Numerous numbers are swirling about the UN climate summit in Glasgow (COP26). These numbers include the global warming targets at 1.5C and 2.0C, recent warming at 1.1C, remaining carbon dioxide budget of 400billion tonnes or atmospheric CO2 of 415 parts per billion.
These numbers are often difficult to understand. The study of ancient climates may help us to appreciate their magnitude in comparison to past natural events. Scientists can also use ancient climate data to improve their predictions for the future by calibrating their models.
Recent research, as summarized in the Intergovernmental Panel on Climate Change's (IPCC) latest report, has enabled scientists to improve their understanding of and measure past climate change.
These changes can be seen in rock outcrops and sediments from the seafloor and lakes.
Scientists are able to use these archives to help us understand the context of climate negotiations and compare current and future climate changes with past events.
One example of the IPCC's most important findings was that the global temperature is currently 1.1 C higher than a preindustrial baseline. This is higher than any time in at minimum the past 120,000 years.
This is because the warmest period between ice age peaks was 125,000 years ago. In contrast to today's warmth, it wasn't driven by CO2 - but rather by changes in Earth’s orbit and spin direction.
Another important finding is the current rate of warming. It is now faster than any time in the past 2000 years, and likely to continue for many more decades.
It is possible to reconstruct past temperatures from the geological records. For example, tiny gas bubbles in Antarctic ice can record atmospheric carbon dioxide concentrations back to 800,000. Scientists can also turn to microscopic fossils found in seabed sediments for further evidence.
These properties, such as the types and elements of fossil shells, are related to the amount of CO 2 in the ocean at the time the fossilized organisms lived. This is also related to the atmospheric CO2.
Recent work has shown that atmospheric CO2 concentrations are higher than ever before in recent years, with current levels at 415 parts per Million (compared to 280ppm in the 1800s).
You can also compare past climate changes to other variables. These include the greenhouse gases methane (now higher than ever in at most 800,000 years), the late summer Arctic sea-ice area (smaller that at any other time in the past 1,000 years), glacier retreat which is unprecedented in at minimum 2,000 years, sea level rising faster than any point in the past 3,000 years, and ocean acidity which is unusually acidic in comparison to the last 2 million years.
Climate models also predict future changes that can be compared with the past. An "intermediate" level of emissions would likely result in global warming of between 2.5C and 4.6C by 2300. This is comparable to the mid-Pliocene warm periods of around 3.2 million years ago.
Extremely high levels of emissions could lead to warming between 6.6C-14.1C. This is just below the temperature of the "Paleocene Eocene Thermal Maximum", which was triggered by large volcanic eruptions around 55 million years ago.
Humanity is on the verge of compressing thousands of years worth of temperature changes into a few centuries.
The distant past can be used to predict the near future
The latest IPCC report makes use of ancient time periods for its climate change projections. This is the first IPCC report to do so. Future projections were produced in previous IPCC reports by simply combining results from all climate models and using their spread to measure uncertainty.
However, this report relied heavily on models that do the best job simulating climate change.
This process was partly based on the model's "climate sensitive" which is the degree to which it heats up when atmospheric CO2 is doubled. There are many lines of evidence that show the "correct" value and uncertainty range of sensitivity. One example is from ancient times when global temperature changes were caused by natural CO2 changes. This could have been due to volcanic eruptions, or a change in the amount carbon dioxide removed from the atmosphere by rocks being eroded.
Scientists can combine temperature and CO2 estimates to determine the "correct" climate sensitivity value. They can also refine future projections by using models that have better climate sensitivities.
Overall, past climates have shown that all aspects of the Earth's system have experienced unprecedented changes in at least thousands years.
Global warming will increase if emissions are not drastically and rapidly reduced. This is something that has never been witnessed in millions of years. Let's all hope that those who attend COP26 will be able to hear the messages of the past.
Dan Lunt is Professor of Climate Science at the University of Bristol. Darrell Kaufman is Professor of Earth and Environmental Sciences at Northern Arizona University.
This article was republished by The Conversation under Creative Commons. You can read the original article.
