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Back to the future: The science of building scenarios

We cannot anticipate everything, but we try to assemble as many of the pieces as possible in order to predict the future. The science – or the art – of building scenarios requires a degree of control over a wide range of factors, all intricately linked. It is like a game, where we have to guess how changing one thing will affect the whole. Some elements appear simple – it is easy to imagine that rising atmospheric temperatures will melt the sea ice and cause sea level to rise, perhaps threatening coastal populations – but at what speed and what intensity and will this start a chain reaction of new calamities?

Choose your own weather
You were dreaming of a perfect future world – longer summers, milder winters, greener grass – maybe the IPCC has invented it for you. They have proposed four sets of scenarios, each with a different answer to the fundamental question: will the 21st century be more and more industrialised, or more and more environmentally friendly?

Inventing new worlds
To invent the future, the references we have are the present – and the past. We build scenarios on the bases of existing or past trends and behaviour, and in this respect, they might teach us more about present processes than about future expectations. For example, we don’t know what is around the corner in terms of new technologies – technologies that could accelerate the impacts or mitigate the effects. However flawed the exercise might be, it is crucial for the climate change debate – as accurate a description of our future landscape as possible.

Scenarios are developed and fine-tuned as more is discovered about the climate system. In 2000, the IPCC proposed new scenarios, described in a Special Report on Emissions Scenarios (Nakicenovic and Swart 2000). These replaced earlier scenarios established in 1992. Observations showed that the predicted changes were occurring much faster than forecast in the 1990s. These newer scenarios also include a range of socio-economic assumptions, such as the population growth, economic development, energy use and environmental concerns envisaged in both global and regional contexts.

The scenario IS92A is from IPCC’s Second Assessment Report (1995) and it assumes that world population grows to 11.3 billions by 2100, economic growth continues at 2.3%-2.9% per annum, and no active steps are taken to reduce CO2 emissions.