Evidence has been reported that rising carbon dioxide levels, primarily caused by the burning of coal, began raising temperatures in the 1830s, much earlier than previously suspected. Although there has been speculation that agriculture may have changed the global climate thousands of years ago, this is the earliest detection of a human-induced signature to temperatures across much of the planet.

“It was an extraordinary finding,” said Dr Nerilie Abram of the Australian National University in a statement. “It was one of those moments where science really surprised us. But the results were clear. The climate warming we are witnessing today started about 180 years ago.”

Abram is the first author of a paper in Nature that looks at oceanic temperatures over the last 500 years, based on changes to corals in the tropical oceans and sediments in non-tropical locations. A recent rapid expansion in the samples collected, and improvements in establishing temperatures at the times these were laid down, has allowed us to gain a fine-grained history of ocean temperatures for the first time.

These measurements produced broadly similar results to the numerous studies of temperatures on land over the same period. The Earth was cooling until the 19th century, something Abram attributes to increased volcanic eruptions, and then warmed afterward.

^{A coral core being collected off north-western Australia to determine the temperature when the coral layers were laid down. Eric Matson/AIMS}

By expanding the field of study, however, Abram was able to determine that the turnaround came as a result of additional carbon dioxide in the atmosphere in the 1830s, rather than simply an easing of volcanic effects. The extra carbon dioxide was tiny by today's standards – an increase from 280 parts per million (ppm) to 295 ppm, compared to the current level of 403 ppm.

Across most of the planet, this produced effects too small to be differentiated from natural fluctuations, at least with existing data. In both Arctic and tropical waters, on the other hand, Abram found a shift beyond what could be explained without the influence of the extra carbon dioxide.

The Arctic has been the site of the largest human-induced warming, something predicted by climatic models. However, Abram told IFLScience that models also predict tropical oceans should show effects particularly quickly because “they have a stable surface layer that absorbs heat.” Other oceans have more mixing between the surface and the depths, causing the additional heat to be drawn into the depths, which take much longer to warm up.

The additional carbon dioxide probably came from multiple sources, including the felling of forests and even oceanic changes brought on by mass whaling. Nevertheless, Abram told IFLScience that the mix of heavy and light isotopes in atmospheric carbon dioxide shows most of the increase came from the burning of fossil fuel as the Industrial Revolution got under way.

Abram said that were it not for human activity, the world would be substantially cooler than the recent record-breaking run. However, she added we can't tell if the centuries of cooling that ended in the early 1830s would have continued, or if there would have been a more modest shift towards warmth.

^{A thin slice of limestone laid down in a cave, which can be used to establish ancient temperatures. Christopher Maupin and Meaghan Gaupin.}