We’ve Been Changing the Climate Since the Beginning of Agriculture

I saw these posts recently on the Real Climate blog, and thought they was interesting enough to repost:

The Early Anthropocene Hypothesis: An Update

An Emerging View on Early Land Use

Apparently, there is increasing support in academia for the theory that human land use clearing and changes after the invention of agriculture were responsible for some abnormal climate trends in the late Holocene, over the last 5000 or so years.

What is the Holocene? The Holocene covers the period from the most recent retreat of the ice until the present day. It isn’t correct to say it started at the end of the last ice age since we’re actually still in one: since scientifically an ice age is when there’s significant ice at th poles, and we still have plenty of ice there for now. But within the current ice age there are interglacial periods when the ice retreats northwards and leaves most of Europe, Asia and North America uncovered, and the Holocene is the most recent of these retreats.

A glacial period, source: Ittiz, Wikipedia

So what’s odd about the Holocene? In previous interglacials, greenhouse gas levels in the atmosphere tended to decrease over time, but in the most recent interglacial this was only true until a few thousand years ago. Then levels of GHGs such as CO2 and methane started rising again. This could be explained by some natural change, or by widespread burning of forests and clearing of land and an increase in methane emitting livestock.

If true, this would demonstrate again the exceptional impact of humans on the global feedback systems that normally operate. We partly reversed the usual increase in tree cover and vegetation that happen during interglacials, and accidentally kept the world slightly warmer than otherwise it would have been. The most recent explosion in human impact would then just be an exponential acceleration of a very, very old trend.

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