Title:Documentation of evidence for IPCC AR5 statement based on past levels of CO2and surface temperature. •IPCCStatement: With medium confidence, global mean surface temperature wassignificantly above pre industrial level during past several periodscharacterised by high atmospheric CO2 concentration. 1. Importance ofstudying past levels of CO2 and surface temperature:Ancient occurrence of greenhouse warming presents usan insight into the coupling of climate and carbon cycle and assists us inforecasting of the aftermath of increased carbon emissions in the future1.”Increasing concentrations of CO2 in sea water are driving aprogressive acidification of the ocean”1.This can adversely affect many marinecalcifying animals, hence, studying the past conditions would help us tounderstand the biodiversity of these marine animals1. Cenozoicglobal archives provide examples of natural climate states globally warmer thanthe present 2.
The Cenozoic era is the most recent of the threemajor subdivisions of the animal history 3. The glacial-interglacialperiods drove CO2 variations of ~100ppm over 420,000 years. Variations inatmospheric CO2 were <20ppm during the last 11,000 years 4. During the Holocene prior to the Industrial Era therewere relatively small variations of atmospheric CO2 recorded in ice cores,despite small emissions from human- caused changes in land use over the lastmillenia 5. The climatereconstructions for the warm period periods of Cenozoic provide an opportunityto assess Earth system and equilibrium climate sensitivity 6.
Thus, itwould give a possible analogue to study future climate (end of the century) andpredict temperature rise 6. Based on the palaeoclimatic evidence, thestate of the biosphere in the Cenozoic era can also be predicted, which wouldfurther assist us in predicting future biosphere condition in a warmer climate 6.Studying the past climate can increase the confidence ability in climate modelsto predict future change 6. Continuedcomparisons of palaeoclimatic data and model simulations are necessary toincrease confidence in future climate projections6. 2. Comparingthe past and the present levels of CO2. 3. Evidence for high carbon dioxide worlds and temperature(Hyperthermals):”The geologicalarchives for Cenozoic era (last 65 Ma) provideexamples of natural climate states globally warmer than the present which areassociated with atmospheric CO2 concentrations above pre industrial level.
Thisrelationship between high CO2 and global warmth are complicated bythe factors such as tectonics and the evolution of the biological system whichplay an important role in the carbon systems”2, 8.Proxy and model data have been available form threeCenozoic warm periods. 1.) Palaeocene –Eocene Thermal maximum (PETM), 2.) EarlyEocene climatic optimum (EECO) 3.) Mid Pliocene warm period2. 3.1 Palaeocene –EoceneThermal maximum (PETM):Proof forglobal temperature increase in the PETM was found in the sedimentary recordswhich included low resolution marine stable isotope records of the PETM and thecarbon excursion isotope and sea floor sediment isotope records8.
Therewas a high-speed and pronounced decrease in the C13/C12 ratioof carbonate and organic carbon8. Prominent drop was noticed in thecarbon 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 buriedrocks and was perhaps liberated as methane8. However, uncertaintiescontinued to exist as processes involving microbial decomposition of organiccarbon which could occur as an additional feedback were unexplored8.PETM was marked by a massive carbonrelease and global acidification 9, 1. According to Zachos, 2005 9, the Palaeocene-Eocenethermal maximum (PETM) was accredited for the release of ~2000 × 109 metrictonnes of carbon in the form of methane9.
This scenario isattributed for lowering of the deep-sea pH9. A study presentedgeochemical data from new five South Atlantic deep-sea sections as theycompelled the timing and extent of large sea-floor carbonate dissolutioncoincident with the PETM9. The benthic foraminiferal extinctionhorizon was characterized by the disappearance of long-lived Palaeocene speciesand major drop in diversity was indicated at the base of the clay layer in eachsite9. The Earth system modelGENIE-1 was used by Panchuk, Ridgwell and Kump10, 2008, to simulatethe preservation of carbonates in deep-sea sediments as a function of bottomwater chemistry. The first spatially resolved model study of ocean and sedimentcarbon cycling was carried out10. It was done by comparing theobserved spatial variations in the CaCO3 wt % of marine PETM sedimentswith predicted changes in CaCO3 for carbon pulses spanning the rangeof distinct sources biogenic methane with ?13 C of 60% tomantle derived volcanic CO2 10. When the global average concentrations of Ca2+ and Mg2+ were set to 18.
2 and 29.2 mmol kg-1a late Palaeocene p CO2 level of 750 ppm gave model average deep oceantemperatures of ~7°C warmer than modern but cooler than temperatures derivedfrom ?18 O ofbenthic foraminifera of ~11-112°10. Carbon release of4500 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 withmassive eruptions of flood basalts and the installation of masses of igneousrocks between older layers of sedimentary rocks and provided important CO2contribution10.
Thesource of the 6800 PgC with ?13 C of 22% could be the large-scaleoxidation of the organic matter10.Although, uncertainties remained as the isotope excursions were insufficient toconstrain 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 thedeep ocean was which could have endangered marine calcifying organisms 1.The non-calcifying deep-sea species and shallow water taxa showed lowerlevels of extinctions whereas, extinction associated with deep-sea calcifierswas noticed at a higher rate1. 3.2Early Eocene climatic optimum (EECO)”The Early Eocene encompasses the warmest climates ofthe past 65 million years11. Evidence shows that there wasoccurrence of frost-intolerant flora and fauna near the higher latitudes, henceit implies that the higher latitude sea water temperatures in deep waterformations in winter could not have gone below 10°C in the early Eocene11.
In 2012, a study was conducted on the sedimentary rocksamples collected from the Palaeocene-Eocene from the Canterbury basin in NewZealand and the benthic and planktic foraminifera for Ca and Mg studies13.A preliminary paleo-calibration for the proxy TEX86 was based onfour multiproxy Eocene records and they represented an SST range of 15-34°C13.This multiproxy was marine temperature history (?18O and Mg/Ca), extendedfrom middle Paleocene to the middle Eocene13.The SST’s which were derived from the proxies exhibited a warm bias that increasedas the TEX86 valuesdecreased. The TEX86 proxy indicated that the southwest Pacific SSTincreased by ~10°C from the middle Paleocene to the early Eocene13. The base of the EECO was poorlydefined in these records13.3.3Mid Pliocene warm period:In Pliocene, there was a long-term increase in theglobal ice volume and decrease in temperature from ~3.
3-2.6 Ma14-16.It marked the onset of continental scale glaciations in the northernhemisphere14-16.
Astudy by Fedorov et al., 201316establishes that about five to four million years ago, in the early Plioceneepoch the Earth had a warm and temperate climate pattern16. In thisstudy, available geochemical proxy records of sea surface temperature were comparedwith that of today16. The ocean temperature records came fromproxies alkenone unsaturation index and the Mg/Ca ratios of planktonicforaminiferal shells which were derived from the material preserved in deep-seasediments16. These proxies were recorded by microorganisms living inthe surface mixed layer of the oceans, therefore, their chemical compositionrepresented their surrounding16. Their study suggested that thecombination of the several dynamic feedbacks which were underestimated in themodel like ocean mixing and cloud albedo may have been the reason for theclimate conditions in the Pliocene16. “Despite largeuncertainties, many proxy data suggest that Pliocene concentrations of CO2were only 50-100 ppm higher than pre-industrial values” 16.
An assessment18 was done of the confidencedetermined for each estimate of the mean annual SST from 95 sites spreadthroughout the mid-Picacenzian global ocean18. The estimates in thisstudy were based on quantitative analysis of planktonic foraminiferal faunasfrom the Deep Sea Drilling Project (DSDP and the Ocean Drilling Programme (ODP).The regional and environmental conditions allowed the inclusions of bioticproxies like molluscs, bryozoa, diatoms, dinoflagellates, radiolaria andostracods on a small scale18. Diatoms were used in the Southern ocean as they wereexcellent indicators of the position of the sea-ice. Marine and shallow-waterregions provided additional geographical coverage18.
To confirm thepalaeo-environmental estimates, independent palaeotemperature methodologieswere conducted using fossil groups18. The North Atlantic group ofsites displayed very high confidence as they illustrated ever-increasing temperatureanomaly with increasing latitude18. The upwelling zones in theNorth, equatorial and South Pacific and in the North Atlantic off North Africashowed warmer than modern SST18. Inability to constrain withcertainty of critical forcing mechanism and boundary conditions that climatemodels require to simulate Pliocene SST’s gave rise to uncertainties18.
4. Relevant work done since the IPCCreport : A new method19was introduced to extract rates of change from asedimentary record based on the relative timing of climate and carbon cyclechanges, without the need for an age model19. The result interpretedthe maximum sustained PETM carbon release rate to less than 1.1 PgC yr -119. The model19 suggestedthat future ecosystem disruptions will mostly exceed the relatively restrictedextinctions detected during PETM19. Cui and Schubert20, 2017used the increase in carbon isotope fractionation by C3 land plantsin response to increase pCO2 20.The uncertainty on each pCO2 estimatedin 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 hyperthermalevents20.Wolfe et al.21, 2017reconstructed temperature, precipitation and CO2 from the latest middle Eocene in sub-arcticCanada21 . “The climatic range and oxygen isotope analysis ofbotanical fossils revealed humid temperate forest ecosystem with mean annualtemperatures of more than 17°C warmer than present”21.
This studyrevealed that reconstructed Delta mean annual temperatures are more than 6°Cwarmer than those produced by Eocene climate models which were forced at 560ppmCO2 21.. The CO2 reconstruction inthis 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 CO2concentrations than previously predictedfor greenhouse climate intervals21. Atmospheric CO2reconstructions based on multi-site boron-isotope records from the latePliocene were done by Martínez-Botí et al. in 201522. It was foundthat the Earth’s climate sensitivity to CO2 radiative forcing washalf as strong during the warm Pliocene as during the cold Pleistocene epoch22.The study concluded that on a global scale, no unexpected climate feedbacksoperated during the warm Pliocene except for the long-term ice albedo feedbacks22.It also interpreted that feedbacks for the Pliocene like future are welldescribed by the current accepted range of 1.5 K to 4.5 K per doubling of CO222.
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 andthermocline sea-water during the PETM was presented23. The plankticforaminifers showed a ~0.
8% decrease in boron isotopic composition along withthe reduction in shell B/Ca in the North Pacific ocean23. Similartrends were present in lower resolution records from the South Atlantic andEquatorial Pacific. The observations were consistent with global acidificationof the surface ocean lasting for ~70 kyr23. The anomalies in theboron records were consistent with an initial surface pH drop of ~0.3 units23.
5. Conclusions:The past events of theCenozoic era give us a glimpse of the state of the planet in a world of higheratmospheric CO2 and higher temperatures. However, uncertainties continueto remain in the implication of certain factors persisting in the warm period.It is important to improve expertise in reducing uncertainties to simulatefeatures of the climate in the three warm periods. Most of the challenges seemto be occurring in the comprehension of the role of positive and negativefeedbacks. A broadened approach is needful to increase model performances toexpand the confidence levels in future.
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