How hot can our planet get? Earth’s climate history holds clues

Antarctica: A continent mostly covered in ice, which sits in the southernmost part of the world.

atmosphere: The envelope of gases surrounding Earth, another planet or a moon.

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.

bacteria: (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals). Bacteria are one of the three domains of life on Earth.

basalt: One of the most common rocks on Earth’s surface, it’s been laid down there by volcanic eruptions. Basalt tends to be very dense (unless volcanic eruptions seeded it with lots of air pockets). 

biology: The study of living things. The scientists who study them are known as biologists.

biome: All of the plants, animals and other living things that naturally dwell together in a forest, the tundra, the open ocean or some other habitat.

carbon: A chemical element that is the physical basis of all life on Earth. (in climate studies) The term carbon sometimes will be used almost interchangeably with carbon dioxide to connote the potential impacts that some action, product, policy or process may have on long-term atmospheric warming.

carbon cycle: A series of processes in which carbon-based molecules are naturally transformed in the environment. For instance, plants may take up carbon dioxide (CO₂) from the air and “exhale” oxygen. Animals or microbes may then feast on the carbon-enriched plants. When these organisms die, their decay will transform the carbon into new carbon-based molecules, such as methane. If the organisms become heated and compressed over eons, they may turn into petroleum, natural gas and other “fossil fuels.” If later burned or released into the environment, the fuels’ carbon atoms again become available to help form the tissues of living things.

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 burns (including fossil fuels like oil or gas). 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.

carbonate: A group of minerals, including those that make up limestone, which contains carbon and oxygen.

chemical: A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H₂O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

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

climate change: Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

continent: (in geology) The huge land masses that sit upon tectonic plates. In modern times, there are six established geologic continents: North America, South America, Eurasia, Africa, Australia and Antarctica. In 2017, scientists also made the case for yet another: Zealandia.

continental United States: Also known as the lower 48 states, these are all U.S. states except Alaska and Hawaii. They all reside below Canada and above Mexico.

Cretaceous: A geologic time period that included the end of the Age of Dinosaurs. Spanning 145 million to 66 million years ago, it was a time when the whole planet had a relatively mild climate, with forests growing near both the North and South Poles. It marked the emergence of flowering plants and massive volcanic eruptions, which triggered big shifts in ocean ecosystems. And some regions cooled, leading dinosaurs there to adapt by evolving a coat of insulating feathers. Finally, it all came to an end 66 million years ago when a giant meteorite crashed into Earth, changing the global climate overnight. This wiped out all the dinosaurs (except those that would survive as birds), along with half of all plant and animal species.

crust: (in geology) Earth’s outermost surface, usually made from dense, solid rock (in planetary science) the outermost surface of rocky planets, dwarf planets and natural satellites.

debris: Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.

dinosaur: A term that means terrible lizard. These reptiles emerged around 243 million years ago. All descended from egg-laying reptiles known as archosaurs. Their descendants eventually split into two lines. Most large dinosaurs died out around 66 million years ago. But some live on. We know them as birds.

disk: A round, flat and usually fairly thin object. (in astronomy) A rotating cloudlike collection of gases, dust or both from which planets may form. Or the structure of certain large rotating bodies in the cosmos, including spiral galaxies.

ecosystem: A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some artificial environment, such as a company, classroom or the internet.

equator: An imaginary line around Earth that divides Earth into the Northern and Southern Hemispheres.

evolution: (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed. Or the term can refer to changes that occur as some natural progression within the non-living world (such as computer chips evolving to smaller devices which operate at an ever faster speed).

extinction: (adj. extinct) The permanent loss of a species, family or larger group of organisms.

feedback: A response or assessment that follows some a particular act or decision. Or a process or combination of processes that propel or exaggerate a change in some direction. For instance, as the cover of Arctic ice disappears with global warming, less of the sun’s warming energy will be reflected back into space. This will serve to increase the rate of Earth’s warming. That warming might trigger some feedback (like sea-ice melting) that fosters additional warming.

field: A term to describe a real-world environment in which some research is conducted, such as at sea, in a forest, on a mountaintop or on a city street. It is the opposite of an artificial setting, such as a research laboratory.

force: Some outside influence that can change the motion of an object, hold objects close to one another, or produce motion or stress in a stationary object.

geologic: An adjective that refers to things that are 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.

gravity: The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

greenhouse effect: The warming of Earth’s atmosphere due to the buildup of heat-trapping gases, such as carbon dioxide and methane. Scientists refer to these pollutants as greenhouse gases. 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.

greenhouse gas: A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.

habitable: A place suitable for humans or other living things to comfortably dwell.

humidity: A measure of the amount of water vapor in the atmosphere. (Air with a lot of water vapor in it is known as humid.)

hydrogen bomb: A nuclear explosive whose violent power comes from the sudden release of energy through the fusion of light nuclei (hydrogen atoms) at very high temperature and pressure, creating helium nuclei.

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.

latitude: The distance from the equator measured in degrees (up to 90). Low latitudes are closer to the equator; high latitudes are closer to the poles.

lava: Molten rock that comes up from the mantle, through Earth’s crust, and out of a volcano.

magma: The molten rock that resides under Earth’s crust. When it erupts from a volcano, this material is referred to as lava.

mantle: (in geology) The thick layer of the Earth beneath its outer crust. The mantle is semi-solid and generally divided into an upper and lower mantle.

marine: Having to do with the ocean world or environment.

mass: A number that shows how much an object resists speeding up or slowing down — basically a measure of how much matter that object is made from.

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 80 times more potent than carbon dioxide is in trapping heat in Earth’s atmosphere, making it a very important greenhouse gas.

migrate: To move long distances (often across many countries) in search of a new home. (in biology) To travel from one place to another at regular times of the year to find food or more hospitable conditions (such as better weather). Species that migrate each year are referred to as being migratory. (in science) To travel from one place to another in an attempt to achieve new conditions that may afford stability.

mineral: Crystal-forming substances that make up rock, such as quartz, apatite or various carbonates. Most rocks contain several different minerals mish-mashed together. A mineral usually is solid and stable at room temperatures and has a specific formula, or recipe (with atoms occurring in certain proportions) and a specific crystalline structure (meaning that its atoms are organized in regular three-dimensional patterns).

model: A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.

molten: A word describing something that is melted, such as the liquid rock that makes up lava.

moon: The natural satellite of any planet.

orb: Something having a spherical shape.

organism: Any living thing, from elephants and plants to bacteria and other types of single-celled life.

oxygen: A gas that makes up about 21 percent of Earth’s atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

Pangaea: (or Pangea) A supercontinent that existed from about 300 to 200 million years ago. It was composed of all of the major continents seen today, squished together.

Permian: A time in the distant geologic past, about 250 million to 300 million years ago. Many reptiles rose to prominence on land; these were not yet dinosaurs. Many large invertebrates ruled the oceans during this period. But most would die off at the end of the Permian, as it gave way to a new geologic period known as the Triassic.

planet: A large celestial object that orbits a star but unlike a star does not generate any visible light.

poles: (in Earth science and astronomy) The cold regions of the planet that exist farthest from the equator; the upper and lower ends of the virtual axis around which a celestial object rotates. (in physics and electrical engineering) The ends of a magnet. (in chemistry) two areas of opposite electrical charge, one positive and one negative.

power plant: An industrial facility for generating electricity.

preindustrial: An adjective that refers to the period before societies had begun to industrialize, using machines and fossil fuels to build products, often with assembly lines or big teams of workers. In the United States, that period began in the mid- to late-1700s.

protein: A compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. Antibodies, hemoglobin and enzymes are all examples of proteins. Medicines frequently work by latching onto proteins.

range: The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which values can vary (such as the highest to lowest temperatures). Also, the distance within which something can be reached or perceived.

recycle: To find new uses for something — or parts of something — that might otherwise be discarded, or treated as waste.

saber-toothed cat: Once popularly referred to as a saber-toothed tiger, this cat (Smilodon fatalis) is not closely related to tigers at all. Adults of this bobtailed species were about 30.5 centimeters (roughly one foot) shorter than today’s lions but would have weighed twice as much. Unlike lions and other big cats of Africa, the saber-toothed cat probably did not chase down its prey, but instead ambushed it from a hiding place. The species died out roughly 10,000 years ago.

sea level: The overall level of the ocean over the entire globe when all tides and other short-term changes are averaged out.

seawater: The salty water found in oceans.

Siberia: A region in northern Asia, almost all of which falls within Russia. This land takes its name from the language of the Tatar people, where Siber means sleeping land. This region is vast. It has become famous for its long, harsh winters, where temperatures can fall to −68° Celsius (−90° Fahrenheit).

solar energy: The energy in sunlight that can be captured as heat or converted into heat or electrical energy. Some people refer to wind power as a form of solar energy. The reason: Winds are driven by the variations in temperatures and the density of the air, both of which are affected by the solar heating of the air, ground and surface waters.

species: A group of similar organisms capable of producing offspring that can survive and reproduce.

stress: (in biology) A factor — such as unusual temperatures, movements, moisture or pollution — that affects the health of a species or ecosystem. (in psychology) A mental, physical, emotional or behavioral reaction to an event or circumstance (stressor) that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative. (in physics) Pressure or tension exerted on a material object.

supercontinent: Any of several large landmasses that divided in the ancient geologic past to form today’s continents. Notable examples of supercontinents include notably Pangaea, Gondwana and Laurasia.

tectonic: Surface activity on a large rocky body (such as a planet or moon) as liquid rock flows up to the surface where it solidifies, then slowly drifts atop molten rock, carrying surface features with it.

Texas: The second largest state in the United States, located along the southern border with Mexico. It is about 1,270 kilometers (790 miles) long and covers an area of 696,000 square kilometers (268,581 square miles).

Theia: (in astronomy) The name of a hypothetical protoplanet, named for the Greek goddess of sight, who was also the supposed mother of the moon goddess Selene. If this protoplanet existed, the Mars-sized rocky world would have died in a violent collision with Earth, some 4.5 billion years ago. Part of the debris from it — and Earth — might have eventually collected to form a new celestial object: Earth’s moon.

theory: (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

thermal: Of or relating to heat.

thermostat: A temperature sensor that allows a system to know when a change — either heating or cooling — is called for.

tipping point: A point at which some action leads to a disproportionately large or irreversible change in a process, impact or attitude.

toxic: Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.

transition: The boundary where one thing (paragraphs, ecosystems, life stage, state of matter) changes or converts into another. Some transitions are sharp or abrupt. Others slowly or gradually morph from one condition or environment to another.

trillion: A number representing a million million — or 1,000,000,000,000 — of something.

tropics: The region near Earth’s equator. Temperatures here are generally warm to hot, year-round.

turbulent: (n. turbulence)  An adjective for the unpredictable fluctuation of a fluid (including air) in which its velocity varies irregularly instead of maintaining a steady or calm flow.

vaporize: To convert from a liquid to a gas (or vapor) through the application of heat.

volcano: A place on the crust of Earth (or another planet) that opens, allowing magma and gases to spew out from underground reservoirs of molten material. The magma rises through a system of pipes or channels, sometimes spending time in chambers where it bubbles with gas and undergoes chemical transformations. The surface around a volcano’s opening can grow into a mound or cone shape as successive eruptions send more lava onto the surface, where it cools into hard rock.

waste: Any materials that are left over from biological or other systems that have no value, so they can be disposed of as trash or recycled for some new use.

water vapor: Water in its gaseous state, capable of being suspended in the air.

weathering: The process of breaking down rocks, soil and chemicals (such as crude oil). Weathering can be chemical, such as by oxidation (rust), or mechanical, such as by water, ice, or wind.

woolly mammoth: A type of extinct mammal that resembled a hairy elephant.