Turning Back Earth’s Clock

Dr. Peteet works in her labs at Goddard Institute for Space Studies (GISS) and Columbia University’s Lamont-Doherty Earth Observatory (LDEO) in New York to study proxies that tell us about past climate. Collecting this data is an exciting and sometimes, dangerous endeavor, taking her on expeditions to remote places around the world.

Some of her most interesting discoveries are about significant, and abrupt shifts in climate that can be compared to fluctuations happening today. Each period offers lessons about the consequences of climate change in our lifetime and for future generations.

During each of these times Earth’s climate changed rapidly and abruptly over the course of decade to a century. Dorothy and many scientists believe these periods provide a present-day climate case study to understand how transitions from cold to warm climates affect our planet and people.

Lessons from the Past

Tools that archeologists found at the Mesa Site
Tools that archeologists found at the Mesa SiteCourtesy Peter Bostrom
The Mesa Site: The Mesa site on Alaska’s North Slope
The Mesa Site: The Mesa site on Alaska’s North SlopeCourtesy Mike Kunz
Rapid climate shifts occurred at the beginning and end of a thirteen hundred year interval known as the Younger Dryas cold event, 12,900 to 11,600 years before the present (BP). Scientists named the era after discovering that the arctic-alpine plant, Dryas, thrived in many parts of Europe during this time, replacing trees. There are other clear signs from different parts of the world that a “Big Freeze” was going on too. Boulders in Scotland deposited by glaciers far away provide evidence of glaciers re-advancing. Fossil pollen collected throughout the northern hemisphere shows the return of arctic and alpine plants replacing plants typical of a warmer climate. In New York scientists see evidence that boreal forest with spruce and fir replaced a mixed boreal-temperate forest including oak and white pine. Scientists conclude a transition of this kind indicates a strong cooling of about four degrees Celsius. But it is the circulation of world’s oceans that tell the biggest story of climate change during the Young Dryas, with cold meltwater discharged from the melting ice into the North Atlantic thought to contribute to the colder temperatures.

About 13,000 years ago on the North Slope of Alaska there lies a story of how a people known as the Mesa Paleoindians endured a large shift in climate. Their home, located 200 feet above the Arctic tundra, was discovered in 1978 by archeologist Michael Kunz. A harsh climate left the Mesa Indian’s home relatively undisturbed, allowing archeologists and paleoclimatologists to unearth evidence about their lives - tools and technology, sources of food, and climate. What Kunz, Peteet and others found is that the Mesa Paleoindians inhabited the North Slope during a period of dramatic climate change. Warming after the last Ice Age supported an environment they utilized, but a gap in their occupation of the site during an abrupt shift back to cooling (Younger Dryas) produced big impacts on the environment and the people. The subsequent warming further changed landscape in ways that likely caused mammoth and bison to disappear, eliminating a main food staple of the Mesa Paleoindians. Unable to adapt or adjust, scientists believe the Paleaoindians either perished and/or abandoned their home.

During the middle ages (1000-1300 AD) in Europe and parts of the northern hemisphere scientists find evidence of a climate period with global temperatures warming on the order of 1-2 degrees Celsius. Archeological discoveries reveal that the Vikings colonized Greenland, even growing wheat along once icy areas. Records collected about past vegetation, ice cover and water temperature also show a warming climate. However, scientists find the warming during this period occurring regionally, mostly in the northern latitudes, not globally. Some of the warming is attributed to greater solar radiation and fewer volcanic eruptions.

Story of Mega Droughts in New York
Dr. Peteet lives and works in and around New York City, one of the most populated and economically important mega cities in the world. Her analysis of pollen cores extracted from lakes, marshes and wetland sediments in New York’s Hudson Valley confirms the MWP in the region and various impacts, including prolonged droughts. Around 850 AD, she found evidence of a drought lasting for about 500 years. By studying these so-called past “mega-droughts,” Peteet and her team are making a significant contribution to help us prepare for potential impacts of climate change impacting our world today in highly populated cities. These impacts included dramatic shifts in vegetation and charcoal layers, along with increases in salt water found in the New York estuary that affected the quality of drinking water.

Following the Medieval Warm Period, scientists find evidence of a subsequent cooling that we now refer to as “The Little Ice Age,” occurring from about 1400-1900 AD. This is Earth’s last record of an Ice Age. Various scientific and historical accounts tell of shorter growing periods and colder winters, along with the advance of glaciers. Some of the causes are decreased solar activity warming the planet’s surface, increased volcanic eruptions spewing cooling aerosol particles into the atmosphere and ocean currents transporting cool water around the globe. Benjamin Franklin famously wrote about “the year without a summer,” establishing a link between cooler climate and volcanoes, like Mt. Tambora that erupted in 1815.

Life in the Last Ice Age
Artwork at the time of the Little Ice Age depicted scenes of people skating and holding “ice fairs” on the Thames River in England, the canals in the Netherlands and the Hudson River in New York. The colder and wetter climate found throughout Europe and North America resulted in poor crop harvests and widespread famine. High prices of food and other staples, led to economic instability and sometimes, social unrest like ensuing riots in France. Moreover, poor nutrition created vulnerable populations, susceptible to diseases like influenza, thatr sometimes develop into dangerous epidemics. In the end, the Little Ice Age provide a cautionary tale about how humans sometimes fail to adapt well to dramatic climate change.