I’m impressed at how easily Professor Yadvinder Malhi can talk about scary scenarios. In a calm and collected tone, he’s discussing the possibility of climate change turning tropical forests from sinks to sources of carbon dioxide. ‘At the moment 40 to 50 per cent of the CO2 we emit is absorbed by either the oceans or the land. Though that may have negative impacts for those ecosystems, particularly in the oceans, it does act as a brake on the rate of global warming,’ he says. ‘If these carbon sinks didn’t exist, the rate of global warming would be double what it is.’
Speeding up, he explains that ‘computer models suggest the land biosphere is particularly vulnerable to climate change – that its carbon sink may actually “switch off” and that forests might absorb less carbon dioxide than they emit. That is one of the biggest uncertainties for climate change projections over this century.’ These gloomy pronunciations are delivered with a small smile, somewhat at odds with the information bombshell. If he were a doctor of medicine, instead of physics and ecosystems, he would have an excellent bedside manner.
In truth, though, Malhi is worried. It’s just that the ‘sink or source debate’ is Global Warming 101 for his field. Troubling facts come with the territory of working at the University of Oxford’s Environmental Change Institute, where the word ‘carbon’ is used as often as ‘coffee’, and ‘work’ often means taking a look under the bonnet of some of the most worrying patterns unfolding in both the Earth and its atmosphere. It’s from Oxford that Malhi leads the Global Ecosystem Monitoring Network (or ‘the GEM’ for short), a team of ecologists collecting monthly carbon samples from sites all over the tropics. In any given year, this data is used to publish a number of reports on climate change and rainforests.
This year, however, will be particularly important because Malhi’s ‘switch off’ has already happened – it was observed during last year’s El Niño.
‘During the El Niño cycle of 2015 and 2016, much of the world warmed by about 1ºC,’ he says. ‘We saw the carbon sink switch off and a surge in the rate of rise of CO2, which seemed to be caused by processes occurring in the tropics.’
Why this is happening scientists haven’t figured out yet. They don’t know if it is due to trees growing less under heat stress (thus taking in less carbon), or if it is because soils are emitting more of it as they warm up. They don’t know if it is a combination of several factors. What they do know, however, is that all hypotheses so far have only been inferred from models – no one has actually measured the processes in the field until now.
It is the type of research the GEM was born to do. In fact, one of Malhi’s main aspirations for setting up the network was to track a major event like an El Niño across the continents. ‘We have about 50 plots in operation from the Amazon and the Andes, to West Africa, the Congo basin and Malaysian Borneo, some with data going back as far as 2005,’ he says. ‘We’ve collected the data and now it’s a case of cleaning it up and working out what’s going on in and around the trees themselves.’
With a view to publishing an initial report in May, he and his team will spend the next few months picking apart the ways El Niño altered the tropical carbon cycle and whether or not it is a sign of climate change to come. ‘We want to see whether this El Niño is some sort of proxy for a future hotter state. If it’s a short-term pulse from the soil heating up then it could just be a temporary blip. But if we find that the temperature is permanently causing stress to trees and slowing their growth, it would be a sign of something more dangerous and ongoing.’
For Malhi, observing the tropical forests during an actual El Niño event is akin to a volcanologist experiencing a major eruption. ‘The phenomenon is troubling,’ he says, ‘but I’m also excited for the opportunity to advance our scientific understanding. We’re using a short term-term phenomenon as a vital opportunity to understand long-term changes.’