A new ice age

The simplest computer-based climate models are energy balance models (EBMs). These models divide the Earth into latitude zones. As latitude is the only dimension used these are one-dimensional models. For each latitude zone the model computes the incoming and outgoing energy. The equations that govern the model are all written in terms of the variable to be predicted, usually surface air temperature. This type of energy balance model came to prominence in the late 1960s when they were used to assess the sensitivity of the climate to a change in the amount of incoming solar radiation. The models predicted that a relatively small decrease in the amount of radiation reaching the Earth could lead to total glaciation. At the same time as these results were published, northern hemisphere temperatures were on a downward trend. This led to speculation that the Earth could be heading for a new 'ice age'. In the 1970s it was global cooling that dominated the text books and grabbed media attention; nor was it all hype. As Grove (1988) points out, the northern hemisphere cooling and glacial advance at this time had distinct financial implications for several northern European countries.

Arrhenious in 1908 and Callendar in 1938 had proposed that increasing carbon dioxide might save the world from a new ice age and these words were echoed again in the 1970s (Fleming, 1998; Mitchell, 1972). The extreme sensitivity of energy balance models, however, turned out to be due to the simple way the model represented the climate and not a true picture of the climate response. Energy balance models have increased in sophistication and are still in use today, mostly to study climates in the geological past. The legacy of these early modelling results remain, however, to haunt the present. It is also true that we are on the downturn of the next glacial cycle and are heading towards a new glacial maximum. Past inter-glacials have lasted between 10 000 and 14 000 years and this one is already 10 000 years old. However, to cite this as a rationale for not worrying about global warming is to forget about time-scales. Global warming will occur over the next few decades to hundred of years. Furthermore, our inadequate understanding of the interplay between radiative forcings and climatic responses should caution against relying on any such simplistic balancing act between global warming and a supposed natural cooling.

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