The US Energy Secretary, Rick Perry, recently denied that CO2 is the main driver behind our changing climate. Nominated by President Trump, his comments are in line with the Administration’s rollback of the Clean Power Plan and withdrawal from the Paris Climate Agreement. The fact is, however, the scientific fundamentals of the greenhouse effect have been understood since the 1800s.
One of the first scientists to look into the planet’s energy balance was Joseph Fourier, a French physicist, in the 1820s. Given the Earth’s distance from the Sun, he was curious to know why its temperature was not cooler. Fourier felt that something other than incoming solar radiation was keeping the planet warm and hypothesized that the atmosphere was somehow acting like an insulating blanket. Working with the limited technology of the day, however, he was unable to make the detailed measurements needed to carry his idea further.
Decades later, in the 1860s, an Irish scientist named John Tyndall picked up Fourier’s theory. An alpine adventurer, he was interested in glaciers and the then controversial idea of ice ages. Wanting to know more about how they formed, he devised an experiment to see if the Earth’s atmosphere was acting like a thermostat. For this, he built a spectrophotometer – an instrument that measures the amount of heat that gases can absorb. His experiments showed that water vapor, carbon dioxide (CO2), and methane were all very efficient at trapping heat. This essentially proved Fourier’s idea of a greenhouse effect.
In the 1890s, Svante Arrhenius, a Swedish physicist, followed up on Tyndall’s idea of an atmospheric thermostat and ran with it. Ruling out water vapor as too transitory, he focused on carbon dioxide, which tends to linger in the atmosphere for a long time. His calculations showed that doubling the amount of carbon dioxide in the atmosphere would raise the average global temperature by 5°C (9°F).
To understand if such a large-scale change in atmospheric CO2 was possible, he turned to Arvid Hogbom, a colleague studying the global carbon cycle. This is the natural geochemical process where volcanic eruptions and the chemical weathering of rocks release CO2, while plants and oceans absorb it. Hogbom confirmed that CO2 levels could change dramatically over long periods of time. However, he also noted that industrial processes were releasing a significant amount of CO2 relatively quickly. Using this information, Arrhenius calculated that human activities, such as burning fossil fuels, could alter the composition of the atmosphere and increase global temperatures. In the 1890’s, however, fossil fuel use was only a fraction of what it is today and he believed it would take more than 1,000 years for the level of atmospheric CO2 to double.
Jumping ahead to the 1950s, Charles David Keeling, a researcher at the Scripps Institution of Oceanography in California, found a way to directly monitor levels of CO2 in the atmosphere. He created an instrument called a gas chromatograph and installed it on top of Mauna Loa in Hawaii. At an elevation of more than 11,000 feet in the middle of the Pacific Ocean, it is removed from both direct CO2 sources like factories and sinks such as forests that could skew the data. Still in operation today, the information recorded at this station is known as the Keeling Curve. It shows the steady increase in CO2 levels in the atmosphere from 1958 to present.
Keeling’s measurements provided solid evidence that CO2 levels were rising and validated the theories of Tyndall and Arrhenius. More recently, scientists were able to extend his curve back in time by analyzing ancient air bubbles trapped in ice cores from Greenland and Antarctica. This lengthy record shows that pre-industrial CO2 levels in the atmosphere were about 280 ppm. Today, they are over 400ppm – the highest they have been in more than 800,000 years.
Seeing this dramatic rise in CO2 and realizing the impact that a warming climate could have on society, the UN formed the Intergovernmental Panel on Climate Change (IPCC) in 1988. They assess the peer-reviewed research of thousands of scientists from around the world and publish a synthesized view of the current state of the science. The latest IPCC report (AR5 published in sections in 2013/2014) unconditionally states that human activities are the main drivers of modern climate change.
Therefore, while it is the nature of all science to evolve with time and research, it is safe to say that role of CO2 does not require further debate. Today, the impacts of different feedback loops within the climate system are an active area of investigation. Science is following the evidence and moving ahead. Politics needs to catch up.