In Heinrich events, glacier-bergs carried sediment and fresh water out to sea. In melting, the bergs left detritus on the ocean floor and the fresh water altered the thermohaline circulation for thousands of years, knocking down global temperature by as much as 2C on average. The two most recent Heinrich events were 14,000 ybp and 8,200 ybp. As global warming proceeds, people watch Greenland and Antarctica and wonder if it could happen again.
NOAA Paleoclimatology Program page on Heinrich and Dansgaard-Oeschger
events:
http://www.ncdc.noaa.gov/paleo/abrupt/data3.html
The 2C estimate is from Elsa Cortijo et al (1997) "Changes in sea surface hydrology associated with
Heinrich event 4 in the North Atlantic Ocean between 40° and 60°N" http://www.sciencedirect.com/science/article/pii/S0012821X96002178
A blog post from Spike this weekend at Climate Progress quotes a sciencedaily on ice saddle melt that is associated with abrupt shifts in thermohaline circulation. Backtracking to the original Grigoire et al. abstract, the authors estimated saddle-melt between domes of continental glaciers was in the order of "nine metres of sea level rise over 500 years."
http://www.ncbi.nlm.nih.gov/pubmed/22785319
From the abstract we might infer that the authors only estimated added ocean volume due to continental surface melt, without further addition of volume to include ocean thermal expansion due to the natural warming at that time. But they might have. In our present circumstances, the ice saddles of both Antarctica and Greenland should be of interest.
In my observation, increased climate instability is knocking down agriculture long before a Heinrich event could be expected to kick the earth's temperature back down a few degrees.
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