The Tiniest Air Raid Sirens

These little guys are members of the Globigerinoides, planktonic foraminifera that have lived in the surface waters of the ocean for a very long time. The picture of their shell on the left (about the size of a grain of sand) and the living organism on the right, was found at Oceanus, the online magazine of the Woods Hole Oceanographic Institution. The composition of their shells when they lived millions of years ago contains a warning for us today.

But first, what is a foraminifera? Well, living things can be divided into the prokaryotes and the eukaryotes. The prokaryotes include the bacteria and other single-celled organisms without internal membranes, while the more complex eukaryotes have internal membranes around their nucleus and other organelles. The eukaryotes include multicellular plants and animals, the fungi, and a fourth group of single-celled organisms called the protists which are neither animals nor plants. Foraminifera are protists with shells. Their name derives from the tiny holes (foramina) that perforate their shells.

The shells of the foraminifera consist of calcium carbonate, and the changing ratio of boron to calcium in these shells indicates the concentration of carbon dioxide when they formed. The proportion of a particular isotope of oxygen (δ18O) indicates the temperature when they formed. Aradhna Tripati and her colleagues at UCLA, in work published in Science magazine on December 4th, 2009, measured the boron:calcium ratio and δ18O ratio in Globigerinoides shells over a 20 million year span of time, extending all the way back to the Miocene Epoch.

Up until now, the 800,000 year Vostok ice core record in Antarctica held the oldest measurements of atmospheric carbon dioxide and temperature, using air trapped in bubbles in the ice. This ice core record shows carbon dioxide and temperature closely tracking each other over 800,000 years, powerful evidence that CO2 influences climate. But now, Tripati, using the tiny shells of foraminifera dug up in layers of sediment at the bottom of the ocean, extends that correlation between CO2 and temperature 25 times further back in history, to 20 million years before the present.

The significance of this longer and older record of temperature tracking CO2 levels lies with the ice sheets of the Miocene and Late Pliocene. Starting 20 million years ago and continuing for five million years, the globe was warmer, no massive ice sheets covered Greenland and Antarctica, and sea levels may have been 25 to 40 meters higher than today. Atmospheric carbon dioxide during this time increased from around 375 ppm to 425 ppm as climate continued to warm.

Then, 14 million years ago, CO2 levels began a steady decrease over a five million year span of time from over 400 ppm down to 250 ppm. The climate cooled, closely tracking the carbon dioxide decrease, as ice sheets grew and sea levels likely dropped as much as 40 meters.

The take home lesson - massive ice sheets may not survive on planet Earth when atmospheric CO2 levels exceed 350 ppm for an extended period of time. Carbon dioxide reached 350 ppm back in the mid-1980s, peaked at 390 ppm in 2009, continues to increase more than 1.5 ppm every year, and that rate of increase is growing.

Without massive ice sheets, global sea levels can rise as much as 40 meters. That will not likely happen in this century, but the last time CO2 levels rose from 350 ppm to over 400 ppm, it took a million years to do so, and it happened in the Miocene Epoch at least 12 million years ago. We will see CO2 concentrations reach 400 ppm by 2015, just 30 years after CO2 passed 350 ppm.

A sea level rise of just a couple of meters in the next 100 years would constitute a major worldwide catastrophe.

The chemical make-up of ancient foraminifera shells suggests we may be headed to or may have already reached a level of carbon dioxide in our atmosphere that cannot maintain the large ice sheets now covering Greenland and Antarctica, making the Globigerinoides the tiniest air raid sirens in history. Listen to them.