PATRICK KELLEY/U.S. COAST GUARD
The average American’s carbon footprint shrinks Arctic sea ice. Not by a little. By a lot — about the area covered by 16 queen-size beds. And that happens each year.
A carbon footprint is the number of tons of carbon dioxide a person produces in their daily activities. The average American produces about 16 metric tons of carbon dioxide each year. Each metric ton of this CO₂ released into the atmosphere directly results in a 3-square-meter (32-square-foot) loss of sea ice cover at summer’s end. That’s similar to losing an area of ice just a bit smaller than a two-seat Smart car, scientists say. They reported their findings online November 3 in Science.
“For the first time now, it is possible to grasp how each one of us contributes to tangible consequences,” says Dirk Notz. He works as a climate scientist at the Max Planck Institute for Meteorology in Hamburg, Germany. He was an author of the new study.
How much CO₂ each person produces leads to a lot of melting ice. And there are some 7 billion people contributing. Altogether, humanity is responsible for the release of some 36 billion metric tons of CO₂ each year. With another trillion metric tons, the Arctic Ocean will have a completely iceless summer.
This might be the first time the summer Arctic has been ice-free in 125,000 years. And it could happen before 2050, Notz estimates. He did the work with Julienne Stroeve. She is a climate scientist in England at University College London. The estimate by these two contradicts many previous studies. Those other studies projected that summertime ice in the Arctic would stick around for far longer.
“Sea ice feels so substantial,” says Cecilia Bitz, especially when its strong enough to land an airplane on. But this new work, she notes, “makes [the ice] feel very fragile.” As an atmospheric scientist, Bitz studies how aspects of Earth’s atmosphere drive weather and climate. She works at the University of Washington in Seattle, and was not involved in the study.
Dwindling ice at the top of the world is important to watch. Its disappearance threatens Arctic species. Its melting also can spread pollution. And, disappearing ice could open the region to polar shipping.
What the new study showed
Each winter, the Arctic sea surface freezes solid. As snow falls on it, more and more accumulates and eventually melts into ice. Some of that ice melts in summer. Still, a great deal of the Arctic retains its ice cover even throughout the summer. But that situation is threatening to change — and in our lifetime.
Satellite data show summertime Arctic ice cover has been falling. In 2012, the area of sea covered by ice hit a record low (since satellite observations began). It was then just a mere 3.39 million square kilometers (1.3 million square miles). That’s well below the average of what had been typical from 1981 through 2010: 6.22 million square kilometers (2.4 million square miles).
How quickly the summer ice would continue to diminish remained unclear. So Notz and Stroeve decided to estimate how soon it could be completely gone.
To do that, they analyzed records of Arctic sea surface temperatures and the minimum sea area covered by summer ice. They looked at records going way back to 1953. The average ice cover by the end of September had dropped. And it fell in lockstep with the rising releases of CO2 from human activities, the team found.
The researchers think they now understand why there is such a simple relationship between CO2 emissions and ice loss. As CO2 builds up in the atmosphere, it strengthens the so-called greenhouse effect. This is where certain gases in the atmosphere, such as carbon dioxide and methane, trap some of the sun’s heat. As CO2 levels climb, less of the sun’s heat bounces back into space. Instead, it remains trapped near Earth’s surface. This increases the amount of ice-warming infrared radiation hitting the Arctic. That causes the outermost edge of the sea ice to retreat northward, moving away from the shore. And that shift northward begins to reduce the total area covered by ice.
The researchers argue that computer models of how Earth’s climate is changing have underestimated this effect. These models use a computer to describe the conditions, functions or appearance of some changing system. The current models don’t accurately re-create how sensitive Arctic ice melting is to rising CO2 levels, the scientists say. There are other factors linked to sea ice loss, though. For instance, the heat entering the Arctic from the Atlantic Ocean can vary. How reflective the region is also can change (allowing more or less light to bounce off ice and back into the atmosphere). These factors were minor over the period the team studied, Notz says. The big factor was the increased heat being trapped in the air by greenhouse gases.
Still, downplaying the role of ocean heating is a mistake, argues Rong Zhang. She’s a scientist with the National Oceanic and Atmospheric Administration. Her work takes place at the Geophysical Fluid Dynamics Laboratory in Princeton, N.J. As an oceanographer, she studies the physical and biological properties of the ocean.
Sea ice cover in the Arctic reaches its peak during winter, she notes. At that time, little light shines on the area. During these months of dark or near-darkness, the greenhouse effect is less important, she explains. But just as the summer minimum of Arctic sea ice has been falling, so has the maximum sea ice present in winter. After declining for decades, it reached a record low in March 2016. Why the ice is declining in the winter is not yet clear, she says. More study is needed.
Scientists need to figure out whether warming from below or above the ice plays a larger role, says Zhang. Indeed, she adds: “There’s not just one explanation.”
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.
Arctic sea ice Ice that forms from seawater and that covers all or parts of the Arctic Ocean.
Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
atmosphere The envelope of gases surrounding Earth or another planet.
average (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.
carbon dioxide (or CO2) A colorless, odorless gas produced by all animals when the oxygen they inhale reacts with the carbon-rich foods that they’ve eaten. Carbon dioxide also is released when organic matter (including fossil fuels like oil or gas) is burned. Carbon dioxide acts as a greenhouse gas, trapping heat in Earth’s atmosphere. Plants convert carbon dioxide into oxygen during photosynthesis, the process they use to make their own food.
carbon footprint A popular term for measuring the global warming potential of various products or processes. Their carbon footprint translates to the amount of some greenhouse gas — usually carbon dioxide — that something releases per unit of time or per quantity of product.
climate The weather conditions prevailing in an area in general or over a long period.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
dynamic An adjective that signifies something is active, changing or moving. (noun) The change or range of variability seen or measured within something.
factor Something that plays a role in a particular condition or event; a contributor.
fluid dynamics The study of liquids and gases in motion.
greenhouse A light-filled structure, often with windows serving as walls and ceiling materials, in which plants are grown. It provides a controlled environment in which set amounts of water, humidity and nutrients can be applied — and pests can be prevented entry.
greenhouse effect A trapping of heat by the presence of certain gases in Earth’s atmosphere, such as carbon dioxide and methane. The greenhouse effect also can occur in smaller environments. For instance, when cars are left in the sun, the incoming sunlight turns to heat, becomes trapped inside and quickly can make the indoor temperature a health risk.
methane A hydrocarbon with the chemical formula CH4 (meaning there are four hydrogen atoms bound to one carbon atom). It’s a natural constituent of what’s known as natural gas. It’s also emitted by decomposing plant material in wetlands and is belched out by cows and other ruminant livestock. From a climate perspective, methane is 20 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
National Oceanic and Atmospheric Administration (or NOAA) A science agency of the U.S. Department of Commerce. Initially established in 1807 under another name (The Survey of the Coast), this agency focuses on understanding and preserving ocean resources, including fisheries, protecting marine mammals (from seals to whales), studying the seafloor and probing the upper atmosphere.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
radiation (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.
reflective Having the quality of reflecting light strongly. Reflective objects can produce a strong bright glare when sunlight bounces off of them. Examples of reflective objects include a mirror, a smooth metal can, a car window, a glass bottle, ice, snow or the watery surface of a lake.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
solid Firm and stable in shape; not liquid or gaseous.
trillion A number representing a million million — or 1,000,000,000,000 — of something.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.