Climate Literacy curriculum

Weather is the condition of the air at a given moment, and climate is the typical weather over a long stretch, roughly several decades. One wild day, or even an odd year, is a single sample, so a big swing in the weather does not by itself mean the climate has shifted.

A normal is the plain average of a weather value over three consecutive decades. The U.S. set now in use covers 1991 to 2020, and the next set, due in 2031, will drop the 1990s. Most of the country was warmer in 1991 to 2020 than in 1981 to 2010, so “above normal” now means above a warmer baseline.

Climate is not set by the air alone. The atmosphere, ocean, ice, land and living things are connected systems that store and swap heat, water and carbon, so a change in one part, such as ice that reflects sunlight shrinking or the ocean taking up carbon, feeds through to the rest.

Nitrogen is by far the most common gas in dry air, with oxygen next, yet neither is on the list of greenhouse gases. The heat trapping is done by small amounts of carbon dioxide, methane, nitrous oxide, ozone and CFCs, plus water vapor, which ranges from near zero in dry desert air to almost 4 percent in the humid tropics.

Air cools with height through the lowest layer, the troposphere, then warms with height in the stratosphere above it. That contrast makes the vertical pattern a test: a brighter Sun would warm every layer, while the observed mix of a warming surface and a cooling stratosphere fits a buildup of heat-trapping gases instead.

Each hemisphere has three circulation cells rather than one. Rising air near the equator brings a low-pressure rain belt, sinking air farther out makes a dry high-pressure belt where many of the world's deserts lie, and a stormy low-pressure band sits near 50 to 60 degrees. Land and sea break the bands into semi-permanent highs and lows.

The tropics heat up more than the rest of the planet because sunlight there arrives most directly. Moving air carries that surplus toward the poles, and so do ocean currents: along the east coasts of continents, warm currents flow from the equator poleward, and an ocean overturning circulation carries warm water north and cold water south.

Parts of the climate system run on very different clocks. Weather changes over hours to weeks, pollution particles stay aloft only days to weeks, and a big eruption's cooling fades in one to two years. Water in the deep overturning circulation takes about 1,000 years to finish its loop, so the deep ocean sets the slowest pace.

Albedo is the share of incoming sunlight that a surface or cloud reflects straight back to space. In NOAA's energy budget about 30 of every 100 units of sunlight are reflected, mostly by clouds. White snow and ice reflect far more than darker ground or open water, so losing them lets the surface absorb more energy, and soot that darkens snow makes it melt faster.

Sunlight arrives mostly as visible and near-infrared light, and about half of it passes through air and clouds to the surface. The cooler Earth sends its energy back out entirely as longer-wave infrared, and most of that is absorbed by the atmosphere, so greenhouse gases matter mainly for the energy leaving, not arriving.

Without its natural greenhouse effect, Earth's average temperature would be about -2 degrees F instead of roughly 57 F, a gap of about 59 F, or 33 C. That natural warming is what keeps the planet habitable; the climate question is how much extra warming added greenhouse gases supply on top of it.

Sunlight absorbed by the ground is given off again as infrared heat. Greenhouse gases absorb most of that infrared and re-radiate it, and part of what they emit is absorbed back at the surface, so heat leaks to space more slowly and more energy stays near the ground than it would with no atmosphere.

Most of the infrared that finally escapes to space is given off by gases and clouds in the atmosphere, not by the ground. In NOAA's energy budget, about 58 of every 100 units leave from the atmosphere and clouds, against 12 straight from the surface, so the gases and clouds above the ground, not the surface itself, do most of the work of shedding Earth's heat.

Earth holds a steady average temperature when the energy arriving from the Sun is matched by the energy it sends back to space. Added greenhouse gases have upset that balance, and while more comes in than goes out the surplus keeps building up as heat, about 90 percent of it in the ocean.

About 90 percent of the excess heat from recent warming has gone into the ocean, because water can store far more heat than air. Most of it sits in the top 700 meters, and as that water warms it expands, which caused about a third of global sea-level rise over the past century.

Radiative forcing is the change in heat a driver adds to Earth's energy budget, expressed in watts per square meter. NOAA puts the extra forcing from long-lived greenhouse gases in 2025 at about 3.6 W/m2, roughly 1.5 percent of the sunlight Earth absorbs, and its forcing estimates carry about 10 percent uncertainty, far less than projections of the warming that follows.

CO2 is long-lived, well mixed and by far the largest contributor to the growth in heating from greenhouse gases, about 81 percent of the increase since 1990 by NOAA's count. Because the extra CO2 keeps concentrations raised for thousands of years, cutting emissions slows the rise in concentration rather than quickly reversing it.

Per ton, methane absorbs far more energy than CO2, yet an emitted molecule stays in the air for only about ten years on average, so how much it counts against CO2 depends on the time horizon chosen. NOAA's carbon-13 measurements show its renewed rise since 2007 comes largely from microbial sources such as wetlands, livestock and rice, not mainly from fossil fuel use.

Nitrous oxide comes mainly from agriculture, especially fertilizer use, with smaller amounts from chemical production and burning fuels. A molecule emitted today stays in the air for more than a century on average, and NOAA measured the global level still climbing in 2025, to about 339 parts per billion, so emissions added now keep acting for generations.

CFCs and other fluorinated gases exist only in tiny amounts, yet per ton they trap substantially more heat than CO2, and some persist for hundreds or thousands of years. Under the Montreal Protocol, the main CFCs were all declining in the air by the mid-2000s, while most HFC substitutes are still rising and now face production controls of their own.

Water vapor is the most abundant greenhouse gas, but temperature sets how much the air holds: warmer air holds more, and extra vapor condenses and rains out. So it amplifies warming that something else started instead of starting it, which is why NASA calls it a feedback rather than a direct cause of climate change.

Global warming potential compares the energy one ton of a gas absorbs over a chosen period with one ton of CO2, which lets inventories add different gases together. The usual period is 100 years; a 20-year GWP ignores impacts after 20 years, so it gives short-lived gases such as methane more weight.

If CO2's heat trapping were already “saturated,” adding more would add no heating. NOAA's yearly index turns measured gas amounts into forcing with radiative transfer calculations, and it shows forcing from long-lived gases still climbing as CO2 rises, with CO2 supplying most of the increase since 1990.

Carbon moves through living things quickly: plants pull CO2 from the air by photosynthesis, and respiration and decay return it. Carbon in limestone and fossil fuels comes from organisms that lived millions of years ago and normally returns very slowly, through erosion, so burning fuel shifts slow-cycle carbon into the air fast.

Most of Earth's carbon is locked in rocks and sediments, and the ocean holds far more than the air. Because the atmosphere is a small store next to those, moving even a sliver of carbon from fossil reserves into it raises the CO2 concentration noticeably, which is why the air is the reservoir whose level swings most.

The yearly zigzag in the CO2 record comes from plants. In the growing season they draw CO2 down for photosynthesis, and in fall and winter decay and respiration return it. The swing is largest in the Northern Hemisphere, which has more land, while the Southern Hemisphere shows a weaker pattern in opposite seasons.

NOAA pages put the ocean's share of the CO2 people emit at roughly a quarter to 30 percent, taken up mainly as a physical response: more CO2 in the air pushes more into seawater. The surface catches up with the air in about a year, but carbon reaches the deep ocean slowly and in only a few places, and warmer water absorbs less.

Forests and soils absorb part of the CO2 people emit, but the service is not guaranteed. Natural sinks on land and in the ocean took about 60 percent of each year's emissions in 1959 and about 55 percent now, and deforestation is turning parts of the Amazon from a deep sink into a seasonal source.

Only about half of the CO2 from burning fuel has stayed in the air; the ocean and, more recently, land ecosystems absorbed the rest. That is why concentration rises more slowly than total emissions, and why weaker natural sinks would leave a bigger share of each year's emissions in the atmosphere.

A pulse of CO2 does not fade on one schedule. The surface ocean trades CO2 with the air within about a year, and about half of emissions has already gone into ocean and land, yet the rise in concentration that emissions cause lasts thousands of years and the warming persists for millennia.

Fossil fuels formed from plants buried for millions of years, so their carbon is low in carbon-13 and holds no carbon-14. A falling carbon-13 share alone could also point to plants and soils, which are low in carbon-13 too; carbon-14 measurements are what pin the decline on fossil fuel burning.

In EPA's U.S. inventory for 2022, transportation is the largest source of direct greenhouse gas emissions, but it drops to second once the emissions from generating electricity are assigned to the sectors that use the power, because buildings use about three-quarters of U.S. electricity. Which sector ranks first depends on where the power plant's emissions are counted.

CO2, mostly from fossil fuel use, is the main greenhouse gas people emit, while methane comes from farming, waste and energy, nitrous oxide mainly from fertilizer, and fluorinated gases from industry and refrigeration. Comparing them needs a common unit, and because methane is short-lived its weight depends on the time horizon chosen.

Fuels release different amounts of CO2 for the same heat. Natural gas, mostly methane, gives about 117 pounds of CO2 per million Btu, while bituminous coal gives about 206, roughly 1.75 times as much. Switching coal to gas cuts CO2 per unit of energy, but gas still emits, and leaked methane is a strong greenhouse gas.

Clearing forests and disturbing soils releases the carbon stored in plants and soil, and it also removes land that would have kept pulling CO2 from the air; NASA notes ongoing deforestation is turning the Amazon from a deep carbon sink into a seasonal source. In the United States, managed forests and other lands remain a net sink, offsetting 13 percent of 2022 emissions.

Some industrial CO2 comes from chemistry, not combustion. Making cement relies on chemical reactions that release CO2 from the raw material itself, so the emissions occur no matter how the kiln is powered, and inventories count these process emissions apart from fuel burned for energy.

Agriculture is the main source of nitrous oxide, chiefly from fertilizer, and a large source of methane. Cattle release methane mostly by belching during digestion, a process called enteric fermentation, with more coming from manure ponds and lagoons, and rice farming is another methane source.

EPA's national inventory, which the United States submits under the UN climate convention, counts emissions from human-made sources inside the country, sorted by source, sector and gas, and subtracts carbon taken up by U.S. forests and soils. That is a territorial boundary: it records where emissions physically occur.

A total shows where the most greenhouse gases are released but ignores how many people release them. EPA put total U.S. emissions in 2022 at about 6,343 million metric tons of CO2 equivalent; dividing a total like that by the population gives a per-person figure, and ranking countries that way can put them in a different order.

Light-colored particles, such as sulfate from burning coal, reflect incoming sunlight and cool the surface. Pollution particles near the ground last only days, so their cooling depends on constant new emissions, unlike sulfate from a large eruption, which reaches the stratosphere and can stay aloft for a few years.

Particles also change clouds. Some pollution aerosols alter the size or lifetime of cloud droplets, and when droplets become smaller a cloud reflects more sunlight, which overall cools the planet. Dust can also affect how ice forms in colder clouds, and these aerosol-cloud effects remain an active research area.

Soot, or black carbon, is made of dark carbon particles from burning fossil fuels, wood and other plant matter. Unlike bright sulfate, it absorbs sunlight and heats the air, and when it settles on snow or ice it darkens the surface and speeds melting. Wildfires, cooking fires, industry and diesel engines are major sources.

An explosive eruption can push sulfur dioxide into the stratosphere, where it forms light-colored sulfate particles that circle the globe and stay aloft for a few years. They block sunlight and cool the planet, an effect that typically fades after one to two years.

Pollution particles that reflect sunlight have offset part of the warming from greenhouse gases, so NASA notes Earth would be even warmer without them. As energy use shifts away from sources that emit them, losing that cooling could bring some temporary warming, but because the change comes gradually over several decades, NASA judges it unlikely to raise temperatures much.

Clouds affect the energy budget in more than one way: they reflect part of incoming sunlight back to space and absorb some as well, and aerosols change their droplets and lifetimes. How these effects shift as the climate changes is still under active research, and such feedbacks are part of why future projections carry large uncertainties.

Ozone near the ground forms when nitrogen oxides and volatile organic compounds from vehicles, power plants and industry react in sunlight. It traps heat and is a pollutant that harms breathing and crops, which makes it a different matter from the protective ozone layer high overhead.

Global temperature records draw on weather stations, ships, moored and drifting buoys, weather balloons and satellites, with buoys mostly replacing ships at sea in recent decades. A few series reach back further, but before about 1880 there are too few measurements to estimate an average for the whole planet.

An anomaly is a reading minus a baseline average for the same place, usually built from 30 or more years. A mountaintop and a nearby valley have very different temperatures but can share a similar anomaly, so dropping one station barely moves an average of anomalies while it would noticeably shift an average of absolute temperatures.

Station records change for reasons that have nothing to do with climate, such as a move or new instruments. Agencies correct for this with peer-reviewed methods: NOAA compares each station with several neighbors to find the one that shifted and size the fix, and independent groups using different techniques get closely matching global results.

Cities really are warmer than the countryside, something observers noticed nearly two centuries ago. Analysts account for it: NASA sorts stations into urban and rural using satellite images of nighttime lights and compares urban records with nearby rural ones, and with that done the urban effect plays no substantial role in global warming.

Separate teams build global temperature records, including NASA, NOAA and the UK Meteorological Office, and each uses its own methods for sparse data and changing sites. Their records agree closely on the warming trend, which shows the result does not depend on any one team's choices.

Because the ocean holds about 90 percent of the excess heat, its heat content is the most direct gauge of the energy Earth is gaining. NOAA has tracked the top 2,000 meters since 1955, and since 1992 a NASA model that blends Argo floats, ship instruments and satellite data has estimated the full water column.

Measurements at Mauna Loa, Hawaii, make up the longest direct record of atmospheric CO2. Under its seasonal wiggle the rise is speeding up: NOAA's global average grew about 1.6 ppm a year in the 1980s and about 2.5 ppm a year from 2016 to 2025.

Different instruments measuring different things point the same way: global sea level is rising, mountain glaciers are losing mass, Arctic sea ice is shrinking in both extent and thickness, and Northern Hemisphere spring snow cover is smaller and melting earlier. They do not rely on thermometers, which is why their agreement is strong evidence.

A proxy is a preserved physical trace, such as tree rings, ice layers, coral bands, pollen or lake and ocean sediment, that stands in for a direct measurement. Many proxies form in layers that mark a fixed span, sometimes a single year, and long records usually show less detail about short-term changes.

Air bubbles in ice cores hold samples of the past atmosphere, so gases such as CO2 and methane are measured directly. The ice records more: snow that fell in colder conditions has a lower oxygen-18 to oxygen-16 ratio, which tracks temperature, and dust and volcanic layers are preserved too.

Where there is a distinct growing season, a tree usually adds one ring a year, wider when warmth or moisture favors growth. Scientists sample many trees, match ring patterns from dead wood to living trees to extend the record, and trust widths as a climate stand-in only where they correlate strongly with nearby weather-station data.

Coral skeletons form seasonal density bands that can be dated to an exact year and season, and their oxygen and trace metals record water temperature. Deep-sea sediment cores hold fossil plankton whose species and shell chemistry let scientists estimate past ocean surface temperatures, including at the last glacial maximum.

Ice ages and warm interglacials have alternated over periods of about 100,000 years, paced by slow changes in Earth's orbit. CO2 and temperature rose and fell together: CO2 was about 185 ppm at the coldest point of the last glacial phase, and warming oceans that hold less carbon help explain the link.

In the early Eocene, about 54 to 48 million years ago, Earth was 9 to 14 C warmer than today with CO2 between 1,000 and 2,000 ppm, likely boosted by volcanic activity. The interglacial about 125,000 years ago was roughly 1 to 2 C above preindustrial, but orbital changes drove that warmth, and they cannot explain recent warming.

Past climate swings were often large but slow. Coming out of the last ice age, CO2 rose about 1 ppm every 125 years; since 1850 it has risen about 1 ppm every 23 weeks, over 300 times faster. NASA puts today's warming at roughly 10 times the average rate of warming after an ice age.

Milankovitch cycles are slow changes in the shape of Earth's orbit, the tilt of its axis and the wobble of that axis. They shift sunlight between seasons and latitudes far more than they change the yearly global total: eccentricity barely alters total annual sunlight, while a larger tilt brings more extreme seasons, felt most at high latitudes.

Orbital changes set off the warming out of each ice age, and CO2 then rose with it, because warmer oceans absorb less carbon and warmer soils release more. That makes CO2 a feedback in those cycles, which fits with it also driving warming today: the same gas can respond to warming and cause it.

The Sun brightens and dims slightly over an 11-year cycle, and satellites watching it for more than 40 years find its output has varied by less than 0.1 percent. Sunlight reaching Earth shows no net increase since the 1950s while global temperature has risen markedly, so NASA judges it extremely unlikely that the Sun caused the recent warming.

If a brighter Sun were warming Earth, every layer of the atmosphere would warm, up through the stratosphere. Instead the surface and lower atmosphere have warmed while the upper atmosphere has cooled, which fits greenhouse gases slowing heat loss from the lower atmosphere rather than more energy arriving from above.

Greenhouse gases warm by slowing heat loss to space, which goes on through the night as well as the day, so comparing daily lows with daily highs is one way to look at the record. U.S. data give a mixed answer: nationally, lows warmed slightly more than highs over the past century, but highs rose a bit faster over the most recent 40 years NOAA analyzed.

Volcanoes do release CO2, but human activity puts out more than 100 times as much as all the world's volcanoes combined. An eruption's main climate effect runs the other way: particles that block sunlight and cool the globe for a year or two, while human greenhouse warming lasts for millennia.

Normally trade winds push warm surface water west toward Asia, cold water upwells off South America, and the thermocline slopes up toward the east. In El Nino the trades relax, the thermocline flattens, upwelling no longer cools the east, and rain follows the warm water east. La Nina, the cool phase, recurs every 3 to 5 years or so.

When El Nino shifts the warmest water and its rising air east, the global atmospheric circulation shifts too, changing weather far from the Pacific. Typical results include more rain in Peru and the southern U.S., drought in Indonesia and Australia, and, in moderate to strong events, far fewer Atlantic hurricane days.

El Nino and La Nina push global temperatures and rainfall up and down from year to year, so single years scatter around the long-term line. Analysts smooth the record with multi-year averages to see the trend, and it keeps rising: the ten most recent years are the warmest on record.

The Pacific Decadal Oscillation is a long-lived, El Nino-like pattern of North Pacific ocean temperatures with warm and cool phases defined by opposite anomalies. Because a phase can persist for years before reversing, a short record that sits inside one phase can look like a trend that a longer record does not show.

1997 to 1998 brought one of the two largest El Nino events on record, so a trend line started in 1998 begins at an unusual peak and can make warming look flat. Longer records show it did not stop: warming continued after 1998, and 2014 through 2020 were all among the hottest years since 1880.

The polar vortex is a large area of cold, low-pressure air that always circles each pole high in the atmosphere and strengthens in winter. When it expands, Arctic air can spill far south with the jet stream, a regular winter event. A severe cold snap in one region is weather, and it says little about the global climate trend.

Water can store far more heat than air, so the ocean has absorbed about 90 percent of the excess heat from recent warming. That heat is carried by currents, and water in the deep overturning circulation takes about 1,000 years to complete its loop, so the ocean adjusts on a far slower clock than the air.

Seawater expands as it warms, so the ocean's heat gain raises sea level without any added water. Expansion caused about a third of global sea-level rise over the past century; melting land ice now supplies about two-thirds of the rise, making it the larger share today.

Deep currents are driven by density, which depends on temperature and salt. Near the poles, forming sea ice leaves its salt behind, so the water around it becomes colder, saltier and denser and sinks, pulling in surface water to replace it. This drives a global conveyor belt that moves only a few centimeters per second.

The Atlantic Meridional Overturning Circulation carries warm water north near the surface and cold water south at depth, bringing warmth to parts of the globe. Research shows it weakened over the past century, but whether it will keep slowing or stop is uncertain; further slowing could shift a rain belt and raise sea level on the U.S. East Coast.

When wind pushes surface water away from a coast or an open-ocean area, colder water rises from below to take its place. That upwelled water is rich in nutrients and fertilizes the surface, so productivity runs high, which is why good fishing grounds tend to lie where upwelling is common.

The ocean is layered: a wind-mixed surface layer sits on top of the thermocline, where temperature drops fast with depth, with colder water below. Surface water reaches the deep only slowly and in a few places, so that layering controls how quickly heat and carbon taken from the air are carried downward.

A marine heatwave is a period of persistent, unusually warm ocean temperatures, judged against the local normal for that month. NOAA flags one when sea surface temperature exceeds a high percentile, such as the 90th, of that place's anomalies. Heat stress can make corals expel the algae in their tissues and turn white, which is called bleaching.

When CO2 dissolves in seawater it combines with water to form carbonic acid, a weak acid that splits into hydrogen ions and bicarbonate. More hydrogen ions mean higher acidity, so as the ocean absorbs more CO2 its pH falls, even though seawater, around pH 8.1, is still basic.

pH is a logarithmic measure of hydrogen-ion concentration, so a change of one pH unit is a tenfold change. That is why a small-looking drop matters: NOAA's education pages call the 0.1-unit fall in surface ocean pH since the industrial revolution about a 30 percent rise in acidity, and NOAA's Ocean Acidification Program puts the rise over 250 years at about 26 percent.

Oysters, corals and other shell builders combine calcium with carbonate ions from seawater. Extra hydrogen ions from absorbed CO2 bond with carbonate, leaving fewer carbonate ions to build and maintain shells and skeletons, and if pH falls far enough those structures can begin to dissolve.

Acidification is a chemical result of the ocean taking up CO2, not a heat effect, which is why NOAA scientists call it the other carbon dioxide problem. Ocean CO2 rises with atmospheric CO2, so the pH keeps falling as long as more CO2 dissolves, a change that does not require the water to warm.

Coastal upwelling, which happens mainly along the western edges of continents, brings deeper, more acidic water to the surface. Off the U.S. West Coast that water last touched the air about 1,000 years ago, and absorbed human CO2 has widened the area it reaches; nutrient runoff from land can add acidity too, as algal blooms die and decay.

Sea ice floats, so it already displaces its own volume of seawater, and melting it does not change sea level. Glaciers and ice sheets rest on land, and when their ice reaches the ocean it adds water; today melting land ice supplies about two-thirds of global sea-level rise.

NOAA's Arctic Report Card describes a region warming far faster than the rest of the planet, and NOAA explains one driver as a melt cycle. Bright sea ice reflects sunlight, so as it melts the darker ocean absorbs more energy and warms, which delays freezing in fall and speeds melting in spring, leaving dark water exposed longer the next summer.

Sea-ice extent counts every area where at least 15 percent of the ocean surface is ice-covered, as seen by satellites, so it says nothing about how thick the ice is. Arctic sea ice has declined in both extent and thickness over recent decades, and its summer minimum extent is shrinking by about 12 percent per decade.

A glacier's mass balance is the snow it gains minus what it loses to melting, runoff and evaporation, plus calving and ocean melt where the ice meets the sea. Most mountain glaciers have shown decades of more losses than gains, so their retreat reflects a lasting imbalance, not one warm summer.

Ice sheets lose mass at the surface, by melt and runoff, and at their edges, where ice calves or thins against warm ocean water. Greenland and Antarctica have both lost mass since 2002, Greenland about twice as fast, and losses cluster where ice flow converges on outlet glaciers, a sign that warming ocean water plays a key role.

Thawing permafrost is a natural source of methane, which NASA measures in Arctic regions, and warmer temperatures release more carbon from soils and permafrost. Because the release is driven by warming itself rather than by any direct human emission, it acts as a feedback that adds more warming.

Mountain snowfields act as frozen reservoirs, holding precipitation from the cool season, when most of it falls, and releasing it in spring and summer; in some western states as much as 75 percent of the water supply comes from snowmelt. Northern Hemisphere spring snow cover has shrunk and is melting earlier, shifting when that water arrives.

Each source works differently. Meltwater from glaciers and ice sheets adds volume, while warming makes seawater expand slightly, an effect that adds up over the ocean’s full depth. Melting floating sea ice, such as the Arctic ice cap, is the look-alike that does not raise sea level, because that ice already displaces its own volume.

A tide gauge records the water's height against a fixed point on nearby land, so it measures relative sea level, which mixes ocean change with any land motion. Satellites measure absolute sea level, the ocean surface's height above Earth's center, over nearly the whole ocean. Gauge records reach back more than a century; satellite records began in the early 1990s.

Land can rise as sediment builds up or as ground rebounds once heavy Ice Age ice is gone, and it can sink through compaction, erosion, natural subsidence or groundwater pumping. That is why, in EPA’s record from 1960 to 2023, relative sea level fell at some Alaska and Pacific Northwest gauges even as the global ocean rose: the land there rose faster.

NOAA expects that, with continued ocean and atmospheric warming, seas will likely rise for many centuries at rates higher than this century’s. Two slow parts of the system carry that momentum: the ocean, which has absorbed more than 90 percent of the added heat and takes a long time to respond, and land ice sheets, whose warming-driven losses NASA describes as raising sea level for centuries to come.

High tide flooding, also called sunny-day or nuisance flooding, happens when a higher sea level combines with ordinary local factors such as strong tides near a full or new moon, or shifts in winds and currents. NOAA reports it is now twice as frequent in U.S. coastal communities as 20 years ago, driven by rising seas, sinking land and lost natural barriers.

Storm surge is the rise in seawater caused solely by a storm, mostly its winds pushing water onshore; storm tide is that surge plus the normal tide. Because both stack on top of mean sea level, NOAA notes that higher seas let deadly and destructive surges push farther inland than they once did.

Observations show atmospheric water vapor rising about 4 percent per degree Fahrenheit of warming, or roughly 7 percent per degree Celsius. More vapor means more water available to rain out when air rises or air masses collide, consistent with the heavier rainfall seen where records are good enough to measure it.

EPA expects many historically wet areas to see more precipitation and flood risk and many historically dry areas to see less precipitation and more drought. Its own U.S. drought record through 2023 shows that kind of split, with large, consistent drying across much of the West while the Midwest and Northeast grew generally wetter. It is a tendency, not a rule for every place.

As temperatures rise, evaporation from soil, lakes and reservoirs and transpiration from plants speed up. That loads the air with water for rain somewhere, but leaves less moisture in the soil in some places, so drought can deepen without less rain: NASA notes droughts are intensifying mainly through drier soils rather than lower rainfall.

In EPA’s U.S. record through 2023, a growing share of precipitation fell in intense single-day events, and nine of the top 10 years for such events came after 1995. That change says nothing by itself about the yearly total, which can hold steady while rain arrives in harder bursts, so intensity and total are separate measurements.

As winters warm, more precipitation falls as rain: in EPA’s record from 1949 to 2024, more than 80 percent of stations in the lower 48 states saw a falling share of winter precipitation arrive as snow. Snow also melts earlier, and where snowmelt matters, winter-spring runoff now comes at least five days earlier than in the mid-20th century at most gauges.

Atmospheric rivers are long, narrow bands of air that carry most of the water vapor moving outside the tropics; an average one carries vapor roughly equal to the flow at the Mississippi River's mouth. Most are weak and supply needed rain and snow, but the strongest can stall over a vulnerable watershed and cause extreme rainfall and floods.

Extremes sit in the tails of the temperature range, so a warmer average pushes more days past old hot thresholds and fewer past cold ones. Studies of single heat waves suggest some became tens to hundreds of times more likely because of human-driven warming, and in EPA’s U.S. record unusually cold winter temperatures have grown less common since the 1980s.

Attribution studies use climate models and observations to ask how much human warming changed an event's likelihood or intensity, such as estimating that about 20 percent of one hurricane's rainfall was attributable to it. Links are strongest for events driven by heat, such as heat waves, and weaker for events that depend mainly on atmospheric dynamics.

Your body cools by evaporating sweat, and a wet-bulb thermometer wrapped in a wet cloth cools the same way, by evaporation, so its reading responds to humidity as well as heat. The Wet Bulb Globe Temperature adds a black globe for sunlight and a shaded air thermometer to gauge heat stress on people working or exercising outdoors.

Flood damage is mostly a question of thresholds, such as the water needed to overtop a levee or overwhelm storm drains. A return period expresses the chance an event is equaled or exceeded in a given year, so when heavy downpours grow more common against unchanged drainage, the overwhelming storm returns more often even if yearly totals barely move.

Drought types differ by what runs short: meteorological drought is a spell of dry weather, agricultural drought is when crops are affected, hydrological drought shows up as low water in the supply system, and socioeconomic drought is when supply and demand for goods are hit. Each is measured differently, so one indicator can signal drought while another does not yet.

Warmer springs, longer dry summers and drier soils and vegetation have lengthened fire seasons, and earlier snowmelt leaves less water in hot months. Climate is not the only driver, though: land use, insect infestations, fuels such as invasive cheatgrass, and fire suppression also shape how often and how intensely fires burn.

In EPA’s Atlantic record through the early 2020s, hurricane counts and U.S. landfalls show no clear trend since 1878 once observing changes are allowed for, yet the storms’ combined intensity rose noticeably over the last 30 years, though the 1950s and 1960s were also active. Warm sea surfaces supply the storms’ energy, and model studies of one hurricane traced part of its rainfall to warming.

A feedback is a process that amplifies or reduces a change as time goes on, as when water vapor increases as Earth warms and adds more warming. Warm periods in the ice-age record show the order: shifts in Earth's orbit changed the sunlight reaching each latitude, and warmer soils and oceans then released carbon or absorbed less of it, so carbon dioxide rose with a warming it did not start.

As the climate warms, the air holds more water vapor; NASA reports an observed rise of about 4 percent per degree Fahrenheit of warming. Because water vapor is itself a greenhouse gas, that extra vapor traps more heat, so it amplifies whatever started the warming, such as added carbon dioxide, without starting a change on its own.

When warming melts sea ice, fewer bright surfaces remain to reflect sunlight, so the ocean absorbs more energy and its temperature rises. Warmer water then delays ice growth in fall and speeds melting in spring, leaving dark water exposed longer the next summer, a cycle of warming and melting. More broadly, a warmer climate has less ice cover, and ice-free surfaces absorb more sunlight.

The Planck feedback is the basic brake: a warmer surface and atmosphere send more infrared out to space. The lapse-rate feedback reflects how warming changes the way temperature falls with height; in the tropics the decline gets gentler, which adds to the brake, while Arctic inversions work the other way. Researchers often combine it with the water vapor feedback, which shrinks the model disagreement that water vapor seems to cause alone.

Equilibrium climate sensitivity is the global-mean surface warming once the climate settles after carbon dioxide doubles from preindustrial levels. Recent community assessments narrowed its estimated range for the first time in decades, largely through paleoclimate records; reconstructions of the Last Glacial Maximum and the mid-Pliocene warm period gave the strongest evidence against very high values.

While added greenhouse gases hold Earth's energy budget out of balance, part of the extra energy goes into storage instead of warming the surface, and beyond about a year nearly all of that storage is the ocean. NASA says the ocean now holds about 90 percent of the heat from human-caused warming, so the warming felt during the transition stays below the value reached once the budget balances.

A tipping element is a large part of the Earth system that could shift to a new state once warming passes a critical threshold, and not easily shift back. Candidates include a major ocean overturning circulation, permafrost and the ice sheets, where an event such as an ice-shelf collapse can heavily influence the rate of ice loss; for boreal forests, the potential to tip is still underexplored.

A climate model is a computer program that uses physical laws to work out how climate has changed and may change, ranging from simple versions that track only essential processes to complex ones linking the atmosphere, ocean, sea ice and land. Scientists use it for experiments, varying solar output, volcanic eruptions or greenhouse gases to see how the climate responds to each.

Some processes, such as how aerosols interact with clouds or how injected aerosol plumes spread, are too small for a global model to resolve, so they are represented through parameterizations, ideally built by first studying them in high-resolution models. Differences in cloud feedback between models have long been argued to cause most of the spread in projected warming, though one 2020 study found much of it offset by other feedbacks.

Models are tested against the record they are meant to explain. NOAA reports that simulations of the last 1,000 years capture some temperature swings from solar and volcanic activity, but neither explains the strong 20th-century warming, and natural forcings alone would have brought a slight cooling since 1960; only adding human greenhouse gases reproduces it.

Running many models under shared scenarios separates two kinds of uncertainty. In a 2024 analysis, nine models given the same concentration scenario projected 2 to 4.5 degrees Celsius of warming by 2100, mostly because their sensitivity to carbon dioxide differs; spread across scenarios instead reflects how much people are assumed to emit.

A projection runs a model under an assumed future. Recent model comparisons drive every model with a shared set of scenarios, the Shared Socioeconomic Pathways, so results are comparable and each one is an if-then statement tied to the pathway it assumed. Check which scenario a warming figure uses before comparing it with another figure.

Weather is the day-to-day state of the atmosphere, while climate is the weather of a place averaged over a long period, usually decades. A climate projection does not try to say what happens on a given day; it is an if-then estimate of how those averages shift under a stated emissions path, which is why NASA frames future emissions as the key question.

The strongest test of a projection is whether later observations follow it. NASA notes that effects scientists long predicted from global warming are now occurring, such as sea ice loss, faster sea level rise and longer, more intense heat waves, and EPA's indicators, as archived in January 2025, recorded 2023 as the warmest year and 2014 to 2023 as the warmest decade on record.

EPA names three factors behind how hard a hazard hits a person: exposure, being where it strikes; sensitivity, such as greater susceptibility in children or people with asthma; and adaptability, the ability to prepare and recover. People with less access to health care, housing and money rebound more slowly, so one storm can be a nuisance in one place and a disaster in another.

Climate pulls harvests several ways at once. EPA, drawing on USDA work, notes changes in temperature, rainfall and frost timing could lengthen growing seasons or allow new crops in some regions while making farming harder in others. Ground-level ozone absorbed by plants slows photosynthesis and growth, and if plants bloom before or after their pollinators emerge, pollination could fall.

In most western U.S. river basins, snowpack stores more water than human-built reservoirs, releasing it as runoff in spring and summer. Higher temperatures make snow melt earlier, which shifts when that runoff arrives, and EPA's record from 1949 to 2024 shows more than 80 percent of stations in the lower 48 states getting a smaller share of winter precipitation as snow.

Species can respond to warming by moving to cooler places: about three in five of the North American bird species EPA tracked shifted their wintering grounds northward since the 1960s, though about a quarter moved south, and many marine fish and shellfish have moved north or into deeper water. Movement has limits; EPA notes birds that need salt water cannot move inland.

Corals depend on algae living in their tissues for food and color. Under heat stress the algae leave and the coral turns white; a bleached coral is not yet dead and can recover if the stress eases, but prolonged stress kills it. Cold snaps and pollution can also cause bleaching, though ocean warming is the leading cause.

Climate reaches health along several routes. Extreme heat is the leading cause of weather-related deaths in the United States, and CDC lists more respiratory and heart disease, injuries and early deaths from extreme weather, shifts in where food- and waterborne and other infectious diseases occur, and threats to mental health. How hard any person is hit depends on exposure, sensitivity and the means to adapt.

Much infrastructure was sized for the water levels and storms of the past. As relative sea level rises, NOAA notes it no longer takes a strong storm to flood the coast: minor high tide flooding already backs up stormwater and closes roads, and moderate flooding can damage homes and businesses. A structure built to an old extreme meets that extreme more often as the baseline shifts.

The communities most vulnerable to climate change are often those that contributed least to causing it. EPA adds that people who are excluded from planning, face racial and ethnic discrimination, or have language barriers are more vulnerable and less able to prepare for and avoid the risks, so the same hazard lands unevenly.

Warming since preindustrial times has grown nearly in proportion to cumulative carbon dioxide emissions, which is why each temperature limit implies a finite total that can be emitted. Its size is uncertain: one 2024 analysis put the cumulative budget for 2 degrees Celsius at about 1,048 billion tons of carbon, not carbon dioxide, with a likely range of 881 to 1,216.

Cutting emissions and stopping them are different targets. While emissions still outpace what natural sinks remove, carbon dioxide keeps accumulating and, because it lasts thousands of years, the warming keeps building. If emissions stopped entirely, NASA says, the rise in temperature would begin to flatten within a few years, then plateau at an elevated level for centuries.

Atmospheric carbon dioxide is a stock that yearly emissions fill and natural sinks drain; those sinks now remove about 55 percent of each year's human emissions, down from about 60 percent in 1959. While the inflow exceeds the drain, the level keeps rising even if the yearly flow shrinks, and the added carbon dioxide lasts for thousands of years.

Removing carbon after passing a temperature limit does not rewind everything. NASA calls the effects to date irreversible on the timescale of people alive today, added carbon dioxide lasts thousands of years, and warming-driven ice losses keep raising sea level for centuries. Removal can still shorten the time spent above a limit, which DOE describes as limiting the harm.

Methane traps far more energy per ton than carbon dioxide but lasts only about a decade on average, so cutting it would slow warming quickly. Carbon dioxide's added concentration lasts thousands of years, so each ton keeps contributing long after methane would have broken down, which is why the two cuts are not interchangeable.

A power source with no smokestack can still carry emissions. Nuclear reactors emit no carbon dioxide while running, but mining and refining uranium and building with metal and concrete take a lot of energy; hydropower dams carry construction emissions and possible reservoir methane; an electric car's emissions depend on the fuel used to make its electricity.

Some climate measures pay off locally too: EPA notes that more efficient buildings, low-carbon transportation and urban green space can also ease urban heat islands and cut air pollution. Trade-offs run the other way as well; DOE lists large land requirements as a key challenge for storing carbon in forests and soils, and raises land-use and energy-use concerns for removal at very large scale.

Capacity is how much power a generator can produce; generation is the energy it actually delivers over time. Capacity factor links the two as actual output divided by potential output, which is why plants above about 70 percent are said to run as base load while peaking units often run below 15 percent.

Wind and solar plants generate only when the wind or sun is available, which EIA labels intermittent. Storage fills gaps by charging when power is available and discharging when needed, but it is a secondary source that returns less electricity than it takes in; as of 2022, pumped-storage hydro and then batteries led U.S. storage capacity.

Grid operators must have plants supply the right amount of electricity at every moment, because supply and demand that fall out of balance can cause local or even widespread blackouts. Balancing authorities do this within regions, using base-load plants that run nearly continuously, intermediate plants that follow demand, and peaking units for the highest loads.

In a photovoltaic cell, absorbed photons knock electrons loose in a semiconductor, and treatment of the cell's front surface draws them there, creating a voltage much like a battery's terminals. Only absorbed photons count; photons that reflect or pass through do nothing, and the best commercial modules now approach 25 percent efficiency, up from under 10 percent in the mid-1980s.

Wind flowing across a blade lowers the air pressure on one side, creating lift, the same force behind an airplane wing, and the lift outweighs drag enough to spin the rotor and generator. Because wind speed increases with height and air higher up is less turbulent, taller towers let turbines capture more energy.

A reactor splits uranium atoms in a controlled chain reaction, producing heat with no carbon dioxide or air pollution while it runs. Its harder questions lie elsewhere: spent fuel and other radioactive waste can stay dangerous for thousands of years and is tightly regulated, and plants require containment structures and armed security.

Each source has its own conditions. Geothermal power is easiest where magma nears the surface at plate boundaries, and its plants emit about 99 percent less carbon dioxide than fossil plants. Hydropower emits no air pollutants directly, though reservoirs can release methane. Biomass counts as low-carbon only while regrowth recaptures its carbon; harvesting wood faster than trees grow causes deforestation.

Electric drives waste less energy than engines: electric vehicles convert over 77 percent of grid electricity into power at the wheels, against about 12 to 30 percent of gasoline's energy for conventional cars. Heat pumps gain by moving existing heat instead of making it, like a refrigerator. Their emissions still depend on how the electricity was generated.

Some emissions are hard to remove with clean electricity alone. Certain industrial processes release carbon dioxide from chemical reactions as well as from the fuel they burn, and DOE names agriculture, aviation and shipping among the hardest sectors to decarbonize, describing carbon removal as one way to address their remaining emissions.

Not emitting and removing are different claims. Capturing carbon at a fossil power plant keeps carbon dioxide out of the air, which reduces emissions; carbon dioxide removal pulls it from the atmosphere and stores it, creating negative emissions. Offsets can rest on either kind of claim, and DOE notes that weak standards make long-term results hard to ensure.

Trees and soils remove carbon dioxide through photosynthesis and can hold it for long periods, but the storage is fragile: wildfires and shifts in land management can release it. DOE lists durable, measurable storage and large land needs as key challenges, and it does not count existing trees, or replanting to make up for wildfire losses, as new removal.

Direct air capture passes ambient air over solid sorbents or liquid solvents that react selectively with carbon dioxide, then usually heats them to release the gas so they can be reused. It counts as removal only when paired with lasting storage such as deep rock, and DOE identifies cutting the cost of capturing straight from the air as its main challenge.

Point-source capture grabs carbon dioxide at a power plant or factory before it escapes, which lowers emissions but removes nothing. BECCS burns plants that drew carbon from the air, then captures and stores the concentrated stream, so the stored carbon came from the atmosphere; capturing carbon from burned plastic waste does not qualify.

Enhanced mineralization speeds up a natural reaction in which carbon dioxide combines with calcium or magnesium in rocks such as basalt to form solid carbonate minerals. It can happen underground, in mined and ground rock, or with rock spread over land and coasts, and like other removal methods it needs ways to verify the carbon actually stays stored.

Solar radiation modification would add a new change, reflecting some sunlight, rather than cancel the effects of greenhouse gases. It would not address ocean acidification, which rising carbon dioxide drives, and if it stopped, temperatures could return rapidly to levels set by cumulative emissions, the risk called termination shock.

A carbon offset is an emission avoided, reduced or removed to make up for emissions elsewhere, so offsets with the same label can rest on very different claims. DOE notes that without strong standards it is hard to confirm an offset reduces emissions for the long term, and stored removals may be more verifiable than promises of avoided emissions.

Mitigation cuts the flow of greenhouse gases into the air or strengthens sinks such as forests and soil, limiting how much the climate changes. Adaptation adjusts to changes already underway or expected, through measures like flood defenses and heat planning. Because some warming is already committed, both are needed, and the faster climate changes, the harder adaptation becomes.

A return period expresses the chance that an event is equaled or exceeded in a given year, and designs are judged against such events. Water systems not designed for intense events are severely challenged as extremes grow, so EPA suggests analyzing or modeling extreme rainfall and checking whether the current design standard still performs.

Coastal responses can be grouped into three families. Protecting builds barriers such as levees, dikes and seawalls; accommodating keeps assets in place but floodproofs them, for instance by raising critical equipment; retreating moves facilities away from the hazard, and EPA notes that relocating treatment plants and pump stations to higher ground reduces their exposure to coastal flooding and erosion.

Natural features can buffer hazards while doing other jobs. Marshes and oyster reefs act as barriers to waves, with 15 feet of marsh absorbing half of incoming wave energy, mangrove roots reduce erosion, living shorelines store carbon for centuries, and trees and other vegetation cool cities by reducing heat islands.

One unusually cold or warm year is climate variability, not climate change, which plays out over decades or longer. Claims that warming stopped after 1998 picked a short window; NASA notes the record shows warming from 1998 on, and EPA’s indicators, as archived in January 2025, list 2014 to 2023 as the warmest decade on record. Longer records separate signal from year-to-year swings.

A temperature anomaly is a difference from the average over a chosen baseline period, such as 1901 to 2000 or 1951 to 1980. Switching baselines shifts every value by the same amount, which, as EPA notes, does not change the shape of the data over time, so compare slopes rather than raw anomaly values from sources using different baselines.

A global average hides large local differences. In EPA’s U.S. temperature record through 2023, the North, the West and Alaska warmed most while parts of the Southeast changed little, and unusually hot summer nights rose even faster than unusually hot summer days. One global number can sit on top of very different regional and day-night changes.

Records keep falling even in a stable climate, but then new highs and new lows would each make up about half of the records set. The informative number is the ratio: since the 1970s, U.S. daily record highs have outnumbered record lows, and the decade from 2000 to 2009 set twice as many highs as lows.

Two causes can both fit a rising temperature line, so scientists ask what pattern each would leave. A more active sun would warm every layer of the atmosphere up through the stratosphere; NASA notes the measured pattern is warming at the surface and cooling in the stratosphere, which fits heat-trapping gases building up near the surface rather than a hotter sun.

A likelihood word can carry a stated probability rather than a tone. In its October 2026 El Nino discussion, NOAA's Climate Prediction Center called a strong-to-very-strong event likely through early 2027 and put that chance above 83 percent. When a source uses words like likely, check whether it attaches or defines a number before reading them as rhetoric.

Climate figures come in different units that measure different things. Carbon dioxide concentration is in parts per million, which EPA put at 419 in 2023; radiative forcing is in watts per square meter, the size of the energy imbalance; and temperature change is usually an anomaly from a baseline rather than an absolute reading.

Earth's climate has shifted before through changes in its orbit, the sun and volcanic activity, and it has sometimes been warmer than now. The question is what explains today's change: NOAA reports that model studies find neither solar nor volcanic forcing can explain the strong warming of the 20th century, and reconstructions of the last 2,000 years show the past 50 years warming at an unprecedented rate.

Weather is the short-term, local state of the atmosphere; climate is weather averaged over decades and usually over large areas. A record cold week in one town reflects variable local weather patterns, and a warmer climate can even bring bigger snowstorms, because warmer air carries more water vapor while it is still cold enough to snow.

Plants build sugars from carbon dioxide, so extra CO2 generally raises photosynthesis in many plants. A 2025 review notes the early boost shrinks as plants acclimate and where nitrogen runs short, and in a 2019 wheat study extra CO2 protected photosynthesis from heat damage but not the grain yield that heat cut. Judge the claim by harvests under real limits, not by leaves alone.

Volcanoes do emit carbon dioxide, but human activities release more than 100 times as much each year, about the output of a Mount Pinatubo-sized eruption twice a day. Large eruptions matter for climate mostly in the other direction: their particles block sunlight and briefly cool the globe, an effect that typically fades after one to two years.

Broad agreement among climate scientists that humans are causing global warming is the result of the evidence rather than a substitute for it. NASA stresses that scientists focus on evidence, not opinions; the consensus exists because measurements and multiple lines of evidence point the same way, and those are what to check.

A projection is conditional on its emissions scenario, so judging it means comparing it with the scenario that actually happened. NASA, for instance, gives its highest 2100 sea-level estimate only for a high-emissions scenario; holding a model to an emissions path the world did not follow tests the assumed path, not the physics.

Before trusting a climate figure, find the dataset behind it and read the fine print: which baseline an anomaly uses, which units a quantity is in, and where the time window starts. NOAA, for example, adjusts station records for instrument changes, station moves and urbanization, so check which version of a record a figure comes from.