The Earth was a very different place some 200 million years ago when the first dinosaurs began to emerge during the Triassic Period. The climate was hot and dry, with atmospheric carbon dioxide levels sitting at about 2,000 parts per million (ppm) – far higher than our current level of about 405 ppm. But a new study suggests that if emissions are left unchecked we could be on track to return to those CO2 levels in the next few centuries, and with a little help from a brighter, hotter Sun, surface temperatures on Earth could soar to new heights.

Climate change skeptics often argue that the atmosphere's greenhouse gas concentrations have always fluctuated. That's true: levels have been known to jump around from about 200 to 400 ppm during colder periods (such as our present time), up to thousands of parts per million in warmer times, but this happens on a scale of millions of years. Our current rate of increase is much faster than growth through natural causes.

A new study by researchers at the University of Southampton built up a picture of the CO2 concentrations over the past 420 million years. The record is compiled from more than 1,200 points of data in the form of fossilized plants and shells, and the carbon isotopic concentration of soil and oceans.

"We cannot directly measure CO2 concentrations from millions of years ago," says Gavin Foster, lead author of the study. "Instead we rely on indirect 'proxies' in the rock record. In this study, we compiled all the available published data from several different types of proxy to produce a continuous record of ancient CO2 levels."

Researchers have used plant fossils, such as this Ginkgo leaf, to determine ancient CO2 concentrations and plot out a record spanning back over 420 million years(Credit: Dana Royer)

With this detailed picture of the planet's atmospheric past, the researchers were able to track the natural CO2 fluctuations, and clearly spot the sudden uptick since the industrial era began. But greenhouse gases don't work alone in creating Earth's climate: our good friend the Sun plays a significant role as well, and in the past these two factors have worked against each other to keep the climate relatively stable.

"Due to nuclear reactions in stars, like our Sun, over time they become brighter," says Dan Lunt, co-author of the study. "This means that, although carbon dioxide concentrations were high hundreds of millions of years ago, the net warming effect of CO2 and sunlight was less. Our new CO2 compilation appears on average to have gradually declined over time by about 3-4 ppm per million years. This may not sound like much, but it is actually just about enough to cancel out the warming effect caused by the sun brightening through time, so in the long-term it appears the net effect of both was pretty much constant on average."

Projecting the data forward, the researchers predict that if humanity burns all fossil fuels that are available, atmospheric CO2 concentrations could shoot past 2,000 ppm by the year 2250. That would return us to levels not seen since the early days of the dinosaurs, but with one big difference: the Sun is brighter nowadays. The two factors would this time work together to heat the Earth's climate to unprecedented temperatures.

"Because the Sun was dimmer back then, the net climate forcing 200 million years ago was lower than we would experience in such a high CO2 future," says Foster. "So not only will the resultant climate change be faster than anything Earth has seen for millions of years, the climate that will exist is likely to have no natural counterpart, as far as we can tell, in at least the last 420 million years."

The research was published in the journal Nature Communications.

Source: University of Southampton via Science Daily

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