For the purposes of this article, it is useful to divide the last millennium into three eras: the Medieval Warm Period, the Little Ice Age, and the Fossil Fuel era, marked by the Industrial Revolution and global warming.
The Medieval Warm Period: 800 – 1300
Most theories invoke a combination of increased solar energy output and decreased volcanism to explain the relatively warm climate of the Middle Ages. These factors do correspond to several warm periods, according to dendrochronologists, who study tree rings, and climate researchers, who study ice core samples. Nevertheless, 500 years is the proverbial blink of an eye on the scale of geologic time. The debate is likely to continue for the foreseeable future.
Regardless of its cause(s), the Medieval Warm Period saw an increase in Europe’s population, the Viking colonization of Iceland and Greenland, and the Inuit expansion from northern Canada to Greenland a few centuries later. In the American Southwest, the Anasazi culture of Chaco Canyon reached its peak around 1100. In the South Pacific, Polynesians reached Easter Island sometime after 800 AD, and the Maori colonized New Zealand around the year 1000.
Compared to the medieval warm period preceding it, global temperatures became cooler and rainfall patterns more unpredictable starting around the year 1300. Theories abound as to the cause of these events, but most focus on one or more of the following factors:
- An increased number of volcanic eruptions led to the reflection of more sunlight back into space, accounting for the colder temperatures. Although volcanic ash does block sunlight from reaching the Earth’s surface, a significant increase in global volcanism is difficult to discern from the historical record. For instance, the eruption of of Mt.Tambora in 1815 was estimated to be many times more powerful than Mt. St. Helens but occurred nearly at the end of this cold period. By the time Krakatoa erupted near Java in 1883, the Little Ice Age had been over for several decades.
- Decreased solar output. Sunspots (a marker of solar activity) wax and wane in a cycle lasting 11 years or so. During the 17th century, however, records indicate that sunspots practically vanished for years on end. No consensus exists as to the cause of this event, known as the Maunder minimum. Interestingly, the coldest decades of the Little Ice Age also occurred during the mid-1600’s, suggesting these two events were more than a mere coincidence.
- Alterations in deep ocean currents. The Gulf Stream and Japanese currents move vast amounts of warm salt water from the equator toward the poles. This process, termed thermohaline circulation, contributes to a mild climate in places as far north as the British Isles and the Pacific Northwest of North America. Without the Gulf Stream, for example, the climate of England would become as frigid as that of the Hudson Bay in northern Canada. Something similar may have happened around the year 1300. In one scenario, the melting of Arctic glaciers during the medieval warm period decreased the North Atlantic Ocean’s salinity to the point that heat convection in the ocean depths practically shut down. The ensuing cold spell lasted for over 500 years.
The consequences of the Little Ice Age were largely negative for Europe and beyond. Cold, damp conditions probably favored the spread of rats that carried the bubonic plague to Europe in the mid-1300’s. The same climate also favored the growth of ergot fungus, which contaminated grain supplies in much of Northern Europe. Ergot poisoning plagued Europe for centuries. To superstitious people living in a pre-scientific era, the frightening, bizarre symptoms of ergot toxicity, most notably burning sensations in the extremities, miscarriages, and hallucinations, was clear evidence of witchcraft. Not surprisingly, hysteria over witches gripped Europe for much of the Little Ice Age.
Cultures in other ecologically vulnerable places declined or collapsed altogether. The Polynesian population on Easter Island declined by 90% in the wake of deforestation and severe drought. The Norse colony in Greenland vanished in the decades after 1410. In Cambodia, the Khmer capital of Angkor Wat was virtually abandoned after 1432. Decades of crop failure in China culminated in the Manchus overthrowing the Ming Dynasty in 1644.
The Fossil Fuel Era and Global Warming: 1820 – Present
For thousands of years, humans have burned small quantities of coal and oil, but that amount was minuscule compared to the massive fossil fuel consumption that powered the Industrial Revolution. As the graph illustrates, the burning of coal, oil, and natural gas has accelerated since 1850 and shows no sign of slowing down today, at least not on a global level.
An inevitable byproduct of burning carbon compounds is the production of far more atmospheric carbon dioxide than green plants and phytoplankton can remove via photosynthesis. Emissions of CO2, methane, and other so called greenhouse gases have skyrocketed, especially after 1900. Annual greenhouse gas emissions are estimated at over 8 billion metric tons worldwide.
Not coincidentally, average global temperatures have increased by 1 or 2 degrees Celsius in that time span. Today, all major glaciers are in retreat. In addition, massive melting of the ice caps in Greenland and Antarctica points to a clear warming trend. Is global warming a human driven phenomenon? Most environmental scientists think the answer is yes, considering the tremendous amount of fossil fuels burned in the last 50 years alone.
Even though world oil reserves are dwindling, the supply of coal and natural gas is expected to last for several more centuries. Even at this stage, however, the consequences of global warming are becoming apparent. They include more frequent droughts in the U.S., Australia, and parts of Africa; more intense hurricanes fed by warmer ocean waters; and a gradual rise in sea levels caused by the melting of glaciers and the polar ice caps.