The Amazon, past and future

I found a paper that tries to present information about the Amazon from when it (arguably) began, up until its present and potential future. It is called “New Views on an old forest: assessing the longevity, resilience and future of the Amazon rainforest”, by Maslin et al., and the authors use palaeoclimate and palaeoecoecological records to determine exactly how far back the Amazon’s existence can be traced.

They argue that the Amazonian rainforest “has been a permanent feature of this continent for at least the last 55 million years”, meaning it has been around since at least the late Cretaceous. However, the definition of what a rainforest is today does not match up with what “rainforests” were millions of years ago, and the paper uses “megathermal moist forests” (MTMF) as a more precise term for discussing the evolution of what would eventually become modern rainforests.

Fig. 1 Reconstructions of the exposed land masses and occurrence of megathermal moist forests for six key periods. Arrows indicate possible dispersal of rainforest taxa

In the past 55 million years, tropical rainforests have existed in more parts of the globe than they do today, and their growth was subject to upper and lower boundaries in temperature and moisture similar to what those of rainforests today. These conditions exist today within the tropics, and in earlier stages of the earth existed in different latitudes and with different expansion. However, the evidence put forth by the authors is that the region where the Amazon rainforest exists today has been continually covered in MTMF throughout the whole period.

There is a third condition (besides temperature and precipitation) that is discussed, and it is levels of atmospheric carbon dioxide. There have been great fluctuations in the quantity of carbon dioxide released into the atmosphere, and the authors cite a paper by Chambers and Silver 2004, that, among others, that show that plants grown under double current carbon dioxide levels grow faster. However, they are careful to note that this does not mean that greenhouse gases are going to necessarily help forests grow. There are certain kinds of plans that are favored by different levels of carbon dioxide, and therefore if carbon dioxide levels were to double, certain plants would probably grow better, while others would be put under tremendous strain. Overall, this would endanger the balance of the Amazonian rainforest, and is unfavorable.

Fig 2. Reconstructions of past atmospheric CO2 levels over the last 600 Ma using modelling, paleosols, algae and foraminifera carbon isotopes, stomata density and boron isotopes. These are compared with global temperatures, mean tropical temperature variation and the extent of continental glaciation.

The article then goes on to discuss the future of the Amazon, and they use what I consider to be one of the most eloquent in summing up our current situation, since “we are entering a non-analogue future”. Global climate change will spur changes within the rainforest, and in turn these changes will affect regional, and consequentially, global climate. However, it is very difficult to predict what may happen. The authors of this paper focus on the same factors that allow the Amazon to exist to try and imagine what can happen in the coming centuries. With rising world temperatures, deforestation, and carbon dioxide levels, the Amazon rainforest runs the risk of disappearing within one century. The primary problem is human impact, but then the reactions of the forest to these impacts are another problem unto their own, and are, as the paper describes, “poorly understood threats”. With no past analogue to base estimates on, the consensus seems to be that mitigating negative human impact is the world’s safest bet.

Sources: Maslin, M.A., Y. Mahli, O. Phillips and S. Cowling “New views on an old forest:assessing the longevity, resilience and future of the Amazon Rainforest.” Transactions of the Institute of British Geographers 30, 4, 390-401 (2005)