Human records are also an important source of information for understanding past climates. What’s more, when climatologists and historians collaborate, they can open up a whole new field of research. For instance, historical maps and paintings can depict frozen rivers, glacier extent, and various plant species. Together, these sources paint a picture of what the climate was like in the past.

Humans as a climate witnesses:

While ice cores can provide insights into the climate of the last 800,000 years, historical data is limited to the period of human documentation. Nevertheless, it is important to gain a deeper understanding of the state of the Earth during our ancestors’ time. In Libya, for example, historians have found cave paintings of elephants, giraffes, and swimmers. These areas are deserts today, but the paintings show that the region wasn’t always so arid. A more humid climate favoring vegetation and water sources must have prevailed for people and animals to live there.

Figure 1: The deserts in northern Africa were once greener. Cave paintings from Libya often show animals that cannot survive there anymore.
Roberto D’Angelo (roberdan) – This image was originally posted to Flickr as DSCN3916

A study by Manning and Timpson (2014) supports this argument. The researchers analyzed over 1,000 bone, wood, charcoal, seed, and other plant and animal remains across the Sahara Desert. Using C14 analysis, they were able to date those findings. They located large clusters of human presence between 10,500 and 5,500 years before present (BP), indicating that the region was much more habitable at that time. This time span is known as the African Humid Period, when the Sahara was greener and had lakes and rivers.

Historical data is rarely found in the form of cave drawings. Most of the available data comes from written records and documentation, which are mostly restricted to regions with a long tradition of writing, such as Europe and Asia. Written historical documentation can take the form of logbooks or harvest records. At best, logbooks are simple descriptions of weather and wind conditions, and harvest documentation often only describes spring and summer temperature anomalies. Therefore, historical data has some limitations, but when a large amount is compiled, it can complement natural climate proxies, allowing us to reconstruct climate fairly accurately.

Although historical records have limitations, such as regional bias and incomplete data, they are still valuable supplements to natural climate proxies. When carefully compiled and interpreted, these human-made sources enrich our understanding of past climates by offering insights into environmental conditions and how societies perceived and adapted to them. Science and history form a powerful partnership in tracing Earth’s climate journey.

1. Disclaimer: This blog entry is the second part of the A Journey Through Time series ︎
2. Image taken from: García-Herrera, R., García, R. R., Prieto, M. R., Hernández, E., Gimeno, L., & Díaz, H. F. (2003). The use of Spanish historical archives to reconstruct climate variability. Bulletin of the American Meteorological Society, 84(8), 1025–1036. https://doi.org/10.1175/BAMS-84-8-1025 ︎

The post A Journey Through Time: Reconstructing Earth’s Climate from History appeared first on Global Change Ecology.

Nature as a climate witness:

Hints on earth’s past climate are hidden everywhere, we just have to know how to find and interpret them. Some of the most fascinating sources used for climate reconstruction come from the most unexpected sources. Tree rings and ice cores are natural archives which can store historical climate patterns in the form of different physical characteristics. By analysing those physical properties scientists can unveil  temperature shifts, precipitation levels and atmospheric conditions. Tree rings show us annual growth trends influenced by climate while ice traps tiny air bubbles that capture the atmospheric conditions of the earth’s past. When taking a deeper look at nature, we can find many more proxies usable for climatic reconstructions. Marine sediments, fossils, lichens, glacial features like moraines, and sedimented pollen are only a few further examples. 

The fact is that if you really want to use natural sources for climate reconstruction, you have to think outside the box and delve deeper. Using annual growth trends or ice cores as climate proxies may sound straightforward, but it’s more complex than you might think.

Let me briefly introduce you to the science behind these ice cores. As well as analysing the content of past greenhouse gases, ice cores provide layers that can be examined. Similar to tree rings, these are formed by seasonal differences in temperature or extraordinary events such as volcanic eruptions. These boundaries are clearly visible in the upper layers and can be used to determine the age of the ice. The thickness of these layers can tell us about precipitation rates and temperature, which are important climate indicators. With the help of geochemists, even deeper ice sheets can be studied. Different oxygen and hydrogen isotopes transfer climatic information.

Figure 1: An ice core drilled from the West Antarctic Ice Sheet Divide project, containing a dark layer of deposited volcanic ash Credit: H. Roop, National Science Foundation

Three common isotopes are found in Antarctic ice: H₂¹⁶O, which makes up almost 99.7% of the precipitated snow, and two rarer ones: H₂¹⁸O and D₂¹⁶O. Changes in the concentration of these isotopes indicate changes in temperature because the proportion of H₂¹⁸O and D₂¹⁶O is linearly and consistently related to temperature in Antarctica. During colder periods, the H₂¹⁶O content of ice is higher. As the climate warms, the proportion of H₂¹⁸O in Antarctic ice cores and the proportion of H₂¹⁶O in oceans increases. The same reasoning can therefore be applied to the analysis of foraminifera, which are shelled microorganisms which trap oxygen in their shells as they form.

1. Disclaimer: This blog entry is the first part of the A Journey Through Time series. ︎
2. Image taken from: https://www.ncei.noaa.gov/news/what-are-proxy-data ︎

The post A Journey Through Time: Reconstructing Earth’s Climate from Nature appeared first on Global Change Ecology.