A "global pattern" of change in the Earth's climate began 2.7 million years ago, say scientists.
Researchers found that, at this point, temperature patterns in the tropics slipped into step with patterns of Ice Ages in the Northern Hemisphere.
They report in the journal Science that atmospheric CO2 could be the "missing link" to explain this global pattern.
The findings, they say, reveal a "feedback process" that could have been magnified by greenhouse gases.
This loop of feedback could have intensified both the Ice Ages in the Northern Hemisphere, and temperature fluctuations in the tropics.
Professor Timothy Herbert from Brown University in Rhode Island, US, led the research.
He and his colleagues, in the US and China, analysed mud cores from the seabed in the four tropical ocean basins - the Arabian Sea, the South China Sea, the eastern Pacific and the equatorial Atlantic Ocean.
These mud cores are laid down over millions of years - as sediments of dead plant and animal material sink to the ocean floor.
So by analysing the chemical composition of this material - specifically the chemical remains of one ancient and tiny marine organism - the scientists were able to produce a timeline of temperature changes.
The team "found a fingerprint in the sequence of temperature changes" - a pattern that began 2.7 million years ago, Professor Herbert explained.
He told BBC News: "The timing and the amplitude of temperature changes [in the Northern Hemisphere] are reproduced in the tropical temperatures. The patterns are incredibly similar."
He added that the study provided the first direct evidence of a global pattern in climate change that dated back almost three million years.
Professor Herbert added that the "best global mechanism" to explain this link was the level of atmospheric greenhouse gases.
Dr Carrie Lear, a palaeoclimate scientist from Cardiff University in the UK, agreed that carbon dioxide was the likely "culprit".
She told BBC News: "This study reveals a feedback process that has magnified climate change since the inception of Northern Hemisphere glaciation 2.7 million years ago.
"It seems the tropical warming caused by high CO2 levels set off a chain of events resulting in additional greenhouse gases, including water vapour, being released to the atmosphere, thus causing further warming."
Dr Lear said that such studies of past climate change were "invaluable in understanding the current climate system, and hence predicting future change".