What is climate change? Causes and effects, plainly

Climate change is a long-term shift in Earth's average temperatures and weather patterns. The climate has always changed, but what scientists mean today is the rapid warming since the late 1800s, driven mainly by people burning coal, oil and gas. Those fuels release carbon dioxide and other gases that trap heat. Averaged over recent decades, Earth's surface is about 1.3 to 1.4°C warmer than in the late 1800s, and the effects show up in heat waves, rising seas and shrinking ice.

In short

• Weather is what happens this week; climate is the average over decades. Climate change is a shift in that average.
• Carbon dioxide in the air has risen from about 280 parts per million before industry to about 423 in 2024, and that extra gas traps extra heat.
• Warming raises sea levels, makes heat waves more common and shifts rain, and it can be slowed by cutting emissions and partly managed by adapting.

How is climate different from weather?

Weather is the state of the air at one place and time: today's rain in Chicago, a cold snap next week. Climate is the pattern of that weather averaged over 30 years or more. A cold winter doesn't disprove warming, any more than one hot day proves it. What matters is the trend, and NASA puts the difference simply in its explainer on weather and climate.

	Weather	Climate
Time scale	Hours to weeks	Decades to millennia
Example	A 35°C day in Paris	How often Paris now gets 35°C days
Forecast with	Weather models, days ahead	Climate models, decades ahead, as ranges
One event tells you	What happened	Almost nothing on its own

Natural swings sit on top of the trend. The biggest from year to year is El Niño and La Niña, a see-saw of Pacific Ocean temperatures that can push one year's global average up or down by a tenth of a degree or more. That is why scientists judge warming by decades, not single years.

How do we know Earth is warming?

The evidence comes from many independent measurements that agree. Thermometers on land and at sea show the surface warming by about 0.06°C a decade since 1850, and about 0.20°C a decade since 1982, according to NOAA. The World Meteorological Organization found that 2024 was the warmest year in a 175-year record, at about 1.55°C above the 1850–1900 average, likely the first calendar year above 1.5°C. Averaged over decades, it estimated warming at 1.34 to 1.41°C, still below that level; single years run hotter or cooler than the trend.

The rest of the planet tells the same story. NASA reports that the top 100 meters of ocean have warmed by about 0.33°C since 1969, and that the ocean has absorbed about 90% of the extra heat. Greenland lost an average of 279 billion tons of ice a year between 1993 and 2019, and Antarctica about 148 billion tons. Global sea level rose about 20 centimeters in the last century, and it is now rising nearly twice as fast as it did then.

Why do scientists blame greenhouse gases?

Some gases in the air let sunlight in but absorb the heat Earth sends back out, which keeps the planet warm enough to live on. This is the greenhouse effect, and the physics behind it was measured in laboratories in the 1800s. Carbon dioxide is the main gas people add. Burning oil, coal and gas releases carbon that plants and plankton locked away millions of years ago.

The rise is measured directly. NOAA reports that global carbon dioxide averaged about 422.8 parts per million in 2024, against 280 or less before the Industrial Revolution. That is a rise of about 51%: (422.8 − 280) ÷ 280 = 0.51. The increase in 2024 alone was 3.75 parts per million, the largest on record.

Other causes have been checked and ruled out as the main driver. The Sun's output, measured by satellites since 1978, shows no net increase since the 1950s, NASA notes. And the pattern of warming points to greenhouse gases: the lower atmosphere is warming while the stratosphere above it is cooling, which is what heat trapped near the ground produces, and the opposite of what a brighter Sun would do.

Hasn't the climate always changed?

It has, and that history is part of how scientists understand it. Over the past million years Earth has swung in and out of ice ages, paced by slow wobbles in its orbit. The difference today is speed and cause.

A rough comparison, using NASA's and NOAA's figures:

Change	Warming	Time taken	Rate per century
Coming out of an ice age	4 to 7°C	about 5,000 years	0.08 to 0.14°C
Since 1982	0.20°C per decade	more than 40 years so far	about 2.0°C

On these numbers, the recent rate is roughly 15 to 25 times the average pace of ice-age recovery. And no orbital change, solar change or volcanic pattern explains the recent rise, while the added greenhouse gases do.

What does climate change do?

The direct effect is more heat: hotter extremes, warmer nights and longer warm seasons. Warmer air holds more water vapor, about 7% more per degree Celsius, so heavy downpours get heavier in many places, while other regions dry out. Seas rise because water expands as it warms and because glaciers and ice sheets melt into the ocean.

Some effects are firmer than others. More frequent heat extremes and rising seas are well established. For storms, research points to a larger share of hurricanes reaching the strongest categories, with heavier rain, rather than more hurricanes overall. For tornadoes, the record is too short and patchy to show a clear trend. Attributing any single storm to climate change takes a dedicated study; it can't be read off one event.

What can be done about it?

There are two kinds of response. Mitigation means cutting the emissions that cause warming: generating electricity from renewable sources and nuclear plants, electrifying cars and heating, using energy more efficiently, and protecting forests that store carbon. Because carbon dioxide stays in the air for centuries, warming stops only when net emissions reach about zero.

Adaptation means living with the change that is already locked in: sea walls and managed retreat from coasts, heat warnings and cooling centers, crops bred for drought, and building codes that expect stronger storms. Most countries pursue both, and the 2015 Paris Agreement set the shared goal of holding warming well below 2°C and pursuing 1.5°C.

Still argued

That people are warming the planet is not in dispute among climate scientists. How much warming each added ton of carbon dioxide causes is still narrowed by research. The IPCC's 2021 report put the warming from doubling carbon dioxide at 2.5 to 4°C as its likely range, with 3°C as the best estimate, tighter than the 1.5 to 4.5°C of its previous report, as the Max Planck Institute for Meteorology summarizes. How fast to cut emissions, and who should pay, are questions of policy and values, argued between countries and parties rather than settled by the science.

Check yourself

• Without looking back: why doesn't one very cold winter disprove global warming?
• What is the evidence that the Sun isn't the main cause of recent warming?
• Why does warming stop only when net emissions reach about zero, not when they start to fall?

Sources

• Evidence — NASA, 2026
• Is the Sun causing global warming? — NASA, 2024
• Global warming — NASA Earth Observatory, 2025
• Climate change: atmospheric carbon dioxide — NOAA Climate.gov, 2025
• Climate change: global temperature — NOAA Climate.gov, 2025
• WMO report documents spiralling weather and climate impacts — World Meteorological Organization, 2025