Who is driving whom? Climate and carbon cycle in perpetual interaction


Date:
October 6, 2020
Source:
MARUM - Center for Marine Environmental Sciences, University
of Bremen
Summary:
The current climate crisis underlines that carbon cycle
perturbations can cause significant climate change. New research
reveals how carbon cycle and global climate have been interacting
throughout the last 35 million years of geologic history, under
natural circumstances.



FULL STORY ==========================================================================
The current climate crisis underlines that carbon cycle perturbations
can cause significant climate change. New research reveals how carbon
cycle and global climate have been interacting throughout the last 35
million years of geologic history, under natural circumstances.


========================================================================== Human-made global heating has long been presented as a relatively simple
chain of cause and effect: humans disrupt the carbon cycle by burning
fossil fuels, thereby increase the concentration of CO2 in the atmosphere, which in turn leads to higher temperatures around the globe. "However, it becomes increasingly clear that this is not the end of the story. Forest
fires become more frequent all over the world, release additional CO2 into
the atmosphere, and further reinforce the global warming that enhanced
forest fire risk in the first place. This is a textbook example of what
climate scientists call a positive feedback mechanism," stresses David
De Vleeschouwer, a postdoctoral researcher at MARUM -- Center for Marine Environmental Sciences at the University of Bremen.

To reveal these kind of climate-carbon cycle feedback mechanisms
under natural circumstances, David De Vleeschouwer and colleagues
exploited isotopic data from deep-ocean sediment cores. "Some of these
cores contain sediments of up to 35 million years old. Despite their respectable age, these sediments carry a clear imprint of so-called
Milankovi? cycles. Milankovi? cycles relate to rhythmic changes in
the shape of the Earth's orbit (eccentricity), as well as to the tilt (obliquity) and orientation (precession) of the Earth's rotational
axis. Like an astronomical clockwork, Milankovi? cycles generate changes
in the distribution of solar insolation over the planet, and thus provoke cadenced climate change," explains David De Vleeschouwer. "We looked
at the carbon and oxygen isotope composition of microfossils within
the sediment and first used the eccentricity, obliquity and precession
cadences as geological chronometers.

Then, we applied a statistical method to determine whether changes
in one isotope system lead or lag variability in the other isotope."
His colleague Maximilian Vahlenkamp adds: "When a common pattern in
both isotope systems occurs just a little earlier in the carbon system
compared to the oxygen isotope system, we call this a carbon-isotope
lead. We then infer that the carbon cycle exerted control over the
climate system at the time of sediment deposition." Paleoclimatologists
and paleoceanographers often use carbon isotopes as an indicator of carbon-cycle perturbations, and oxygen isotopes as a proxy for changes
in global climate state. Changes in the isotopic composition of these
deep-sea microfossils may indicate, for example, an increase in the
continental carbon storage by land plants and soils, or global cooling
with a growth of ice caps.

"The systematic and time-continuous analysis of leads and lags between
carbon cycle and climate constitutes the innovative character of this
study. Our approach allows to sequence Earth's history at high resolution
over the past 35 million years," says Prof Heiko Pa"like. "We show that
the past 35 million years can be subdivided in three intervals, each
with its specific climate- carbon cycle modus operandi." On average, the authors found oxygen isotopes to lead carbon isotope variations. This
means that, under natural conditions, climate variations are largely
regulating global carbon cycle dynamics.

However, the research team focused on times when the opposite was
the case.

Indeed, De Vleeschouwer and colleagues found a few examples of
ancient periods during which the carbon cycle drove climate change on approximately 100,000- year timescales, just as it is the case now on much shorter timescales -- "but then of course without human intervention,"
states Pa"like.

During the oldest interval, between 35 and 26 million years ago, the
carbon cycle took the lead over climate change mostly during periods
of climate stability. "Periods of climate stability in the geologic
record often have an astronomical cause. When the Earth's orbit around
the sun is close to a perfect circle, seasonal insolation extremes are truncated and more equable climates are enforced," explains David De Vleeschouwer. "Between 35 and 26 million years ago, such astronomical configuration would have been favourable for a temporal expansion of the Antarctic ice sheet. We propose that under such a scenario, the intensity
of glacial erosion and subsequent rock weathering increased. This is
important, because the weathering of silicate rocks removes CO2 from
the atmosphere, and thus ultimately controls the greenhouse effect."
But around 26 million years ago, the modus operandi radically changed. The carbon cycle took control over climate at times of climate volatility,
not stability. "We believe this change traces back to the uplift of
the Himalayan mountains and a monsoon-dominated climate state. When
seasonal insolation extremes are amplified through an eccentric Earth
orbit, monsoons can become truly intense. Stronger monsoons permit more chemical weathering, the removal of CO2 from the atmosphere and thus
a carbon-cycle control over climate." The mechanisms proposed by the
authors not only explain the observed patterns in carbon and oxygen
isotopes, they also provide new ideas as to how the climate system
and the carbon cycle interacted through time. "Some hypotheses need
further testing with numerical climate and carbon cycle models, but
the process-level understanding presented in this work is important
because it provides a glimpse at the machinery of our planet under
boundary conditions that are fundamentally different from today's,"
says De Vleeschouwer. Moreover, this work also provides scenarios that
can be used to evaluate the ability of climate-carbon cycle models when
they are pushed to the extreme scenarios of the geologic past.


========================================================================== Story Source: Materials provided by MARUM_-_Center_for_Marine_Environmental_Sciences,
University_of_Bremen. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. David De Vleeschouwer, Anna Joy Drury, Maximilian Vahlenkamp, Fiona
Rochholz, Diederik Liebrand, Heiko Pa"like. High-latitude biomes
and rock weathering mediate climate-carbon cycle feedbacks on
eccentricity timescales. Nature Communications, 2020; 11 (1) DOI:
10.1038/s41467-020- 18733-w ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201006114221.htm

--- up 6 weeks, 1 day, 6 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)