Humans Push Earth to Tipping Point: Small Changes Could Create Dramatic Results

The term, "tipping point," became popular with the 2000 release of Malcolm Gladwell's book, "The Tipping Point: How Little Things Can Make a Big Difference." More recently, however, "tipping point" has become a warning cry from atmospheric scientists and geologists who see the Earth coming ever closer to damaging, irreversible and human-caused changes.

Simply defined, a tipping point marks the moment at which even a very small change leads to rapid and dramatic results. The difference between 33 degrees Fahrenheit and 32 degrees Fahrenheit, for example, is a tipping point for water. At the first temperature, water remains liquid. One degree lower, though, and water shifts to solid form ... it freezes.

Increasingly over the past few years, scientists are now identifying potential tipping points related to climate change. As humans continue pumping additional amounts of carbon dioxide -- a greenhouse gas -- into the air, average global temperatures have grown warmer, more sea ice and land ice have melted, and sea levels have little by little crept upward.

Such gradual changes might not continue into the future, however, scientists warn. As warming increases, so too does the risk of reaching one of several global tipping points that could lead to sudden and dramatic shifts.

"Society may be lulled into a false sense of security by smooth projections of global change," wrote a team of researchers just this week in an article published in the Proceedings of the National Academy of Sciences (PNAS).

Just last spring, researchers at NASA and the Columbia University Earth Institute reported that even moderate continued warming of the atmosphere could create a tipping point that would start an irreversible disintegration of Arctic sea ice and the West Antartic ice sheet.

Scientists last year observed signs that such changes were already in motion, as evidenced by rapid ice melting in Greenland and Antarctica, as well as by a record-low extent of Arctic summer sea ice that opened the Northwest Passage for the first time in recorded human history.

And last November, the Intergovernmental Panel on Climate Change released its fourth climate synthesis report that warned continued human-caused warming could lead to "some impacts that are abrupt or irreversible." Such possible impacts, the report said, could include rapid rises in sea level, dramatic changes in ocean chemistry and marine life, and mass extinctions of many life forms on a global scale.

In this week's PNAS paper, researchers with the Potsdam Institute for Climate Impact Research and the British University of East Anglia studied findings from leading climate experts to compile a list of the top nine elements that could push the Earth to a climate tipping point. They include:

1. Arctic sea ice.
2. The Greenland ice sheet.
3. The West Antarctic ice sheet.
4. The Earth's northern, or Boreal, forests.
5. The Amazon rainforest
6. The El Niño phenomenon
7. The West African monsoon.
8. The Indian summer monsoon.
9. The Atlantic thermohaline circulation (THC), also known as the ocean "conveyor belt" that brings warm tropical waters and moderate temperature to Europe.

Of these elements, the ones that are most sensitive and most likely to lead to a climatic tipping point are the Greenland ice sheet and Arctic sea ice. The most uncertain and less sensitive elements include the West Antarctic ice sheet, the northern forests and Amazon rainforest, El Niño and the monsoons. And the Atlantic conveyor belt was described as low sensitivity and intermediate uncertainty.

"Both the loss of Greenland ice and Arctic sea-ice would increase freshwater input to the North Atlantic, which would weaken the THC," said researcher Stefan Rahmstorf. "This could result in a chain reaction: vanishing Arctic sea ice leads to amplified high-latitude warming and accelerates loss of Greenland ice. The large freshwater input could then at some point shut off the Atlantic deep-water formation and disrupt the North Atlantic current which is part of the THC."