Editing Eocene (section)

==== Equable climate problem ====

One of the unique features of the Eocene's climate as mentioned before was the equable and homogeneous climate that existed in the early parts of the Eocene. A multitude of [[Proxy (climate)|proxies]] support the presence of a warmer equable climate being present during this period of time. A few of these proxies include the presence of fossils native to warm climates, such as [[crocodile]]s, located in the higher latitudes,<ref name="Sloan12"/><ref name="Huber13"/> the presence in the high latitudes of frost-intolerant flora such as [[palm trees]] which cannot survive during sustained freezes,<ref name="Huber13"/><ref name="Huber14"/> and fossils of [[snakes]] found in the tropics that would require much higher average temperatures to sustain them.<ref name="Huber13"/> [[TEX86|TEX<sub>86</sub>]] BAYSPAR measurements indicate extremely high [[sea surface temperature]]s of {{cvt|40|C}} to {{cvt|45|C}} at low latitudes,<ref>{{cite journal |last1=Grossman |first1=Ethan L. |last2=Joachimski |first2=Michael M. |date=27 May 2022 |title=Ocean temperatures through the Phanerozoic reassessed |journal=[[Scientific Reports]] |volume=12 |issue=1 |page=8938 |doi=10.1038/s41598-022-11493-1 |pmid=35624298 |pmc=9142518 |bibcode=2022NatSR..12.8938G |s2cid=249128273 }}</ref> although [[clumped isotopes|clumped isotope]] analyses point to a maximum low latitude sea surface temperature of {{cvt|36.3|C}} ± {{cvt|1.9|C}} during the EECO.<ref>{{cite journal |last1=Evans |first1=David |last2=Sagoo |first2=Navjit |last3=Renema |first3=Willem |last4=Cotton |first4=Laura J. |last5=Müller |first5=Wolfgang |last6=Todd |first6=Jonathan A. |last7=Saraswati |first7=Pratul Kumar |last8=Stassen |first8=Peter |last9=Ziegler |first9=Martin |last10=Pearson |first10=Paul N. |last11=Valdes |first11=Paul J. |last12=Affek |first12=Hagit P. |date=22 January 2018 |title=Eocene greenhouse climate revealed by coupled clumped isotope-Mg/Ca thermometry |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=115 |issue=6 |pages=1174–1179 |doi=10.1073/pnas.1714744115 |pmid=29358374 |pmc=5819407 |bibcode=2018PNAS..115.1174E |doi-access=free }}</ref> Relative to present-day values, bottom water temperatures are {{cvt|10|C-change}} higher according to isotope proxies.<ref name="Huber14"/> With these bottom water temperatures, temperatures in areas where deep water forms near the poles are unable to be much cooler than the bottom water temperatures.{{citation needed|date=February 2022}}

An issue arises, however, when trying to model the Eocene and reproduce the results that are found with the [[Proxy (climate)|proxy data]].<ref name="Sloan15"/> Using all different ranges of greenhouse gasses that occurred during the early Eocene, models were unable to produce the warming that was found at the poles and the reduced seasonality that occurs with winters at the poles being substantially warmer. The models, while accurately predicting the tropics, tend to produce significantly cooler temperatures of up to {{cvt|20|C-change}} colder than the actual determined temperature at the poles.<ref name="Huber14"/> This error has been classified as the "equable climate problem". To solve this problem, the solution would involve finding a process to warm the poles without warming the tropics. Some hypotheses and tests which attempt to find the process are listed below.{{citation needed|date=February 2022}}

===== Large lakes =====

Due to the nature of water as opposed to land, less temperature variability would be present if a large body of water is also present. In an attempt to try to mitigate the cooling polar temperatures, large lakes were proposed to mitigate seasonal climate changes.<ref name="Sloan16"/> To replicate this case, a lake was inserted into North America and a climate model was run using varying carbon dioxide levels. The model runs concluded that while the lake did reduce the seasonality of the region greater than just an increase in carbon dioxide, the addition of a large lake was unable to reduce the seasonality to the levels shown by the floral and faunal data.{{citation needed|date=February 2022}}

===== Ocean heat transport =====

The transport of heat from the tropics to the poles, much like how ocean heat transport functions in modern times, was considered a possibility for the increased temperature and reduced seasonality for the poles.<ref name="Huber17"/> With the increased sea surface temperatures and the increased temperature of the deep ocean water during the early Eocene, one common hypothesis was that due to these increases there would be a greater transport of heat from the tropics to the poles. Simulating these differences, the models produced lower heat transport due to the lower temperature gradients and were unsuccessful in producing an equable climate from only ocean heat transport.{{citation needed|date=February 2022}}

===== Orbital parameters =====

While typically seen as a control on ice growth and seasonality, the orbital parameters were theorized as a possible control on continental temperatures and seasonality. Simulating the Eocene by using an ice free planet, [[Orbital eccentricity|eccentricity]], [[obliquity]], and [[precession]] were modified in different model runs to determine all the possible different scenarios that could occur and their effects on temperature. One particular case led to warmer winters and cooler summer by up to 30% in the North American continent, and it reduced the seasonal variation of temperature by up to 75%. While orbital parameters did not produce the warming at the poles, the parameters did show a great effect on seasonality and needed to be considered.<ref name="Sloan18"/>

===== Polar stratospheric clouds =====

Another method considered for producing the warm polar temperatures were [[polar stratospheric cloud]]s.<ref name="Sloan19"/> Polar stratospheric clouds are clouds that occur in the lower stratosphere at very low temperatures. Polar stratospheric clouds have a great impact on radiative forcing. Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and trap outgoing longwave radiation. Different types of polar stratospheric clouds occur in the atmosphere: polar stratospheric clouds that are created due to interactions with nitric or [[sulfuric acid]] and water (Type I) or polar stratospheric clouds that are created with only water ice (Type II).{{citation needed|date=February 2022}}

Methane is an important factor in the creation of the primary Type II polar stratospheric clouds that were created in the early Eocene.<ref name="Sloan6"/> Since water vapor is the only supporting substance used in Type II polar stratospheric clouds, the presence of water vapor in the lower stratosphere is necessary where in most situations the presence of water vapor in the lower stratosphere is rare. When methane is oxidized, a significant amount of water vapor is released. Another requirement for polar stratospheric clouds is cold temperatures to ensure condensation and cloud production. Polar stratospheric cloud production, since it requires the cold temperatures, is usually limited to nighttime and winter conditions. With this combination of wetter and colder conditions in the lower stratosphere, polar stratospheric clouds could have formed over wide areas in Polar Regions.{{citation needed|date=February 2022}}

To test the polar stratospheric clouds effects on the Eocene climate, models were run comparing the effects of polar stratospheric clouds at the poles to an increase in atmospheric carbon dioxide.<ref name="Sloan19"/> The polar stratospheric clouds had a warming effect on the poles, increasing temperatures by up to 20&nbsp;°C in the winter months. A multitude of feedbacks also occurred in the models due to the polar stratospheric clouds' presence. Any ice growth was slowed immensely and would lead to any present ice melting. Only the poles were affected with the change in temperature and the tropics were unaffected, which with an increase in atmospheric carbon dioxide would also cause the tropics to increase in temperature. Due to the warming of the troposphere from the increased [[greenhouse effect]] of the polar stratospheric clouds, the stratosphere would cool and would potentially increase the amount of polar stratospheric clouds.

While the polar stratospheric clouds could explain the reduction of the equator to pole temperature gradient and the increased temperatures at the poles during the early Eocene, there are a few drawbacks to maintaining polar stratospheric clouds for an extended period of time. Separate model runs were used to determine the sustainability of the polar stratospheric clouds.<ref name="Kirk21"/> It was determined that in order to maintain the lower stratospheric water vapor, methane would need to be continually released and sustained. In addition, the amounts of ice and condensation nuclei would need to be high in order for the polar stratospheric cloud to sustain itself and eventually expand.{{citation needed|date=February 2022}}