A drier rainforest?

When researching the possible effects of climate change to the Amazon Rainforest one issue keeps coming up time and again. The possibility of the Amazon experiencing a drying out is discussed in many academic journals, but one article by Mahli et al (2008) caught my eye.

In my research into the Amazon I assumed that if there was to be any drying of the Amazon basin then this would be linked to global greenhouse gas emissions and climate change. However, Mahli et al. discusses the fact that rather than climate change, the current deforestation of the Amazon is one of the main reasons behind the potential reduction in rainfall, which is something I had not yet considered.

Mahli et al. highlights that nearly 25-50% of rainfall that falls in the Amazon basin is recycled from the basin itself. This is largely a result of trees extracting water from the soil and, through the process of transpiration, returning it to the atmosphere to fall once again as rain. The article highlights that while small scale forest loss can in some cases enhance convection and rainfall, this is only one of 2 stable states the forest can exist in. With the removal of 30-40% of the Amazons trees the basin (or specific areas of it) could shift to a permanently drier state.

Clearly this could spell disaster for not only the indigenous people reliant on the land, but also the general economy for the area as a whole as numerous farms and pastures could be affected by the potential lowering of precipitation.

Another point to Malhi et al.’s article is the great lack of certainty in the figures surrounding such a major issue such as this. Although they put forward solid mechanisms for why there could be a reduction in rainfall, when trying to calculate the actual possibility of a decrease in precipitation, the figures used to calculate this are ‘crude’ and produce much uncertainty. Even more alarming is that these figures do not even take into account the potential reduction in rainfall due to the afore mentioned deforestation.  To me, this level of uncertainty is a cause for concern. With this in mind and a pinch of scepticism, take a look at the spatial reduction in precipitation maps produced.

The probability of a decrease in precipitation of more than 0%, 20% and 50% using moderate greenhouse gas emission models.

Taken from Mahli et al. (2008)

If these projected figures do indeed amount to a reduction of rainfall in the Amazon this could be disastrous. These figures demonstrate that there will be significantly more drying in the dry season (which starts in June) and it is in fact this season that is most critical for vegetation patterns, and are therefore most important to the tribe people tending the land.

If there is one thing this article has highlighted to me, it is the great level of uncertainty that comes with predictions of climate and changes that come with the Amazon, and it is something I will carry with me and remember when researching future issues.

Malhi, Y., J. T. Roberts, R. A. Betts, T. J. Killeen, W. Li and C. A. Nobr (2008) Climate Change, Deforestation, and the Fate   of the Amazon.  Science. 319 pp. 169-172