An old pollen seed can predict tomorrow's climate

3D illustration of different pollen
3D illustration of different pollen
3D illustration of different pollen

Is it possible that a tiny pollen dredged up from a European lake can hold answers about both our past and our future? Researchers at Lund University use pollen as old as 12 000 years to predict our future climate, and to study ecological and historical change.

Researchers Esther Githumbi and Johan Lindström use pollen from the ice age to the present to inform vegetation models and find crucial answers about how vegetation and land cover can impact our climate in the future.

– The pollen from past epochs in Europe can give us many answers about our climate, since the pollen we retrieve from lakes or bogs are the result of the climate at the time the pollen fell into the lake. It is a testament of what was happening at a specific time in history. We want to find out why certain trees grew in a certain place at a certain time. Why is the vegetation changing through different time-periods? says Esther Githumbi, researcher at Physical Geography and Ecosystem Science at Lund University.

Esther Githumbi analyses very large pollen data sets that have been collected by palynologists from different places in Europe, from Crete/Malaga in the South to Kirkenes/Tromsö in Norway in the North. The pollen is collected by drilling up sediment from lakes, wetlands and bogs. The further down you dredge, the older is the pollen.

– By studying pollen from 12 000 years ago to the present, we can see how tree cover has moved across countries. For example, we can see that Europe was covered to a large extent by woodland until about 6000 years ago, when we observe a major deforestation event. However, if we compare these areas with how they are today, we can see that in some places there are forests and in others there are no forests. That is a sign that a change has been happening.

Changes in vegetation cover can be both related to climate and/or manmade explains Esther Githumbi. One example of a temperature change is how the tree border in Europe moved north after the latest ice age. With increasing temperatures, some trees such as linden cover expanded 10,000 years ago in the Mediterranean region, and from about 3000 years ago in the boreal region which includes the areas of Northern Europe that consists primarily of coniferous forests and wetlands. A manmade change in contrast, is the expansion of open woodland from the Bronze Age onwards because of increased farming.

– I see many uses for these pollen sets. They give us a track-record of how temperature has impacted vegetation, and conversely, how vegetation has impacted the climate. Since we cover such a long time period, we can see how vegetation changes over swathes of time, instead of just 50-100 years. It can also shed light on how our ancestors have used the landscape in terms of farming and industrialization, says Esther Githumbi.

How can ancient pollen from trees and herbs shed light on our future climate?

Research on pollen sets to gain information about our ecological past is widespread. What is less common is to use the data to inform and improve climate models of our future. This is what sets the project at Lund University apart.

– Climate models are often created with data about our present vegetation and temperature, coupled with projections of future climate, as well as demographical and technological developments. What we want to do is to add historical vegetation changes into the mix, says Johan Lindström, researcher at the Centre for Mathematical Sciences, Lund University.

He explains that data of historical vegetation cover, and how that vegetation has been impacted by changes in temperature, can help yield important answers about what we could expect in the future. For example: if a certain species of tree disappeared from a region as the temperature got hotter in the past, could that mean we can expect the same development in the future?

– Take the changing tree border as an example. We know that trees migrated North after the ice age. If we look at the rate that the expansion happened, and how it interacted with the climate, can we say something about how rapidly we can expect the trees to migrate today? For Sweden, with our tree border in the North, this type of research is very important since it will have implications for fauna, mainly the open mountain areas, and animals such as reindeer.

Another benefit of using pollen data in climate modelling is that one can get more information about how deforestation might change not just the temperature, but also other aspects of our ecosystems.

– We know that deforestation can lead to increases in temperature since trees absorb and store carbon dioxide. However, less tree cover also gives rise to other side effects that are too seldom taken into account? Deforestation may imply heat reflection to the atmosphere from the light deforested land area (instead of heat absorption by the dark forest) and decreases in temperature. The net effect of all processes at the interface between vegetated land and atmosphere for a specific change in vegetation is still under debate. These are questions we might be able to answer if we create more inclusive climate models, with past changes, our present ecological status, and future projections all merged together, concludes Johan Lindström.

The project is a collaboration under the research environment MERGE, a Strategic Research Area working with modelling the regional and global climate system, with focus on vegetation and terrestrial ecosystems. MERGE covers advanced research, disciplinary and interdisciplinary research studies, and dialogue with the wider society on scientific knowledge and research needs.