Title:
Documentation of evidence for IPCC AR5 statement based on past levels of CO2
and surface temperature.
•IPCC
Statement: With medium confidence, global mean surface temperature was
significantly above pre industrial level during past several periods
characterised by high atmospheric CO2 concentration.
1.
Importance of
studying past levels of CO2 and surface temperature:
Ancient occurrence of greenhouse warming presents us
an insight into the coupling of climate and carbon cycle and assists us in
forecasting of the aftermath of increased carbon emissions in the future1.
“Increasing concentrations of CO2 in sea water are driving a
progressive acidification of the ocean”1.This can adversely affect many marine
calcifying animals, hence, studying the past conditions would help us to
understand the biodiversity of these marine animals1. Cenozoic
global archives provide examples of natural climate states globally warmer than
the present 2. The Cenozoic era is the most recent of the three
major subdivisions of the animal history 3. The glacial-interglacial
periods drove CO2 variations of ~100ppm over 420,000 years. Variations in
atmospheric CO2 were <20ppm during the last 11,000 years 4. During the Holocene prior to the Industrial Era there
were relatively small variations of atmospheric CO2 recorded in ice cores,
despite small emissions from human- caused changes in land use over the last
millenia 5. The climate
reconstructions for the warm period periods of Cenozoic provide an opportunity
to assess Earth system and equilibrium climate sensitivity 6. Thus, it
would give a possible analogue to study future climate (end of the century) and
predict temperature rise 6. Based on the palaeoclimatic evidence, the
state of the biosphere in the Cenozoic era can also be predicted, which would
further assist us in predicting future biosphere condition in a warmer climate 6.
Studying the past climate can increase the confidence ability in climate models
to predict future change 6. Continued
comparisons of palaeoclimatic data and model simulations are necessary to
increase confidence in future climate projections6.
2.
Comparing
the past and the present levels of CO2.
3.
Evidence for high carbon dioxide worlds and temperature
(Hyperthermals):
"The geological
archives for Cenozoic era (last 65 Ma) provide
examples of natural climate states globally warmer than the present which are
associated with atmospheric CO2 concentrations above pre industrial level. This
relationship between high CO2 and global warmth are complicated by
the factors such as tectonics and the evolution of the biological system which
play an important role in the carbon systems"2, 8.
Proxy and model data have been available form three
Cenozoic warm periods. 1.) Palaeocene –Eocene Thermal maximum (PETM), 2.) Early
Eocene climatic optimum (EECO) 3.) Mid Pliocene warm period2.
3.1 Palaeocene –Eocene
Thermal maximum (PETM):
Proof for
global temperature increase in the PETM was found in the sedimentary records
which included low resolution marine stable isotope records of the PETM and the
carbon excursion isotope and sea floor sediment isotope records8. There
was a high-speed and pronounced decrease in the C13/C12 ratio
of carbonate and organic carbon8. Prominent drop was noticed in the
carbon content of the marine sediment deposited at a large depth. According to Zachos,
Dickens and Zeebe8 2008, the sources of carbon remained uncertain8.
They8 mention that the carbon might have come from deeply buried
rocks and was perhaps liberated as methane8. However, uncertainties
continued to exist as processes involving microbial decomposition of organic
carbon which could occur as an additional feedback were unexplored8.
PETM was marked by a massive carbon
release and global acidification 9, 1. According to Zachos, 2005 9, the Palaeocene-Eocene
thermal maximum (PETM) was accredited for the release of ~2000 × 109 metric
tonnes of carbon in the form of methane9. This scenario is
attributed for lowering of the deep-sea pH9. A study presented
geochemical data from new five South Atlantic deep-sea sections as they
compelled the timing and extent of large sea-floor carbonate dissolution
coincident with the PETM9. The benthic foraminiferal extinction
horizon was characterized by the disappearance of long-lived Palaeocene species
and major drop in diversity was indicated at the base of the clay layer in each
site9.
The Earth system model
GENIE-1 was used by Panchuk, Ridgwell and Kump10, 2008, to simulate
the preservation of carbonates in deep-sea sediments as a function of bottom
water chemistry. The first spatially resolved model study of ocean and sediment
carbon cycling was carried out10. It was done by comparing the
observed spatial variations in the CaCO3 wt % of marine PETM sediments
with predicted changes in CaCO3 for carbon pulses spanning the range
of distinct sources biogenic methane with ?13 C of 60% to
mantle derived volcanic CO2 10. When the global average concentrations of Ca2+ and Mg2+
were set to 18.2 and 29.2 mmol kg-1
a late Palaeocene p CO2 level of 750 ppm gave model average deep ocean
temperatures of ~7°C warmer than modern but cooler than temperatures derived
from ?18 O of
benthic foraminifera of ~11-112°10. Carbon release of
4500 to 6800 PgC over 5 to 20 kyr was estimated, which led to emissions rate of
~0.5 to 1.0 PgC yr/110. The timing of the PETM coincided with
massive eruptions of flood basalts and the installation of masses of igneous
rocks between older layers of sedimentary rocks and provided important CO2
contribution10. The
source of the 6800 PgC with ?13 C of 22% could be the large-scale
oxidation of the organic matter10.
Although, uncertainties remained as the isotope excursions were insufficient to
constrain the potential sources of carbon addition10.
Using the Earth system model GENIE-1, Ridgwell and Schmidt 20101 found under saturation with respect to carbonate in the
deep ocean was which could have endangered marine calcifying organisms 1.
The non-calcifying deep-sea species and shallow water taxa showed lower
levels of extinctions whereas, extinction associated with deep-sea calcifiers
was noticed at a higher rate1.
3.2
Early Eocene climatic optimum (EECO)
"The Early Eocene encompasses the warmest climates of
the past 65 million years11. Evidence shows that there was
occurrence of frost-intolerant flora and fauna near the higher latitudes, hence
it implies that the higher latitude sea water temperatures in deep water
formations in winter could not have gone below 10°C in the early Eocene11.
In 2012, a study was conducted on the sedimentary rock
samples collected from the Palaeocene-Eocene from the Canterbury basin in New
Zealand and the benthic and planktic foraminifera for Ca and Mg studies13.
A preliminary paleo-calibration for the proxy TEX86 was based on
four multiproxy Eocene records and they represented an SST range of 15-34°C13.
This multiproxy was marine temperature history (?18O and Mg/Ca), extended
from middle Paleocene to the middle Eocene13.
The SST's which were derived from the proxies exhibited a warm bias that increased
as the TEX86 values
decreased. The TEX86 proxy indicated that the southwest Pacific SST
increased by ~10°C from the middle Paleocene to the early Eocene13. The base of the EECO was poorly
defined in these records13.
3.3
Mid Pliocene warm period:
In Pliocene, there was a long-term increase in the
global ice volume and decrease in temperature from ~3.3-2.6 Ma14-16.
It marked the onset of continental scale glaciations in the northern
hemisphere14-16. A
study by Fedorov et al., 201316
establishes that about five to four million years ago, in the early Pliocene
epoch the Earth had a warm and temperate climate pattern16. In this
study, available geochemical proxy records of sea surface temperature were compared
with that of today16. The ocean temperature records came from
proxies alkenone unsaturation index and the Mg/Ca ratios of planktonic
foraminiferal shells which were derived from the material preserved in deep-sea
sediments16. These proxies were recorded by microorganisms living in
the surface mixed layer of the oceans, therefore, their chemical composition
represented their surrounding16. Their study suggested that the
combination of the several dynamic feedbacks which were underestimated in the
model like ocean mixing and cloud albedo may have been the reason for the
climate conditions in the Pliocene16. "Despite large
uncertainties, many proxy data suggest that Pliocene concentrations of CO2
were only 50-100 ppm higher than pre-industrial values" 16.
An assessment18 was done of the confidence
determined for each estimate of the mean annual SST from 95 sites spread
throughout the mid-Picacenzian global ocean18. The estimates in this
study were based on quantitative analysis of planktonic foraminiferal faunas
from the Deep Sea Drilling Project (DSDP and the Ocean Drilling Programme (ODP).
The regional and environmental conditions allowed the inclusions of biotic
proxies like molluscs, bryozoa, diatoms, dinoflagellates, radiolaria and
ostracods on a small scale18. Diatoms were used in the Southern ocean as they were
excellent indicators of the position of the sea-ice. Marine and shallow-water
regions provided additional geographical coverage18. To confirm the
palaeo-environmental estimates, independent palaeotemperature methodologies
were conducted using fossil groups18. The North Atlantic group of
sites displayed very high confidence as they illustrated ever-increasing temperature
anomaly with increasing latitude18. The upwelling zones in the
North, equatorial and South Pacific and in the North Atlantic off North Africa
showed warmer than modern SST18. Inability to constrain with
certainty of critical forcing mechanism and boundary conditions that climate
models require to simulate Pliocene SST's gave rise to uncertainties18.
4.
Relevant work done since the IPCC
report :
A new method19
was introduced to extract rates of change from a
sedimentary record based on the relative timing of climate and carbon cycle
changes, without the need for an age model19. The result interpreted
the maximum sustained PETM carbon release rate to less than 1.1 PgC yr -1
19. The model19 suggested
that future ecosystem disruptions will mostly exceed the relatively restricted
extinctions detected during PETM19.
Cui and Schubert20, 2017
used the increase in carbon isotope fractionation by C3 land plants
in response to increase pCO2 20.
The uncertainty on each pCO2 estimated
in this experiment is low20. The results represent first pCO2 proxy estimates directly attached to the Eocene hyperthermals20.
Thus, the results signified that the early Eocene pCO2 was assisted by the background pCO2 less than ~3.5×pre-industrial levels20.
The results also symbolized that pCO2
>1000 ppmv perhaps had occurred only briefly during hyperthermal
events20.
Wolfe et al.21, 2017
reconstructed temperature, precipitation and CO2 from the latest middle Eocene in sub-arctic
Canada21 . “The climatic range and oxygen isotope analysis of
botanical fossils revealed humid temperate forest ecosystem with mean annual
temperatures of more than 17°C warmer than present”21. This study
revealed that reconstructed Delta mean annual temperatures are more than 6°C
warmer than those produced by Eocene climate models which were forced at 560ppm
CO2 21.. The CO2 reconstruction in
this study was lower than inferences of ~800-1000 ppm from alkenone ?13 C between 39 and 37 Ma and 650
+/-110 ppm at 68% confidence.21 Thus, the study supported lower CO2
concentrations than previously predicted
for greenhouse climate intervals21.
Atmospheric CO2
reconstructions based on multi-site boron-isotope records from the late
Pliocene were done by Martínez-Botí et al. in 201522. It was found
that the Earth’s climate sensitivity to CO2 radiative forcing was
half as strong during the warm Pliocene as during the cold Pleistocene epoch22.
The study concluded that on a global scale, no unexpected climate feedbacks
operated during the warm Pliocene except for the long-term ice albedo feedbacks22.
It also interpreted that feedbacks for the Pliocene like future are well
described by the current accepted range of 1.5 K to 4.5 K per doubling of CO2
22.
In a study done by Penman et al23.,
2014 Boron based proxies for surface ocean carbonate chemistry were used23.
The first observational evidence for a drop in the pH of surface and
thermocline sea-water during the PETM was presented23. The planktic
foraminifers showed a ~0.8% decrease in boron isotopic composition along with
the reduction in shell B/Ca in the North Pacific ocean23. Similar
trends were present in lower resolution records from the South Atlantic and
Equatorial Pacific. The observations were consistent with global acidification
of the surface ocean lasting for ~70 kyr23. The anomalies in the
boron records were consistent with an initial surface pH drop of ~0.3 units23.
5.
Conclusions:
The past events of the
Cenozoic era give us a glimpse of the state of the planet in a world of higher
atmospheric CO2 and higher temperatures. However, uncertainties continue
to remain in the implication of certain factors persisting in the warm period.
It is important to improve expertise in reducing uncertainties to simulate
features of the climate in the three warm periods. Most of the challenges seem
to be occurring in the comprehension of the role of positive and negative
feedbacks. A broadened approach is needful to increase model performances to
expand the confidence levels in future.
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