Climate and the Carbon Cycle

People often mix these two words up, but scientists use them for different things. NOAA puts it in one line: climate is what you expect; weather is what actually happens.

That difference matters for this lesson. One cold week, or one warm winter, doesn't tell you much about climate. A climate trend shows up only when you average many years of measurements over many places.

Almost all of Earth's energy comes from the sun. At the top of the atmosphere, sunlight delivers about 1,360 watts per square meter to a surface facing the sun. Averaged over the whole spinning globe, day and night, that works out to about 340 W/m².

Earth's temperature depends on a balance. If the planet takes in more energy than it gives off, it warms. If it gives off more than it takes in, it cools. NASA's energy budget tracks where each part of the incoming sunlight goes.

About 29% of sunlight is reflected straight back to space: roughly 23% by clouds and the air itself, and roughly 7% by bright surfaces such as snow and sea ice. The other 71% is absorbed: 23% by the atmosphere and 48% by the land and ocean. Absorbed sunlight warms the surface, and the warm surface gives off energy as invisible infrared radiation, the kind you feel as heat from a warm sidewalk.

Here is where the greenhouse effect comes in. Some gases in the air, called greenhouse gases, absorb infrared radiation and give it off again in all directions, so part of it goes back down to the surface. NASA lists them as carbon dioxide (CO₂), methane, ozone, nitrous oxide, chlorofluorocarbons (CFCs) and water vapor. Nitrogen and oxygen, which make up most of the air, don't absorb infrared this way.

The greenhouse effect is natural and necessary. NASA says it keeps Earth's average surface temperature at about 15 °C (59 °F), which is part of what makes the planet livable. The concern is not that the greenhouse effect exists but that adding more greenhouse gases strengthens it.

Carbon is stored in reservoirs: the atmosphere, the ocean, plants, soils, permafrost, fossil fuels and rock. It moves between them in fluxes, such as photosynthesis pulling CO₂ out of the air or the ocean absorbing it. Scientists measure reservoirs in billions of tonnes of carbon and fluxes in billions of tonnes per year.

In the slow cycle, rain that is slightly acidic from dissolved CO₂ wears away rock. Rivers carry the dissolved carbon to the sea, where it can end up in shells and limestone on the sea floor. Volcanoes return some of it to the air. Fossil fuels (coal, oil and natural gas) formed from ancient buried life and are also part of the slow cycle. Burning them moves carbon from the slow cycle to the fast one in a few generations.

The diagram uses numbers from the Intergovernmental Panel on Climate Change (IPCC), which reviews published research. They describe the decade 2010–2019.

Notice two things. First, the natural flows are huge: about 142 billion tonnes of carbon a year go into plants, and about 137 come back out. Second, they nearly cancel out. Human activity adds about 9.4 billion tonnes a year from fossil fuels and cement and 1.6 from clearing land, which is small next to photosynthesis but is added on top of a cycle that was close to balanced.

Where does that extra carbon go? The IPCC found that from 2010 to 2019, human activity released about 10.9 billion tonnes of carbon a year. About 46% stayed in the atmosphere, 23% went into the ocean and 31% into plants and soils on land. That's why the atmosphere's carbon keeps growing, by about 5.1 billion tonnes a year, even though land and ocean absorb more than half of what we add.

NOAA notes that the ocean holds about 50 times more carbon than the atmosphere, and it is the planet's largest active carbon sink. CO₂ dissolves into surface seawater, and currents slowly carry it into the deep ocean. According to the IPCC, the ocean has taken up about 170 of the 685 billion tonnes of carbon that people released between 1750 and 2019.

The ocean is a slow sink. NASA explains that over thousands of years it may absorb up to 85% of the extra carbon from fossil fuels, but mixing surface water down to the depths takes a long time. The IPCC also projects that ocean and land sinks will take up a smaller share of emissions if emissions keep rising.

Absorbing CO₂ changes seawater's chemistry. CO₂ and water combine to form carbonic acid (H₂CO₃), a weak acid that breaks apart into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻):

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻

More hydrogen ions means a lower pH. NOAA reports that since the Industrial Revolution the pH of surface ocean water has dropped by about 0.1 units. That sounds tiny, but the pH scale is logarithmic, so it means about a 30% increase in acidity. The IPCC concludes that human CO₂ emissions are "virtually certain" to be the main driver.

The extra hydrogen ions also bond with carbonate ions (CO₃²⁻), leaving fewer for animals such as oysters, corals and tiny sea snails called pteropods to build shells and skeletons. If the pH drops far enough, shells can even begin to dissolve.

Climate science rests on several independent lines of evidence. Each one is measured a different way, by different teams, and they all point in the same direction.

The Mauna Loa record is the longest continuous record of CO₂ measured directly in the air. C. David Keeling of Scripps Institution of Oceanography collected the measurements from March 1958 to April 1974; NOAA's own measurements continue the record. The yearly averages below are NOAA's data, not estimates.

Note: after Mauna Loa erupted in late November 2022, NOAA paused measurements at Mauna Loa Observatory. From December 2022 to July 4, 2023 the readings came from a site at the Maunakea Observatories, about 21 miles north. Mauna Loa measurements resumed in July 2023 (NOAA Global Monitoring Laboratory).

Two patterns stand out. The line climbs every year, and it climbs faster over time: NOAA Climate.gov reports an average rise of about 0.8 ppm a year in the 1960s and about 2.6 ppm a year from 2015 to 2024. The yearly zigzag is the planet "breathing." Most land plants are in the Northern Hemisphere, so CO₂ drops each spring and summer as they grow and rises each fall and winter as leaves drop and decay.

Mauna Loa is one station. NOAA also averages many stations around the world, and NOAA Climate.gov reports that global CO₂ set a new record in 2024 and is now about 50 percent higher than before the Industrial Revolution.

Earth's climate has always changed. NASA notes eight cycles of ice ages and warmer periods in the last 800,000 years; the last ice age ended about 11,700 years ago. So scientists ask a fair question: could natural causes explain today's warming? They test each candidate against the evidence.

The IPCC puts the pieces together. Its 2021 assessment estimates that natural drivers changed global temperature by between −0.1 °C and +0.1 °C from 1850–1900 to 2010–2019, while human-caused aerosols cooled it by 0.0 to 0.8 °C. Its headline conclusion: "It is unequivocal that human influence has warmed the atmosphere, ocean and land."

A feedback is a chain of effects that loops back on itself. A positive (reinforcing) feedback makes a change bigger; a negative feedback makes it smaller. Two reinforcing feedbacks are especially well understood.

These are changes that have already been measured, not predictions. The numbers come from the IPCC's 2021 assessment and from NASA.

Science can describe the choices, but deciding among them involves values, costs and trade-offs that communities and governments weigh differently. NASA describes two broad kinds of response, and most plans use some of each.

The IPCC finds a nearly straight-line relationship between the total CO₂ people emit and how much the planet warms. That link, which connects the carbon cycle in section 3 to the temperature record in section 5, is why scientists track carbon so closely.