Study may answer longstanding questions about Little Ice Age

From the press release,

The study, led by the University of Colorado Boulder with co-authors at the National Center for Atmospheric Research (NCAR) and other organizations, suggests that an unusual, 50-year-long episode of four massive tropical volcanic eruptions triggered the Little Ice Age between 1275 and 1300 A.D. The persistence of cold summers following the eruptions is best explained by a subsequent expansion of sea ice and a related weakening of Atlantic currents, according to computer simulations conducted for the study.

The study, which used analyses of patterns of dead vegetation, ice and sediment core data, and powerful computer climate models, provides new evidence in a longstanding scientific debate over the onset of the Little Ice Age. Scientists have theorized that the Little Ice Age was caused by decreased summer solar radiation, erupting volcanoes that cooled the planet by ejecting sulfates and other aerosol particles that reflected sunlight back into space, or a combination of the two.

“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.”

What it says: The Little Ice Age was initiated by a series of volcanic eruptions then ocean circulation and sea ice changes caused a positive (ie cooling) feedback that lasted for centuries.

What it means: The Little Ice Age and Medieval Warm Period are major problems for the Forcing model or theory of climate change, because the changes in radiative forcings were too small to cause either the LIA or the MWP. Something else was required. This study proposes internal climate system feedbacks (ocean circulation and sea ice) that turn short term volcanic cooling into much longer term cooling.

What I think: Coincidentally I floated a similar theory at Watts Up With That a couple of weeks ago. I argued that we know that shifts from warm to cool can occur rapidly over a few decades, the start of the Younger Dryas cold period is an example, when average temperatures fell as much as 5C in a short period of perhaps a few decades. I’m sceptical that ocean circulation changes could occur quickly enough to provide the necessary feedback and think land albedo changes are more likely, at least initially.

Consider the following scenario. A series of large volcanic eruptions occurs at mid-latitudes, for example, in the US West or Japan. Large volcanic eruptions typically erupt at a lower level for several weeks to months before ending with an ash and gas column that reaches the stratosphere. Climate science focuses on the material injected into the stratosphere which causes climate cooling, and mostly ignores the much larger amount of material injected into the troposphere. Tropospheric aerosols seed clouds and cause increased precipitation. So a large volcanic eruption causes both cooling and increased precipitation. Clearly snow fall areas and amounts will increase along mid-latitudes increasing the planet’s albedo for a period. If more eruptions occur roughly a year apart the cooling effect will increase, and start to cool the oceans causing the increased sea ice the paper proposes.

Note that the oceans lose heat to the atmosphere largely as a result of the ocean surface being warmer than the atmosphere (approximately 2C warmer on a global basis). Decrease atmospheric temperatures and heat flows from the oceans will increase due to the increased temperature difference and thus cool the oceans.

In conclusion, strong positive cooling feedbacks are needed in the climate system to produce the rapid cooling we know has occurred in the past, and stratospheric volcanic aerosol cooling combined with (I argue) increased precipitation as snow from tropospheric volcanic aerosols and then subsequent ocean cooling and increased sea ice appears a plausible mechanism. However, the existence of strong positive cooling feedbacks tells us nothing about the positive warming feedbacks the IPCC says exist.

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