Although improving models of factors such as atmospheric chemistry and vegetation cover will help enormously, the process is also complicated by the increasing impact of man. "Humans have become a geophysical force," said Will Steffen of the Australian National University, speaking on earth system dynamics in the Anthropocene at the IUGG meeting. And it's not just the carbon cycle that humans have altered by adding fossil sources of carbon to the mix. They have affected the nitrogen cycle by fixing nitrogen to create fertilizers and built dams that change hydrological cycles. In fact, Steffen says that humans have affected the land surface even more than the atmosphere, through processes such as cultivating land and introducing grazing.

So what's the best way to predict the effects of future human acts on the environment? People are likely to instigate new energy technologies and bring in systems such as carbon taxes and carbon trading, all of which will influence carbon emissions to an extent that's not yet clear. According to Steffen, there are two approaches to modelling human effects on climate – either include anthropogenic factors such as greenhouse gases, aerosols and land use in conventional climate models, or use an economics approach. But both types of approach are highly unbalanced, he says. And while economists tend to model the next 10 or 20 years of behaviour, that's a small timescale for geophysical models.

Guy Brasseur of the US National Center for Atmospheric Research, convenor of the IUGG session, agrees. "A big challenge is to link physical and social models," he told environmentalresearchweb. "The two communities need to understand each other's language. Physicists tend to generalize, while social scientists do local case studies." Similarly, physicists "like equations and a deterministic approach", while social scientists "know you can't use thermodynamic load to understand human behaviour".

Steffen feels one solution could be to focus on economics and land-use change. "Should we focus on modelling or scenario development?" he asked. So far researchers have taken a scenario development approach, but it's impossible to model contingent factors such as wars. "The future will depend on the nature of human aspirations… and how can we ever model those?" said Steffen.

Liquid matters

Joe Alcamo of the University of Kassel, Germany, also called for a change in the modelling approach in the field of water resources. Perugia seemed a particularly appropriate place to speak about water, as Leonardo da Vinci came to the town to draw up plans for a canal, later using his sketches of the Perugia valley as a backdrop for the Mona Lisa. To date, there have been two approaches to the analysis of water resources – detailed local studies and watershed modelling. Alcamo, who is co-chair of the Global Water System project, believes that climate change means that we must now look at water on a global basis.

He proposed a list of new tasks for the water research and management community: develop global water assessment capabilities on a par with the current IPCC assessments for climate; expand monitoring and models; introduce a regular reporting system similar to that of the IPCC; carry out a comprehensive freshwater biodiversity survey to act as a benchmark; provide technical tools such as databases, scenarios and conflict research to reconcile competing water uses; and expand the focus on local basics and watersheds to a global basis.

There could be busy times ahead for the modelling community.