The cold winters of the past few years raise interest in the Little Ice Age, a period of relatively colder temperatures in Europe that was sandwiched between the Medieval Warm Period (lasting from about 900 CE to 1300 CE) and the mid-19th century. In this period, mountain glaciers expanded and winters in Europe and in North America were markedly colder than in the 20th century.
The height of the LIA occurred between roughly 1600 and 1800, when, especially near the end of this period, industrialization and associated air pollution began to increase. Explaining the end of the LIA has been somewhat of a dilemma as glaciers in the Alps started retreating in the mid-19th century when, according to weather records, they should have advanced until about 1910. However the 19th century was also a period of rapid increases in pollutant emissions and downwind deposition of soot (black carbon powder produced from incomplete combustion of fuels). When deposited on ice and snow, soot increases the absorption of sunlight raising the surface temperature. Using a computer simulation model, Painter et al found that both soot levels and associated snowmelt were sufficient to explain the rate of retreat of Alpine glaciers. The soot deposition rate continued to rise in the 20th century, correlating with observed increases in carbon emissions and continued retreat of many mountain glaciers.
What about the cause of the Little Ice Age? A coinciding period of low sunspot activity which is associated with lower radiative energy output from the sun is one likely factor. In fact, during the “Maunder Minimum” of the 17th century, there were essentially no sunspots, an uncommon occurrence that would lower heating of the earth. Also, during the Little Ice Age, the weather pattern known as the North Atlantic Oscillation was more often in a negative mode resulting in the irruption of cold air from the Arctic southward over Europe. The NAO, you may recall, caused the very cold winters in the past few years in Europe and eastern North America but also produced milder temperatures in Greenland. Another contributing factor, although usually restricted to periods of only a few years, is volcanism. The famous “year without a summer”, 1816, when frigid weather led to widespread crop failures and famine in Europe, was attributed largely to a major volcanic irruption in Indonesia.
It now appears that black carbon not only affects mountain glaciers, but also is a larger contributor to global warming than it was thought to be until the last year or so. In fact, although getting much less attention than CO2 as a greenhouse gas, its quantitative contribution to global warming may be as much as two-thirds that of CO2 and greater than that of methane, previously thought to be the second most common greenhouse gas.
In the developing world, the principal global sources of soot are the burning of wood and other plant material for cooking and heating, and the setting of fires to clear land for grazing and agricultural crops.
In the industrialized countries, the combustion of coal and diesel fuels and the burning of agricultural waste are major sources. Most biomass burning appears to occur in Africa where burning to clear lands for grazing and burning of fuels for cooking and heating occurs over large swaths of this huge continent. Future soot emissions are likely to change and may very well rise given the planned increase in traditional coal-fired power plants in rapidly developing countries like China and India.
Once soot is released into the atmosphere it may travel great distances before settling out. For example, in one of the largest fire episodes of recent years, vast areas of tropical forest were destroyed by intentionally set fires in Indonesia (primarily Kalimantan) in 1997. About 45,000 km2 (17,375 square miles) was burned, an area equivalent to the states of Rhode Island, Connecticut, Delaware and New Jersey. Most of this forest was growing on peat (organic) soils which yield dense clouds of particulate matter when burned and hold more combustible carbon than the forests themselves. Smog from the fires covered all of southeast Asia for weeks causing health problems for millions. Emissions from the fires were detected as far away as Hawaii. Similarly, emissions from heavy industry and wildfires in Russia (including Siberia) are regularly tracked as they spread north over the Arctic and are deposited in northern Canada and Alaska.
Interestingly, despite the longer-term trend in soot deposition in northern latitudes, soot levels have actually declined since the 1990s at High Arctic study sites in Alaska, Canada and Norway, and probably more widely. Here much of the deposited soot is traceable back to emissions from the (former) Soviet Union, Europe, East Asia (especially China) and North America. The most likely cause of the decline appears to be the sharp economic slow-down that followed the dissolution of the Soviet Union and reduced emissions from industrial and transportation sectors. (climate.gov)
Could another “Little Ice Age” occur? Well, in fact, we are presently entering a period of reduced sunspot activity that may produce a cooling period similar to that which occurred during the LIA. Present solar activity, in fact, is at its lowest in 100 years. In response, colder winters might be expected although the cooling, if it occurs, should not be interpreted as signaling an end to the current global warming trend but rather as a short-term reversal such as has frequently occurred in the past. However, the earth and its atmosphere are very different today than during the LIA, so the climate of several centuries ago many not be a reliable model of current climate.
Some literature sources:
Painter T.L., et al (6 authors). End of Little Ice Age forced by industrial black carbon 2013 PNAS.org 10.1073/pnas.1302570110 PNAS Sept. 3, 2013)