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The mid-Cretaceous hothouse is a time period characterized by warm air temperatures, warm ocean temperatures, and no ice volume, even in polar regions, that occurred from 90 to 120 million years (Ma) ago. In association with the geologic timescale, the Cretaceous period took place from approximately 145 Ma to 65 Ma, and was part of the Mesozoic Era. The period of the mid-Cretaceous was characterized by an equable climate, meaning the lack of a distinct temperature gradient between the tropics and the poles. Many scientists believe that the cause of this equable climate and the associated globally warm temperatures during the mid-Cretaceous can possibly be attributed to increased greenhouse gases to enhance the greenhouse effect and changes in planetary albedo due to natural forcings at the time. However, the major factor that forced the equable climate of the Cretaceous is now thought to be increased greenhouse gas concentrations, mainly carbon dioxide.

Equable Climate and the Associated Paradox
An equable climate is characterized by a weak meridional temperature gradient between the equator and the poles at sea level. Today, the sea-level temperature difference is about 50 degrees Celsius in the northern hemisphere, 90 degrees in the southern hemisphere, from the equator to the poles, and in the time of the mid-Cretaceous, this difference ranged from 30 degrees to as little as 24 degrees Celsius in both hemispheres. In order to warm the polar regions and reduce the meridional temperature gradient an increase in poleward heat transport is required (Hay, 2008). However, a reduced meridional temperature gradient results in less poleward heat transport, which means this is an unlikely force in the creation of an equable climate. This is a major climate paradox that many scientists are still researching today when looking into the mid-Cretaceous climate. Some believe that the key to an equable climate still lies with the factor of higher greenhouse gas concentration that evidence dictates did occur during that time period.

Another problem with the reduced meridional temperature gradient is that the tropics would have “overheated,” in a sense, in this type of situation. Some scientists believe that a combination of three mechanisms, high CO2 content, increased oceanic heat flux, and geography might have compensated for this and prevented the tropics from overheating.

In addition to the weak equator to pole temperature gradient, oceans at the time of the mid-Cretaceous were also characterized by a limited temperature gradient between surface water temperatures and deep water temperatures. Based on oxygen isotopic data, this may have lead to a disruption in the thermocline and thermohaline circulations of the ocean.

The Greenhouse Gas Effect
Another one of the main factors that may have contributed to the warm temperatures linked to the mid-Cretaceous hothouse was the high greenhouse gas concentration of carbon dioxide and methane, reinforced by high water vapor content in the atmosphere due to very warm temperatures that enhanced the greenhouse effect. Carbon dioxide values during the Mid-Cretaceous were estimated to be approximately 2 to 4 times greater than values today, which have a substantial influence on the amount of infrared radiation that is absorbed and reemitted back to the planet. Research conducted in the Gyeongsang Basin in Korea revealed evidence indicating that pCO2 levels, or the partial pressure of carbon dioxide, were at their peak between the early and late Cretaceous, which corresponds to high temperature values previously determined in paleoclimate temperature data.

An increase in volcanic activity globally, as seen in the remains of LIPS, or large igneous provinces, located in what is now the Caribbean and Madagascar, is also considered to have been a major contributor to the high amount of carbon dioxide that was released into the atmosphere to correspond with the spike in temperatures during the hothouse. High oceanic crust production and expansion of the seafloor is another potential trigger, albeit small factor, in causing high pCO2 values during the mid-Cretaceous. Both of these processes would have occurred very slowly over a long period of time in order to cause a large increase in select greenhouse gases.

Vegetation
When modeling the mid-Cretaceous, general circulation models (GCMs) are used to show “winter surface temperatures that are too cold in continental interiors to explain the distribution of cold-sensitive fossil plants”. Vegetation is generally overlooked when it comes to mechanisms that would play a role in land maintaining warmth at higher latitudes. Vegetation plays an important role in regulating fluxes of energy, water vapor, and momentum between the land surface and atmosphere. Vegetation also plays a key role in the absorption of solar radiation, which in turn decreases the surface albedo of the Earth. Because of the extensive land area that vegetation covers due to stems and leaves, there is an increase in aerodynamic drag or surface roughness which in turn increases mass transfer as well as energy transfer between the Earth’s surface and its atmosphere. “...vegetation played an important role in maintaining warm polar climates during periods such as the mid-Cretaceous, because high-latitude forests occupied extensive regions that today are occupied by tundra or glacial ice”.

At high latitudes, a forest vegetation presence sets in motion a feedback between the albedo, surface temperature, and snow and ice cover, which leads to a much warmer climate than one would expect. In a comparison of models, researchers have found that in high latitude regions (60°-90°N) covered in vegetation, the surface albedo will reduce throughout the year, but it is most effective in the first few months of the year. Between trees, forest cover, and the growth of leaves in the late spring to early summer, there is an absorption of radiation and these plants have lower albedoes than bare soil.

One of the models used for simulations of the Cretaceous hothouse time period is run by the National Center for Atmospheric Research (NCAR). This model is known as GENESIS Global Climate Model v. 102 and is used to study atmosphere, ocean, and land surface interactions. The atmosphere submodel incorporates thermodynamics, fluid motion equations, radiative processes, and more. The modified Community Climate Model from NCAR also uses water vapor transport along with convection, clouds, and others. The atmosphere is divided vertically into 12 different levels. The ocean submodel has mixed-layer ocean data up to 50 m deep that is then coupled with a thermodynamic model of sea-ice. This portion of the model “crudely captures the seasonal heat capacity of the ocean mixed layer but ignores salinity, upwelling, and energy exchange with deeper layers”. The third submodel, land surface, combines “a land surface transfer component (LSX) to account for the physical effects of vegetation, a six-layer soil component, and a three-layer thermodynamic snow component”. Within the model, soil and vegetation are displayed differently. Soil is shown with color and texture while vegetation is divided into an upper and lower layer depending on the type of vegetation.

Lack of Ice Volume and Sea Level
Scientists used a planetary albedo model that suggested that the greater extent of low albedo water and reduced area of more reflective land during the mid-Cretaceous could be a significant factor in explaining Cretaceous warmth. This would have occurred due to the fact that the land mass at the time was closer together, leaving the rest of the planet to the open ocean, which has a much lower albedo. Model simulations showed that no one type of effect (i.e. sea level, continental displacement, or topography) can fully account for the warming that was occurring in the polar oceans, nor can they account for the higher temperature of the mid-Cretaceous. Higher sea levels at the time of the mid-Cretaceous had even more of an impact on lowering the planetary albedo, to increase the amount of solar insolation taken in rather than reflected back out of the planet. Also, due to the fact that there was a lack of ice sheets during the mid-Cretaceous, the planetary albedo was lowered even more because there was no ice-albedo feedback effect, which allowed for even more warming of the planet.

Comparisons to Today’s Climate
A few scientists speculate that the climate of today, and the impact of humans on the atmosphere and climate, may resemble similar aspects of factors that lead to the warm atmospheric and oceanic temperatures that comprised the climate of the mid-Cretaceous hothouse. The primary forcing that generates a more equable climate is greenhouse gases, specifically CO2, and current trends have CO2 values being 8 times the pre-industrial values by the year 2300, which will dramatically increase the melting of sea ice. In the future, there will likely be no arctic sea ice cover during the summer months. This will lead to changes in atmospheric circulations, by shifting the areas of rising and sinking motion, changes of the locations of frontal boundaries associated with the major planetary atmospheric circulations, and changes in oceanic circulations. There will be little to no ice-albedo feedback, which will lead to even more warming of the planet as higher albedo ice disappears and lower albedo sea level increases.