Climate Change Explained: What Is Driving It and What Has Already Changed

Climate change is a lasting rise in global temperature, with shifts in ice, sea level and weather extremes that follow from it. The IPCC calls human responsibility for warming unequivocal, and 2025 data from WMO, NOAA, NASA and the Global Carbon Project show the trend continuing. This article lays out the evidence, where sources differ and what the research does not yet cover.

Climate change is a lasting shift in the planet's conditions, with a rise in global temperature at its center. The IPCC's Sixth Assessment Report Synthesis Report, published in March 2023, concluded that human activities, principally emissions of greenhouse gases, have unequivocally caused global warming. By the report's accounting, global surface temperature in 2011–2020 stood 1.1°C above its 1850–1900 level.

The question matters because the measurements have kept coming in since that report. The World Meteorological Organization (WMO), in its State of the Global Climate 2025 report released on March 23, 2026, put 2025 at about 1.43°C above the 1850–1900 average. Meanwhile, the Global Carbon Project projects that fossil carbon dioxide (CO2) emissions reached a record in 2025, and the budget of CO2 that remains for holding warming near 1.5°C is small.

More than a warming thermometer

The phrase global warming names the central signal: the rise in average surface temperature. Climate change is the wider term, because a warmer planet also alters precipitation, ice, sea level, ocean chemistry and the frequency and intensity of extremes. The IPCC's wording supports this broader framing, and it is the one used here.

Trends, not single years, carry the evidence. WMO reports that 2025 ran cooler than 2024 month by month, yet it still ranks among the warmest years on record, so the dip does not contradict the long-term rise. The reverse also holds: one calendar year above 1.5°C is not the same as breaching the long-term 1.5°C goal, which concerns warming sustained over decades.

Natural variability still shapes individual years. According to WMO, monthly temperatures in 2025 were generally lower than in late 2023 and 2024 because the tropical Pacific shifted from a strong El Niño to neutral or weak La Niña conditions. Even so, they were generally higher than in any year before 2023, which WMO's report says made 2025 the warmest year on record without El Niño conditions.

How far the planet has warmed

The long-term picture

The IPCC's 2023 synthesis offers two related figures. Observed global surface temperature averaged 1.1°C above 1850–1900 during 2011–2020. Separately, the report estimates the human-caused portion of warming from 1850–1900 to 2010–2019 at a best estimate of 1.07°C, with a likely range of 0.8°C to 1.3°C.

These numbers come from an assessment that relies on scientific literature up to about 2021. They therefore predate the temperature spike of 2023 and 2024, and the report does not account for it. For recent years the better reference is WMO's annual report.

The latest year

WMO's 2025 figure is a synthesis of nine global temperature datasets: 1.43°C above the 1850–1900 average, with an uncertainty of ±0.13°C at a 90% interval. Whether 2025 ranks as the second or third hottest year depends on which dataset is used. WMO also reports that 2015 through 2025 are the 11 hottest years in the record.

Where the heat goes

The 2025 report treats Earth's energy imbalance as a key indicator for the first time, and it shows a record high in 2025. The imbalance is the gap between the energy the planet absorbs and the energy it sheds. According to WMO, more than 90% of the excess heat is stored in the ocean, which has taken up the equivalent of about eighteen times annual human energy use each year for two decades.

Ice shows the same direction. WMO reports that annual Arctic sea ice extent was at or near a record low in 2025 and that glacier melt continued. Antarctic annual extent was 10.81 ± 0.26 million square kilometers, the third lowest since satellite records began in 1979.

How scientists know humans are responsible

The attribution rests on a chain of evidence. The IPCC states that the rise in well-mixed greenhouse gas concentrations since about 1750 is unequivocally caused by human emissions. Emissions accounting and atmospheric measurements then line up with the temperature record.

The concentration record is the clearest link. According to the Global Carbon Budget 2025, pre-industrial CO2 was about 278 parts per million (ppm) in 1750. NOAA data reported by climate.us put the 2025 global average at 425.62 ppm, a record and 53% above the pre-industrial level. Direct measurement at Mauna Loa in Hawaii began in 1958, when CO2 was already about 315 ppm.

The two laboratories in the last row use different methods, which explains the small gap between their May 2026 peaks. NOAA's figure was 1.8 ppm above the previous May. Mauna Loa also sits at 3,400 meters in the northern subtropics, so its readings can differ somewhat from the global surface average.

The source of the extra carbon is identified as well. Dr. Josep Canadell of the Global Carbon Project, quoted by Australia's CSIRO, said fossil fuels account for about 90% of all CO2 emissions. The remainder comes mainly from land-use change, such as clearing forests.

Confidence in the overall conclusion is high, but its independence has limits. The IPCC, WMO, NASA and NOAA agree on direction, and they draw partly on shared datasets and cite one another. The independent strength comes from different kinds of measurement: surface thermometers, satellites, ocean heat content, CO2 air samples, tide gauges and satellite altimeters, all pointing the same way.

Emissions: how much, who and how fast

A record that is growing more slowly

The Global Carbon Budget, an annual assessment by the Global Carbon Project published in Earth System Science Data, estimates 2024 fossil CO2 emissions at 10.3 ± 0.5 billion tonnes of carbon (GtC). Land-use change added 1.3 ± 0.7 GtC, for a total of 11.6 ± 0.9 GtC, equal to 42.4 ± 3.2 billion tonnes of CO2. The two units differ by a factor of about 3.67, so carbon and CO2 figures cannot be compared directly.

For 2025, the project projects fossil CO2 emissions of 38.1 billion tonnes, a record, and land-use emissions of 4.1 billion tonnes, which CSIRO sums to 42.2 billion tonnes. The projected rise in fossil CO2 is about 1%: the published paper gives 1.0% (range 0.2% to 1.7%), while earlier press materials gave 1.1%. Coal rose 0.8%, oil 1% and gas 1.3%, according to the Global Carbon Project.

Growth is slowing, not reversing. Total CO2 emissions grew 0.3% per year over the past decade, compared with 1.9% per year in the decade before. Because 2025 figures are projections from partial-year data, their uncertainty ranges are wide.

Who emits

According to the Global Carbon Project, China accounted for 32% of global fossil CO2 in 2025, the United States for 13%, India for 8% and the European Union for 6%. A separate CSIRO page gives the EU share as 7%, so that figure differs by source. The IPCC adds that historical and ongoing contributions are unequal across regions, countries and individuals.

The US trend turned upward in 2025. Emissions in the United States and the EU had declined in recent years, but the Global Carbon Project projected a 1.9% rise in US emissions in 2025, citing colder weather and other factors. Elsewhere, fossil CO2 fell in 35 economies with growing GDP during 2015–2024; together they represent 27% of world fossil CO2.

The remaining carbon budget

The IPCC says limiting human-caused warming requires net-zero CO2 emissions, and that cumulative emissions until that point, together with cuts this decade, largely decide whether warming stays at 1.5°C or 2°C. It also says that projected emissions from existing fossil fuel infrastructure would exceed the remaining 1.5°C budget.

The Global Carbon Budget 2025 puts the remaining budget from 2025 at 170 billion tonnes of CO2 for 1.5°C, about four years at current emissions, at a 50% likelihood. For 1.7°C the figure is 525 billion tonnes (12 years), and for 2°C it is 1,055 billion tonnes (25 years). These are approximations that depend on assumptions, not deadlines.

The estimates also get revised. This year's assessment found the natural land CO2 sink substantially smaller than previously estimated, while net land-use emissions were revised upward. Land-use emissions carry a large uncertainty, ±0.7 GtC on an estimate of 1.3 GtC.

Effects already measured

Rising seas

Sea level offers one of the clearest records. NASA's satellite altimetry shows the rate of rise increasing from 0.20 centimeters per year in 1993 to an expected 0.44 centimeters per year in the current estimate. NASA Earth Observatory describes an earlier path: about 1.5 millimeters per year across the 20th century, 2.5 millimeters per year by the early 1990s and 3.9 millimeters per year over the past decade.

The year 2024 stood out. NASA reported a rise of 0.59 centimeters, against an expected 0.43 centimeters, and noted that two-thirds of that year's rise came from thermal expansion of warming water. Over the satellite era as a whole, two-thirds of the rise came from melting land ice. Total rise since 1993 is about 10 centimeters (4 inches), according to NASA.

Over a longer span, NOAA's climate.gov reports that global average sea level has risen 8–9 inches (21–24 centimeters) since 1880. The rates in these sources come from different methods and eras, with tide gauges earlier and satellites since 1993, so they should not be read as a single seamless series. Totals since 1993 also vary by source and end-year: roughly 91 millimeters, 10 centimeters or 11 centimeters (the WMO figure). Dataset versions and recalibration may explain the gaps, though that could not be confirmed. Local sea level further depends on land rising or sinking and on ocean currents, so some places see more than the global average.

Extremes and health

The European Commission's Knowledge4Policy page, summarizing the IPCC, says human-caused climate change is already affecting many weather and climate extremes in every region. It reports with very high confidence that extreme heat has increased human mortality and illness in all regions. Every additional increment of warming intensifies multiple hazards, and the likelihood of abrupt or irreversible changes rises at higher warming levels.

A Royal Meteorological Society summary of the WMO report adds two figures: 1.2 billion workers are exposed to heat stress each year, and about half the world's population is at risk of dengue. Both come from a secondary summary and have not been checked against the WMO report itself, so they are best treated as indicative.

The US policy context

On February 12, 2026, the Environmental Protection Agency finalized the rescission of its 2009 Greenhouse Gas Endangerment Finding. According to a Davis Polk analysis, that finding was the legal foundation for most federal greenhouse gas regulation since 2010. The rule was published in the Federal Register on February 18 and took effect on April 20, 2026. It also repeals federal greenhouse gas emission standards for vehicles and engines going back to model year 2012.

The action is a legal one, not a scientific finding. EPA's own page says that without the finding it lacks authority under Section 202(a) of the Clean Air Act to set vehicle greenhouse gas standards. A Kirkland & Ellis analysis says the rule acknowledges scientific uncertainty but rests on legal conclusions, and a Harvard Salata Institute commentary, which has a stated viewpoint, likewise says EPA does not rely on or evaluate the science. The rule does not amount to a government conclusion that climate change is not occurring.

Its reach has limits. According to a Jones Day note, the rule does not repeal the power plant greenhouse gas rules, oil and gas methane regulations, aircraft engine standards, greenhouse gas permitting requirements or the Greenhouse Gas Reporting Program. Legal challenges have begun and are expected to reach the Supreme Court, but the current court status could not be verified for this article.

What the numbers leave open

One more distinction matters for reading the news. A slower rise in CO2 growth is not a decline in CO2. A secondary tracker citing NOAA reports Mauna Loa growth of 3.32 ppm in 2023 and 3.33 ppm in 2024, easing to 2.23 ppm in 2025, but that is a single secondary source. NOAA's preliminary data show a September 2026 value of 426.07 ppm against 424.37 ppm a year earlier, so the concentration kept climbing. The Met Office forecast a 2026 Mauna Loa annual average of 429.4 ± 0.6 ppm, and growth swings from year to year with El Niño and La Niña through land and ocean uptake.

What to watch is whether slower emissions growth turns into actual decline while the 1.5°C budget is measured in single-digit years. The next WMO and Global Carbon Budget editions will show whether 2025 was a ceiling or a pause, and the courts will decide how much federal machinery remains to act on it.

Frequently asked questions

What is the difference between climate change and global warming?

Global warming refers to the rise in average surface temperature, the core signal. Climate change is broader and also covers changes in precipitation, ice, sea level, ocean chemistry and extremes, consistent with the IPCC's framing.

How much has the Earth warmed?

The IPCC reports 1.1°C above 1850–1900 for 2011–2020. WMO's synthesis of nine datasets puts 2025 at about 1.43°C above that baseline, with an uncertainty of ±0.13°C at a 90% interval.

What causes climate change?

The IPCC states that human activities, principally greenhouse gas emissions, have unequivocally caused global warming. Fossil fuels account for about 90% of CO2 emissions, according to the Global Carbon Project, with land-use change making up most of the rest.

Is the sea level really rising?

Yes. NASA's satellite record shows about 10 centimeters (4 inches) of rise since 1993, with the rate increasing over time, and NOAA's climate.gov reports 8–9 inches since 1880. Totals differ somewhat by source and end-year, and local sea level varies.

Did the EPA decide climate change is not real?

No. In 2026 the EPA rescinded its 2009 endangerment finding on legal grounds about its statutory authority. Law-firm analyses say the rule does not rest on scientific conclusions, and court challenges are under way.

Sources

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2. AR6 Synthesis Report: Climate Change 2023 , European Commission Knowledge4Policy
3. State of the Global Climate 2025 , World Meteorological Organization (2026-03-23)
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5. WMO State of the Global Climate 2025 report , UN-Water
6. State of the Global Climate 2025 , Royal Meteorological Society
7. Global Carbon Budget 2025 , Earth System Science Data (Copernicus) (2026)
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17. Mauna Loa carbon dioxide forecast for 2026 , Met Office (2026-02-04)
18. Global Mean Sea Level , NASA
19. NASA Analysis Shows Unexpected Amount of Sea Level Rise in 2024 , NASA (2025)
20. Tracking 30 Years of Sea Level Rise , NASA Earth Observatory
21. Climate Change: Global Sea Level , NOAA Climate.gov
22. EPA's Endangerment Finding in danger: Key takeaways and uncertainties of repeal , Davis Polk (2026)
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26. The legal reasoning behind the endangerment rescission , Salata Institute for Climate and Sustainability, Harvard

This article was drafted with the help of AI and reviewed by Jaroha editors before publication.