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I imagine that human agriculture had a major effect on the composition of flora across the planet's surface.

Did the transition from gathering to growing have a measurable effect on climate? If so, what was it?

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Raising herd animals would likely contribute more than mere farms. Many farms simply produce feed for animals anyway, and this can lead to an inflated population which can wreck havoc when released in the wild to graze. That said, with the exception of Egypt and a few rare cases, every agricultural society eventually failed because they could either could not keep up with the deposits of silt clogging their water works, or they could not contend with the the rising salinity of soil, caused by prolonged irrigation. The evaporation of water leaves behind an excess of mineral salts, and eventually a field's production drops and the desert takes over. The usual method people employ to contend with these issues is to build more canals and irrigate more land elsewhere. So for a few thousand years agriculture societies have been converting land to desert and this no doubt this would have effected the climate. But probably only a scant iota when compared to the last 100 years of agriculture . . .

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    $\begingroup$ Exactly what I was going to say: It probably had a bigger effect on runoff, river deposition, soil composition, etc. than it did on, say, global mean temperature. It comes down to how broadly you think the definition of "climate" is. I would include in climate the ecosystems that can survive on that land with the given soil, rivers, and weather patterns (a la the Koppen classification). We definitely changed at least the first two with the advent of agriculture. $\endgroup$ – Jareth Holt Jul 6 '15 at 14:58

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