In the midst of an unprecedented heatwave gripping vast regions of the world, people are grappling with the question of whether such scorching temperatures have ever been witnessed before. As the year 2023 bears witness to some of the hottest days in recent history, curiosity lingers about the distant past, long before the advent of weather stations and satellites.
Certain news outlets have made claims that daily temperatures have reached heights not seen in the past 100,000 years. However, as a paleoclimate scientist specializing in studying past temperatures, I find these headlines somewhat imprecise and cringe-worthy. While the assertion might hold some truth, the lack of detailed temperature records spanning 100,000 years means we cannot state it with absolute certainty.
Nonetheless, there are established facts about the current state of Earth’s climate and its comparison to the past. Several years ago, scientists determined that the planet had entered a new climate state, unlike any observed in the last 100,000 years. The Intergovernmental Panel on Climate Change (IPCC) issued a report in 2021, stating that the Earth was already over 1 degree Celsius (1.8 Fahrenheit) warmer than preindustrial times, with greenhouse gas levels in the atmosphere ensuring prolonged elevated temperatures.
Even in the most optimistic scenarios for the future, wherein human activities drastically reduce greenhouse gas emissions, global temperatures are projected to remain at least 1 degree Celsius above preindustrial levels for several centuries, and potentially higher. This new climate state, characterized by a multi-century warming trend of 1 degree Celsius and above, provides a solid basis for comparison with temperature reconstructions from ancient eras.
To gauge past temperatures, paleoclimate scientists rely on information stored in natural archives, such as lake and ocean sediment cores dating back thousands of years. However, these records have certain limitations, including the potential for sediment mixing and imprecise timelines, which may blur short-term temperature spikes and fluctuations.
Throughout history, Earth has experienced alternating glacial and interglacial periods, lasting around 100,000 years each, driven primarily by shifts in Earth’s orbit and greenhouse gas concentrations in the atmosphere. Currently, we find ourselves in an interglacial period that commenced approximately 12,000 years ago, following the retreat of ice sheets and a rise in greenhouse gases.
Taking into account this 12,000-year interglacial period, it appears that global temperatures might have peaked roughly 6,000 years ago. Nevertheless, the IPCC report suggests that the global warming level did not exceed 1 degree Celsius at that time. However, ongoing research indicates that global average temperatures continued to rise throughout the interglacial period, an area still subject to active investigation.
To identify a time that might have been as warm as the present, we must venture even further into the past. The last glacial episode, lasting nearly 100,000 years, provides no evidence of global temperatures reaching preindustrial levels during that extended period. By going back even further to the previous interglacial epoch, which peaked approximately 125,000 years ago, we do find evidence of warmer temperatures. This period likely saw a long-term average temperature of no more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial levels – not significantly higher than our current global warming level.
However, the future trajectory remains a cause for concern. Unless humanity rapidly and consistently reduces greenhouse gas emissions, current projections indicate that by the end of the century, Earth’s temperatures could soar to approximately 3 degrees Celsius (5.4 degrees Fahrenheit) above preindustrial levels, and possibly even higher. Such a scenario would necessitate looking millions of years into the past to find a comparable climate state. In this case, we would be transported back to the Pliocene epoch, a distant era that witnessed the emergence of agriculture and civilization as we know it.