley Centre model is HadCM3 which stands for Hadley Centre Coupled Model version 3. HadCM3 was developed in 1999 and was the first unified model climate configuration not to require flux adjustments (artificial adjustments applied to climate model simulations to prevent them drifting into unrealistic climate states).
HadCM3 is also the climate configuration used in the following projects:
- UK Climate Projections ( UKCP09)
- the operational decadal forecasting system (DePreSys) until 2012
- the regional modelling system PRECIS
The massive increases in computer power since the 1970s are used in the following ways in the Met Office Hadley Centre:
Much higher resolution is used to give more regional detail. The changes between the 1970s and the present day outlined above required 256 times more computer power.
Representations of all the key processes identified as important for climate change are included in the latest version of the model.
Much longer predictions are run, typically reproducing the last 150 years and predicting the next 100 to 1,000 years.
Far more experiments are run with different versions of the models so that we can quantify the uncertainty in our predictions.
The latest climate models predict similar possible global average temperature changes to models used 10 to 15 years ago, ranging from 1.6-4.3C (2.9-7.7F) in the current best estimates using a mid-range emissions scenario.
However, we are much more confident about these ranges. Using Met Office models we have even been able to start to assign probabilities to more dangerous high temperature changes at the upper end of this range that could arise if climate turns out to be very sensitive to increased greenhouse gases.