The Amazon is losing its capacity to absorb carbon. Or, why we can’t rely on forests to solve climate change

By Chris Lang

A paper published this week in Nature concludes that the Amazon is losing its capacity to absorb carbon. In the past decade, the carbon absorbed by the Amazon each year has decreased by about one-third.

The study, titled, “Long-term decline of the Amazon carbon sink”, is the most extensive land-based study of the Amazon to date. The 30-year survey of trees in the Amazon involved a team of nearly 100 researchers, led by the University of Leeds. The authors write:

While this analysis confirms that Amazon forests have acted as a long-term net biomass sink, we find a long-term decreasing trend of carbon accumulation. Rates of net increase in above-ground biomass declined by one-third during the past decade compared to the 1990s.

Dieback or Fertilization?

There are two predictions of how tropical forests respond to climate change: “dieback” and “fertilization”. The dieback theory predicts that climate change will bring more extreme seasons, more droughts and more tree deaths. The fertilization theory predicts that more carbon dioxide in the atmosphere will mean faster tree growth and therefore more carbon storage in forests.

The authors write that,

Some earlier modelling studies predicted a large-scale dieback of the Amazon rainforest, while more recent studies predict a carbon sink well into the twenty-first century due to a CO2 fertilization effect. The realism of such model predictions remains low owing to uncertainty associated with future climate and vegetation responses in particular changes in forest dynamics.

Direct observations are therefore crucial, to determine how forests are reacting to climate change and how they might react in the future. The paper in Nature is based on measurements of trees on the ground from 321 inventory plots dating back to the 1980. The inventory research is coordinated by the Amazon Forest Inventory Network (Rainfor).

Trees starting to live fast, die young

Lead author Roel Brienen and co-author Oliver Phillips wrote a piece on The Conversation explaining the research. They explain that the Amazon has acted as a carbon sponge, because trees are growing faster than they are dying. This has “helped to put a modest brake on the rate of climate change” by absorbing two billion tonnes of carbon dioxide each year.

“However”, Brienen and Phillips write,

we also found a long and sustained increase in the rate of trees dying in Amazon forests that are undisturbed by direct human impacts. Tree mortality rates have surged by more than a third since the mid-1980s, while growth rates have stalled over the past decade. This had a significant impact on the Amazon’s capacity to take-up carbon.

As Phillips puts it in a press release, “With time, the growth stimulation feeds through the system, causing trees to live faster, and so die younger.”

Recent droughts and high temperatures in the Amazon have made matters worse, but tree mortality rates started to increase before the intense drought in 2005.

We can’t rely on forests to solve the carbon problem

In the Mirror, Phillips is quoted as saying,

“All across the world even intact forests are changing. Forests are doing us a huge favour, but we can’t rely on them to solve the carbon problem. Instead, deeper cuts in emissions will be required to stabilise our climate.”

Obviously, this has serious implications for REDD.

While there are many good reasons for reducing deforestation, to address climate change we need to reduce emissions from fossil fuels. And the best way of reducing emissions from fossil fuels is to leave them underground.