Climate change is a big issue. Windmills, solar panels and electric vehicles are becoming common as people try to reduce greenhouse gas (GHG) emissions, which are thought to be causing the Earth’s atmosphere to get warmer.
But as farmers and ranchers know, predicting and determining the causes of weather and climate is not simple. Scientists have learned a lot about climate change, but its magnitude and causes are still not certain. There has been warming over the last century, especially in the Arctic, which has had considerable loss of sea ice. Human (and livestock) GHGs are thought to contribute to warming, but there is uncertainty over the contribution of natural factors, such as variation in the sun’s radiation, the Earth’s orbit, clouds and volcanoes.
A perspective on natural climate variation—free of human GHGs—is provided by considering the climate before the Industrial Revolution in the 1800s. There are extensive, complex scientific studies of past climates using ice cores from Greenland and Antarctica, sediment cores from seas and lakes, tree rings and other geological measures. Two periods in the past have received particular attention: the Medieval Warm Period and the Little Ice Age, which I describe below.
Table 1. Times of past climate change
Time Period
Dates
Last major glaciation
70,000 to 20,000 years ago
Melting of last major glaciation
20,000 to 8,000 years ago
Medieval Warm Period
1,220 to 520 years ago (A.D. 800 to 1500)
Little Ice Age
520 to 120 years ago (A.D. 1500 to 1900)
We are now in the geologic time period known as the Holocene, which began about 11,500 years ago. Geologists call this 11.5 kiloannum (ka). For perspective, the Egyptian pyramids were built about 4,500 years ago, and of course, Jesus lived 2,000 years ago. The Holocene was preceded by the ice ages, which had several cold glacial periods and warm interglacial periods that alternated for more than a million years.
The last major glacial period lasted from about 70,000 years ago to the Last Glacial Maximum 20,000 years ago, after which the glaciers melted rapidly until 8,000 years ago (see Table 1). Ice sheets more than a mile thick covered North America as far south as Long Island, NY, Wisconsin and northern Montana during this glacial period. The transition from the glacial period to the interglacial period had an increase in average atmospheric temperature of 9 F (5 C). We are in this relatively warm interglacial period now, although large ice sheets remain in Greenland and Antarctica.
Now shift your thinking to a more recent time frame. A period of 700 years, from A.D. 800 to 1500, is known as the Medieval Warm Period (also called the Medieval Climatic Anomaly). Some researchers suggest the Medieval Warm Period was shorter, from A.D. 900 to 1300. During this period, the North Atlantic Ocean, Europe and other areas had relatively warm temperatures, although the warming was not consistent worldwide.
The temperatures in Europe during the Medieval Warm Period may have been 1.8-3.6 F warmer than the temperatures in the early 1900s. It is thought that the warmer conditions resulted in good agricultural production in Europe and allowed Vikings to colonize Iceland, southern Greenland and Newfoundland.
The Medieval Warm Period was followed by a colder 400-year period from about 1500 to 1900 A.D., known as the Little Ice Age. During this period, a series of short-term cooling events occurred, atmospheric temperatures fell and alpine (mountain) glaciers grew and advanced in Europe, Alaska, New Zealand and the southern Andes Mountains in South America. The glaciers that are now in Glacier National Park, MT, were formed during the Little Ice Age.
The Little Ice Age was colder than the Medieval Warm Period, although cooling and glacier growth was not uniform across the Earth, and some areas were relatively warm. Alpine glaciers expanded and destroyed farms and towns, winters were cold, and summers were cool and wet, which resulted in crop failures and famines in Europe. Sea ice expanded in the North Atlantic, isolating the Viking colonies in Greenland, which were abandoned.
Glaciers also advanced in Japan, with temperatures in winter decreasing by about 6 F. The Little Ice Age may have been caused by reduced radiation from the sun, changes in the atmosphere and volcanoes. Following the Little Ice Age, the Earth entered the present period of warming that began in the late 1800s and early 1900s.
The natural variation in the Earth’s climate can be substantial and affect human welfare as indicated by the Medieval Warm Period and Little Ice Age. Scientists recognize the need to consider natural climate influences when assessing the impact of human GHG emissions and are doing research accordingly.
Claims that humans are the primary cause of climate change must be tempered with the extensive science that shows other influences on climate. This approach will result in better policies to deal with climate change. — Dr. Matthew Cronin
Matthew Cronin was a research professor at the University of Alaska and is now a scientist with Northwest Biology Company LLC in Bozeman, MT. He can be reached at croninm@aol.com.
1. Cronin, Thomas M. 2010. Paleoclimates. Columbia University Press, New York.
2. Pielou, E. C. 1991. After the Ice Age. University of Chicago Press, Chicago.
3. Past Global Changes Project: 2k Network Projects | PAGES (pastglobalchanges.org)
4. U.S. Global Change Research Program: Fifth National Climate Assessment | GlobalChange.gov
5. Medieval Warm Period: medieval warm period | climatology | Britannica
6. Little Ice Age: Little Ice Age | geochronology | Britannica
7. Last major glaciation: Wisconsin Glacial Stage | Time, Map, & Facts | Britannica
