Cool tropics paradox

Last updated August 19, 2023

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.

Origin of the paradox

Proxy-based reconstructions of paleotemperature appeared to predict a low temperature gradient between the tropics and poles. Data from surface-dwelling foraminifera 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

The only way the model could be "tweaked" was by changing 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] Other feedbacks, including increased poleward heat transport by the oceans, and vegetational responses at high latitudes, were proposed, but these did not 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 foraminiferal 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 foraminiferal temperatures, suggesting that foraminifera 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.

Related Research Articles

The Cretaceous is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of the entire Phanerozoic. The name is derived from the Latin creta, "chalk", which is abundant in the latter half of the period. It is usually abbreviated K, for its German translation Kreide.

The Cenozoic is Earth's current geological era, representing the last 66 million years of Earth's history. It is characterised by the dominance of mammals, birds, conifers and flowering plants, a cooling and drying climate, and the current configuration of continents. It is the latest of three geological eras since complex life evolved, preceded by the Mesozoic and Paleozoic. It started with the Cretaceous–Paleogene extinction event, when many species, including the non-avian dinosaurs, became extinct in an event attributed by most experts to the impact of a large asteroid or other celestial body, the Chicxulub impactor.

The Eocene Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name Eocene comes from the Ancient Greek ἠώς and καινός and refers to the "dawn" of modern ('new') fauna that appeared during the epoch.

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich from his studies of marine beds in Belgium and Germany. The name comes from the Ancient Greek ὀλίγος and καινός, and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period.

The Paleogene is a geologic period and system that spans 43 million years from the end of the Cretaceous Period 66 million years ago (Mya) to the beginning of the Neogene Period 23.03 Mya. It is the beginning of the Cenozoic Era of the present Phanerozoic Eon. The earlier term Tertiary Period was used to define the span of time now covered by the Paleogene Period and subsequent Neogene Period; despite no longer being recognized as a formal stratigraphic term, "Tertiary" still sometimes remains in informal use. Paleogene is often abbreviated "Pg".

Paleoclimatology is the scientific study of climates predating the invention of meteorological instruments, when no direct measurement data were available. As instrumental records only span a tiny part of Earth's history, the reconstruction of ancient climate is important to understand natural variation and the evolution of the current climate.

The Paleocene–Eocene thermal maximum (PETM), alternatively "Eocene thermal maximum 1" (ETM1), and formerly known as the "Initial Eocene" or "Late Paleocene thermal maximum", was a time period with a more than 5–8 °C global average temperature rise across the event. This climate event occurred at the time boundary of the Paleocene and Eocene geological epochs. The exact age and duration of the event is uncertain but it is estimated to have occurred around 55.5 million years ago (Ma).

Foraminifera are single-celled organisms, members of a phylum or class of amoeboid protists characterized by streaming granular ectoplasm for catching food and other uses; and commonly an external shell of diverse forms and materials. Tests of chitin are believed to be the most primitive type. Most foraminifera are marine, the majority of which live on or within the seafloor sediment, while a smaller number float in the water column at various depths, which belong to the suborder Globigerinina. Fewer are known from freshwater or brackish conditions, and some very few (nonaquatic) soil species have been identified through molecular analysis of small subunit ribosomal DNA.

In the study of past climates ("paleoclimatology"), climate proxies are preserved physical characteristics of the past that stand in for direct meteorological measurements and enable scientists to reconstruct the climatic conditions over a longer fraction of the Earth's history. Reliable global records of climate only began in the 1880s, and proxies provide the only means for scientists to determine climatic patterns before record-keeping began.

The geologic temperature record are changes in Earth's environment as determined from geologic evidence on multi-million to billion (10⁹) year time scales. The study of past temperatures provides an important paleoenvironmental insight because it is a component of the climate and oceanography of the time.

TEX₈₆ is an organic paleothermometer based upon the membrane lipids of mesophilic marine Nitrososphaerota (formerly Marine Group 1 Crenarchaeota).

Paleoceanography is the study of the history of the oceans in the geologic past with regard to circulation, chemistry, biology, geology and patterns of sedimentation and biological productivity. Paleoceanographic studies using environment models and different proxies enable the scientific community to assess the role of the oceanic processes in the global climate by the re-construction of past climate at various intervals. Paleoceanographic research is also intimately tied to paleoclimatology.

In geochemistry, paleoclimatology and paleoceanography δ¹⁸O or delta-O-18 is a measure of the ratio of stable isotopes oxygen-18 (¹⁸O) and oxygen-16 (¹⁶O). It is commonly used as a measure of the temperature of precipitation, as a measure of groundwater/mineral interactions, and as an indicator of processes that show isotopic fractionation, like methanogenesis. In paleosciences, ¹⁸O:¹⁶O data from corals, foraminifera and ice cores are used as a proxy for temperature.

The Eocene–Oligocene extinction event, also called the Eocene-Oligocene transition or Grande Coupure, is the transition between the end of the Eocene and the beginning of the Oligocene, an extinction event and faunal turnover occurring between 33.9 and 33.4 million years ago marked by large-scale extinction and floral and faunal turnover. Most of the affected organisms were marine or aquatic in nature. They included the last of the ancient ungulates, the "condylarths".

The Cretaceous Thermal Maximum (CTM), also known as Cretaceous Thermal Optimum, was a period of climatic warming that reached its peak approximately 90 million years ago (90 Ma) during the Turonian age of the Late Cretaceous epoch. The CTM is notable for its dramatic increase in global temperatures characterized by high carbon dioxide levels.

The climate across the Cretaceous–Paleogene boundary is very important to geologic time as it marks a catastrophic global extinction event. Numerous theories have been proposed as to why this extinction event happened including an asteroid known as the Chicxulub asteroid, volcanism, or sea level changes. While the mass extinction is well documented, there is much debate about the immediate and long-term climatic and environmental changes caused by the event. The terrestrial climates at this time are poorly known, which limits the understanding of environmentally driven changes in biodiversity that occurred before the Chicxulub crater impact. Oxygen isotopes across the K–T boundary suggest that oceanic temperatures fluctuated in the Late Cretaceous and through the boundary itself. Carbon isotope measurements of benthic foraminifera at the K–T boundary suggest rapid, repeated fluctuations in oceanic productivity in the 3 million years before the final extinction, and that productivity and ocean circulation ended abruptly for at least tens of thousands of years just after the boundary, indicating devastation of terrestrial and marine ecosystems. Some researchers suggest that climate change is the main connection between the impact and the extinction. The impact perturbed the climate system with long-term effects that were much worse than the immediate, direct consequences of the impact.

Cyclonic Niño is a climatological phenomenon that has been observed in climate models where tropical cyclone activity is increased. Increased tropical cyclone activity mixes ocean waters, introducing cooling in the upper layer of the ocean that quickly dissipates and warming in deeper layers that lasts considerably more, resulting in a net warming of the ocean.

Hantkenina is a genus of planktonic foraminifera that lived from the Middle Eocene up to late Eocene, cirka 49 Ma-33.9 Ma. There have been 11 morphospecies described, including one of Cribrohantkenina

The Atlantic meridional overturning circulation (AMOC) is a large system of ocean currents, like a conveyor belt. It is driven by differences in temperature and salt content and it is an important component of the climate system. However, the AMOC is not a static feature of global circulation. It is sensitive to changes in temperature, salinity and atmospheric forcings. Climate reconstructions from δ¹⁸O proxies from Greenland reveal an abrupt transition in global temperature about every 1470 years. These changes may be due to changes in ocean circulation, which suggests that there are two equilibria possible in the AMOC. Stommel made a two-box model in 1961 which showed two different states of the AMOC are possible on a single hemisphere. Stommel’s result with an ocean box model has initiated studies using three dimensional ocean circulation models, confirming the existence of multiple equilibria in the AMOC.

Global paleoclimate indicators are the proxies sensitive to global paleoclimatic environment changes. They are mostly derived from marine sediments. Paleoclimate indicators derived from terrestrial sediments, on the other hand, are commonly influenced by local tectonic movements and paleogeographic variations. Factors governing the earth climate system include plate tectonics, which controls the configuration of continents, the interplay between the atmosphere and the ocean, and the earth's orbital characteristics. Global paleoclimate indicators are established based on the information extracted from the analyses of geologic materials, including biological, geochemical and mineralogical data preserved in marine sediments. Indicators are generally grouped into three categories; paleontological, geochemical and lithological.

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.
1 2 3 4 5 6 7 8 9 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.
1 2 3 4 5 6 7 8 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<0983:ROCETT>2.0.CO;2.
1 2 3 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.

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[D'hondt1996-1] 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.

[Crowley2000-2] 1 2 3 4 5 6 7 8 9 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.

[Kobashi2001-3] 1 2 3 4 5 6 7 8 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<0983:ROCETT>2.0.CO;2.

[Lunt2007-4] 1 2 3 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.

[Tripati2003-5] 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.

[Pearson2001-6] 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.

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