Humans Move Most Earth, Says Study

Dec. 15, 2006 — Human changes to landscapes are now on par with the wasting power of weather and tectonic uplift.

A new take of the scale of human changes to the face of the Earth shows that by farming alone, humans have now managed to move a thousand times more earth than the annual sediment loads of all the world's rivers combined. That's enough soil to cover the state of Rhode Island nearly two miles deep in dirt.

And the rate of human changes to the land is increasing.

"We move a lot more per year than natural processes," said geologist Brandon McElroy of the University of Texas in Austin. He and Syracuse University's Bruce Wilkinson tackled the matter by looking at the geologic record of how fast sediments built up in past times on Earth, and comparing them to recent decades.

"As sedimentologists, we knew we had the sediment record to show that," explained McElroy.

The two published a report on the matter in the latest issue of the Bulletin of the Geological Society of America.

Pre-human rates of erosion tended to move far less soil. Over much of the last 545 million years natural processes moved an average of about five billion tons of soil per year, the researchers report. That increased to 16 billion tons in the Pliocene, 5.3 to 1.8 million years ago, due to changes in climate.

The melting of ice sheets from the last Ice Age pushed the erosion numbers up to about 21 billion tons. But all those numbers are dwarfed by the current global soil shifting by humans of about 75 billion tons per year.

What's more, we move soil from entirely different places than nature does.

Through the eons, soil was shifted from the highest ten percent of the land surface. This is where rain and snow and wind are the most vigorous. The geologic record shows these features contributed more than 80 percent of sediments.

But because farmlands are primarily on lowlands, the gigantic sediment influx to rivers — caused by simply clearing and plowing land and making soil more vulnerable to washing away — is now coming from places like the Great Plains of the North America. That difference has caused yet another change — the overall lowering of the Earth’s landmasses by some 2 1/2 inches.

That last number is not particularly scary, given the mean elevation of Earth's land masses is 2,750 feet above sea level, the researchers reported.

But given the possibility that it's the most productive soil that's being shifted away from croplands, and that the need for food will increase as the human population grows, "the impact is significant," they concluded.

"One of the big questions that (they) address is ‘How sustainable is this?’" said geologist Roger Hooke of the University of Maine at Orono. Hooke is among many scientists who have been pondering the human contribution to erosion for many years.

That said, the biggest contribution of the new study, he said, is that it adds the perspective of Earth's sedimentation rates long before humans showed up. That provides a basis for comparison that hadn't been part of the puzzle until now.