Thawing mosses tell a climate change tale | Science News for Students

Thawing mosses tell a climate change tale

Dating of plants suggests summer is hotter now than it’s been in at least 45,000 years
Nov 21, 2017 — 6:30 am EST
Arctic moss

Geologist Kurt Refsnider, now at Prescott College in Arizona, collects mosses that were revealed by retreating ice on Canada’s Baffin Island. Radiocarbon dates for the mosses suggest the region has less ice cover now than it did tens of thousands of years ago.

G. Miller

SEATTLE, Wash. — Some mosses in Canada’s eastern Arctic have long been entombed in ice. But now, rising temperatures have been melting that ice, bringing those mosses to light. By dating how long ago those mosses were buried, scientists now conclude that summertime temperatures in this region are the warmest they’ve been in tens of thousands of years.

“We were stunned,” said Gifford Miller of the University of Colorado Boulder. As a paleoclimatologist, he studies ancient climate. At some sites he studied, the emerging plants last saw the sun no less than 45,000 years ago. Some may have been buried for up to 115,000 years.

Miller described his findings October 22, here, at the Geological Society of America’s annual meeting.

As the planet warms, ice has been retreating on Baffin Island, in Canada’s far North. Miller’s team has been probing the plants unburied by this melting.

They collected an impressive number of samples, and their findings are very compelling, said Lee Corbett. She’s a geologist at the University of Vermont in Burlington who was not involved in the study. “It truly is an indication that humans are pushing the climate into a new regime,” she said. She points out it’s “one that modern, agriculture-based civilizations have never witnessed.”

Dating mosses

Miller’s group wanted to track the growth and retreat of Baffin Island’s ice cover. To do this, they have been hunting for remnants of scraggly mosses along the edges of the island’s retreating ice sheets.

The newly exposed mosses are dead. (At least for a year or two. Then they come alive again. That’s why Miller calls them “zombie” mosses.) The researchers could tell when the mosses were last exposed to the atmosphere and using sunlight to perform photosynthesis by looking at their carbon. They used a technique called radiocarbon dating.

Some elements, such as carbon, come in forms that have the same number of protons but different masses. Each of these is a different isotope. Carbon has three isotopes. Only one of them — carbon-14 — is radioactive. It has a half-life of about 5,730 years. After that period, about half of the carbon-14 atoms in the original sample will have decayed into a new isotope. Radiocarbon dating compares the ratio of carbon-14 to carbon-12 in some sample. This ratio changes as carbon-14 decays. And the ratio corresponds to when the moss was last alive.

After about 45,000 to 50,000 years, though, almost all of an objects’ radiocarbon will have decayed. The radiocarbon dating technique won’t work for anything older than that.

So far, Miller’s team has determined the ages for 370 different plant samples collected on Baffin Island. Those ages tend to cluster into groups. Each represents a time when the ice expanded across the island to entomb the affected plants. One large group dates to around 3,700 years ago. Another is from around 900 years ago. A third dates to around A.D. 1450, corresponding to a cold period known as the Little Ice Age.

But in a few regions, the plants were so old that they had no carbon-14 in them. Mosses with the “dead” radiocarbon were found at high elevations. They were on pedestals of rock with persistent ice caps that are now slowly melting. Because the radiocarbon clock stops at about 50,000 years, it’s not possible to see exactly when those spots were last ice-free. But an ice core collected in nearby Greenland suggests that the planet experienced continuous cold from 40,000 to about 115,000 years ago. That’s when the planet’s previous warm interglacial period ended, Miller noted.

Originally, the researchers expected to find plants dating to medieval times. That would have suggested that the region is the warmest it’s been since the Middle Ages, which ended in the 14th or 15th centuries. But finding 3,700-year-old plants was a surprise, Miller said. And “we never anticipated we’d find plants 40,000 years old,” he added.

Indeed, he says, “It’s a bit spooky because it provides quantitative evidence that the magnitude of summer warmth is already sufficient to melt all ice in the eastern Canadian Arctic. It’s just a matter of time now.”

Power Words

(more about Power Words)

agriculture     The growth of plants, animals or fungi for human needs, including food, fuel, chemicals and medicine.

annual     Adjective for something that happens every year. (in botany) A plant that lives only one year, so it usually has a showy flower and produces many seeds.

Arctic     A region that falls within the Arctic Circle. The edge of that circle is defined as the northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice.

atmosphere     The envelope of gases surrounding Earth or another planet.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

carbon     The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

decay     The process (also called “rotting”) by which a dead plant or animal gradually breaks down as it is consumed by bacteria and other microbes. (for radioactive materials) The process whereby a radioactive isotope — which means a physically unstable form of some element — sheds energy and subatomic particles. In time, this shedding will transform the unstable element into a slightly different but stable element. For instance, uranium-238 (which is a radioactive, or unstable, isotope) decays to radium-222 (also a radioactive isotope), which decays to radon-222 (also radioactive), which decays to polonium-210 (also radioactive), which decays to lead-206 — which is stable. No further decay occurs. The rates of decay from one isotope to another can range from timeframes of less than a second to billions of years.

element     (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

elevation     The height or altitude at which something exists.

geological     Adjective to describe things related to Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists.

Greenland     The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically part of North America (sitting just east of Northern Canada), Greenland has been linked more politically to Europe. In June 2009, Greenland became an independent nation. Ice covers roughly 80 percent of Greenland. Indeed, the Greenland ice sheet is the world’s largest. Although this is the 12th biggest nation (based on surface area), Greenland averages the fewest people per square kilometer of its surface area.

half-life    The time it takes for half of some quantity of a radioactive isotope to decay (transform) into a new isotope.

ice age     Earth has experienced at least five major ice ages, which are prolonged periods of unusually cold weather experienced by much of the planet. During that time, which can last hundreds to thousands of years, glaciers and ice sheets expand in size and depth. The most recent ice age peaked 21,500 years ago, but continued until about 13,000 years ago.

ice sheet     A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.

interglacial    An adjective that responds to a period of relatively mild climate that exists between two successive ice ages. Earth is now in an interglacial period, one that began roughly 11,700 years ago.

isotope     Different forms of an element that vary somewhat in mass (and potentially in lifetime). All have the same number of protons but different numbers neutrons in their nucleus.

Little Ice Age    Following a so-called Medieval Warming Period (which spanned from roughly A.D. 900 to 1300), global temperatures chilled demonstrable. Mountain glaciers expanded through about 1850 (starting from between A.D. 1300 to 1500, depending on which authority is cited). This Little Ice Age ended around the latter part of the 1800s, as a planet-wide warming got underway. That warming trend continues through today.   

medieval    An adjective referring to the historical period known as the European Middle Ages. It roughly spanned from A.D. 500 to about 1500.

moss     A small, flowerless green plant that lacks true roots. It tends to grow as carpets or rounded cushions in damp habitats. It can reproduce asexually, through fragmentation, or by means of spores released from stalked capsules.

persistent     An adjective for something that is long-lasting.

proton     A subatomic particle that is one of the basic building blocks of the atoms that make up matter. Protons belong to the family of particles known as hadrons.

radioactive     An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy is by a process known as radioactive decay.

radiocarbon dating     A process to determine the age of material from a once-living object. It is based on comparing the relative proportion, or share, of the carbon-12 to carbon-14. This ratio changes as radioactive carbon-14 decays and is not replaced.

ratio     The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).

remnant     Something that is leftover — from another piece of something, from another time or even some features from an earlier species.


Meeting:​​ ​ G.H. Miller et al. The dead speak: Tracking the cryospheric response to contemporary warming in Arctic Canada with entombed vegetation and in situ 14C in adjacent rocks. Geological Society of America annual meeting, Seattle, October 22, 2017.