The climate of Earth has always been dynamic, showing a meteoric rise in temperatures over millions of years. It is now being learned through new research that our planet could actually be hotter during its 500-million-year history. These represent the very important context not only of Earth’s past climate but also of today’s human-induced climate change and its ramifications for life. These ancient periods of warming have allowed scientists to piece together the role of greenhouse gases and the pace of climate change, emphasizing the unprecedented rate of contemporary global warming.
Understanding Earth’s Climate History
Through the last 500 million years, Earth’s temperature has seesawed between froze ice ages and scorching greenhouse periods. Led by University of Arizona and Smithsonian researchers, a new study offers the most detailed view available of how Earth’s surface temperature has varied over the past 485 million years. The timescale, derived through both geologic records and climate models, reveals great fluctuations in temperature amplitude out of range for today.
Over this time, the Earth’s temperature has ranged from 52°F to 97°F (11°C to 36°C), and the periods of warmth seem to be intimately linked to very high levels of atmospheric CO2. This record, derived by using a technique called data assimilation that incorporates real-world data and climate simulations, represents the best understanding to date of how ancient climates have behaved.
“Carbon dioxide is the dominant control on global temperatures across geological time,” says Jessica Tierney, a co-author of the study. “When CO2 levels are high, the planet warms significantly, and when they’re low, cooling occurs.” It confirms once again the role of CO2 in forcing past climate changes, be it during a natural period of warming or during the rapid climate changes we are experiencing today.
The Role of Greenhouse Gases in Ancient Climate Change
During much of the Phanerozoic Eon, the period of time from about 485 million years ago to the present day, atmospheric levels of CO2 were much higher than those today. This strongly corresponds with global warming. For example, the intense greenhouse period in the Paleocene-Eocene Thermal Maximum, or PETM, some 55 million years ago saw temperatures skyrocket and the eventual extinction of several species. Several such periods of extreme heat have occurred, most linked to mass extinctions and major turnovers in ecosystems on Earth.
Until now, scientists had to rely on proxy records of those temperatures—such things as ancient shells and tree rings when they are fossilized. And then they combined that information with the most sophisticated climate models to generate an incredibly accurate temperature curve that gives them an unprecedented view of Earth’s temperature variability over half a billion years.
What sets this study as particularly applicable to the modern discussion on climate is the contrast in the rate of temperature fluctuation. While the ancient shifts were mostly enabled by natural causes, such as volcanic eruptions or asteroid impacts, today’s changes are caused primarily by human activity. Greenhouse gases, especially CO2, are being emitted at a pace that far outstrips any natural processes observed in Earth’s history.
Lessons from the Past for Modern Climate Change
The new temperature timeline isn’t just a tool to look into the distant past–it’s a stark warning for the future. “If you’re studying the last couple of million years, you won’t find anything that looks like what we expect in 2100 or 2500,” warns Scott Wing, a paleobotanist at the Smithsonian and one of the study’s co-authors. Science therefore has to backtrack and begin its research from these very ancient periods, where global temperatures were much higher than they are today, to understand the future of climate trajectories.
The study emphasizes that Earth’s climate can—and has—changed dramatically in response to CO2. However, the pace of modern warming is cause for alarm. The current rate of global warming linked to human activities has no precedent in the geological record. Whereas ancient warming events unfolded over thousands or even millions of years, today’s warming is happening on a scale of mere decades.
“This is the fastest rate of climate change we’ve seen in the geologic record,” Tierney says. This heating up had already begun to drastically affect ecosystems, now changing species’ ranges, melting polar ice caps, and raising sea levels. Whereas these changes in earlier times were at a pace that gave time for ecosystems to adjust, today’s species, including humankind, find it very problematic to adapt to such drastic shifts.
What’s in Store for the Future: Are We Headed toward Another Hot House?
Like in today’s context, the Earth recently was both much hotter and much colder than it is now; however, scientists are concerned about how quickly temperatures are increasing today. Whereas human civilization has evolved during a time of relatively stable climate—an “icehouse” condition and one characterized by cooler global temperatures than now to boot. The last time the Earth was this warm was 3 million years ago, before humans existed.
Currently, the global average temperature is about 59°F, which is much cooler than those previous warm periods on Earth. However, as outlined above, the rapid rise of CO2 levels due to fossil fuels is leading to a much greater rate of temperature rise than ever occurred in the last 500 million years. If emissions are not halted, the IPCC needs to impose a greater than or equal to 3°C increase in average temperatures, with catastrophic potential impacts on ecosystems of the planet and human societies by the end of the century.
Conclusion: Global Wake-Up Call
The investigation into Earth’s ancient temperatures provides important information about the natural forces that have molded our planet’s climate over the years. Yet, it is also an instructive lesson in another way: whereas Earth itself can endure extremely hot conditions, human civilization may not be in a position to adapt to the rapid changes we are experiencing at the moment. As greenhouse gas emissions continue to rise, so do the risks of a warmer, more unstable planet.
Though the past 500 million years show that Earth’s climate has always been in transition, the unprecedented rate of change today makes this era an anomaly. And far from what it will take to avoid the worst impacts of climate change, dramatic action is needed—more today than ever.
The temperature of Earth during the past 500 million years was considerably hotter than previously believed—an important lesson for modern, human-driven climate change.
