The cool tropics paradox is the apparent difference between modeled estimates of tropicaltemperatures during warm, ice-free periods of the Cretaceous and Eocene, and the colder temperatures which proxies suggested were present. The long-standing paradox was resolved when novel proxy derived temperatures showed significantly warmer tropics during past greenhouse climates. The low-gradient problem, i.e. the very warm polar regions with respect to present day, is still an issue for state-of-the-art climate models.
The cool tropics paradox. The geological evidence appeared to constrain temperatures to the red/blue band, whereas models produced the brown line taking just continental configuration into account, and the blue line when they included increased atmospheric CO2. Vertical axis: Temperature; horizontal:latitude.
Proxy-based reconstructions of paleotemperature appeared to predict a low temperature gradient between the tropics and poles. Data from surface-dwelling foramanifera suggested that during the late Cretaceous, an unusually warm period, sea surface temperatures were cooler than today's.[1] The term was later applied to similar situations, for example during the Eocene.
Climate models which worked during the Tertiary failed to produce this low temperature gradient; in order to match the observed data, they predicted that the tropics should be 40 °C or more - much hotter than the proxies said they were, and much hotter than the tropical surface temperatures observed today, which average around 25 °C (77 °F). To attempt to match the data, bizarre models involving unreasonable eddies were required.
Models
Models were developed to predict and explain the lack of ice during the warm periods of the Cretaceous and Eocene. Models are developed according to the fundamental principle that they should be kept as simple as possible. Consequently, the first models attempted to explain the lack of ice using solely the different continental configuration.[2] These could not produce an ice-free state without using an increased atmospheric concentration of CO2; this assumption was checked against the evidence and found to be valid.[2] This introduced a new difficulty: more CO2 would produce warmer tropical sea temperatures, and the evidence suggested they were the same or even colder than today's.[2]
Data supporting cool tropical oceans
Foraminiferal data, suggesting tropical temperatures cooler than today's, disagreed with terrestrial proxies, which spoke of warmer temperatures[3] - although most of the terrestrial figures are based on extrapolation of data from outside the tropics.[4]
Sources of error
Analytical error is around 2−3 °C for individual specimens, but this drops to 0.5−1.0 °C when a sample is analysed - not enough to explain the discrepancy.[2] Other factors mean that any pristine sample can be considered to have an associated error of up to 3 °C.[2] Changes in salinity, kinetic and diagenesis, can also confound analysis: the latter two are each estimated to reduce estimated temperatures by 1−2 °C, and are difficult to quantify.[2]
Reconciling the data with the model
Taking the data to be true, how could they be reconciled with the predictions of the model? The only way the model could be "tweaked" was by fiddling with the parameterisation of clouds, one of the most unpredictable aspects of any model. The model was adjusted to assume that the higher CO2 levels produced more tropical cloud-cover, shielding these regions from the sun's heat.[2] However, there was no evidence for this behaviour, and still left problems. The poles were still warmer than the models predicted.[2] Further feedbacks, including increased poleward heat transport by the oceans, and vegetational responses at high latitudes, were proposed, but these didn't fully explain behaviour in the southern hemisphere, and winter, respectively.[2]
Unravelling the paradox
Hints of warmth - terrestrial proxies
Data from terrestrial proxies suggested that the equator may have reached 30 °C[4] - however, this figure is based upon extrapolation of data found outside the tropics.[4] This would imply that the foramaniferal proxies were wrong - the tests may perhaps have been overprinted by diagenesis. Researchers turned to shallow marine molluscs as it is easy to determine whether their shells had been altered by diagenesis.
Detecting diagenesis in molluscs
Many mollusc shells are constructed from aragonite, a mineral that is quickly replaced by calcite by diagenetic alteration.[3] Also, near-shore molluscs preserve seasonal variability in their shells, a feature that would be lost in the presence of a diagenetic signal.[3] This removes ambiguity about whether or not a shell has been affected by post-deposition processes.
Data from molluscs
Evidence from the molluscs suggested a cooling between the Eocene and Oligocene.[3] Taken from the Mississippi embayment, they recorded temperatures of around 26 °C in the Eocene, and 22 °C in the Oligocene; this cooling was markedly seasonal, with reconstructed water temperatures being 5° cooler in the summer, but just 3° cooler in winter.[3] This trend fits best if CO2 was the dominant force for cooling.[3]
The winter temperatures of molluscs match well with the foramaniferal temperatures, suggesting that foramanifera predominantly grew during the winter months.[3] The overall temperatures corresponded well with terrestrial and modelled estimates of a sea surface temperature around 4–5° warmer than today's.[3]
Reassessment of the foraminiferal record
The magnesium/calcium paleothermometer is a recently developed alternative to the δ18O method, and avoids many of the uncertainties inherent in the latter method. Use of this technique generates results more consistent with those expected, in contrast to the original δ18O records from the same sites.[5] Further painstaking studies targeted solely those foraminifera which could be demonstrated not to have undergone any diagenesis in fact give a δ18O signature similar to that expected,[6] suggesting that poor preservation was responsible for the original confusion.
References
^D'hondt, S.; Arthur, M.A. (1996). "Late Cretaceous Oceans and the Cool Tropic Paradox". Science. 271 (5257): 1838. Bibcode:1996Sci...271.1838D. doi:10.1126/science.271.5257.1838. S2CID 129699980.
^ abcdefghiCrowley, T.J.; Zachos, J.C. (2000). "Comparison of zonal temperature profiles for past warm time periods". Warm Climates in Earth History. Cambridge University Press. pp. 50–76. ISBN978-0-521-64142-5. Retrieved 2008-04-24.
^Tripati, A.K.; Delaney, M.L.; Zachos, J.C.; Anderson, L.D.; Kelly, D.C.; Elderfield, H. (2003). "Tropical sea-surface temperature reconstruction for the early Paleogene using Mg/Ca ratios of planktonic foraminifera". Paleoceanography. 18 (4): 25. Bibcode:2003PalOc..18d..25T. doi:10.1029/2003PA000937.
^Pearson, P.N.; Ditchfield, P.W.; Singano, J.; Harcourt-brown, K.G.; Nicholas, C.J.; Olsson, R.K.; Shackleton, N.J.; Hall, M.A. (2001). "Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs". Nature. 413 (6855): 481–487. Bibcode:2001Natur.413..481P. doi:10.1038/35097000. PMID 11586350. S2CID 4344258.
January 07, 2023
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This article reads more like a story than an encyclopedia entry To meet Wikipedia s quality standards and conform to the neutral point of view policy please help to introduce a more formal style and remove any personally invested tone April 2008 The cool tropics paradox is the apparent difference between modeled estimates of tropical temperatures during warm ice free periods of the Cretaceous and Eocene and the colder temperatures which proxies suggested were present The long standing paradox was resolved when novel proxy derived temperatures showed significantly warmer tropics during past greenhouse climates The low gradient problem i e the very warm polar regions with respect to present day is still an issue for state of the art climate models The cool tropics paradox The geological evidence appeared to constrain temperatures to the red blue band whereas models produced the brown line taking just continental configuration into account and the blue line when they included increased atmospheric CO2 Vertical axis Temperature horizontal latitude Contents 1 Origin of the paradox 1 1 Models 1 2 Data supporting cool tropical oceans 1 3 Sources of error 1 4 Reconciling the data with the model 2 Unravelling the paradox 2 1 Hints of warmth terrestrial proxies 2 2 Detecting diagenesis in molluscs 2 3 Data from molluscs 2 4 Reassessment of the foraminiferal record 3 ReferencesOrigin of the paradox EditProxy based reconstructions of paleotemperature appeared to predict a low temperature gradient between the tropics and poles Data from surface dwelling foramanifera suggested that during the late Cretaceous an unusually warm period sea surface temperatures were cooler than today s 1 The term was later applied to similar situations for example during the Eocene Climate models which worked during the Tertiary failed to produce this low temperature gradient in order to match the observed data they predicted that the tropics should be 40 C or more much hotter than the proxies said they were and much hotter than the tropical surface temperatures observed today which average around 25 C 77 F To attempt to match the data bizarre models involving unreasonable eddies were required Models Edit Models were developed to predict and explain the lack of ice during the warm periods of the Cretaceous and Eocene Models are developed according to the fundamental principle that they should be kept as simple as possible Consequently the first models attempted to explain the lack of ice using solely the different continental configuration 2 These could not produce an ice free state without using an increased atmospheric concentration of CO2 this assumption was checked against the evidence and found to be valid 2 This introduced a new difficulty more CO2 would produce warmer tropical sea temperatures and the evidence suggested they were the same or even colder than today s 2 Data supporting cool tropical oceans Edit Foraminiferal data suggesting tropical temperatures cooler than today s disagreed with terrestrial proxies which spoke of warmer temperatures 3 although most of the terrestrial figures are based on extrapolation of data from outside the tropics 4 Sources of error Edit Analytical error is around 2 3 C for individual specimens but this drops to 0 5 1 0 C when a sample is analysed not enough to explain the discrepancy 2 Other factors mean that any pristine sample can be considered to have an associated error of up to 3 C 2 Changes in salinity kinetic and diagenesis can also confound analysis the latter two are each estimated to reduce estimated temperatures by 1 2 C and are difficult to quantify 2 Reconciling the data with the model Edit Taking the data to be true how could they be reconciled with the predictions of the model The only way the model could be tweaked was by fiddling with the parameterisation of clouds one of the most unpredictable aspects of any model The model was adjusted to assume that the higher CO2 levels produced more tropical cloud cover shielding these regions from the sun s heat 2 However there was no evidence for this behaviour and still left problems The poles were still warmer than the models predicted 2 Further feedbacks including increased poleward heat transport by the oceans and vegetational responses at high latitudes were proposed but these didn t fully explain behaviour in the southern hemisphere and winter respectively 2 Unravelling the paradox EditHints of warmth terrestrial proxies Edit Data from terrestrial proxies suggested that the equator may have reached 30 C 4 however this figure is based upon extrapolation of data found outside the tropics 4 This would imply that the foramaniferal proxies were wrong the tests may perhaps have been overprinted by diagenesis Researchers turned to shallow marine molluscs as it is easy to determine whether their shells had been altered by diagenesis Detecting diagenesis in molluscs Edit Many mollusc shells are constructed from aragonite a mineral that is quickly replaced by calcite by diagenetic alteration 3 Also near shore molluscs preserve seasonal variability in their shells a feature that would be lost in the presence of a diagenetic signal 3 This removes ambiguity about whether or not a shell has been affected by post deposition processes Data from molluscs Edit Evidence from the molluscs suggested a cooling between the Eocene and Oligocene 3 Taken from the Mississippi embayment they recorded temperatures of around 26 C in the Eocene and 22 C in the Oligocene this cooling was markedly seasonal with reconstructed water temperatures being 5 cooler in the summer but just 3 cooler in winter 3 This trend fits best if CO2 was the dominant force for cooling 3 The winter temperatures of molluscs match well with the foramaniferal temperatures suggesting that foramanifera predominantly grew during the winter months 3 The overall temperatures corresponded well with terrestrial and modelled estimates of a sea surface temperature around 4 5 warmer than today s 3 Reassessment of the foraminiferal record Edit The magnesium calcium paleothermometer is a recently developed alternative to the d18O method and avoids many of the uncertainties inherent in the latter method Use of this technique generates results more consistent with those expected in contrast to the original d18O records from the same sites 5 Further painstaking studies targeted solely those foraminifera which could be demonstrated not to have undergone any diagenesis in fact give a d18O signature similar to that expected 6 suggesting that poor preservation was responsible for the original confusion References Edit D hondt S Arthur M A 1996 Late Cretaceous Oceans and the Cool Tropic Paradox Science 271 5257 1838 Bibcode 1996Sci 271 1838D doi 10 1126 science 271 5257 1838 S2CID 129699980 a b c d e f g h i Crowley T J Zachos J C 2000 Comparison of zonal temperature profiles for past warm time periods Warm Climates in Earth History Cambridge University Press pp 50 76 ISBN 978 0 521 64142 5 Retrieved 2008 04 24 a b c d e f g h Kobashi T Grossman E L Yancey T E Dockery D T 2001 Reevaluation of conflicting Eocene tropical temperature estimates Molluskan oxygen isotope evidence for warm low latitudes Geology 29 11 983 986 Bibcode 2001Geo 29 983K doi 10 1130 0091 7613 2001 029 lt 0983 ROCETT gt 2 0 CO 2 a b c Lunt D J Ross I Hopley P J Valdes P J 2007 Modelling Late Oligocene C4 grasses and climate Palaeogeography Palaeoclimatology Palaeoecology 251 2 239 253 Bibcode 2007PPP 251 239L doi 10 1016 j palaeo 2007 04 004 Tripati A K Delaney M L Zachos J C Anderson L D Kelly D C Elderfield H 2003 Tropical sea surface temperature reconstruction for the early Paleogene using Mg Ca ratios of planktonic foraminifera Paleoceanography 18 4 25 Bibcode 2003PalOc 18d 25T doi 10 1029 2003PA000937 Pearson P N Ditchfield P W Singano J Harcourt brown K G Nicholas C J Olsson R K Shackleton N J Hall M A 2001 Warm tropical sea surface temperatures in the Late Cretaceous and Eocene epochs Nature 413 6855 481 487 Bibcode 2001Natur 413 481P doi 10 1038 35097000 PMID 11586350 S2CID 4344258 Retrieved from https en wikipedia org w index php title Cool tropics paradox amp oldid 1061008227, wikipedia, wiki, book, books, library,
