Over the last 485 million years, the Earth’s surface temperatures have fluctuated more than previously thought. This is shown by a study that combined geological data with climate models to look far into the past. According to the study, the global average temperature was significantly higher than it is today for most of the time – and always showed a close connection to the CO2 concentration in the atmosphere. The results can help to better understand the history of life on Earth and make more precise predictions for future climate changes.
The Cambrian explosion of species around 540 million years ago produced the ancestors of almost all living creatures today and marked the beginning of the Phanerozoic, the “age of visible life”. The fossils from this period include, for the first time, not just microscopic single-celled organisms, but complex multicellular organisms. But under what climatic conditions did life develop in the millions of years that followed? Fossil mussel shells, among other things, provide information about this. Depending on how warm the ocean was at the time, they stored different ratios of oxygen isotopes. However, reconstructions based on such data alone are subject to great uncertainty.
Large temperature fluctuations
A team led by Emily Judd from the Smithsonian National Museum of Natural History in Washington has now combined such geological data with climate models to create the most accurate temperature curve to date for the past 485 million years. “This method was originally developed for weather forecasting,” explains Judd. “Instead of using it to predict future weather, we are using it here to draw conclusions about past climate conditions.” It was shown that the Earth’s surface temperatures fluctuated more in the past than previously thought.
According to the reconstructions, global average temperatures ranged from 11 to 36 degrees Celsius – and were above today’s value of around 15 degrees Celsius for long periods of time. According to the researchers, the results are also important for understanding current climate change. “If you look at the last few million years, you won’t find anything that looks like what we expect in 2100 or 2500,” says Judd’s colleague Scott Wing. “You have to go back even further, to the periods when the earth was really warm, because that’s the only way we can better understand how the climate might change in the future.”
Close connection to CO2 concentration
The analyses show a strong connection between the global average temperature and the CO2 concentration in the atmosphere. “The consistency of this relationship is surprising, because on this time scale we would have expected a stronger influence of solar radiation,” write the researchers. However, the current data suggest that CO2 is also the strongest influencing factor on geological time scales. “When the CO2 content is low, the temperature is cold; when the CO2 content is high, the temperature is warm,” summarizes co-author Jessica Tierney from the University of Arizona.
Current human greenhouse gas emissions are causing the Earth to warm faster than in all of the climate history studied to date. “Our entire species and the species we share the planet with evolved in a cold climate phase and are adapted to it,” says Tierney. “We are changing the climate in a way that makes it completely unsuitable for humans. The planet has been warmer before and can get warmer again – but humans and animals cannot adapt that quickly.” In the past, major climate changes have led to mass extinctions.
The researchers point out that while the current reconstruction of the past climate is the best available to date, it needs to be validated and refined by further research. “We all agree that this is not the last curve,” says Judd’s colleague Brian Huber. “Research will continue to discover new clues about the deep past that will help revise this curve later.”
Source: Emily Judd (Smithsonian National Museum of Natural History, Washington) et al., Science, doi: 10.1126/science.adk3705