New
research reveals how the arrival of the first plants 470 million years ago
triggered a series of ice ages. Led by the Universities of Exeter and Oxford, the study is
published in Nature Geoscience.
The
team set out to identify the effects that the first land plants had on the
climate during the Ordovician Period, which ended 444 million years ago. During
this period the climate gradually cooled, leading to a series of ‘ice ages’.
This global cooling was caused by a dramatic reduction in atmospheric carbon,
which this research now suggests was triggered by the arrival of plants.
Among
the first plants to grow on land were the ancestors of mosses that grow today.
This study shows that they extracted minerals such as calcium, magnesium,
phosphorus, and iron from rocks in order to grow. In so doing, they caused
chemical weathering of the Earth’s surface. This had a dramatic impact on the
global carbon cycle and subsequently on the climate. It could also have led to
a mass extinction of marine life.
The
research suggests that the first plants caused the weathering of calcium and
magnesium ions from silicate rocks, such as granite, in a process that removed
carbon dioxide from the atmosphere, forming new carbonate rocks in the ocean.
This cooled global temperatures by around 5 C.
In
addition, by weathering the nutrients phosphorus and iron from rocks, the first
plants increased the quantities of both these nutrients going into the oceans,
fuelling productivity there and causing organic carbon burial. This removed yet
more carbon from the atmosphere, further cooling the climate by another two to
3 C. It could also have had a devastating impact on marine life, leading to a
mass extinction that has puzzled scientists.
The
team used the modern moss, Physcomitrella
patens for their study. They placed a number of rocks, with or
without moss growing on them, into incubators. Over three months they were able
to measure the effects the moss had on the chemical weathering of the rocks.
They
then used an Earth system model to establish what difference plants could have
made to climate change during the Ordovician Period.
One
of the lead researchers, Professor Tim Lenton of geography at the University of Exeter said: “This study demonstrates
the powerful effects that plants have on our climate. Although plants are still
cooling the Earth’s climate by reducing atmospheric carbon levels, they cannot
keep up with the speed of today’s human-induced climate change. In fact, it
would take millions of years for plants to remove current carbon emissions from
the atmosphere.”
Professor Liam Dolan of Oxford University,
one of the lead researchers, said: “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. 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.”
