Initial ISI-MIP simulation showing the effects on vegetation productivity at the highest emissions scenario (reduction: red to yellow; increase: green to blue)

A community-driven modelling effort aims to quantify one of the gravest of global uncertainties: the impact of global warming on the world’s food, health, vegetation and water.

Each model has its own strengths and weaknesses. That’s why the comparison process is so valuable – no single model is sufficient but together we can reduce the uncertainty.

Dr Andrew Friend

How different will the world be if it’s 2°C, 3°C or 4°C warmer? Ask this question of the multitude of different climate change impact models – each built by researchers interested in different aspects of global warming – and the likelihood is that you will get a multitude of answers. Modelling the global impact of climate change is an extremely complex process, and yet it’s absolutely essential if policy makers are to understand the consequences tomorrow of emissions policies adopted today.

Earlier this year, an international group of researchers initiated a joint project to attempt the first systematic quantification of some of the uncertainties surrounding climate change impacts to agriculture, health, biomes and water. Uncertainties such as: to what extent will the world’s vegetation change? Which regions will succumb to drought or flood? What will be the impact on global food crops? And how will the spread of human diseases be affected?

The Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP), coordinated by the Potsdam Institute for Climate Impact Research in Germany and the International Institute for Applied Systems Analysis in Austria, involves two- dozen research groups from eight countries.

Dr Andrew Friend from Cambridge’s Department of Geography is coordinating the analysis of results concerning changes to the world’s biomes – the communities of plants, animals and soil organisms that are  characterised by a similar structure and climatic requirement.

It’s a fast-track programme. All of the teams are working to a tight deadline and, by January 2013, they hope to be ready to publish their findings on the likely impacts of climate change predictions. The collective results will contribute towards the next report of the Intergovernmental Panel on Climate Change (IPCC), the leading international body that provides a clear scientific view on the current state of knowledge in climate change and its potential environmental and socioeconomic impacts.

Each group is using their own model, together with the very latest climate predictions produced by leading climate modelling groups around the world, to run comparable simulations for four different warming scenarios – conservative, drastic and two in between –  from 1950 to 2099.

For Friend, this means Hybrid, the model he first developed 15 years ago. At its heart is a set of equations that dynamically model global vegetation: “It works by simulating the dynamics of individual trees and an underlying herbaceous layer. You assume the plants compete within patches, and then scale these up to 50-km grid boxes. We use data to work out the mathematics of how the plant grows – how it photosynthesises, takes-up carbon and nitrogen, competes with other plants, and is affected by soil nutrients and water – and we do this for different vegetation types. Effectively, the whole of the land surface is understood in 2,500 km2 portions. We then input real climate data up to the present and look at what might happen every 30 minutes to 2099.”

For the most extreme scenario of climate change being modelled, Friend expects to see significant impact: “this scenario could show the whole of the Amazon rainforest disappearing this century, depending on the climate model. The circumpolar Boreal forest, which began to emerge at the end of the last ice age, could migrate into the tundra and perish at its southern limit. By contrast, Russia may benefit from an increased ability to grow crops in regions that were previously too cold, and this greater productivity would help absorb atmospheric carbon.”

Modelling impacts is complex, as Friend explained: “the increase in CO2 in the atmosphere from the burning of fossil fuels creates global warming. CO2 can act on vegetation, increasing their rate of photosynthesis and therefore productivity. However, in heatwaves, ecosystems can emit more CO2 than they absorb from the atmosphere. We saw this in the 2003 European heatwave when temperatures rose 6°C above mean temperatures and the amount of CO2 produced was sufficient to reverse the effect of four years of net ecosystem carbon sequestration.”

One of the greatest uncertainties in climate change is the feedback from changes in terrestrial carbon cycling. “Many scientists think that if soil warms it will release carbon because of the increased breakdown of dead organic matter by bacteria and fungi,” added Friend. “But there’s a lot of debate over whether this stimulation will be sustained over a long time – if it is, then you end up releasing enormous amounts of CO2 into the atmosphere, causing further global warming.”

Working with PhD student Rozenn Keribin, Friend is using Darwin, the University’s High Performance Computing Service’s supercomputer, to run the simulations; what takes a month to perform on a PC can be easily accomplished overnight on Darwin.

As the results of each group’s simulations become available over the coming months, the data will be assembled and compared. Friend fully expects that this process will reveal differences: “each equivalent model has its own strengths and weaknesses. That’s why the comparison process is so valuable – no single model is sufficient but together we can reduce the uncertainty.”

Why is this so important? “To make policy you need to understand the impact of decisions,” said Friend. “There hasn’t been a coordinated impacts project for IPCC across sectors before, and now this is covering four key sectors across four climate change scenarios from multiple climate models. The idea is to understand at what point the increase in global temperature starts to have serious effects across all the sectors, so that policy makers can weigh up the probable impacts of allowing emissions to go above a certain level, and what mitigation strategies are necessary to avoid significant risk of dangerous climate change.”

For more information, please contact Louise Walsh ( at the University of Cambridge Office of External Affairs and Communications.

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