Tropical rainforest revenge for a warming planet

Glen Paul: G’day, and welcome to CSIROpod. I’m Glen Paul.

Warming is the one thing scientists know with most certainly will occur under climate change in the tropics. But what has been less understood is that rising temperatures have a corresponding increase in the release or carbon dioxide from tropical forests and ocean ecosystems.

New research has revealed that unlike that other parts of the planet, changes in temperature over the tropics act inconstant on both photosynthesis, which is absorption of carbon dioxide, and respiration, which is the release of carbon dioxide, the two important mechanisms that naturally regulate year to year changes in atmospheric carbon dioxide concentrations.

The study, which appears in the journal Proceedings of the National Academy of Sciences, was co-authored by CSIRO’s Dr Pep Canadell, who joins me on the phone. Pep, does this mean tropical rainforests are set now to be put into a cycle of releasing more carbon dioxide as the planet warms?

Dr Canadell: This is the intent of the research. The intent of the research was to study this interannual variability of temperature going up and temperature going down due to normal climate anomalies, like when we have El Niño years we have a warmer planet, and when we have big volcanic eruptions we have cooler years. And so what we tried to do is to study what we can learn from studying those transitions from cooler to warmer that would then help us to understand the future of a warmer planet. And so the answer is that, yes, we’ll actually find less carbon uptake coming from tropical forests in warmer temperatures than currently.

Glen Paul: Who was involved in the study, and how was it undertaken?

Dr Canadell: The study is a NASA led study by Doctor Weile Wang at the NASA Ames Centre in California, a number of international collaborators from other parts of the U.S., Europe, and Australia. So the idea was to bring together these large datasets that exist on atmospheric CO₂ concentration globally, and a number of products from NASA on temperatures and precipitation tried to correlate these variables to really understand how much the tropical forests are actually driving those CO₂ atmospheric concentration changes year to year.

Glen Paul: It’s great to hear that NASA and CSIRO have been working together on this. The data itself, is that reflective of information that’s been collected over a period of time?

Dr Canadell: The data has been put together by several global networks. We have NOAA taking measurements, and collecting measurements from all over the world on CO₂ concentrations. We have large global datasets also from NASA that look into the long term record of temperature and precipitation. We have also the infrastructure to run multiple biospheric models to actually help us to understand and attribute the various changes we see in terms of photosynthesis and respiration which determines how much carbon gets up taken by a tropical forest.

And so this was, you know, all put together to try to bring this new picture of how we think that information coming from the year to year variability can inform us in the long term response of tropical forests as the globe warms.

Glen Paul: And you mentioned volcanic eruptions, so you’re able to use the normal impacts of those along with El Niño years as a means of replicating the effects of climate change on the tropics, for the purposes of the study?

Dr Canadell: Yes. So really the objective of the study was to try to look at the big departures of temperature and precipitation that occur naturally due to a number of climate modes or system phenomenon. The two most important for us was that when have El Niño years we have a warmer planet, and when we have big volcanic eruptions like El Chichón in the ’80s, and Pinatubo in the ’90s, because the large injection of aerosol in the troposphere, and even into the lower stratosphere, the whole planet cools off.

So by looking at the record over the last 50 years of CO₂ concentrations going up and down annually because of all these phenomena, then we were able to try to do the correlations between temperature and precipitation, at the same time to run the models to then reveal what was happening. And what we saw is that temperatures over the tropics are really controlling to a large extent the global CO₂ ups and downs that we see annually happening because of the climate variability.

And that’s really revealing in the sense that these are just small anomalies, one year anomaly, it may last a little more than one year, but when these anomalies disappear the earth system recovers quite well, which is incredible, and it shows how resilient the earth is to these perturbations. The key question is that what happens when we apply a sustained perturbation, like the one on climate change due to anthropogenic emissions, as we have now. And the answer is that most likely we’re going to see a similar type of response from the ones we see year to year, that is a big decline of the capacity of the tropical forest to actually remove atmospheric CO₂.

Glen Paul: Hmm, a bit to think about there. And the report is available in the Proceedings of the National Academy of Sciences. Is that available online?

Dr Canadell: Yes, yes. You can actually just find it at www.pnas.org

Glen Paul: Great. Thanks, Pep, I appreciate your time today.

Dr Canadell: OK. Thank you so much.

Glen Paul: Dr Pep Canadell. And to find out more about the study, or to find us on other social media, just visit www.csiro.au.