Wednesday, February 1, 2012

The Little Ice Age Was More Than A Solar Experience

The term Little Ice Age (LIA) was coined by François E. Matthes in 1939 in a paper published by the American Geophysical Union.  It described a period of unusually cool temperatures roughly from the 16th to the 19th century.

Ten reconstructions of temperature variations during the last 2000 years. The Medieval Warm Period and Little Ice Age are shown at the period when they are proposed to have occured.  Image credit:  Robert A. Rohde / Global Warming Art

The start and end of the period has been debated.  Wikipedia has a number of start dates culled from several published papers:

"Any of several dates ranging over 400 years may indicate the beginning of the Little Ice Age:
  • 1250 for when Atlantic pack ice began to grow
  • 1275 to 1300 based on radiocarbon dating of plants killed by glaciation
  • 1300 for when warm summers stopped being dependable in Northern Europe
  • 1315 for the rains and Great Famine of 1315–1317
  • 1550 for theorized beginning of worldwide glacial expansion
  • 1650 for the first climatic minimum.
The Little Ice Age ended in the latter half of the 19th century or early in the 20th century."

A number of causes have been proposed for the Little Ice Age.  The most frequently used are the sunspot minimum, increased volcanism, and changes in ocean circulation.  Some have used the sunspot minimum as an example of the sun's powerful influence over climate change in order to argue against anthropogenic global warming.  However, the changes in the solar irradiance do not fully explain the cooling then nor the warming now.

Now a paper has been published that examines the time period in detail.  The paper was published this week in the AGU journal Geophysical Research Letters.  A subscription is required to see the entire paper, but here is the abstract:

Northern Hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes. However, the causes of superposed century-scale cold summer anomalies, of which the Little Ice Age (LIA) is the most extreme, remain debated, largely because the natural forcings are either weak or, in the case of volcanism, short lived. Here we present precisely dated records of ice-cap growth from Arctic Canada and Iceland showing that LIA summer cold and ice growth began abruptly between 1275 and 1300 AD, followed by a substantial intensification 1430–1455 AD. Intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium. A transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times, and that cold summers can be maintained by sea-ice/ocean feedbacks long after volcanic aerosols are removed. Our results suggest that the onset of the LIA can be linked to an unusual 50-year-long episode with four large sulfur-rich explosive eruptions, each with global sulfate loading >60 Tg. The persistence of cold summers is best explained by consequent sea-ice/ocean feedbacks during a hemispheric summer insolation minimum; large changes in solar irradiance are not required.

This is one of the most comprehensive studies I have seen on the subject.  The team of international scientists have built a compelling case to argue in favor of increased volcanism combined with sea-ice/ocean feedbacks.  It does not mean that the sun did not play a role in the LIA, but it does not appear to have been a main cause.

"This is the first time anyone has clearly identified the specific onset of the cold times marking the start of the Little Ice Age," says lead author Gifford Miller of the University of Colorado Boulder. "We also have provided an understandable climate feedback system that explains how this cold period could be sustained for a long period of time. If the climate system is hit again and again by cold conditions over a relatively short period-in this case, from volcanic eruptions-there appears to be a cumulative cooling effect."

"Our simulations showed that the volcanic eruptions may have had a profound cooling effect," says NCAR scientist Bette Otto-Bliesner, a co-author of the study. "The eruptions could have triggered a chain reaction, affecting sea ice and ocean currents in a way that lowered temperatures for centuries."

See the corresponding article on Climate Matters.

Citation: Miller, G. H., et al. (2012), Abrupt onset of the Little Ice Age triggered by volcanism and sustained by sea-ice/ocean feedbacks, Geophys. Res. Lett., 39, L02708, doi:10.1029/2011GL050168.