The thin blue line

01The air

Everything happens inside this line

From orbit, all of the air is the glowing band along the horizon. Half of it lies below 5.5 km, and 99% within 31 km of the ground.¹ The Earth is 6,371 km from surface to centre.³

Where this page began

The opening view is from the height of the International Space Station, about 420 km up.⁴ On the basketball that is 8 mm above the surface. Its crew see about 16 sunrises and 16 sunsets every day.⁵

Why it glows blue

Air molecules scatter blue light far more strongly than red, so the sky above us is blue and the band of air seen from orbit glows blue. Near the horizon sunlight crosses so much air that most of its blue is scattered away, and the Sun turns orange and red.⁶

How we know

Air pressure at any height is the weight of all the air above it. At 31 km the pressure is 1% of its value at sea level, so 99% of the air lies below that height.¹

02Life

A planet that breathes

Every spring the north turns green, and the air feels it. Plants on land and plankton in the sea turn about 105 billion tonnes of carbon a year into new growth, 56 on land and 49 in the sea.⁷ Watch a year of satellite data: forests green and fade, deserts stay bare and the cold seas bloom.

You can see it in the air

At Mauna Loa in Hawaii, carbon dioxide usually peaks in May and falls to its lowest in September or October as the northern forests grow, a swing of about 6 parts per million.^{9, 10} The north has most of the land, so it sets the beat.

The green mass of the world

Forests cover 4.14 billion hectares, 32% of the land.¹¹ Plants hold about 450 of the 550 billion tonnes of carbon in all living things.¹² The ocean's plankton make about as much oxygen as marine life uses up.⁸

How we know

Leaves absorb red light and reflect near-infrared strongly, so satellites measure both and take their normalised difference (NDVI) for vegetation.¹³ At sea, chlorophyll turns the water from blue toward green, which ocean-colour instruments read as the concentration of plankton.¹⁴

03Energy

Sunlight in, heat out

Spread over the whole turning globe, sunlight arrives at 340 watts on every square metre. About 100 of them bounce straight back from clouds, the air itself, ice and deserts, and the planet sheds 239 as invisible infrared heat.¹⁵ These maps are what NASA's CERES instruments measure leaving the Earth.¹⁶

The blanket

Water vapour, carbon dioxide and methane let sunlight through but absorb part of the heat rising from the ground and send some of it back down. The ground receives 342 W/m² of infrared from the sky, more than twice the 160 W/m² of sunlight it absorbs.¹⁵

First measured

In 1856 Eunice Foote set glass cylinders of different gases in the sunshine and found the one holding carbon dioxide grew hottest. She wrote that an atmosphere of that gas would give the Earth a high temperature.¹⁸

Out of balance

The Earth now takes in about 1 W/m² more than it sheds, averaged over 2006 to 2025, more than twice the imbalance of 1976 to 1995.¹⁹ In 2023 satellites measured 1.8 W/m².²⁰ Since 1971, ninety-one per cent of the extra heat has gone into the ocean.²¹

04Carbon dioxide

800,000 years in one line

Bubbles of ancient air sealed in Antarctic ice record carbon dioxide for 800,000 years. Through eight ice ages it never rose above 300 parts per million and never fell much below 175.²² Then, in a geological instant, it left that range behind.

The first reading

Charles David Keeling's first measurement at Mauna Loa, on 29 March 1958, was 313 ppm. In May 2026 the air there peaked at 432 ppm,²⁵ 55% above the 278 ppm of 1750.²⁶

How we know it is ours

Four fingerprints. The carbon in the air is getting lighter, since fossil carbon is low in carbon-13. The new carbon holds no carbon-14, since fossil fuels are too old to keep any. Oxygen is falling as the fuel burns. And carbon dioxide is highest in the northern hemisphere, where most fuel is burned.²⁴

Where it goes

Of every 10 tonnes of carbon dioxide released from 2015 to 2024, 5 stayed in the air, about 3 went into the ocean and about 2 into plants and soils.²⁷

The last time

The last time the air held about this much carbon dioxide for long, about 3 million years ago, the world was 2.5 to 4 °C warmer than before industrialisation²⁶ and the sea 5 to 25 m higher.²¹

Doubling carbon dioxide warms the Earth by about 3 °C once the oceans catch up, over centuries.²¹ Each further doubling adds about as much again, probably a little more.¹⁵ Choose a level.

05Temperature

A warming world

Scroll from 1880 to today through thermometers on land and at sea. Each colour is a place's yearly temperature against its own 1951 to 1980 average.²⁸ The Arctic turns red first and furthest: since 1979 it has warmed nearly four times as fast as the planet as a whole.²⁹

The record

2024 was the warmest year in NASA's record, 1.28 °C above the 1951 to 1980 average, and 2025 was effectively tied with 2023 for second.^{30, 31} The eleven years from 2015 to 2025 are the eleven warmest in all eight datasets the WMO uses.³²

Past 1.5 °C

Measured against 1850 to 1900, 2024 was likely the first calendar year more than 1.5 °C warmer, at 1.55 ± 0.13 °C. The Paris goal is measured over decades, so a single warm year does not break it.³³

The reefs

From January 2023 to September 2025, heat strong enough to bleach coral reached 84% of the world's reef area, the largest bleaching event on record.^{34, 35}

How we know

NASA combines more than 25,000 weather stations on land with ship and buoy measurements of the sea surface.^{28, 31} The land has warmed faster than the sea: by 2011 to 2020, 1.59 °C against 0.88 °C since 1850 to 1900.²¹

06Ice

The shrinking cap

Each September the Arctic's floating sea ice shrinks to its smallest. From 1979 to 1990 it averaged about 7 million km² that month.³⁶ In 2012 it fell to a record 3.39 million km² on its smallest day.³⁷ Scroll through the Septembers.

The trend

Since 1979 the Arctic has lost September ice at about 74,000 km² a year, 11.6% a decade.³⁸ The 20 smallest summer minimums have all come in the last 20 years.³⁷

A mirror that melts

Bare sea ice reflects 50 to 70% of sunlight and snow-covered ice up to 90%. Open ocean reflects just 6% and soaks up the rest.³⁹ Less ice means more sunlight absorbed, more warming and less ice again.

On land

Since 2002 Greenland has lost about 265 billion tonnes of ice a year and Antarctica about 135 billion.⁴⁰ Melted completely, Greenland's ice would raise the sea about 7.4 m and Antarctica's about 58 m.⁴¹ The sea has already risen about 10 cm since 1993.⁴²

07Us

Eight billion lights

The night side shows us. These are NASA's Black Marble lights, cloud-free nights in 2016 seen by the Suomi NPP satellite.⁴³ About 8.3 billion people live on Earth in 2026.⁴⁴

What we release

In 2025 burning fossil fuels and making cement released an estimated 38.1 billion tonnes of carbon dioxide, a record, plus about 4.1 billion tonnes from clearing forests and other changes to the land.⁴⁵

What is changing

In 2025 renewables made 33.8% of the world's electricity and overtook coal, at 33.0%, for the first time in 100 years. Solar generation grew by a record 636 TWh, up 30% on 2024.⁴⁶

The stars we lose

Our lights now hide the Milky Way from more than a third of humanity.⁴⁷

08Repair

We are fixing the sky

In 1985 scientists reported deep losses of ozone over Antarctica each spring.⁴⁸ Two years later the world agreed the Montreal Protocol to phase out the chemicals destroying it, and in 2009 it and its parent treaty, the Vienna Convention, became the first treaties in UN history ratified by every country.⁴⁹

The hole

The ozone hole's largest single day was 9 September 2000, at 29.9 million km².⁵² In 2025 it averaged 18.7 million km², the fifth smallest since 1992, and broke up nearly three weeks early.⁵³

The world avoided

Without the treaty, two-thirds of the ozone layer would have been destroyed by 2065.⁵⁴ The extra ultraviolet light could have stunted plants so badly that the lost carbon sink alone could have added 0.5 to 1.0 °C of warming by the end of this century.⁵⁵

09One atmosphere

One sky, shared

Every country breathes the same air. Carbon dioxide released anywhere spreads through the whole atmosphere, so what one place adds, every place keeps. That is what makes climate change hard. It is also why the ozone story matters: the whole world measured a problem in the sky, agreed on it and is fixing it.