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Plant invasion triggered ice ages

Pete Wilton

They may have looked more like a green carpet than a forest but the first land plants really did change the world.

New research led by scientists from Oxford University and Exeter University has shown that the invasion of the land by plants in the Ordovician Period (488-443 million years ago) cooled the climate and triggered a series of ice ages.

I asked Liam Dolan of Oxford University’s Department of Plant Sciences, one of the leaders of the research reported today in Nature Geoscience, about the work and what it reveals about yesterday’s - and tomorrow’s - climate:

OxSciBlog: What sort of plants were there during the Ordovician?
Liam Dolan: The fossil record tells us that the first plants to grow on land appeared sometime before 475 million years ago during the Ordovician Period.

We only have fossilised remains of small fragments of plants but we don’t know how the bits fit together – a bit like a jigsaw puzzle with a lot of the bits missing. It is safe to say that these plants were very small and probably looked like liverworts and mosses - their closest living relatives.

OSB: How did the climate change during this period?
LD: Climate changed dramatically during the late Ordovician Period. It changed from a climate that was warmer than today (with no ice) into an ice age. Ice ages are pretty rare in Earth history and what gave rise to the Ordovician glaciation has always been a mystery.

OSB: How did you test whether plants triggered this change?
LD: While increasing the amount of carbon dioxide causes global warming, removing carbon dioxide from the atmosphere causes global cooling. One of the dominant mechanisms for removing carbon dioxide from the atmosphere is silicate weathering: the chemical reaction between silicate minerals of rocks and carbon dioxide in the atmosphere.

We tested the hypothesis that non-vascular plants (mosses) increase rates of silicate weathering. To our amazement we found that these simple plants did in fact increase the weathering of silicate minerals. We then incorporated these measurements of silicate weathering rates into computer models of Ordovician Period climate.

When we re-ran the models with our new data, we discovered that the appearance of the first land plants in the Ordovician plants would have caused a dramatic decrease in atmospheric carbon dioxide which would have brought about climate cooling and contributed to the initiation of the late Ordovician ice age. 

OSB: What do your results reveal about how plants influence climate?
LD: We know that plants play a critical role in climate systems by pulling carbon dioxide out of the atmosphere in two ways: Firstly, plants carry out photosynthesis, which converts carbon dioxide into plant biomass that store carbon. Secondly, plants increase rates of silicate weathering, the chemical reaction that breaks down rocks, and in so doing removes carbon dioxide from the atmosphere.

We knew that the dramatic cooling of the planet between 300 and 200 million years ago was the result of the evolution of large plants with large rooting systems that caused huge changes in both of these processes. In the results we published today we showed that the appearance of the first land plants had an effect much earlier - 100 million years earlier.

For me the most important take home message is that the invasion of the land by plants - a pivotal time in the history of the planet - brought about huge climate changes. It should also remind us that the removal of large areas of the world’s vegetation, which act as carbon stores, will increase atmospheric carbon dioxide levels and cause dramatic climate change.

OSB: What can they tell us about plants and climate change today?
LD: Our discovery emphasizes that plants have a central regulatory role in the control of climate: they did yesterday, they do today and they certainly will in the future. This study warns us that if we continue destroy the Earth’s vegetation, by felling forests and draining wet lands, we will suffer dramatic climate change: the opposite of an ice-age. That’s called global warming.

Professor Liam Dolan is based at Oxford's Department of Plant Sciences.