There is no denying humanity’s indelible contribution to climate change. After millions of years of relative stability, just a few hundred years of greenhouse gas emissions will charge the Earth and its creatures toward unprecedented warming. The finding is published in the journal Nature Communications.

If unabated in the next 100 to 200 years, carbon dioxide (CO₂) concentrations in Earth's atmosphere will rise to the highest since the Triassic 200 million years ago. If CO₂ continues to rise even further, the next 200 to 300 years will surge to a warming state without geological precedent in the last 420 million years.

For the study, the team gathered over 1,241 estimates of atmospheric CO₂ concentrations from 112 published studies to create a record dating back 420 million years. Since a direct measurement of ancient CO₂ concentrations is not possible, researchers rely on indirect “proxies” to construct a record. This included published data on fossilized plants, the carbon isotopic composition of ancient soil samples, and the boron isotopic composition of fossil shells.

The cumulation of these proxies reveal a startling truth: While Earth’s climate has fluctuated in the past, the current speed of climate change is uniquely swift. 

^{Living Ginkgo leaf (left) and fossil (right). Density of stomata in such leaves is a proxy of atmospheric CO2 in past. Dana Royer}

Atmospheric CO₂ levels depend on a variety of factors, including volcanism, metamorphism, organic carbon weathering, human activity, and more. There have, of course, been fluctuations in the climate record throughout history, but the climate had remained relatively stable for millions of years until the Industrial Revolution. 

Before the Industrial Revolution, carbon dioxide concentrations were around 280 parts per million (ppm). Today, that figure has risen to 400 parts per million. By 2250, that could surge past 2,000 ppm if no efforts are made to mitigate emissions – levels not seen since the Triassic (220-200 million years ago), with the climate reaching a state of warmth not seen since the Devonian (around 400 million years ago). The increase in temperature is partly due to the addition of a future hotter Sun.

Millions of years ago, the Sun was much dimmer than it is today. That means back then its output was less – over time, it got brighter and its intensity slowly increased. Yet, if this is the case, why is there little evidence to suggest a similar warming of the climate? This, the researchers say, is down to a delicate balance between a brightening Sun and declining atmospheric carbon dioxide levels. 

"Due to nuclear reactions in stars, like our Sun, over time they become brighter," said co-author Dan Lunt of the University of Bristol in a statement. "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."

He explained that their new CO₂ concentration record showed an average decline over time of about 3-4ppm per million years, which he says acted as a counter to the increasing brightness of the Sun.

"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," he added.

That balance is now breaking, with humans' short but powerful industrial impact in the last couple hundred years. This, however, does not mean we should throw our hands up and declare all is lost. There are possible solutions to mitigate such a climate future. Renewable energy, a cut in fossil fuel emissions, environmental protection, innovative research, and the Paris agreement are all avenues in which to mitigate climate change. The challenge now is to enact this future before the damage is irreversible.