Global climate change results from a combination of periodic external and internal forcing mechanisms, and a complex series of interactive feedbacks within the climate system itself. These climate changes occur over a whole range of time scales from a few years to hundreds of millions of years.
The traditional view of climate change has been one of cause and effect, where the climate system responds, generally in a linear fashion to climatic forcing. Only now is humanity beginning to realise that this view is too simple. In recent years climate models have re-emphasised the complex nature of the climate system and our limited understanding of its behaviour. The very nature of the feedback processes highlights the non-linearity of the climatic system.
External forcing sets the pace of climate change; but the internal (non-linear) dynamics of the climate system modulate the final response. This is true for all time scales. Galactic variations impose external variations in insolation, whilst tectonic movements through mantle convection regulate the climate changes over hundreds of millions of years. Milankovitch orbital variations act as a pacemaker to the internal variations of ocean circulation and atmospheric composition.
Today we may be witnessing one of the most profound climatic changes in the Earth's history. Certainly, larger changes in global climate have occurred in the past, but over much longer time periods. The danger facing the global society today is that anthropogenic global warming may be too fast to allow humans, and other species, to adapt to its detrimental impacts. In addition, through enhanced greenhouse forcing, we may be pushing the climate system towards a bifurcation point, where climatic responses may become highly non-linear through complex feedback processes, driving the system to a completely different, and most probably, inhospitable state for humankind.
The challenge for scientists is to understand the climate system, and ultimately predict changes in global climate. To this end, greater collaboration is required between modellers, empiricists and policy makers. Ultimately, the climate system may be too complex to simulate reliably, and the study of global climate change will remain an imprecise science.
In light of this, the precautionary approach to mitigating the threats of anthropogenic global climate change must be fully recognised and adopted by the international community. In addition, increased emphasis will need to focus on the study and modelling of the impacts of future global warming. Greater integration between scientific and policy scenarios will be beneficial to the management and control of future impacts to society. Greater emphasis on impact scenarios at the regional level is also needed, if society is truly to "think globally" and "act locally". Indeed, the challenge for society as a whole is to respond to current dangers regarding global warming, and ultimately to "manage" the climate system in a sustainable and responsible manner.