Medieval Climate Change

Essay 20: Climate Change over the Past Thousand Years For the purposes of understanding global climate change, 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 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

As the graph in the reference section illustrates, the burning of coal, oil, and natural gas has accelerated since 1850 and shows no sign of slowing down today, at least not at the global level. An inevitable byproduct of burning carbon compounds is the production of far more atmospheric carbon dioxide than green plants and phytoplankton can extract 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