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Sometimes a piece of the puzzle won’t fit, but overall the picture is coming together. Dave Ginsberg

150 years and counting: confidence in climate science

In the lead up to the release next month of the Intergovernmental Panel on Climate Change’s (IPCC’s) Fifth Assessment Report we are exploring concepts of confidence and certainty in climate science. The first article is here

Building any scientific theory is like putting together pieces of a jigsaw puzzle. Pieces of evidence are assembled in order to resolve the complete picture.

And the picture has never been clearer for the puzzle of human-induced climate change.

The theory that additional carbon dioxide (CO2) in the atmosphere would increase global temperatures, and cause other changes to Earth’s climate, is not new. That puzzle box was opened nearly 200 years ago.

Joseph Fourier, who made the initial hypothesis of a greenhouse effect, identified the very first piece of the climate change puzzle in 1824. In 1859, John Tyndall identified the “greenhouse gases” and their role in the atmosphere.

In 1896, Svante Arrhenius made the first suggestion that humans could influence the climate. Even well over 100 years ago, he postulated that global temperatures would rise by 5-6C if the CO2 concentration of the atmosphere was doubled.

More puzzle pieces were added when Brooks reported increases in temperatures in the 1920s. But it was Guy Callendar who, through meticulous examinations undertaken from the late 1930s to 1960s, identified not only increases in global temperatures, but also suggested that these were caused by human emissions of greenhouse gases.

By the late 1950s and into the 1960s the atmosphere was being studied in increasing detail with the sudden expansion of observations that were associated with the International Geophysical Year. In addition, scientists gathered yet more puzzle pieces with the advent of computers that could accurately model the physics of the atmosphere.

In 1960, Charles Keeling first published what is now known as the “Keeling Curve”, showing consistent rises in observed atmospheric CO2.

By now, the scientific puzzle was well and truly taking shape. There was clear evidence that CO2. was increasing in our atmosphere and that temperatures were increasing concurrently. Many independent lines of evidence were consistent with Arrhenius’ theory from over 60 years prior.

Throughout the latter half of the 20th century, this evidence continued to grow, increasing confidence in the theory that humans were affecting the climate. Scientists used high accuracy instruments to observe changes, reconstructed past climatic changes and also modelled the climate system using the fundamental laws of physics and chemistry.

At the same time, the number of alternative hypotheses about the observed warming declined. While scientists were neatly putting together these puzzle pieces, they were also examining whether any other known process, besides human activities, could be responsible for the observed changes. Solar variations, volcanoes and other natural cycles were all systematically discounted.

From 1991 to 2011 alone, more than 4,000 additional pieces of the climate change puzzle were gathered. An estimated 97% of them fit the puzzle and were consistent with previous evidence. So of these 4,000 pieces, 3,880 pieces demonstrated that humans were having a noticeable and significant influence on our climate.

Trying to construct a puzzle is difficult and time consuming. You might lose a few pieces along the way, and sometimes some pieces just don’t fit. But as with any large puzzle, there comes a point where there are enough pieces, enough consistent evidence, to be able to resolve the picture.

The result of nearly 200 years of scientific endeavour is now a clear and recognisable picture of humans influencing the climate through the emission of greenhouse gases.

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