Giving the Earth a Cool Shower–Is Massive Irrigation Hiding the Greenhouse Effect Around the World?

A few months ago I reported on a study that suggested that human beings were depleting groundwater resources at such a staggering rate that it was actually contributing to a rise in global sea levels—yet another signal of just how enormous the impact of human agriculture is on everything from the hydrological cycle to the global ecosystem.

Another study that came out last fall adds a different twist. According to research published in Climate Dynamics by Benjamin I. Cook, Michael Puma and Nir Krakauer (of Columbia Earth Institute’s Lamont-Doherty Earth Observatory, the Center for Climate Systems Research and the City College of New York, respectively) it is possible that expected warming trends from greenhouse gasses are being “masked” in places around the world by irrigation.

The researchers found that this cooling effect is largest over North America, India, the Middle East and East Asia. This is not surprising, given that those areas contain some of the world’s most heavily irrigated regions—regions that are breadbaskets to the world.

The study finds that under increased greenhouse gas climate forcing, the irrigation cooling effect disappears over North America, remains unchanged for India, and intensifies over China and the Middle East. However, as the researchers astutely point out, that scenario assumes that irrigation practices remain the same. Given that non-renewable groundwater supply is rapidly depleting in most of those same regions, and surface reservoir water storage is itself sensitive to it is highly unlikely that current irrigation patterns can be maintained.

According the report, “roughly 40 percent of the global land surface area has been directly modified by human activities, primarily through conversion of natural ecosystems to croplands and pastures.” Some 2 percent of total land area is irrigated. Finally, the localized impact of land cover changes (presumably including massive irrigation) can cause local climate changes that are “orders of magnitude larger than the signal from well mixed greenhouse gasses.”

Because global warming is such a monumental crisis and challenge, it’s easy to give it de facto priority over all other resource or environmental crises. But the alarming fact is that human beings have dramatically altered the world in ways that go well beyond the production of atmospheric carbon—alternations that will, in fact, strongly impact the way climate change manifests itself and the ways in which we must adapt.

Here’s to “thinking and acting both globally and locally” –there really is no other answer.

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