Alex Powell journeys through deep time to examine how our atmosphere has altered over evolutionary history, and whether our knowledge of this can help us to understand the potential effects of the modern climate crisis.
Artist’s interpretation of a landscape during the Pliocene epoch. Image Credit: John Clark Ridpath on Wikimedia Commons.
Atop the Mauna Loa volcano in a remote corner of Hawaii resides a centre of unmatched importance in our understanding of the Earth’s climate. Collecting data multiple times a week since the 1950s, the Mauna Loa Observatory has played a pivotal role in determining how human activity has influenced atmospheric composition. In 2016, measurements from this facility documented the surpassing of a threshold feared by many climate experts: for the first time in millennia, levels of carbon dioxide (CO2) in our atmosphere reached 400 parts per million. This recording marked a 50% increase in the air’s greenhouse gas content in less than 200 years.
Figure showing surface average CO2 concentrations from 1960-2020. Image Credit: Friedlingstein et al., on Wikimedia Commons.
The measurements taken at the Mauna Loa Observatory evidence the disproportionate impact humans have had on the climate in such a short time. However, current carbon dioxide concentrations pale in comparison to levels at certain points in the Earth’s history. As life evolved and increased in complexity, changes to carbon storage, sequestration and emissions continuously altered the climate long before humans began to drive changes of our own.
During the Earth’s hottest period around 50 million years ago, when temperatures were over 10℃ warmer than they are today, CO2 concentrations were four times higher than modern levels. The Arctic was a tropical haven, too hot for ice sheets to form, but providing the perfect conditions for palm trees to thrive. How CO2 levels rose quite this high remains uncertain, with suggestions ranging from volcanic activity to widespread wildfires. What we do know however is that, following this peak, greenhouse gas levels gradually began to decrease. This enabled more familiar ecosystems to develop, and life as we know it to evolve.
Current climate projections are dwarfed in magnitude by some of the conditions our planet has seen in its 4.5 billion year-long history. Even the worst-case scenario set out by experts of an almost 5℃ temperature increase by the end of the century seems meagre relative to the Earth’s hottest days. This begs the question as to whether we really need to be concerned about the changes humans have caused, and are continuing to cause, to the climate; our planet has faced far more extreme conditions than those we currently find ourselves in, and has lived to tell us the tale.
Though life may have persisted in a greenhouse world, conditions like those we appear set to face have never been experienced by our species, or many of the species we share our planet with. Looking to the past could help us to determine what the world might look like in a warmer future. Palaeoclimatologists have been able to reconstruct prehistoric climates using everything from the shells of marine species to pollen trapped deep within ice sheets. By examining what life was like when CO2 levels were last as high as they are today, we can obtain an insight into how the changes we are causing could affect our most important ecosystems.
400 parts per million was most recently surpassed during the Pliocene epoch, roughly 5.3 to 2.6 million years ago, when the planet looked very different to our modern world. Large regions of Europe and the USA consisted of barren arctic tundra, whilst rainforests and jungles slowly became confined to the equator. A great number of evolutionary changes within the animal kingdom also occurred during this time period; as many of the Earth’s continents were connected by land bridges, animals were free to migrate and develop adaptations to life in new areas.
Many famed prehistoric mammals such as the woolly mammoth, sabre-toothed tiger and giant sloth therefore began to walk the land. Whilst many modern marine species continued on their evolutionary trajectories, the Megalodons dominated the seas. At up to 18m long, this group of sharks was the largest to ever live. Extremely high sea levels, 10-40m above what they are today, and complex patterns of glaciation created further distinctions between this world and the planet we are familiar with.
Despite atmospheric similarities, global temperatures were around 4℃ higher during the Pliocene than they are currently. This is largely due to the fact that, in contrast to the rapid global warming we are currently experiencing, both temperatures and CO2 levels were gradually decreasing. Much of this epoch can therefore be considered a reversed version of modern-day climate change. The recognition of this similarity has been incredibly useful in creating estimates of how changes driven by humans are likely to affect our planet; by determining the sensitivity of prehistoric climates to fluctuations in greenhouse gas concentrations, we can loosely predict how our planet is likely to respond to current trends in emissions.
During the Pliocene, a doubling of atmospheric carbon dioxide levels prompted an approximately 3℃ rise in global surface temperature. Since the great industrialisation of the 18th and 19th centuries, humans have driven a 50% increase in the amount of this greenhouse gas in our atmosphere, yet temperatures have been elevated by little more than 1℃. Parallels between the Pliocene and the present day indicate that we may not yet be experiencing the complete consequences of modern atmospheric change. The fact that we are continuing to emit vast quantities of greenhouse gases makes this finding all the more troubling.
Though the study of prehistoric climates can provide great insight into what may be to come, it is vital to consider how the dynamic nature of our Earth has influenced life in the past. Changes to natural cycles, such as the size and extent of our ice sheets, have always had an extensive influence on our environment. This means that patterns observed throughout deep time might not always be applicable to the world we live in today.
A Pandora instrument in the Mauna Loa observatory, measuring trace gases in the atmosphere. Image Credit: NASA, Wikimedia Commons.
However, though we as humans are facing a set of environmental conditions never before seen by our species, our planet has survived a changing climate countless times over. Looking to the past could therefore prove incredibly useful as we prepare to confront an uncertain future. As the Mauna Loa Observatory now records atmospheric carbon dioxide levels in excess of 420 parts per million in 2024, little else can provide a comparable window into what may be to come.
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