Middle Miocene Climatic Optimum

The Middle Miocene Climatic Optimum (MMCO), sometimes referred to as the Middle Miocene Thermal Maximum (MMTM), or Miocene Climatic Optimum (MCO) was an interval between around 17 and 14 million years ago when the climate was approximately 3 °C warmer than today.

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 superparamagnetic 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.

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 extended. The latitudinal climate gradient was about 0.3 °C per degree of latitude. During orbital eccentricity maxima, which corresponded to warm phases, the ocean's lysocline shoaled^{[clarification needed]} 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 an overall MAT no cooler than 17.4 °C. Central Europe's 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. The Ebro Basin of northeastern Spain was nonetheless humid. 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 respectively in the present; precipitation amounts at this site were 9–24 mm in winter and 204–236 mm in summer. Unusually, the bottom waters of the Vienna Basin show a marked cooling during the MMCO. Southwestern Anatolia was dominated by forests.

The Northern Hemisphere summer location of the Intertropical Convergence Zone (ITCZ) shifted northward; because the ITCZ is the zone of maximum monsoon rainfall, the precipitation brought by the East Asian Summer Monsoon (EASM) increased over southern China while simultaneously declining over Indochina, although this finding is contradicted by other studies suggesting that the East Asian hydroclimate was planetary during the MMCO rather than monsoonal. The northwestern Junggar Basin had a warm and humid climate. The Tibetan Plateau likewise was overall wetter and warmer. East Asian hydroclimate during the MMCO was predominantly governed by the 405-kyr eccentricity and 173-kyr obliquity bands.

Overall, Western North America north of 40° N was wetter than south of 40° N. The interior Pacific Northwest experienced a dramatic increase in precipitation during the MMCO around 15.1 Ma. In contrast, 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.