Middle Miocene Climatic Optimum

The Middle Miocene Climatic Optimum (MMCO), sometimes referred to as the Middle Miocene Thermal Maximum (MMTM), was an interval of warm climate during the Miocene epoch, specifically the Burdigalian and Langhian stages.

Duration
Based on the magnetic susceptibility of Miocene sedimentary stratigraphic sequences in the Huatugou section in the Qaidam Basin, the MMCO lasted from 17.5 to 14.5 Ma; rocks deposited during this interval have a high magnetic susceptibility due to the production of supermaramagnetic and single domain magnetite amidst the warm and humid conditions at the time that defines the MMCO.

Estimates derived from Mg/Ca palaeothermometry in the benthic foraminifer Oridorsalis umbonatus suggest the onset of the MMCO occurred at 16.9 Ma, peak warmth at 15.3 Ma, and the end of the MMCO at 13.8 Ma.

Climate
Global mean surface temperatures during the MMCO were approximately 18.4 °C, about 3 °C warmer than today and 4 °C warmer than preindustrial. The latitudinal zone of tropical climate was significantly greatened. During orbital eccentricity maxima, which corresponded to warm phases, the ocean's lysocline shoaled by approximately 500 metres.

The Arctic was ice free and warm enough to host permanent forest cover across much of its extent. Iceland had a humid and subtropical climate.

The mean annual temperature (MAT) of the United Kingdom was 16.9 °C. In Central Europe, the minimal cold months temperature (mCMT) was at least 8.0 °C and the minimal warm months temperature (mWMT) was about 18.3 °C, with a total MAT of no cooler than 17.4 °C. Central Europe's mean annual precipitation range was 1050–1600 mm, based on data from Hevlín Quarry in the Czech Republic. Climatic data from Poland and Bulgaria suggest a minimal latitudinal temperature gradient in Europe during the MMCO. Dense, humid rainforests covered much of France, Switzerland, and northern Germany, while southern and central Spain were arid and contained open environments. In the North Alpine Foreland Basin (NAFB), hydrological cycling intensified during the MMCO. The Austrian locality of Stetten had a mean winter temperature of 9.6-13.3 °C and a mean summer temperature of 24.7-27.9 °C, contrasting with -1.4 °C and 19.9 °C in the present, respectively; precipitation amounts at this site were 9–24 mm in winter and 204–236 mm in summer.

The Northern Hemisphere summer location of the Intertropical Convergence Zone (ITCZ) shifted northward; because the ITCZ is the zone of maximal monsoonal rainfall, the precipitation brought by the East Asian Summer Monsoon (EASM) increased over southern China while simultaneously declining over Indochina. The Tibetan Plateau was overall wetter and warmer.

Overall, Western North America north of 40 °N was wetter than south of 40 °N. The Mojave region of western North America exhibited a drying trend. Along the New Jersey shelf, the MMCO did not result in any discernable climatic signal relative to earlier or later climatic intervals of the Miocene; temperatures here may have been kept low by an uplift of the Appalachian Mountains.

Northern South America developed increased seasonality in its precipitation patterns as a consequence of the ITCZ's northward migration during the MMCO. The Bolivian Altiplano had a MAT of 21.5-21.7 ± 2.1 °C, in stark contrast to its present MAT of 8-9 °C, while its MMCO precipitation patterns were identical to those of today.

In Antarctica, average summer temperatures were about 10 °C. The East Antarctic Ice Sheet (EAIS) was severely reduced in area, and it may have occupied as little as 25% of its present volume. However, despite its diminished size and its retreat away from the coastline of Antarctica, the EAIS remained relatively thick. Additionally, Antarctica's polar ice sheets exhibited high variability and instability throughout this warm period.

Modelling of ocean circulation shows that the Atlantic Meridional Overturning Circulation (AMOC) was strengthened by the greater inflow of waters from the Pacific and Indian Oceans due to more open Panama and Tethys Seaways. This stronger AMOC in turn resulted in a deeper mixed layer. The Antarctic Circumpolar Current (ACC) became stronger as westerly wind stress increased and Antarctic sea ice diminished in extent.

Causes
The global warmth of the MMCO resulted from its elevated atmospheric carbon dioxide concentrations relative to the rest of the Neogene. Boron-based records indicate pCO2 varied between 300 and 500 ppm during the MMCO. A MMCO pCO2 estimate of 852 ± 86 ppm has been derived from palaeosols in Railroad Canyon, Idaho. The primary cause of this high pCO2 is generally accepted to be elevated volcanic activity. Hydrothermal alteration by magmatic dikes and sills of sediments rich in organic carbon further contributed to rising pCO2. The activity of the Columbia River Basalt Group (CRBG), a large igneous province in the northwestern United States that emitted 95% of its contents between 16.7 and 15.9 Ma, is believed to be the dominant geological event responsible for the MMCO. The CRBG has been estimated to have added 4090–5670 Pg of carbon into the atmosphere in total, 3000-4000 Pg of which was discharged during the Grande Ronde Basalt eruptions, explaining much of the MMCO's anomalous warmth. Carbon dioxide was released both directly from volcanic activity as well as cryptic degassing from intrusive magmatic sills that liberated the greenhouse gas from existing sediments. However, CRBG activity and cryptic degassing does not sufficiently explain warming before 16.3 Ma. Enhanced tectonic activity led to increased volcanic degassing at plate margins, enabling high background warmth to occur and complementing CRBG activity in driving temperatures upwards.

Albedo decrease from the reduction in Earth's surface area covered by deserts and the expansion of forests was an important positive feedback enhancing the warmth of the MMCO.

The increase in organic carbon burial on lands submerged by rising sea levels resultant from the increased warmth were an important negative feedback inhibiting further warming. This positive carbon excursion is called the Monterey Carbon Excursion, which is globally recorded but mainly in the circum pacific belt. The Monterey Excursion seems to envelop the MMCO, meaning this carbon excursion started just before the climatic optimum and it ended just after it.

Climate modelling has shown that there remain as-of-yet unknown forcing and feedback mechanisms that had to have existed to account for the observed rise in temperature during the MMCO, as the amount of carbon dioxide known to have been in the atmosphere during the MMCO along with other known boundary conditions are insufficient in explaining the high temperatures of the Middle Miocene.

The strengthening of West African offshore winds and enhancement of continental weathering in North Africa caused oxygen minimum zones to expand in the Atlantic off the coast of West Africa.

Biotic effects
The world of the MMCO was heavily forested; trees grew across the Arctic and even in parts of Antarctica. Tundras and forest tundras were absent from the Arctic.

Northern North America was dominated by cool-temperate forests. Western North America was mostly composed of warm-temperate evergeen broadleaf and mixed forest. In spite of the climatic changes, the niches of Oregonian equids were unchanged throughout the MMCO. What is now the Mojave Desert was a grassland dominated by C3 grasses during the MMCO. Central America was tropical, as it is today.

In Europe, the MMCO witnessed the northward expansion of thermophilic plants. Along the northwestern coast of the Central Paratethys, mixed mesophytic forest vegetation predominated. At the Stetten locality, spruces and firs increased in abundance during transgressive phases of precessionally forced transgressive-regressive cycles, while marshes, many of them saline, dominated by Cyperaceae and swamps dominated by Taxodiaceae prevailed during sea level lowstands. Because of the dense, humid forests covering central eastern France and northern Germany, the species richness of these areas was high and the mammal community dominated by small taxa, while the more arid Iberian Peninsula had a lower species richness and a relative absence of medium-sized mammals. Along the western margin of the Central Paratethys, primate diversity exploded, likely because of the unique mosaic of different habitats it hosted. Europe also contained an abundance of ectothermic vertebrates due to its much warmer climate in the MMCO compared to the present. In the Paratethys, marine biodiversity peaked at the culmination of the MMCO.

The MMCO may have favoured the spread of pongines into Asia by creating continuous stretches of subtropical forest that enabled the migration of these apes from Africa into Eurasia. A simultaneous dispersal of rhizomyine and ctenodactyline rodents occurred along this same corridor.

Northern South America possessed tropical evergreen broadleaf forests. The Atacama Desert already existed along the western coast of central South America and graded into temperate xerophytic shrubland and temperate sclerophyll woodland and shrubland to the south. In eastern South America south of 35 °S, warm-temperate evergreen broadleaf and mixed forest predominated, alongside temperate grassland. The MMCO played a major role in the partitioning and diversification of South America's land mammal faunas.

In Africa, rapid speciation in Bicyclus representing the continent's largest radiation of satyrine butterflies occurred amidst the climatic changes of the MMCO.

Comparison to present global warming
The MMCO's temperature estimates of 3-4 °C above the preindustrial mean are similar to those projected in the future by mid-range forecasts of anthropogenic global warming conducted by the Intergovernmental Panel on Climate Change (IPCC). Estimates of future pCO2 are also remarkably similar to those derived for the MMCO. Because of these many similarities, many palaeoclimatologists use the MMCO as an analogue for what Earth's future climate will look like. Arguably, it is the best of all possible analogues; the pCO2 of the cooler Pliocene has already been exceeded, while the warmer Eocene had global temperatures and carbon dioxide levels so high that reaching them would require scenarios that are no longer considered realistic or likely to occur.