Man-made greenhouse gases began to nudge up the Earth's temperatures almost 200 years ago as the Industrial Revolution gathered pace, far earlier than previously thought, scientists say.
Greenhouse gas emissions from industry left their first traces in the temperatures of tropical oceans and the Arctic around 1830, they said, challenging widespread views that man-made climate change began only in the 20th century.
The Industrial Revolution began around 1750 in Britain, with a surge in the use of coal to power factories, ships and railways, and gradually spread around the world.
Greenhouse gases at the time were only a fraction of those now blamed for trapping excessive levels of the sun's heat in the atmosphere, stoking more droughts, floods, heat waves and rising sea levels.
"Our findings show that the climate can respond very quickly to changes in greenhouse gases," lead author Nerilie Abram, of the Australian National University, told Reuters of the findings published in the journal Nature.
The scientists detected a rise in temperatures in the 19th century by studying the growth of old trees, corals, the makeup of lake sediments and air trapped in ice cores in Antarctica.
Their computer models showed that natural factors - such as changes in the sun's energy output or the Earth's orbit - could not fully explain the warming trend.
The rising heat only made sense when factoring in an early dose of man-made greenhouse gases, they wrote.
Previously, many scientists have reckoned a small rise in 19th century temperatures was a rebound after a sun-dimming volcanic eruption of Tambora in Indonesia in 1815.
"This is further evidence that the climate has already changed significantly since the pre-industrial period," said Ed Hawkins, a climate scientists at Reading University who was not involved in the study.
Last year, almost 200 nations agreed at a Paris summit to shift from fossil fuels and set a goal of limiting rises in average surface temperatures to "well below" 2degC above pre-industrial times, ideally below 1.5degC.
The Paris deal does not define pre-industrial. Temperatures this year, likely to set new records, are just over 1degC above levels in the 1880s, a widely used baseline in climate science.
Abram said using a baseline of 1800 would make the Paris Agreement harder to achieve by adding perhaps 0.2degC.
"We are frighteningly close already to 1.5," she said.