Little Ice Age that is known as the Medieval Warm Period. There is some concern that a shutdown of thermohaline circulation could happen again as a result of the present warming. More recent research indicates that the overall Atlantic Meriditional Overturning Circulation may already be weaker now than it was during the Little Ice Age, or perhaps even over the past millennium. While there is still a robust debate about the present-day AMOC strength, these findings make the link between AMOC and the Little Ice Age unlikely. However, some research instead suggests that a far more localized disruption of the North Subpolar Gyre convection was involved in the Little Ice Age. This is potentially relevant for the near future, as a minority of climate models project a permanent collapse of this convection under some scenarios of future climate change. Decreased human populations Some researchers have proposed that human influences on climate began earlier than is normally supposed (see Early anthropocene for more details) and that major population declines in Eurasia and the Americas reduced that impact and led to a cooling trend. The Black Death in Europe The Black Death is estimated to have killed 30% to 60% of the European population. In total, the plague may have reduced the world population from an estimated 475 million to 350–375 million in the 14th century. It took 200 years for the world population to recover to its previous level. William Ruddiman proposed that those large population reductions in Europe, East Asia, and the Middle East caused a decrease in agricultural activity. Ruddiman suggests that reforestation took place as a result, causing additional carbon dioxide uptake from the atmosphere, leading to the cooling noted during the Little Ice Age. Mongol invasions A 2011 study by the Carnegie Institution's Department of Global Ecology asserts that the Mongol invasions and conquests, which lasted almost two centuries, contributed to global cooling by depopulating vast regions and replacing cultivated land by carbon-absorbing forest. Destruction of native populations and biomass of the Americas William Ruddiman further hypothesized that a reduced population in the Americas after European contact started in the 16th century could have had a similar effect. In a similar vein, Koch and others in 1990 suggested that as European conquest and disease brought by Europeans killed as many as 90% of Indigenous Americans, around 50 million hectares of land may have returned to a wilderness