Climate simulation models are usually 250 years in scope – 150 years into the past and 100 years into the future. The models have time intervals of 20 minutes. Many millions of calculations have to be undertaken for every 20 minute time-step, so huge super-computers are required to do the job. Even with such super-computers, as at the UK’s Met Office, it still takes 3 months to run a complete simulation.
To test a climate model, a simulation is undertaken of the observed 0.8 degree Celsius warming of the globe over the past 150 years – a process called fingerprinting. In this way, the simulation’s predictions can be tested against observed data. The models are built using the factors that climate scientists think have affected climate in that time: natural factors such as volcanoes and variations in the output of the sun; and human factors, most particularly the increase of carbon dioxide resulting from burning fossil fuels and deforestation.
When the models are run using only only natural factors, they can reproduce the observed climate data, but only until about 1970. After 1970 the models and the observed data diverge, and in fact the models tend to show a period of planetary cooling. When the human factors, particularly carbon dioxide emissions, are included in the models, then the real, observed global warming and that predicted by the simulation, coincide.
This ability to pull out the effects of natural and human factors not only tests the models, but also allows scientists to accurately attribute the effects that human activity is having on the Earth’s climate. This process of fingerprinting therefore allows the Intergovernmental Panel on Climate Change to make such definitive statements about the effect we are having on our climate. Their fourth report states that “There is at least a 90% chance that the observed increase in temperature globally is due to man-made greenhouse gases”.
The challenge then is to work out how to project climate change into the future, which is a far more challenging process as there are so many variables, including:
- How will the land and sea sinks behave when concentrations of atmospheric carbon dioxide increase?
- How will human population grow?
- How will human behaviour change, regarding levels of carbon dioxide output?
To model future climate change, scientists create a number of different scenarios for these unknown factors. These seem to result in a spread of possible future global warming over the next 100 years from 2-6 C. The current worst case of 6C constitutes a warming of more than that between the last ice age and now, but 100 times faster. And this has the ability to tip us over the edge into a series of horrific scenarios, such as the slowing of the Gulf Stream, and the melting of the Antarctic and Greenland ice sheets, the latter resulting in a sea level rise of more than 10m.