History of climate change science

The history of the scientific discovery of climate change began in the early 19th century when scientists first noticed changes in Earth's climate, such as ice ages, and began to understand the greenhouse effect. They realized that gases in the atmosphere, like greenhouse gases, could trap heat and change the planet's temperature.

Important scientists like Joseph Fourier, Claude Pouillet, Eunice Newton Foote, and John Tyndall made early discoveries about how these gases work. Later, Svante Arrhenius in 1896 made the first detailed prediction of how increasing carbon dioxide in the atmosphere could warm the Earth.

By the 1960s, scientists had strong evidence that human activities were affecting the climate. They also learned that some pollutants could cool the planet, a phenomenon called global dimming. In the 1990s, scientists using advanced computer models confirmed that human-caused increases in greenhouse gases were leading to global warming.

Today, climate science continues to grow, with researchers studying how climate change affects weather patterns and extreme events. Reports from the Intergovernmental Panel on Climate Change help share this important knowledge with the world.

Prior to the 20th century

People have long wondered if the climate of a place could change over many years. For example, a student of a famous ancient thinker noticed that draining swamps made areas colder, and thought cutting down forests might warm places up. Chinese scholars noticed that some plants found far underground grew in warmer places, suggesting climate changes over time.

In the 1800s, scientists began studying Earth’s past climates. They found signs that huge glaciers had moved across places that are now warm. One scientist noticed that giant rocks in valleys looked like they had been pushed there by glaciers and shared this idea with others. This idea was met with doubt at first, but later became widely accepted. Another scientist, Joseph Fourier, discovered that Earth’s air keeps it warmer than it would be in space. He thought human activities might change the climate, especially by changing how land is used.

Later, a scientist named Svante Arrhenius calculated that if the amount of a certain gas in the air doubled, Earth’s temperature would rise significantly. He also thought that human activities, like burning coal, could slowly change the climate over many years.

20th century onwards

Early scientists began to understand that human activities might change the Earth’s climate. In the early 1900s, they studied ancient climates found in lake mud and tree rings, noticing patterns linked to changes in the Sun’s activity. Some thought sunspots caused climate changes, but others were unsure.

By the 1950s, better tools showed that carbon dioxide in the air could trap heat without being canceled out by water vapor. Scientists found that oceans didn’t absorb all extra carbon dioxide quickly. In the 1960s, studies of deep-sea cores and ancient corals suggested that Earth’s climate could shift quickly with small changes.

In the 1970s, some worried about cooling from pollution, but evidence grew that the Earth was warming. Scientists used computers to predict that doubling carbon dioxide would raise global temperatures. By the 1980s, more agreed that human activities were warming the planet. International efforts began to study and address climate change.

Discredited theories and reconciled apparent discrepancies

See also: Climate change denial

Early scientists discovered that the Earth warms up because of a process called radiation trapping, similar to how a greenhouse works. This led to the idea of the "greenhouse effect."

Some scientists, like Prof. R. W. Wood in 1909, thought greenhouses warmed only because of air movement, not radiation trapping. But later experiments showed that radiation trapping is the main reason greenhouses heat up, so the idea of the greenhouse effect in the atmosphere is correct.

There have been debates about where weather stations are placed and how they measure temperature. Some people thought these stations might give wrong results, but studies show that these effects are small and do not change the overall warming trend. Scientists have carefully checked these measurements and found that the Earth is warming as expected.

Scientists also looked at temperature changes in the upper atmosphere and found that they match what we would expect from human-caused climate change, not just natural changes like the Sun's activity. Studies show that the Sun’s energy has not increased since 1978, so it cannot explain the recent warming. Models that include human influences match what we see happening today.

Past estimates of greenhouse gas emissions and temperature rises

Previous estimates for the year 2020

In 2011, the United Nations Environment Programme studied how world emissions might change up to the year 2020 based on different choices. They brought together 55 scientists and experts from 28 groups in 15 countries. If no new steps were taken to cut emissions, they thought global emissions in 2020 could be about 56 gigatonnes of CO₂-equivalent, with a range of 55 to 59 gigatonnes.

If countries followed the most ambitious promises made in the Copenhagen Accord, emissions would still be around 50 gigatonnes by 2020. If trends continued without strong action, there was a concern of a 3°C temperature rise by the end of the century, which could cause serious problems for nature, money, and society.

The report also looked at how different plans to fight climate change could change emissions. Stronger efforts could bring emissions down to between 49 and 52 gigatonnes, with a middle estimate of 51 gigatonnes. Weaker efforts could leave emissions between 53 and 57 gigatonnes, with a middle estimate of 55 gigatonnes.