Penn StateNASA

Module 2: Recent Climate Change

Life was good for the ancient Mayans of Central America. From 300 BC to 640 AD they built temples and palaces, including the magnificent Tikal temple in Guatemala and the Xunantunich temple in Belize. Hieroglyphic inscriptions on these and other buildings point to a thriving agricultural society.

Tikal Temple in Guatemala
Tikal Temple, Guatemala
Credit: Work found at Wikipedia CC BY-SA 3.0 (Creative Commons)

New dynasties were founded and society was peaceful. But the following three centuries were marked by war between city states, leading to collapse of the once flourishing civilization. Archeologists have puzzled over the cause of this monumental collapse for decades. Now research by Penn State Department of Archeology Professor Douglas Kennett and his colleagues have unlocked a potential answer. A 20-centimeter long stalactite from a cave in Belize has yielded a remarkable proxy record of climate in the Mayan heartland. Oxygen isotope data show that the centuries in which the civilizations expanded and thrived had plentiful rainfall, but in 900 AD, a devastating century-long drought began. Note that as oxygen isotopes are fractionated by evaporation, water in rainfall has a different oxygen isotope composition from water vapor in the atmosphere--they are sensitive proxies for rainfall. Thus Kennett and his colleagues proposed that severe drought caused the political upheaval that led to the ruin of the Mayans.


Cave covered in stalactites
Yok Balum Cave, Belize showing stalactites similar to the one that provided the climate record
Credit: Douglas Kennett

The Mayans, and other ancient dynasties including the Ming Dynasty of China, are of great interest because they demonstrate how climate change has impacted society in the past. As we will see in Module 10, drought and food shortages in modern times have caused great conflict in Africa, so it is a very powerful lesson to see just how devastating drought has been before.

Now fast forward just a little and we have multiple accounts and proxy records of two significant changes in climate that impacted medieval societies, not to the same degree that climate impacted the Mayans — the Medieval Warm Period (AD 950 to 1250) and the Little Ice Age (AD 1450-1850). The Medieval Warm period is famous because of its connection to some interesting events in the European and North Atlantic regions. During this time, it appears that wine production in Great Britain was abundant, even though in today's climate, wine grapes struggle at this high northern latitude. This is also the period of time when the Vikings colonized Greenland (they originally called it "Vinland"), indicating that this region was warmer than it is today. In a recent examination of climate proxy records from around the world, Michael Mann and his colleagues determined that the temperatures in the North Atlantic region were indeed warmer than the 1961 - 1990 period, but globally, the climate was not as warm as today, as can be seen in the below figure.

Graph showing temperature anomalies from the year 500 until 2000
The blue line is the climate record reconstructed from multiple proxies, including tree rings, coral data, and cave deposits. The red line is the CRUTEM4 instrumental record that only goes back to 1850. Also shown are the approximate time ranges of the Medieval Warm Period and the Little Ice Age.
Credit: David Bice

The Little Ice Age is similarly famous for its connections to European history. During this period, the winters in Europe were cold enough that the canals in the Netherlands froze over, allowing for skaters to travel through the countryside on these frozen pathways — this activity is recorded in some of the masterpieces of Dutch painters such as Bruegel. The Little Ice Age was a time of minor advances in many of the Alpine glaciers, and it also signaled the end of the Greenland colonization experiment. This was generally a difficult period in European history, marked by plagues, famine, fighting, and political turmoil. The cause of the Little Ice Age, like the cause of the Medieval Warm Period, is not entirely settled, but it does coincide with a period of volcanic eruptions, which should cool the climate, and a period of decreased solar activity known as the Maunder Minimum. It has also been suggested that a slowdown in the thermohaline circulation in the oceans (see Modules 3 and 6) may have contributed to this cooling.

In both the Mayan and Medieval accounts, we can see that proxies and historic and archeological information are consistent. But the recent warming in the iconic Hockey Stick most certainly stands out in terms of its magnitude and its abruptness.

Recent Climate Change

In this module, we take a look at the wide range of observations that give us a sense for how the climate has been changing in the last 100 to 2000 years. As you might expect, there is plenty of high-quality data for recent times, but as we move into the past, the details of the global climate are a bit harder to decipher; nonetheless, a lot of hard work by climate scientists has put together a pretty clear picture of the global climate further into the past. This work is providing a much more solid understanding of key times like the neck of the "Hockey Stick" that we discussed briefly in Module 1.

The central idea of this module is that by knowing how the climate has changed recently, we are in a better position to predict what the future holds.