Written by Ben McNeil
5 min read
The Consensus: Humans Are the Primary Driver of Recent Global Warming
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Earth’s average surface temperature has warmed ~0.7°C since 1951. This warming is not caused by natural processes like solar radiation or volcanic activity. An increase in greenhouse gases via human activity is highly likely to be the primary driver of recent global warming. This consensus is based on 7 experts answers from this question: Are humans the primary cause of recent global warming?
“l’effet de serre”
The idea that humans could alter the temperature of our planet is a huge claim and it’s one that dates back to the 19th century. In 1824, French physicist and mathematician Joseph Fourier made an unexpected discovery. The sun gives earth all our warmth (or so Fourier thought). By knowing how much energy the sun emits and the distance to the sun, Fourier calculated what the temperature of earth should be. He calculated the temperature to be a life-stopping -18°C (-64°F). That’s odd.
The oceans weren’t frozen, life was blooming and the average temperature on earth was a balmy 15°C (59°F). Something was wrong with his assumption. Something must be trapping the heat Fourier thought. He called this trapping of heat l’effet de serre – the greenhouse effect. But it took a while for others to work out what the invisible ingredient could be.
In 1859 Irish physicist John Tyndall experimented on how different atmospheric gases absorbed heat in the lab. Tyndall noticed that some gases let sunlight pass through while trapping heat. Those gases included carbon dioxide and water vapour. These “greenhouse gases” are critical to very existence. They make up a tiny 0.3% of the atmosphere, yet warm the planet by 33°C (123°F). It’s unlikely for humans to have evolved on a frozen planet – thankyou greenhouse effect!
In 1896 Swedish chemist Svante Arrhenius, took Tyndall’s results to predict future warming on earth. He calculated the planet would warm by ~4-6°C if atmospheric CO2 concentrations doubled. But humans could never the levels found in the atmosphere – could they?
Fun Fact: Svante Arrhenius was not worried about global warming. In fact, coming from chilly Sweden, Arrhenius wanted to set all the coal mines on fire to speed up the process.
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Humans as climate-makers
The industrial revolution was taking off in the 19th century – powered by oil and coal. But we never knew if concentrations of atmospheric CO2 were changing. Enter Charles Keeling, an atmospheric chemist from the USA. Keeling started taking precise CO2 measurements on top of a Hawaiian volcano. He found a consistent one percent increase each year. Arrhenius’s work was finally dusted clean and took on a whole new meaning.
In 1972, a British atmospheric scientist named J. S. Sawyer calculated the expected warming for earth by the year 2000. He predicted atmospheric CO2 concentrations to be 25% higher, leading to warming of ~0.6°C. The evidence shows CO2 concentrations grew ~15% and earth warmed ~0.5°C by 2000. Pretty good prediction Dr Sawyer!
By 1979, the United States government took notice. They invested in climate monitoring, sensors and satellites all over the world. From the land to the ocean to mountains, the theory was not speculation anymore. It became an observable fact over the last few decades.
Temperatures have warmed about 0.25°C for each decade since 1979. Ice on land and in the sea is retreating. Sea-levels are rising. The evidence is overwhelming. But how do scientists know humans are causing it aside from the theory? Via deduction.
There are only a few things that can change earths temperature: the sun, the ocean or volcanoes. Scientists can track the radiation we get from the sun. Over the past few decades, we’ve got less energy from the sun. In other words, we should be cooling if controlled only by the sun. Volcanoes and oceans do cause variations, but nothing like the warming trend we’ve seen since the 1950s.
Are humans the primary driver? Dr Michael Wehner, a senior climate scientist at Lawrence Berkeley National Laboratory in the USA says “Near Certain” adding that humans as the most likely driver of recent climate warming is “supported by a vast body of peer reviewed scientific literature spanning several decades”.
The theory, data and observations confirm the strong consensus we found amongst climate scientists that humans are likely to be the primary driver of the climate warming over the past 40 years.
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