(image from Schwarzschild 2012)
UPDATE April 13 2012: Anthony Watts alerted us to the guest post at WUWT by Willis Eschenbach titled Dronning Maud Meets the Little Ice Age. Willis raises quite very substantive issues with the paper. I urge him to submit his analysis for peer review.
Every once in a while. a nugget of new research insight appears that adds to our understanding of the climate system, and its complexity. One article of this type has appeared
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
with the abstract [highlight added]
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.
The Kep Points listed in the Miller et al 2012 paper are
- Little Ice Age began abruptly in two steps
- Decadally paced explosive volcanism can explain the onset
- A sea-ice/ocean feedback can sustain the abrupt cooling
The Miller et al article is summarized in
Schwarzschild, Bertram M., 2012:The triggering and persistence of the Little Ice Age. Physics Today. April 2012. page 15 http://dx.doi.org/10.1063/PT.3.1506
The abstract of the Physics Today article reads
“A mere half century of volcanism seems to have initiated a chill lasting half a millennium”.
Extracts from the article are
For more than 500 years until the middle of the 19th century, much of the Northern Hemisphere experienced the “Little Ice Age,” the most extended period of anomalous cold—winter and summer—in 8000 years. Picturesque aspects of the LIA are familiar from paintings of winter scenes in northern Europe. But more somber manifestations include numerous famines in Europe and Asia and the extinction of the Norse settlements in southern Greenland.
The LIA’s start and finish dates, as well as its cause, have long been subjects of debate and puzzlement. Variations in solar irradiation and volcanic eruptions have been invoked as possible causes. But the one seems too weak and the other too ephemeral.
Unlikely or not, four major tropical eruptions in the late 13th century are identified as the LIA’s triggering mechanism in a recent paper, by Gifford Miller (University of Colorado at Boulder) and coworkers in the US and Iceland. They present precise new carbon-14 dating results and a model simulation of reinforcing feedbacks to pinpoint the LIA’s abrupt onset and understand its duration.
An important manifestation of the LIA was the expansion of glaciers and year-round icecaps at high latitudes and elevations. Expansion chronology is often measured by 14C dating of biological debris swept into glacial moraines. But much of the vegetation in those dumping grounds of glacial scouring was killed decades or centuries after the initial shift to colder summers and the consequent advance of perennial ice cover. So Miller and company chose to concentrate on small, localized icecaps that would have reacted much faster and preserved rooted vegetation precisely where it was killed by the expanding ice.
At several dozen such highland sites across Baffin Island in the Canadian Arctic where the surface has only now been exposed after centuries of icy entombment, the team obtained precise 14C kill dates for more than 100 clusters of freshly exposed moss (see figure 1). Recalibrating raw 14C ages for known temporal variations of the atmosphere’s 14C concentration, the team was able to reveal and date prominent kill-rate peaks with almost decadal resolution…
The team concludes that the peak near 1300 AD marks the LIA’s sudden onset, and that it was triggered by the four major volcanic explosions in the previous half century, shown in figure 2b. The figure estimates the global stratospheric load of sulfate aerosol based on dateable sulfate concentrations in Arctic and Antarctic ice cores. Though each eruption would have produced just a few cold summers before its aerosol precipitated out, the millennium’s greatest eruption, followed in quick succession by three more, seems to have initiated the very long chill…”
Long stretches with low kill rates on Baffin Island—for example, the three centuries preceding the 1300 peak—might reflect either a warm period with receding perennial ice or unrelenting cold with continuous maximal ice cover. To resolve such ambiguities, the team dated and measured thicknesses of annual sediment layers under a glacier-fed lake in central Iceland. The more massive the icecap that drives the glacier, the greater (on average) is the thickness of the debris layer it deposits each summer. In that way the team was able to conclude that the pre-1300 trough was indeed a warm spell and that the falloff after the great kill peak around 1450 marks the onset of continuous maximal ice coverage through the end of the 19th century. The 1450 peak is thought to be associated with the well-documented 1452 Kuwae eruption on an island east of New Guinea.
”The problem with volcanic explanations for the LIA has always been that the sulfate aerosol is gone after three years,“ says Miller. “But the sea-ice data have impelled us to undertake a model simulation to see if decadal volcanic triggering, reinforced by interaction between expanding sea ice and ocean currents, might do the trick.”
If eruptions recur faster than surface water temperatures can recover, the cumulative ocean cooling could be much greater than from any single eruption. In the Arctic Ocean, the anomalously cold surface water might inhibit summer ice melt enough to cause an extended southward expansion of sea ice into the Atlantic.
”Model simulation is always tenuous,” cautions Miller, “but this one does suggest a plausible mechanism for the centuries-long persistence of a decade-scale perturbation. I’m particularly pleased that the empirical evidence for the sudden accumulation of ice off Iceland’s north coast around 1300 supports the simulation’s massive export of arctic ice into the North Atlantic and the consequent disruption of warming ocean circulation.”
This study provides evidence that the climate system responds to perturbation (in this case from volcanic eruptions) that is quite nonlinear as a result of atmospheric-ocean-land interactions. It also illustrates a challenge to skillful modeling on decadal and longer time scales as such volcanic eruptions cannot be predicted.
There is also an interesting observation in figure 2 of the Schwarzschild 2012 article. I reproduced this figure 2 at the begining of this weblog post. In this figure, cool temperature anomalies are higher on the left hand axis. It clearly shows the little ice age effect that is discussed in the Schwarzschild and Miller et al articles (with the coldest actually ~1850-1900!). However it also shows that the warmest period was in the mid 20th century and it has actually cooled since then.