Title: characterised by high atmospheric CO2 concentration. 1.



Documentation of evidence for IPCC AR5 statement based on past levels of CO2
and surface temperature.


Statement: With medium confidence, global mean surface temperature was
significantly above pre industrial level during past several periods
characterised by high atmospheric CO2 concentration.

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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 1000 ppmv perhaps had occurred only briefly during hyperthermal

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

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.



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.






1.) Ridgwell, A. and Schmidt, D.
(2010). Past constraints on the vulnerability of marine calcifiers to massive
carbon dioxide release. Nature Geoscience, 3(3), pp.196-200.

2.) IPCC Fifth
Assessment Report (AR5). (2013). Geneva: WMO, IPCC Secretariat.

3.) Eicher, D. (1982). Geologic
time. Englewood Cliffs: Prentice-Hall.

4.) Field, C. (2012). The
global carbon cycle. United States: Island Press.

5.) Pongratz, J., Reick, C., Raddatz,
T. and Claussen, M. (2009). Effects of anthropogenic land cover change on the
carbon cycle of the last millennium. Global Biogeochemical Cycles,
23(4), p.n/a-n/a.

6.) Marci Robinson
and Harry Dowsett, U.S. Geological Survey 926A National Center, Reston, VA

7.) National Oceanic and Atmospheric Administration,
Earth System Research Laboratory, Global Monitoring Division.

Zachos, J., Dickens, G. and Zeebe, R. (2008). An
early Cenozoic perspective on greenhouse warming and carbon-cycle
dynamics. Nature, 451(7176), pp.279-283

9.)Zachos, J. (2005). Rapid Acidification
of the Ocean During the Paleocene-Eocene Thermal Maximum. Science,
308(5728), pp.1611-1615.

 10.) Panchuk, K., Ridgwell, A. and Kump, L.
(2008). Sedimentary response to Paleocene-Eocene Thermal Maximum carbon
release: A model-data comparison. Geology, 36(4), p.315.

11.) Huber, M. and
Caballero, R. (2011). The early Eocene equable climate problem revisited. Climate
of the Past, 7(2), pp.603-633.


12.) Lunt, D., Dunkley
Jones, T., Heinemann, M., Huber, M., LeGrande, A., Winguth, A., Loptson, C., Marotzke,
J., Tindall, J., Valdes, P. and Winguth, C. (2012). A model-data comparison for
a multi-model ensemble of early Eocene atmosphere-ocean simulations:
EoMIP. Climate of the Past Discussions, 8(2), pp.1229-1273.


13.) Hollis, C., Taylor,
K., Handley, L., Pancost, R., Huber, M., Creech, J., Hines, B., Crouch, E.,
Morgans, H., Crampton, J., Gibbs, S., Pearson, P. and Zachos, J. (2012). Early
Paleogene temperature history of the Southwest Pacific Ocean: Reconciling
proxies and models. Earth and Planetary Science Letters, 349-350,


14.) Lisiecki, L. and
Raymo, M. (2005). A Pliocene-Pleistocene stack of 57 globally distributed
benthic ?18O records. Paleoceanography, 20(1), p.n/a-n/a.


15.) Mudelsee, M. and
Raymo, M. (2005). Slow dynamics of the Northern Hemisphere glaciation. Paleoceanography,
20(4), p.n/a-n/a.


16.) Fedorov, A.,
Brierley, C., Lawrence, K., Liu, Z., Dekens, P. and Ravelo, A. (2013). Patterns
and mechanisms of early Pliocene warmth. Nature, 496(7443),


17.) Haywood, A., Hill,
D., Dolan, A., Otto-Bliesner, B., Bragg, F., Chan, W., Chandler, M., Contoux,
C., Jost, A., Kamae, Y., Lohmann, G., Lunt, D., Abe-Ouchi, A., Pickering, S.,
Ramstein, G., Rosenbloom, N., Sohl, L., Stepanek, C., Yan, Q., Ueda, H. and
Zhang, Z. (2012). Large-scale features of Pliocene climate: results from the
Pliocene Model Intercomparison Project. Climate of the Past Discussions,
8(4), pp.2969-3013.


18.) Dowsett, H.,
Robinson, M., Haywood, A., Hill, D., Dolan, A., Stoll, D., Chan, W., Abe-Ouchi,
A., Chandler, M., Rosenbloom, N., Otto-Bliesner, B., Bragg, F., Lunt, D.,
Foley, K. and Riesselman, C. (2012). Assessing confidence in Pliocene sea
surface temperatures to evaluate predictive models. Nature Climate
Change, 2(5), pp.365-371.


19.) Zeebe, R., Ridgwell, A.
and Zachos, J. (2016). Anthropogenic carbon release rate unprecedented during
the past 66 million years. Nature Geoscience, 9(4), pp.325-329.


20.) Cui, Y. and Schubert, B.
(2017). Atmospheric p CO 2 reconstructed across five early Eocene global
warming events. Earth and Planetary Science Letters, 478,


Wolfe, A., Reyes, A., Royer, D., Greenwood, D., Doria, G.,
Gagen, M., Siver, P. and Westgate, J. (2017). Middle Eocene CO2and climate
reconstructed from the sediment fill of a subarctic kimberlite maar. Geology,
45(7), pp.619-622.

22.) Martínez-Botí, M., Foster, G., Chalk,
T., Rohling, E., Sexton, P., Lunt, D., Pancost, R., Badger, M. and Schmidt, D.
(2015). Addendum: Plio-Pleistocene climate sensitivity evaluated using high-resolution
CO2 records. Nature,
526(7573), pp.458-458

23.) Penman, D., Hönisch, B., Zeebe, R., Thomas, E. and
Zachos, J. (2014). Rapid and sustained
surface ocean acidification during the Paleocene-Eocene Thermal Maximum. Paleoceanography,
29(5), pp.357-369.