Earth’s next big chill may be on hold. Earth should be nearing another frosty period — with a year-to-year expansion of polar and mountaintop ice. But that is not happening. And the reason seems to be a growing buildup of chemicals that warm the atmosphere, the so-called greenhouse gases. Chief among them: carbon dioxide, better known as CO 2 .

Ice comes and goes. For the last 11,700 years, Earth has been between cold periods when the amount of ice on Earth’s surface expands. Scientists call these icy periods glaciations. The warm times between them are known as interglacial periods.

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A similar warm interglacial occurred around 790,000 years ago. It lasted only about 10,800 years, a new study finds. That would suggest that our current warm period should be wrapping up.

The fact that it’s not is likely due to the extra CO 2 that has been entering Earth’s atmosphere from human activities. The burning of fossil fuels has spewed much of that CO 2 . As a result, people may be postponing the next glaciation, researchers now propose. They described their reasoning online May 19 in Geology.

“Most probably, the next glaciation should have already started,” says coauthor Paul Renne. He is a geoscientist at the Berkeley Geochronology Center in California. (Geo means of or relating to the Earth. Chronology refers to the order and timing of past events.) “We are experiencing a climate that has been modified by human activity,” Renne says.

Earth’s climate naturally warms and cools. Slow, periodic changes in our planet’s orbit and tilt create these long cycles. That orbit and planetary tilt affect how much sunlight reaches Earth’s upper atmosphere. With less solar warming, Earth’s climate cools. An ice age is when Earth is cold enough for ice to cover parts of Earth’s surface year-round. By this definition, Earth is currently in an ice age.

During an ice age, when things are chilly enough, more ice builds up in winter than can melt away in summer. This prompts glaciers and ice sheets to grow. This is how a glaciation commences. It ends when Earth warms enough that the ice cover stops expanding. That ice may even begin to melt back, as has been happening increasingly for decades, now, on glaciers around the planet. Indeed, once-stable Antarctic glaciers are now melting rapidly, a May 22 study points out.

The new calculations

Earth’s current orbit and tilt closely match its orientation about 790,000 years ago. That similarity is useful. It allows researchers to estimate when Earth’s next glaciation may start. But figuring out when a warm period may end can be tricky.

Renne was part of a research team led by geologist Biagio Giaccio. Giaccio works at the Institute of Environmental Geology and Geoengineering in Rome, Italy. These experts examined sediments from an ancient lake in Sulmona, Italy, to learn more about that historical interglacial period.

Hundreds of thousands of years ago, volcanoes in the area spat ash into the air containing radioactive potassium. Some of that ash settled in the lake. Over time, the ash left distinct layers in the sediments on the lake floor. By measuring how much potassium decayed into argon, the research team could calculate the relative age of each layer.

Oxygen atoms in each layer revealed the local climate when that layer formed. The scientists focused on “heavy” oxygen. (Also known as oxygen-18, this form has two extra neutrons in its nucleus.) As Earth’s northern hemisphere cooled, the concentration of oxygen comprised of heavy atoms increased.

Those changes let researchers figure out when the ancient warm period began and ended. Those calculations show it lasted around 10,800 years. That’s about 1,700 years less than experts had thought. It also was about 1,000 years shorter than today’s warm period.

Extra CO 2 in the atmosphere may be extending the current warmth, the researchers now say. When that historic interglacial period ended 790,000 years ago, levels of CO 2 in the atmosphere were around 250 parts per million. (CO 2 levels have been above 260 parts per million for several thousand years now. In fact, the global average recently rose above 400 parts per million.) A 2007 study had estimated that CO 2 levels should now be 245 parts per million. That is, it would have been without the extra CO 2 that people have added to the atmosphere. That lower level also would be within the range favorable to a new period of ice expansion.

“We basically short-circuited the beginning of a glaciation,” concludes climatologist Stephen Vavrus. He works at the University of Wisconsin–Madison. “The future is murky now,” he says, because “humans are exerting such a big influence on the climate system.”

Power Words

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argon An element first discovered on August 13, 1894, by Scottish chemist Sir William Ramsay and English physicist John William Strutt, better known as Lord Rayleigh. Argon was the first “noble” gas, meaning one that does not react chemically with other elements. For its discovery, Ramsey would receive the 1904 Nobel Prize in chemistry and Rayleigh the 1904 Nobel Prize in Physics.

atmosphere The envelope of gases surrounding Earth or another planet.

carbon dioxide 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.

climate The weather conditions prevailing in an area in general or over a long period.

fossil fuels Any fuel — such as coal, petroleum (crude oil) or natural gas — that has developed in the Earth over millions of years from the decayed remains of bacteria, plant or animals.

geology The study of 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. Planetary geology is the science of studying the same things about other planets.

glacier A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.

heavy oxygen An isotope, or alternative form of the element, that has two extra neutrons in its nucleus. So instead of being the normal oxygen-16, the heavy isotope form is oxygen-18.

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 an ice age, which can last millions of years, glaciers and ice sheets cover parts of Earth throughout the year. The most recent ice age began around 2.6 million 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.

orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

oxygen A gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.

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.

radioactive decay A process by which an element is converted into a lighter element through the shedding of subatomic particles (and energy).

sediment Material (such as stones and sand) deposited by water, wind or glaciers.

solar Having to do with the sun, including the light and energy it gives off.