University of Tasmania Professor of Zoology and leader of the Tasmanian Aquaculture and Fisheries Institute Marine Environment programme, Craig Johnson says up to 80 per cent of the kelp forests have vanished in some areas of the East Coast.
Tasmania has Australia's largest forests of the giant kelps - Macrocystis pyrifera and angustifolia - which can grow up to 30m tall.
Prof. Johnson studied aerial photographs from the past 50 years of the dense floating kelp canopies along the East Coast.
The survey showed that between 20 and 80 per cent of the cover that existed in the 1940s has been lost.
Other studies have shown that kelp beds have declined from about 120sqkm in 1954 to 0.5sqkm in 1988-89.
The most probable cause was climate change, Prof. Johnson said.
Global warming is affecting the behaviour of ocean currents.
The East Australian current that runs down the coast of Australia has changed behaviour over the last few decades, pushing further south more frequently and making bigger incursions into Tasmanian waters.
Eddies that spin off that current are increasingly influencing the Tasmania coastline.
"The East Coast used to be much more dominated by subantarctic water," he said.
"The difference between the two water masses is not just temperature but also the nutrient load. And it's probably that nutrient load that is affecting the giant kelp. The warmer water doesn't have enough nutrients in it for the kelp."
Prof. Johnson said the main cause for alarm was that kelp loss was symptomatic of a more severe problem: global warming.
"Climate change is on us and it's real and it's having real effects. We're feeling those effects not just in Tasmania but in the rest of the world and yet there's a lack of political will to really do anything seriously about addressing it," he said.
"It's all part of the global climate change, the result of all the things we're doing to the world's atmosphere."
Losing kelp forests was like chopping down tree forests, he said.
Not only was there the loss of plant growth which releases carbon that gets passed up the food web but also the loss of the physical structure that attracts hundreds of species.
"If the rate of production of material by plants drops, then the food web that depends on those plants has got to drop as well," he said.
Giant kelp forests are habitats of outstanding ecological and economic significance, representing areas of high biodiversity and productivity.
They are key habitats for hundreds of species including fish, shellfish, other seaweeds and, in particular, the tiny invertebrates associated with the kelp canopy, an important part of the ecosystem.
"When the giant kelp forests are removed we will see a very significant shift in the ecosystem," Prof. Johnson said.
Another contributing factor to kelp loss in some specific locations is thought to be lobster fishing, which has led to an increase in sea urchin which eat all of the algae and seaweeds leaving barren rock.
Kelp forests are also affected by storms.
"Fishermen say this kelp comes and goes all the time and they're absolutely right but the other side of the coin is, on top of that there is also a downward trend," Prof. Johnson said.
What can be done about the loss of the giant kelp forests?
"We've got to recognise that the system has changed and we therefore might have to change the way we use it. It may not be as productive as it was. We might have to manage some species a little differently."
But localised management practices would have little effect against global changes.
"If it really is an effect of water masses, until we do something about climate change nothing else we can do is going to have much impact.
"The kelp loss is symptomatic of a much bigger scale problem, a global problem, yet our Government and the US Government, for example, refuse to sign the Kyoto Protocol.
"This is a global problem and it requires a global solution and therefore a global political will."