Human geoengineering is being linked to past climate change.

That idea, debated for the past several years by climate scientists, holds that the introduction of large-scale rice agriculture in Asia, coupled with extensive deforestation in Europe began to alter world climate by pumping significant amounts of greenhouse gases — methane from terraced rice paddies and carbon dioxide from burning forests — into the atmosphere. In turn, a warmer atmosphere heated the oceans making them much less efficient storehouses of carbon dioxide and reinforcing global warming. . .

"Between 5,000 and 8,000 years ago, both methane and carbon dioxide started an upward trend, unlike during previous interglacial periods," explains Kutzbach. Indeed, Ruddiman has shown that during the latter stages of six previous interglacials, greenhouse gases trended downward, not upward. Thus, the accumulation of greenhouse gases over the past few thousands of years, the Wisconsin-Virginia team argue, is very likely forestalling the onset of a new glacial cycle, such as have occurred at regular 100,000-year intervals during the last million years. Each glacial period has been paced by regular and predictable changes in the orbit of the Earth known as Milankovitch cycles, a mechanism thought to kick start glacial cycles.

"We're at a very favorable state right now for increased glaciation," says Kutzbach. "Nature is favoring it at this time in orbital cycles, and if humans weren't in the picture it would probably be happening today."

I wonder if this human induced warming accounts to any significant degree for the Medieval Warm Period (~800-1300 AD), and if so why did it end? Perhaps that too is human influenced.

Stanford University researchers have conducted a comprehensive analysis of data detailing the amount of charcoal contained in soils and lake sediments at the sites of both pre-Columbian population centers in the Americas and in sparsely populated surrounding regions. They concluded that reforestation of agricultural lands-abandoned as the population collapsed-pulled so much carbon out of the atmosphere that it helped trigger a period of global cooling, at its most intense from approximately 1500 to 1750, known as the Little Ice Age.

"We estimate that the amount of carbon sequestered in the growing forests was about 10 to 50 percent of the total carbon that would have needed to come out of the atmosphere and oceans at that time to account for the observed changes in carbon dioxide concentrations" . . .

What they found was a record of slowly increasing charcoal deposits, indicating increasing burning of forestland to convert it to cropland, as agricultural practices spread among the human population-until around 500 years ago: At that point, there was a precipitous drop in the amount of charcoal in the samples, coinciding with the precipitous drop in the human population in the Americas.

So perhaps the Little Ice Age (~1400-1900 AD) that followed the Medieval Warm Period was also human influenced. It's as if the old and new worlds were separately working to warm the world and fight off the overdue ice age, developing and increasing until they met and swapped spit. Then half of them died from disease and the fields reverted to uncultivated wilderness in the Americas, allowing the ice to advance again.

This is a land use narrative driven by agriculture rather than an industrial narrative driven by fossil fuels. It's interesting that the effects have been beneficial for life on earth since ice ages are generally bad for living things. There are other factors - Milankovitch cycles, solar variability, volcanoes, long duration circulation cycles of atmosphere and oceans - that complicate the narratives and make the effects highly contingent. It explodes the simple minded narrative of a return to a pre-industrial equilibrium, while reinforcing the concept of human influence on climate.

21st century intentional geoengineering seems less radical given that we have always done unintentional geoengineering, but we need a great deal more information in order to have some hope of predictable outcomes.