(Translated from English)
In the near future, the situation on Earth could become similar to the warm period of the mid-Pliocene several million years ago. Many of the figures mentioned at the UN Climate Change Conference (COP26 in Glasgow) relate to the climate. For example, 1.5 °C and 2.0 °C – global warming limits; 1.1 °C – recent warming; the remaining CO₂ budget of 400 billion tonnes; or the current atmospheric CO₂ level, which is 415 parts per million (ppm, parts per million).
The practical significance of these numbers is often difficult to grasp. However, research into climatic conditions in the distant past can provide an idea of the scale of current changes by making comparisons with natural events of the past. Knowledge of climate change in antiquity allows scientists to adjust research models and methods, thereby improving the accuracy of future forecasts.
A recent study*, included in the latest Intergovernmental Panel on Climate Change (IPCC) report, has enabled scientists to improve their understanding and assessment of past climate change. These are visible in rock outcrops, deep ocean and lake sediments, polar ice, and other more ephemeral natural “archives”, such as tree rings and corals. As scientists discover more of these “natural archives” and improve the way this information is used, we are better able to compare recent and expected climate change with that of the past, providing a scientific explanation for the numbers mentioned in climate discussions.
For example, one of the key findings of the IPCC report is that global temperature (currently 1.1 °C above pre-industrial levels) is higher than at any time in at least the last 120,000 years (approximately). As is known, the last warm period between ice ages reached its peak approximately 125,000 years ago – unlike today, the heat at that time was not determined by CO₂, but by changes in the Earth’s orbit and its axis of rotation. Another finding relates to the current rate of warming, which is the fastest in the last 2,000 years (possibly even longer).
Geological information allows for the reconstruction of more than just temperature. For example, tiny gas bubbles trapped in Antarctic ice can point to CO₂ concentrations in the atmosphere 800,000 years ago. Furthermore, scientists can study microscopic fossils preserved in sediments on the seabed. The results of such studies (e.g., elements making up fossil shells) are linked to factors such as the amount of CO₂ in the ocean at the time the fossilised organisms were alive; this, in turn, is linked to the amount of CO₂ in the atmosphere. As we have become better at using such “proxies” (information carriers or repositories) to determine atmospheric CO₂, a recent study has shown that current atmospheric CO₂ concentrations (approximately 415 parts per million (ppm) (compared to 280 ppm before the industrial boom of the early 19th century)) are higher than at any time in at least the last 2 million years.
Other climate variables can also be compared with past changes. For example, greenhouse gases – methane and nitrous oxide (now at higher levels than at any time in the last 800,000 years), Arctic sea ice cover at the end of summer (lower than at any time in the last 1,000 years), glacial retreat (which has no parallel in at least the last 2,000 years), sea levels (rising faster than at any time in at least the last 3,000 years), and ocean acidity (unusually acidic compared to the last 2 million years).
Changes predicted by climate models can also be compared with the past. For example, “intermediate” emission levels will likely cause global warming of between 2.3 °C and 4.6 °C by 2300, which is similar to the mid-Pliocene warm period approximately 3.2 million years ago. Very high emissions would cause warming of between 6.6 °C and 14.1 °C, which to some extent matches the warmest period since the extinction of the dinosaurs – the “Paleocene-Eocene Thermal Maximum” – caused by powerful volcanic eruptions approximately 55 million years ago. Thus, humanity is currently on track to disrupt temperature changes that have spanned millions of years in just a few centuries.
The distant past can help predict the near future
For the first time in the history of IPCC reports, the latest report has used ancient time periods to refine climate change projections. In previous IPCC reports, future projections were prepared by calculating the average results from all climate models, using their variance as a measure of uncertainty. In the new report, projections for temperature, precipitation, and sea levels are based more on those models that best reflected known climate changes.
Part of this process was based on the “climate sensitivity” of each individual model – the amount “it would warm” if atmospheric CO₂ doubled. The “correct” value of sensitivity (and the range of uncertainty or error) is known from several lines of evidence, one of which is derived from specific periods in the distant past when global temperature changes were determined by natural CO₂ changes caused, for example, by volcanic eruptions, or by changes in the amount of carbon removed from the atmosphere as rocks weathered. Therefore, combining ancient CO₂ and temperature estimates allows scientists to determine the “correct climate sensitivity value”, thereby refining future projections and placing greater reliance on climate models with more accurate “climate sensitivity”.
Overall, climate conditions in the past, as far as we know them, show that recent changes in all aspects of the Earth system have no parallel in at least thousands of years. If emissions are not reduced rapidly and substantially, global warming will reach levels not seen for millions of years. Let us hope that the messages from the past will be heeded by more than just the participants of COP26.