For perspective, let's begin with some recent geological history. Due to changes in Earth's orbital behavior around the sun, there have been many glacial periods during the Pleistocene Epoch of the past 2.6 million years (a very small fraction of Earth's 4.54-billion-year age). The most recent glacial period ran from 115,000 to 11,700 years ago when our current non-glacial period began (with human agriculture beginning 2,000 years later).
The preceding warmer non-glacial period ran from 131,000 to 115,000 years ago. Geologists have known for years, based on evidence of ancient shorelines, that oceans were 25 feet higher during that non-glacial period than they are today. They have also found that global temperatures then were only one degree Celsius (1.8 degrees Fahrenheit) warmer than today. What caused the flooding?
Scientists are now zeroing in on the probable culprit: collapse of the West Antarctic ice sheet. Since climate disruption is predicted to raise temperatures several more degrees by 2100, a similar collapse and flood could begin within a few decades. Collapse of the West Antarctic ice sheet would alone raise global sea levels by at least 8 feet per century for the next few centuries as the ice sheet vanishes, swamping coastal towns and cities. But Greenland and the far larger East Antarctic Ice Sheet will also be melting, so flooding could be much worse.
Although climate disruption is fearsome, the science is ironically beautiful. To begin with, how can scientists determine temperatures 125,000 years ago? Climate scientist James Hansen describes the process in his 2009 book Storms of My Grandchildren.
Geologists determine ancient temperatures by pushing long hollow pipes straight down into the ocean floor, capping them, pulling them back up, and extracting (in segments) their long "cores" of ocean sediment. These sediments can extend back thousands or millions of years. The sediments contain, among other things, shells of ancient microscopic animals. The oxygen atoms in these shells come in two varieties, namely "O-16" plus a very small fraction of "O-18." The numerical value of this small fraction varies with the temperature at the time the animal lived, so by measuring this fraction scientists can determine the temperature at that time. That date can be determined roughly from the depth of the core, but more accurately from the known historical changes in the strength and direction of Earth's magnetic field, changes that are recorded in certain magnetic minerals within the core.
The journal Science, on Page 1339 of its 21 December 2018 issue, tells the remaining remarkable details. Acquisition of this knowledge rivals the very best detective stories.
Whence came this ancient flood? Scientists had long suspected Greenland's melting ice, but Oregon State University glacial geologist Anders Carlson and his team disproved this hypothesis. Using methods similar to the temperature determinations described above, Carlson's group drilled an ocean core off the southern tip of Greenland and showed that throughout the interglacial period, ice continued grinding away at Greenland's bedrock and depositing bits of that rock in sediments off of Greenland. Thus Greenland's ice could not have entirely melted.
Scientists then turned to the world's other giant repository of perennial ice: Antarctica. Carlson's team applied their sediment core techniques to three likely regions of melting West Antarctic ice, namely the narrow peninsula stretching northward toward Argentina (check it out on a map), a region just south of the peninsula containing the vulnerable Pine Island Glacier, and a region just south of that called Amundsen Province. Sediment core data from just offshore showed ice continued grinding away on the peninsula, but vanished from the Pine Island Glacier region early during the last interglacial period, and vanished from the Amundsen region a little later. Carlson's team reported all this last December at an American Geophysical Union meeting in Washington, D.C.
The data is still provisional, but additional certainty should come soon when a new voyage begins drilling five additional Antarctic cores. I suspect the results will support the conclusion that massive amounts of Antarctic ice melted during the last interglacial. After all, 25 feet of sea-level rise had to come from somewhere.
Looking at the broader picture, carbon dioxide emissions rose by 2.7 percent in 2018 due mainly to India, China and the USA (up by 2.5 percent). Carbon dioxide concentrations reached 405 parts-per-million in 2017, matching the record high concentrations of the 3-degrees-warmer Pliocene era 4 million years ago (before the ice ages). At that time, oceans were 70 feet higher than they are today.
We must stop burning fossil fuels.
Commentary on 02/05/2019