Caldeira Lab Research:Land Plants, Carbon, and Climate
Coupled climate–carbon simulations indicate minor global effects of wars and epidemics on atmospheric CO2 between AD 800 and 1850
Julia Pongratz, Ken Caldeira, Christian H. Reick, and Martin Claussen
We investigate whether events such as wars and epidemics during the last millennium have been responsible for decreases in atmospheric CO2 reconstructed from ice cores. We find that the Black Death and the Ming Dynasty collapse did not last long enough for forests to fully regrow and soil carbon pools to come into equilibrium, but CO2 was drawn down from the atmosphere during long-lasting events such as the Mongol invasion and the conquest of the Americas. The strongest event, the Mongol invasion, caused forests to absorb 0.7 gigatons of carbon. A measurable decrease of the global CO2 concentration is still unlikely because of substantial emissions from continued clearing of forests elsewhere in the world.
Figure: The Mongol invasion and the fall of the Ming Dynasty in China, the Black Death in Europe, and the Conquest of the Americas all lead to reforestation on abandoned agricultural area and have a lowering tendency on the global CO2 concentration (black lines, uncertainty of maximum CO2 drawdown indicated by yellow bars). The blue range indicates the importance of concurrent emissions from the rest of the world, outside the region, where clearing of forest for agricultural purposes continues. If the CO2 decrease caused by the region reached values below the blue range, the event would decrease global CO2, otherwise the effect of the event would be overwhelmed by the CO2 increase caused by emissions from the rest of the world. Only the Mongol invasion lowers the regional CO2 fluxes sufficiently strongly to overcompensate the CO2 increase resulting from concurrent emissions from the rest of the world.
Abstract
Historic events such as wars and epidemics have been suggested as explanation for decreases in atmospheric CO2 reconstructed from ice cores because of their potential to take up carbon in forests regrowing on abandoned agricultural land. Here, we use a coupled climate–carbon cycle model to assess the carbon and climate effects of the Mongol invasion (~1200 to ~1380), the Black Death (~1347 to ~1400), the conquest of the Americas (~1519 to ~1700), and the fall of the Ming Dynasty (~1600 to ~1650). We calculate their impact on atmospheric CO2 including the response of the global land and ocean carbon pools. It has been hypothesized that these events have contributed to significant increases in land carbon stocks. However, we find that slow regrowth and delayed emissions from past land cover change allow small increases of the land biosphere carbon storage only during long-lasting events. The effect of these small increases in land biosphere storage on global CO2 is reduced by the response of the global carbon pools and largely offset by concurrent emissions from the rest of the world. None of these events would therefore have affected the atmospheric CO2 concentration by over 1 ppm. Only the Mongol invasion could have lowered global CO2, but by an amount too small to be resolved by ice cores.
Press coverage: Interview on BBC World Service (Jan 27, 2011)