Climate models are overly optimistic: rainforests do not store more CO₂ when it increases in the atmosphere
Many climate models assume that if the amount of CO₂ in our atmosphere increases, tropical rainforests will store more of it. This turns out to be less than hoped for, shows paleoecologist William Gosling (IBED). Moisture and forest fires appear to play a more significant role than greenhouse gases.
“Many climate models assume that plants grow faster when there is more CO₂ in the atmosphere,” William Gosling of the UvA Institute for Biodiversity and Ecosystem Dynamics (IBED) explains over the phone. “It has been proven several times that if you give a plant in a climate chamber more CO₂, it grows better. But now we have studied how this works in an ecosystem and the results are different.”
To do this, Gosling, together with an international research team from the Netherlands, the United States and the United Kingdom, looked at Lake Bosumtwi, a crater lake in Ghana that is millions of years old. Gosling: "What is new about this study is that we not only looked at the vegetation, but also at many other factors such as animals, forest fires and water management within a radius of 10 to 20 kilometres around the lake. We examined how the system functions and what factors cause changes.”
Ice cores
To find out, the researchers used ice cores (cylindrical samples of ice from a glacier or ice sheet) to look back 500,000 years in time. There, they observed changes of 100 parts per million (ppm) in atmospheric CO₂ at different points in history. Gosling says: “At that time, carbon dioxide in the air was not as high as it is now, but the level changed just as much. We wanted to know how those spikes affected changes in vegetation.”
No matter how the researchers examined the data, they found no evidence that the level of CO₂ in the air affected the vegetation. But they did discover that the amount of moisture, forest fires and the presence of herbivores play a major role in changes in vegetation. Gosling: “It's really quite simple: if you have a lot of CO₂ in the air and the forest burns to the ground every year, then no forest will grow. While many global climate models include the CO₂ effect when they model the future, they don't include forest fires.”
This does not mean that we should throw existing climate models overboard, says Gosling. But this research does give us an indication of how we can improve these models. For instance, we should add the parameters of forest fires, humidity and the number of animals in the landscape.
Lake Bosumtwi - besides being a fantastic archive of how the world has changed over millions of years - is just one research site. Gosling would like to extend this research to other locations in Africa that may not go as far back in time. The researchers were also inspired by the paper of tropical biologist Caroline Lehmann at the University of Edinburgh. She studied forest ecosystems in Africa, South-East Asia and South America and discovered that the ecological relationships differ from continent to continent. Gosling says: “We would like to use the same approach to see if we find the same results in lakes in South America and South-East Asia.”
8,000 kilometres of rainforest
What does this study teach us about climate change? Gosling explains: “We cannot assume that nature will save us and that tropical rainforests will absorb the CO₂ again. We know that seasonal influences will increase in the tropics, causing a dry season and therefore the chance of forest fires. There is no point in planting or restoring tropical rainforests in areas that will become increasingly dry over the next 50 years.”
With the latter, Gosling refers to the forest-planting initiative the Great Green Wall. This billion-dollar project, sponsored by the UN, aims to plant 8,000 kilometres of rainforest from West Africa to East Africa to prevent the expansion of the Sahara Desert. Gosling says: “If you draw lessons from our study, you can ask yourself whether this is the right place to plant new forest. Especially if you consider that in 50 years' time, when a tropical tree has reached maturity, the drought will only have worsened.”
Yet Gosling would like to end on a positive note, "otherwise it becomes a bit depressing". His research provides some guidance on what can be done to reverse the negative feedback loop. “We can focus more on actions to conserve moisture and prevent forest fires. If forests are planted in humid environments, the trees can absorb CO₂ and have a positive effect on the local moisture balance. That way the ecological cycle can be closed.”
The researchers published their results in the scientific journal Science.
