The inadvertent introduction of smallpox and other Old World diseases into the New World by the Spanish Conquistadors apparently produced so many deaths among native Americans that populations locally collapsed and agricultural activity dramatically decreased in some areas. Native American farming often occurred in small clearings within the forest, and as these clearings became revegetated enough trees grew to pump down global atmospheric CO2 by ca. seven parts per million by the year 1610, an event that has recently been been named the "Orbis Spike". The Orbis Spike is defined as something that happened in the year 1610, but when the last 1000 years of CO2 concentrations in the atmosphere are examined, its clear that the decline in CO2 was not restricted just to the year 1610, but instead was a more prolonged event that began about 1550 and lasted until ca. 1700.
Human activities caused two brief episodes of lower CO2 during the last 800 years
A similar short-lived period of decreasing CO2 began about the year 1300 and continued until 1450, which approximates the timing of the Mongol conquest of much of Asia and the introduction of the Bubonic Plague into Europe and Asia.
Prof. Nate Ruddiman first suggested that the introduction of Old World diseases into the Americas in the 16th century and the spread of the Bubonic Plague across Asia and Europe by the Mongols in the 13th century were responsible for the two small downward excursions in CO2 between the years 1300-1450 and 1550-1700. If this linkage is valid, then these two events constitute useful tests of the "Green" Geoengineering idea that it is possible to fix the Greenhouse Warming problem by planting trees to pump down CO2 from the atmosphere.
Prior to the earlier drop in CO2, Genghis Khan conquered much of Asia and easternmost Europe. China reportedly suffered a drastic decline in population during the 13th and 14th centuries. Before the Mongol invasion, Chinese dynasties reportedly had approximately 120 million inhabitants; after the conquest was completed in 1279, the 1300 census reported roughly 60 million people. Historians record the complete destruction of at least 92 Chinese cities and the massacre of all their inhabitants, accounting for loss of ca. 45 million people. The total population of Persia may have dropped from 2,500,000 to 250,000 following the Mongol invasion there. About half the population of Ukraine and Hungary were killed. Similar devastation occurred through the Moslem Caliphate centered in Baghdad, which was sacked and the population massacred in 1258. At the same time as the Mongol invasion, the Bubonic plague spread widely in Asia and Europe, including to areas not invaded by the Mongols. The double whammy of the Mongol invasion and Bubonic plague so decimated the population of Europe and Asia that agricultural work diminished and fields from China to Hungary were abandoned and reoccupied by native forests. This inadvertent "Green Geoengineering" reduced atmosphere CO2 by about 4-5 ppm.
In the later event, the Spanish conquered and destroyed the Aztec and Inca Empires in the 16th century, and the inadvertent introduction of Old World diseases like smallpox, typhus, measles, influenza, bubonic plague, cholera, malaria, tuberculosis, mumps, yellow fever, and whooping cough produced devastation and mass loss of life across much of the rest of the Americas. While population numbers in the Americas prior to contact with Europe are very poorly known, the population may have been about 50 million, falling to ca. 10 million about 150 years. This hemispheric devastation may've produced a CO2 drop of ca. 7 ppm.
The decreases in atmospheric CO2 of 4-7 ppm seen in these two events should be considered upper limits, as other factors like solar variability and volcanic eruptions are also thought to have influenced atmospheric CO2 content during these episodes. Nonetheless, they constitute useful tests of the effectiveness of the "Green Engineering" concept that planting trees can reduce atmospheric CO2 content. The historical record suggests that even with massive population declines and concomitant hemisphere-wide increases in tree growth, a maximum 4-7 ppm of CO2 reduction is all that can be accomplished by this Green Engineering method.