Seeing forest for the trees - via satellite

Understanding and predicting climate change could be easier thanks to hi-tech radar imaging being developed at the University of Adelaide.

Staff at the university's Centre of Expertise in Microwave Radar (CoEMR) are developing a new technique of radar sensing to provide global 3-D maps of vegetation structure.

It is hoped that the technique will help reduce uncertainties surrounding the use of the carbon cycle and its impact on long-term climatic trends - and ultimately improve predictions for climate change.

The timing could not be more appropriate with many more nations, including Australia, preparing to sign the Kyoto Protocol. The Kyoto Protocol seeks to impose political constraints on nations based on the connection between climate change and increased emission of greenhouse gases such as carbon dioxide. It allows for trade in carbon-credits between nations, but requires new technology for monitoring and verification.

Shane Cloude, Professor of Microwave Radar with the university's School of Electrical and Electronic Engineering, said changes in vegetation structure, induced by climatic conditions, natural disturbance and human activities, can have a substantial impact on carbon storage and the exchange of greenhouse gases such as CO2 with the atmosphere.

"Fossil fuel burning and deforestation, which release CO2, are believed to be the two dominant contributions to the rise of atmospheric carbon over the past 50 years," Professor Cloude said.

"Trees store carbon in their biomass and so forests have the potential to act as carbon sinks to re-absorb some of the excess CO2. Given the importance of forests in both scientific and political terms, there remain surprisingly large uncertainties in our global knowledge of vegetation biomass.

"In addition, the dynamic processes of carbon flux due to changes in vegetation and its interaction with the wider carbon cycle call for a better quantitative understanding of the spatiotemporal variations in biomass," he said.

With the erosion of large scale land-based observation networks, interest is turning to remote sensing technologies to make a big impact in this area and establish reference global maps of biomass for input to climate change models.

"One key technology is microwave radar imaging from space satellites," Professor Cloude said.

"It was realised more than 20 years ago that microwaves have the advantage over optical sensors of being able to penetrate dense forests and provide a signal related to the total biomass of the vegetation.

"However, radar brightness alone is insufficient to provide the kind of accuracies required.

"Therefore, at the CoEMR, we are developing a new approach that relies on measurement of forest height by using a radar technique called polarimetric interferometry, or POLInSAR, which is both well suited to satellite technology and can provide the kind of accuracies required for global mapping.

"This method employs two passes of the satellite in orbit over the same scene and by very accurate measurement of the phase or time shift between the two signals in different polarisations, provides an estimate of the mean vegetation height.

"Height is then used to estimate biomass via growth models or allometric relations."

Initial experiments with this mode of the satellite will take place in 2005/06 using the new Japanese ALOS satellite, with a possible global mapping mission to follow by the end of the decade.

The University of Adelaide will establish a POLInSAR calibration site in South Australia and work with data from a well-characterised forest test site near Injune in Queensland to validate the technology.