With climate change expected to bring more heatwaves, violent storms and heavy rainfall, the impact on human society is likely to be significant. As if we didn’t have enough to adapt to, a new paper suggests extreme weather could further contribute to rising carbon dioxide levels by reducing how much carbon plants draw out of the air.

Growing plants take carbon dioxide out of the atmosphere and store it as living plant matter. The world’s ecosystems have absorbed about a third of the carbon dioxide humans have put into the atmosphere through fossil fuel burning. In effect, by reducing the amount of carbon dioxide in the atmosphere, plants keep global temperature below what it might otherwise be.

The new Nature review paper suggests extreme weather events – especially heatwaves, droughts and fires – weaken ecosystems’ ability to act as a buffer against climate change. And the situation is likely to worsen as temperatures rise further and extreme events become more frequent or more severe, the authors say.

Carbon losses

The scorching heatwave in central Europe in 2003 gave scientists a chance to look closely at how water scarcity affects the amount of carbon dioxide forests take up. And the results suggested the consequences are much more serious than first thought.

Soon afterwards, scientists from eight different countries launched an international project to investigate the connection between extreme events and the carbon cycle.

The impacts of extreme weather can be pretty immediate if vast swathes of vegetation are killed off by drought or wildfire and stop taking up carbon dioxide, say the authors. But there are less obvious effects that take a while to appear. Changes in temperature or rainfall extremes can make trees and plants less resistant to pests and diseases, for example.

While most of these impacts are relatively well known, the new paper is the first to put a figure on how much less carbon dioxide ecosystems absorb as a result of extreme events.

Different types of extreme event affect the carbon cycle in different ways. Downward arrows are where the land absorbs carbon dioxide (carbon sinks), upward arrows are where it’s released (carbon sources). Yellow represents fast responses while some take much longer to take effect, marked in purple. Source: Reichstein et al., (2013)

11 billion tonnes

The international team of scientists, led by Dr Markus Reichstein from the Max Planck Institute in Germany, analysed nearly 30 years worth of global satellite data.

The scientists used images between 1982 and 2001 to estimate how much light plants in particular areas absorbed. From that, they worked out how the amount of carbon-rich biomass the plants produced changed during and after extreme events. The scientists also used carbon dioxide measurements from a global network of 500 recording stations on land.

Overall, they worked out that 11 billion fewer tonnes of carbon dioxide were taken up by plants during each year of the 30-year period, compared to a world without extreme events.

That’s a huge amount – roughly equivalent to the amount of carbon dioxide taken up each year by the world’s ecosystems. The implication is that without extreme events, plants could reduce carbon dioxide concentrations by twice as much as they are now.

Carbon fertilisation

In the past few decades, global ecosystems have taken up about 30 per cent of the carbon dioxide in the atmosphere, even though emissions have been gradually rising.

That’s because the higher atmospheric carbon dioxide levels have had a beneficial effect on plants, encouraging them to grow and use water more efficiently. This is something scientists call carbon dioxide fertilisation.

But the new research suggests that even a small shift in the frequency or severity of extreme events in the future could see the negative effects partially – or even completely – offset any potential benefit from carbon dioxide fertilisation.

As Dr Chris Huntingford from the UK Centre for Ecology and Hydrology explains to us:

“What this paper shows is that [extremes events] could negate quite a large fraction of [carbon dioxide] fertilisation … [T]his is a potentially unwelcome human-climate system feedback, causing more carbon dioxide to instead remain in the atmosphere.”

In other words, as carbon dioxide emissions rise, plants could end up absorbing less and less of it. Add to that other factors that reduce ecosystems’ ability to take up carbon, such as deforestation, and you have an even less optimistic picture.

And as Reichstein explains, there are knock on effects for extreme weather. He says:

“As extreme climate events reduce the amount of carbon that the terrestrial ecosystems absorb and the carbon dioxide in the atmosphere therefore continues to increase, more extreme weather could result. It would be a self-reinforcing effect.”

Important steps

The new study is the first attempt to put a figure on how extreme events affect the world’s ecosystems – and there are still a lot of uncertainties. For one thing, Reichstein says his team would like a new generation of experiments to look closely at how responses vary across specific ecosystems, such as forests, bogs, grasslands and croplands.

But it’s still a big step forward in unravelling how ecosystems might change in response to short, sharp events as well as gradual changes in the climate – and it’s important climate models get both of these right.