BIOGEOCHEMISTRY

Climatic plant power

"Yves Goddéris and Yannick Donnadieu"

Levels of atmospheric carbon dioxide constrain vegetation types and thus

also non-biological uptake during rock weathering. That’s the reasoning used

to explain why CO2 levels did not fall below a certain point in the Miocene.

The world is currently at risk of overheating

in response to all the carbon dioxide being

pumped into the atmosphere from the use of

fossil fuels: the current atmospheric concentration

of CO2 is about 385 parts per million

(p.p.m.), compared with a ‘pre-industrial’

level of around 280 p.p.m. But overheating is

an atypical menace in the recent history of the

Earth. Over most of the past 24 million years,

it was the possibility of cooling that posed the

main threat to life. Cooling, however, did not

reach the levels of severity that might have

been expected, and on page 85 of this issue

Pagani et al.1 put forward a thought-provoking

case as to why that was so.

Since the end of the Eocene, around 40

million years ago, Earth’s climate has been

naturally getting colder. In temporal terms,

the cooling has sometimes occurred in discrete

steps, sometimes as a long-term trend2.

Over the same interval, levels of atmospheric

CO2 have fallen from around 1,400 p.p.m. at

the end of the Eocene to possibly as low as

200 p.p.m. during the Miocene3 — the geological

period between around 24 million and 5 million

years ago.

This long-term history of atmospheric CO2

is the result of the interplay between several

pro cesses. The degassing of the Earth through

magmatic activity (for instance volcanic

eruptions) is the main source of CO2, and

the dissolution of continental rocks captures

atmospheric CO2, which is eventually stored

as marine carbonate sediments4. The efficiency

of the dissolution process — chemical

weathering — is heavily dependent on climate,

but also depends on vegetation and physical

erosion. The last two parameters boost CO2

uptake by rock weathering. In particular, land

plants promote rock dissolution through the

mechanical action of roots, and by acidifying

the water in contact with rocks5. Acidification

occurs through the release of organic acids and

the large-scale accumulation of CO2 in soil

through root respiration. Removing plants,

particularly trees, may strongly decrease the

dissolution rate of rocks and the ability of this

process to consume atmospheric CO2.

In their paper, Pagani and colleagues1 consider

the potential role of the rise of many mountainous

regions (orogens) over the past 40 million

years, especially in the warm and humid

low-latitude areas. In these mountain ranges,

physical erosion would break down rocks and

expose them to intense chemical weathering.

The uptake of atmospheric CO2 would consequently

increase, as indicated by the levels of

CO2 measured, which could have declined to

the lowest levels since multicellular life evolved

on Earth some 500 million years ago.

But what could stop this CO2 uptake pump?

Degassing through magmatic activity was

probably declining at the same time (at best it

remained constant), and tectonic activity accelerated

mountain uplift over the past 24 million

years. According to this line of evidence, CO2

levels should have plunged to below 200 p.p.m.,

with ice ultimately covering large surfaces of

the Earth as a consequence. But that was not

the case. Earth even experienced a warm spell

between 18 million and 14 million years ago2.

Pagani et al.1 propose an exciting hypothesis

to explain why, 24 million years ago, CO2

might have levelled off at about 200 p.p.m., and

then stuck there. They suggest that, when CO2

levels became too low, forests became starved

and were progressively replaced by grasslands,

particularly in the low-latitude orogens. Grasslands

exert a much less vigorous effect on rocks

than do trees. In consequence, runs the thinking,

CO2 consumption due to weathering

declined because of changing terrestrial ecosystems,

which in turn stabilized atmospheric

CO2 at around 200 p.p.m.

Overall, the authors’ model provides an

elegant twist on several ideas about the Earth

system that emphasize the role of vegetation

in dynamically regulating and fixing the lower

limit of atmospheric CO2. But it also raises contentious

issues.

First, in the model1, forest starvation is triggered

by the low level of atmospheric CO2,

and by elevated temperature. But do proxy

estimates of conditions at that time confirm

this paradoxical combination? Proxy measurements

of CO2 levels include marine carbonate

boron isotopes6, carbon-isotope values

of alkenones produced by oceanic algae3 and

the density of stomata — a measure of gas

exchange — in fossil leaves7. Unlike the geochemical

proxy records3,6, the more recent

estimates based on the stomatal index7 depict

a highly variable CO2 trend over the Miocene

(in good agreement with climatic fluctuations),

rather than a CO2 level stuck at 200 p.p.m.

Furthermore, the estimates show that CO2

concentrations are above the forest-starvation

level most of the time, oscillating between 300

and 500 p.p.m.

Second, in their model Pagani et al. assume

that rock weathering generated by mountain

uplift would have continuously consumed

atmospheric CO2 until it reached the foreststarvation

level. But there is evidence that the

extra consumption of CO2 due to the Himalayan

uplift, the most important orogeny of

the recent past, occurred mainly through the

burial of organic matter in the Bengal fan,

and not through rock weathering8,9. In addition,

the tectonic history of the past 24 million

years is still subject to debate, and the timing

of the uplift of the main mountain ranges, such

as the Himalaya and Andes, is far from fully

constrained10.

Finally, the link between weathering and

continental vegetation is well recognized. But

it is complex. Apart from acidifying water and

mechanical effects, land plants also control the

hydrology of soils. In humid tropical environments,

about 70% of the rainfall is absorbed by

land plants and then evaporates through their

leaves. This effect should inhibit weathering

reactions by limiting the amount of water available

for rock dissolution. Also, in equatorial

uplifted areas, intense erosion occurs through

landslides triggered by heavy rainfall11. These

landslides bring fresh rock material in contact

with water by removing the soil mantle, promoting

weathering and CO2 consumption. The

role of vegetation cover in these systems might

not be as significant as Pagani et al. suggest.

The authors themselves acknowledge some

of these limitations, and all in all have put

forward a bold and provocative hypothesis.

But accounting for all of the processes and

constraints involved is probably beyond the

capabilities of the first-order global models

that Pagani et al. used, and more-complex and

process-based modelling12,13 will be required to

test their conclusions. Whatever the outcome,

that should prove to be a fruitful exercise for

carbon-cycle modellers intent on understanding

the processes that drove climate and CO2

oscillations during the Miocene.

Yves Goddéris is at the LMTG-Observatoire

Midi-Pyrénées, CNRS, Université de Toulouse III,

Toulouse F-31400, France. Yannick Donnadieu

is at LSCE, CNRS-CEA, Gif-sur-Yvette F-91191,

France.

e-mails: godderis@lmtg.obs-mip.fr;

yannick.donnadieu@lsce.ipsl.fr