In his new book, A World Without Ice, geophysicist Henry Pollack explains the complex influences that Earth's ice has had on human survival, and that population growth and industrialization are now having on the survival of Earth's ice. Following is an excerpt.
By Henry Pollack
Just as the international financial system surprised the world with a major collapse in 2008, the global climate system, with its human component, is equally capable of serious surprises.
Lurking in the shadows of climate change is the possibility that the accelerations we now observe in the climate system are portends of approaching tipping points.
Tipping points represent changes in the a system that occur when the system passes form one mode of behavior to another, sometimes imperctibly, sometimes suddenly. A simple analogy is the process of paying off a home mortgage.
Each monthly mortgage payment comprises both interest and principal. In the early years of the mortgage, the payoff of the loan principal is painfully slow and annoyingly incremental, as most of the monthly payment goes to paying the interest on the loan.
In a typical 30-year home mortgage, a homeowner, after 10 years of payments, has paid off only 10 percent of the loan. After 21 years of payments, the monthly check is finally split evenly between interest and principal, a tipping point that typically passes without recognition or acknowledgment. But beyond that tipping point, the reduction of the unpaid balance accelerates, and, as the mortgage approaches payoff, there is a rapid erosion of the remaining unpaid loan. At the end, there is another tipping point, impossible not to notice — a very abrupt transition to a new state in the homeowners’ personal finances, when there is no mortgage payment to make at all.
In the climate system, there are several possible tipping points: major realignments of oceanic and atmospheric circulation, rapid releases of greenhouse gases now trapped in permafrost and in the ice that exists at shallow depths beneath the ocean floor, and sudden changes in sea level. All these possibilities are related to changes in Earth’s ice.
What role does ice have in taking the climate across a tipping point?
The average temperature of a planet’s surface depends directly on the amount of incoming solar energy absorbed by the surface. But not all the solar radiation delivered to Earth is absorbed — some is reflected back into space.
Snow and ice are both highly reflective substances, and so the fraction of Earth’s surface covered by snow and ice is a big determinant of Earth’s average surface temperatures. The more radiation that is reflected away, the less energy remains to warm the planet. Currently, Earth reflects about 30 percent of the arriving solar radiation back into space.
When the amount of snow and ice cover changes over time, so does the balance between reflection and absorption of solar energy. As ice increases on Earth, more solar energy is returned to space and less is absorbed, thus lowering the surface temperature. More ice promotes a cooler planet, and a cooler planet encourages the accumulation of even more ice. This interaction is called a positive feedback, and leads to an ever-faster acceleration of climate change.
Diminishing ice cover also drives a similar feedback, but in the other direction: As Earth becomes darker and less reflective, more solar radiation is absorbed, and the planetary surface grows warmer, and a warmer planet leads to even less ice cover and a further acceleration in warming.
How do the ice-climate feedbacks lead to tipping points in the climate?