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Hot topic: Discovering the greenhouse effect

Published: 11 March 2021

Our current knowledge of Earth’s atmosphere is the product of centuries of observations, experiments and insights by many scientists . Discover how we came to understand the greenhouse effect, and what it means for our warming climate

The importance of Earth's atmosphere

Earth’s average surface temperature has risen by more than 1°C since preindustrial times (which scientists define as the average temperature between 1850 and 1900). Fundamental to this warming is the way in which certain gases within the atmosphere trap heat—a principle known as the greenhouse effect.

Viewed from space, the atmosphere looks wafer-thin relative to the size of the Earth. But its composition, and the behaviour of the gases within it, play a crucial role in making the planet hospitable for life as we know it. 

Without it, Earth’s average temperature would be a chilly -15°C—approximately 30°C colder than it is today. Brrr! 

However, human activities are changing the concentrations of certain gases in the atmosphere, and this is driving a rise in Earth’s temperature. Warming of 1°C doesn’t sound like much, but relatively small changes in global average temperature have dramatic consequences for climate. During the last ice age, the planet’s average temperature was only about 5°C cooler than today, but conditions were very, very different. 

Earth seen from space NASA Johnson Space Center, CC BY-NC 2.0 Image source for Earth seen from space
The thin layer of Earth's atmosphere, seen from space

What is the greenhouse effect?

The Sun’s energy reaches the Earth as sunlight. As the planet warms, some of its heat radiates back out to space. The balance of energy coming in versus energy lost determines the planet’s temperature.

Certain gases in the atmosphere—especially carbon dioxide, methane and water vapour—absorb some of the energy radiated by the Earth, and re-emit it again in random directions. Some of it is directed back down towards the surface. This means that on balance, less energy is lost to space, and Earth’s temperature becomes warmer than it would otherwise be in the absence of these so-called 'greenhouse gases'.

Find out more about greenhouse gases and their influence on Earth’s temperature

The gases responsible for this warming effect are only present in the atmosphere at very low concentrations. Carbon dioxide makes up about 0.04% of the atmosphere and methane about 0.017%. The concentration of water vapour varies widely but on average is about 1%. Despite this, they are extremely important in creating the conditions for life on Earth as we know it.

Over the last two centuries, human activities—especially the burning of coal, oil and gas—have released greenhouse gases into the atmosphere in sufficient quantities to affect Earth’s temperature.

Poster on climate change with figures including a schematic of the greenhouse effect and the Keeling curve of atmospheric carbon dioxide concentration.
Science Museum Group Collection More information about Poster on climate change with figures including a schematic of the greenhouse effect and the Keeling curve of atmospheric carbon dioxide concentration.

Discovering the greenhouse effect

It’s tempting to think of the history of science as a series of ‘Eureka!’ moments, but more often than not, so-called discoveries are long and protracted sagas. The greenhouse effect is no exception. Even the origins of the term are hard to pinpoint exactly; it was apparently first used in 1907,  although some scientists had described aspects of Earth’s physics as being reminiscent of a greenhouse or hot-house before this. 

The insights which together have contributed to our current understanding of the greenhouse effect have been the work of numerous individuals over several centuries. Though some of the protagonists now seem remarkably prescient, it was only in the latter decades of the 20th century that we came to understand the significance of the greenhouse effect in relation to human-caused climate change. 

Scientists involved in this story:

These individuals are just a few in a large cast of characters who have helped us understand the physical and chemical properties of gases in the atmosphere. However, it wasn’t until later in the 20th century that the greenhouse effect was drawn into research that sought to understand humankind’s impact on the climate.

Humans and the greenhouse effect

Dave Keeling receiving the Medal of Science, 2001

Geophysical research expanded dramatically during the Cold War. Knowledge of the Earth—its atmosphere, oceans and structure—had wide-ranging military applications, and this unlocked unprecedented financial support for research programmes and environmental monitoring initiatives. 

Against this backdrop, geochemist Charles David ‘Dave’ Keeling began work at the Scripps Institution of Oceanography in California, tasked with establishing the baseline concentration of carbon dioxide in the atmosphere. This would provide a basis against which any future changes could be measured. Keeling established a station at Mauna Loa Observatory in Hawaii where observations were taken daily. 

Keeling flask

Specially designed round flasks were used for the observations. First, the air inside them was removed—the tape prevented the glass from shattering. Outside, the observer held their breath to avoid contaminating the sample, and released the stopcock to allow the atmospheric air to rush in. The samples were analysed in a laboratory using delicate instruments called gas manometers. 

Keeling’s first measurement, taken on 29 March 1958 at Mauna Loa, revealed there were 313 molecules of carbon dioxide within every million molecules of air. But the concentrations appeared to fluctuate wildly. Only once the first year of observations were complete did trends begin to emerge. 

A building with two silver domed roofs, with a mountain in the background University Corporation for Atmospheric Research, Public domain, via Wikimedia Commons Image source for A building with two silver domed roofs, with a mountain in the background
Mauna Loa observatory on the Big Island of Hawaii

The graph—often referred to as the Keeling curve—has a distinctive saw-tooth pattern, which is caused by the activities of plants. When plants photosynthesise, they consume carbon dioxide from the air in order to grow. Since much of Earth’s vegetation is concentrated in the northern hemisphere, carbon dioxide concentrations in the atmosphere fall during northern hemisphere spring and summer. 

Graph of data showing a curve from the bottom left to top right Scripps Institute of Oceanography
This graph of carbon dioxide concentrations in the atmosphere is generally has become known as the ‘Keeling curve’

Despite these fluctuations, Keeling’s observations soon revealed a relentless upward trend. The observations have continued almost uninterrupted until the present day, with stations established in a variety of locations around the globe, and provide a stark visual record of human influence upon the atmosphere. In 2013 concentrations reached 400 ppm for the first time in nearly 3 million years,  and show no signs of slowing.

British Antarctic Survey ice core drill used in research into past climates, 1985
Science Museum Group Collection More information about British Antarctic Survey ice core drill used in research into past climates, 1985

How do we know about carbon dioxide concentrations from the distant past, when we have only been making direct observations since the 1950s? Scientists use tiny air bubbles trapped in polar ice to analyse Earth’s atmosphere in the distant past—even as it was hundreds of thousands of years ago. Specially designed drills are used to collect cores of ice for these observations—the deeper the ice, the older the atmospheric air trapped inside.

Local knowledge, global knowledge

Keeling chose to sample air at Mauna Loa because its Pacific location and high altitude would be relatively free of contamination from local sources, which is one of the key reasons this series is taken as an indicator of global baseline carbon dioxide concentrations. 

The Mauna Loa observatory where Keeling worked is one of several in the Hawaiian islands. The agendas, questions and methods of scientists are sometimes in conflict with the interests and values of people living where science is done, as tensions around some of Hawaii’s scientific sites have revealed. For example, opposition to the planned Thirty Meter Telescope atop neighbouring peak Mauna Kea demands that scientists respect the importance of that land to local Hawaiian communities.

As well as establishing global concentrations, scientists are also curious about how carbon dioxide moves around the atmosphere from the point of emission. The Greenhouse Gas Laboratory at Royal Holloway University of London has monitored greenhouse gases since the 1990s; its location in Egham, to the south-west of London, provides a rather different kind of monitoring site to Mauna Loa. Depending on the wind direction, the site's instruments either capture relatively 'clean' air from the western Atlantic, or air rich in human pollution from London.

Greenhouse Gas Lab objects

The greenhouse effect and the climate crisis

Our knowledge of the complex behaviour of the atmosphere and the gases within it has been pieced together over hundreds of years, by investigators using a whole range of scientific techniques, including experimentation, mathematical modelling and meticulous observation. 

Since the late 20th century, the greenhouse effect has assumed new significance as a cornerstone of our understanding of Earth’s climate and the ways in which we humans are disrupting it. The 19th-century researchers who first began to study the atmosphere had no idea that their research would be at the heart of one of humanity’s greatest challenges, but their insights are fundamental to shaping how we will respond. We now know that unless we rapidly reduce our greenhouse gas emissions—perhaps even find ways of removing carbon dioxide already emitted—we will not be able to avoid irreversible and catastrophic changes to our climate.

Find out more

Books

  • James R Fleming, Historical Perspectives on Climate Change, 1998
  • Joshua P Howe, Behind the Curve: Science and the Politics of Global Warming, 2014
  • Joshua P Howe (ed.), Making Climate Change History: Documents from Global Warming’s Past, 2017

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