The cool tropics paradox refers to an apparent difference between modelled estimates of tropical

Tropics

The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

 temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

s during warm, ice-free periods of the Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 and Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

, and the colder temperatures which proxies suggested were present. The long-standing paradox was resolved when it became clear that the proxies were misleading, meaning that tropics were warmer than previously believed.

Origin of the paradox

Proxy

Proxy (climate)

In the study of past climates is known as paleoclimatology, climate proxies are preserved physical characteristics of the past that stand in for direct measurements , to enable scientists to reconstruct the climatic conditions that prevailed during much of the Earth's history...

-based reconstructions of paleotemperature appeared to predict a low temperature gradient between the tropics and poles

Polar region

Earth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...

. Data from surface-dwelling foramanifera suggested that during the late Cretaceous, an unusually warm period, sea surface temperature

Sea surface temperature

Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...

s were cooler than today's. 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. These could not produce an ice-free state without using an increased atmospheric concentration of ; this assumption was checked against the evidence and found to be valid. This introduced a new difficulty: more would produce warmer tropical sea temperatures, and the evidence suggested they were the same or even colder than today's.

Data supporting cool tropical oceans

Foramaniferal data, suggesting tropical temperatures cooler than today's, disagreed with terrestrial proxies, which spoke of warmer temperatures - although most of the terrestrial figures are based on extrapolation of data from outside the tropics.

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. Other factors mean that any pristine sample can be considered to have an associated error of up to 3°C. 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.

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 levels produced more tropical cloud, shielding these regions from the sun's heat. However, there was no evidence for this behaviour, and still left problems. The poles were still warmer than the models predicted. 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.

Hints of warmth - terrestrial proxies

Data from terrestrial proxies suggested that the equator may have reached 30°C - however, this figure is based upon extrapolation of data found outside the tropics. This would imply that the foramaniferal proxies were wrong - the tests may perhaps have been overprinted by diagenesis

Diagenesis

In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...

. Researchers turned to shallow marine molluscs

Mollusca

The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

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. Also, near-shore molluscs preserve seasonal variability in their shells, a feature that would be lost in the presence of a diagenetic signal. 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. 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. This trend fits best if was the dominant force for cooling.

The winter temperatures of molluscs match well with the foramaniferal temperatures, suggesting that foramanifera predominantly grew during the winter months. The overall temperatures corresponded well with terrestrial and modelled estimates of a sea surface temperature around 4-5° warmer than today's.

Reassessment of the foramaniferal 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. Further painstaking studies targeted solely those foramanifera which could be demonstrated not to have undergone any diagenesis in fact give a δ¹⁸O signature similar to that expected, suggesting that poor preservation was responsible for the original confusion.