Estimates released by the Intergovernmental Panel on Climate Change (IPCC) in a report match levels of carbon dioxide in the atmosphere millions of years ago, according to a study carried out by researchers from the University of Southampton, Australian National University and University of Bristol.
Published in the journal Nature, the study analyzed records that showed carbon dioxide content in the Earth's atmosphere now is comparable to what the planet had millions of years ago during the Pliocene era.
Back then, the temperature was about 35.6 degrees Fahrenheit warmer than what the world is dealing with today, with atmospheric carbon dioxide at 350 to 400 parts per million, a figure similar to levels already reached in the last few years.
The researchers pointed out that studying the association between climate change and levels of carbon dioxide during a time when the Earth was warmer would help in estimating how the climate would react to higher carbon dioxide levels. This parameter is referred to as climate sensitivity. How climate sensitivity can change over a long period of time is also showed in the study.
Dr. Gavin Foster, one of the study's authors, explained that the Earth is still in the process of adjusting to the quick rise in carbon dioxide in the atmosphere today while records of Pliocene era temperatures already reflect a full response to warming related to carbon dioxide.
With climate sensitivity estimates well within ranges provided by the IPCC in its climate report, researchers said the research community already has an idea of what the Earth would be like as the planet moves towards Pliocene-like environment warmed by greenhouse gases.
"Our new records also reveal an important change at around 2.8 million years ago, when levels rapidly dropped to values of about 280 ppm, similar to those seen before the industrial revolution. This caused a dramatic global cooling that initiated the ice-age cycles that have dominated Earth's climate ever since," added Dr. Miguel Martinez-Boti, the study's lead author.
Researchers clarified the difference in climate sensitivity during warmer and colder periods in the Earth's history as being affected by the growth and depletion of continental ice sheets developed during the ice ages. Ice sheets reflect sunlight and their growth boosts the impact of changes in carbon dioxide level.
By accounting for the effect ice sheets have, researchers were able to confirm that climate on Earth changed with a sensitivity similar to overall forcing observed during both colder and warmer periods.