Secrets of the forest

What will happen to the Amazon when global warming gathers pace? An island off the Canadian coast may hold the answer. Kate Ravilious reports

Walk through the Amazon rainforest today and you will find it is steamy, warm, damp and lush. But if you had been around 15,000 years ago, during the last ice age, would it have been the same? For more than 30 years, scientists have been arguing about how rainforests like the Amazon might have responded to the cold, dry climates of the ice ages, but until now, no one has come up with a convincing answer. Some scientists believe that the Amazon shrivelled up into small surviving pockets of rainforest (called refugia) separated by huge expanses of savannah grassland. Others think that the Amazon was tougher than that, continuing to flourish as a rainforest throughout every ice age.

Walk through the Amazon rainforest today and you will find it is steamy, warm, damp and lush. But if you had been around 15,000 years ago, during the last ice age, would it have been the same? For more than 30 years, scientists have been arguing about how rainforests like the Amazon might have responded to the cold, dry climates of the ice ages, but until now, no one has come up with a convincing answer. Some scientists believe that the Amazon shrivelled up into small surviving pockets of rainforest (called refugia) separated by huge expanses of savannah grassland. Others think that the Amazon was tougher than that, continuing to flourish as a rainforest throughout every ice age.

Rainforests like the Amazon are important for mopping up carbon dioxide from the atmosphere and helping to slow global warming. Currently the trees in the Amazon soak up around 500 million tonnes of carbon dioxide each year: equivalent to all of the UK's carbon dioxide emissions for one year. But how will the Amazon respond to future climate change? If it gets drier will it still survive and continue to draw down carbon dioxide? Scientists hope that by understanding how rainforests reacted to climate change in the past they will be able to predict how the rainforest will manage in the future.

Unfortunately, getting into the Amazon rainforest and gathering data is very difficult. To study past climate scientists need to look at fossilised pollen, preserved in lake muds. Going back to the last ice age means drilling deep down into lake sediments, which requires specialised equipment and heavy machinery. There are very few roads and paths, or places to land helicopters and aeroplanes. Rivers tend to be the easiest way to access the forest, but this still leaves vast areas between the navigable rivers completely unsampled. To date, only a handful of cores have been drilled that go back to the last ice age and none of them provide really conclusive evidence of how the Amazon rainforest responds to climate change.

Now a geologist from Bristol University may have discovered how rainforests react to climate change, by leaping to a different location and going back even further in time, to the Carboniferous period, 300 million years ago. Howard Falcon-Lang has been digging fossils out of a sea cliff in a remote corner of Canada to see how ancient rainforests coped when the Earth's thermostat went up and down.

Along the north-east coast of Cape Breton Island in Nova Scotia, eastern Canada, tremendous cliffs tower over the beaches. Inside these cliffs lie the secrets of an ancient rainforest. About 300 million years ago, Cape Breton Island was part of a supercontinent called Pangea. This huge landmass sat over the equator, covered by tropical rainforest and crawling with gigantic insects, including millipedes that were nearly a metre long. Carboniferous literally means "coal-bearing", and most of the coal seams that we mine today were created from the thriving tropical rainforests of that time. But as the Cape Breton cliffs show, life didn't always flourish and the weather wasn't always warm and moist.

Taking a closer look at Carboniferous rocks reveals distinct cycles in the rock layers, each covering around 100,000 years. The cycles usually start with limestone layers, full of shells, which formed in shallow seas. Next comes a muddy layer, which was deposited from an ancient delta system, a bit like the present day Mississippi delta. Finally the cycle is topped off by a red-coloured rock, which formed in riverbeds in very dry conditions. After the red layer the cycle slides back in the opposite direction, going into the muddy layer and then the limestone. Each of these cycles represents an ice age, with ice caps expanding and contracting and a corresponding fall and then rise of sea level, by up to 100m each time. Meanwhile, the climate swung from being damp and tropical to cool and dry.

Previously paleobotanists have tended to ignore the glacial plants (in the red beds) in Carboniferous rocks because they are poorly preserved. Most paleobotanists assumed that the glacial plants would be similar to the interglacial plants. But better microscopes and new techniques to analyse rock cycles have meant that Dr Falcon-Lang has been able to show that there was more to the Carboniferous period than just lush rainforests and giant millipedes.

He set to work with his hammer and chisel, collecting thousands of plant fossils from the rock cycles that he could see along the Cape Breton coast. Back home he spent many hours analysing each of these plant fossils under the microscope. To his delight he was able to see clear changes between the types of plants growing in tropical regions during the warm and wet phases, compared with those growing in the cool and dry phases. "Between the ice-ages, when it was warm and wet, I found lots of fossilised clubmosses [lycopods], which were part of a dense coal-forming rainforest," he says.

Today's relatives of these plants are tiny, but in the Cape Breton cliffs the clubmosses were up to 30m tall. "When the climate swung into an ice age, turning cool and dry, the clubmoss-dominated rainforest collapsed and was replaced by shrubby, fire-prone types of vegetation that are extinct relatives of today's conifers," he explains. Without a doubt these tropical rainforests responded to climate change and were unable to survive an ice age unscathed.

From just 50km of coastline Dr Falcon-Lang has been able to sample an area of about 1,000 square km (about the size of Moscow) and cover a time period of around 1.5 million years (13 ice-age cycles). "It is unusual to be able to learn more about really ancient times than the recent past, but that is exactly the case when it comes to studying how climate change affects tropical rainforests," he says.

To make sure that he isn't just looking at an unusual pocket of Carboniferous rainforest, Dr Falcon-Lang now wants to investigate other rock sequences over a larger area. Next he plans to work with the United States Geological Survey, studying long sequences of Carboniferous rocks dotted up and down the Appalachian Mountains, in the north-east of America. "I'm hoping to match up some of the glacial/interglacial rock cycles over very long distances so that I can see what was happening to rainforests right across the supercontinent of Pangea," he explains.

Dr Falcon-Lang believes that the dramatic changes in plant species and climate that he has seen in the Cape Breton cliffs support the idea of the rainforest "refugia" hypothesis. Applying this idea to more recent times leads him to think that the Amazon rainforest may have responded to climate change and ice ages. "It is certainly possible that the Amazon rainforest contracted into smaller refugia during the last glacial maximum, but this needs to be tested by sampling cores from more areas that were likely to have been drier at this time," he says.

So what lies in store for the Amazon? Although Dr Falcon-Lang's work can't tell us how rainforests might respond to human interference, it does tell us that rainforests are likely to change quite dramatically with large swings in climate. Global climate models suggest that the Amazon rainforest area will become much warmer and drier in the future. The added impact of increased carbon dioxide in the atmosphere and massive logging programmes are unlikely to spell good news for the forest. "It is a fragile environment and the rapid changes occurring right now are probably going to be disastrous," says Dr Falcon-Lang. Unless we take drastic action soon, fossilised plants may be all that remain of the Amazon rainforest in the not-too-distant future.