As this blog winds down, this post will summarise what the blog has discussed. It began with an overall assessment of abrupt climate changes, where they have occurred in the Earth's recent history and the effects they have presented.
From that point, research on the Younger Dryas and 8,200 year abrupt climate events illustrated the effect of the thermohaline circulation (THC) - Broecker's "Great Ocean Conveyor". The mechanisms behind the THC were described in "Short circuiting the thermohaline circulation". In the same post, the Younger Dryas (YD) stadial was introduced, a period of abrupt cooling through a period of deglaciation. After much research on the possible trigger-points for the YD, I reached a general consensus that an abrupt freshening of the North Atlantic, caused by meltwater outbursts from the glacial Lake Agassiz was the likely cause. However, on reflection, there is a significant time-lag between the outbursts and the climate change suggesting that the outbursts may have weakened the THC for another factor to cause its collapse. Evidence presented in "Point to Ponder, Perhaps?" from Steffenson et al (2008) and others, showed that the YD collapsed to an extremely abrupt (years-decades) increase in temperature of c. 7 C over just 50 years.
In a natural progression, the 8200-year event was discussed in "Fusing the thermohaline circulation - the 8200-Year Event". Evidence for a huge, much greater, series of meltwater outbursts occurred in coincidence with a 400-year cooling event with its trough at c. 8,200 years BP. Despite a larger freshwater forcing than the YD, the shorter event occurred when Northern Hemisphere insolation levels were higher because of orbital forcing thus being able to recover faster than the YD.
From this point, the early Holocene was left behind to discuss the centennial-scale variability which has occurred in the last 2000 years. The Medieval Warm Period (MWP) (800-1200AD), Little Ice Age (LIA) (1200-1850) and the transition between these two events were discussed through multi-proxy research. Although the literature has not come to a consensus on the overall cause of these events, the global nature of the temperature fluctuations cannot easily be explained by a reduction to the THC. Instead, the possible transition between Holocene ~1,500 year solar insolation cycles would explain the global evidence for the MWP and LIA.
Finally, in my penult post, I brought my research to the present in an assessment of whether abrupt climate change events could occur within the 21st century. Through analysis of the USGS 2008 report on the likelihood Abrupt Climate Change. It is unlikely that a large scale THC/AMOC collapse would occur within the next century partly because there is no Lake Agassiz from which freshwater forcing could travel. However, some papers, including the study from Schwartz and Randall (2003) suggests that AMOC collapse could potentially occur causing 5-10F temperature change over just a decade. Conversely, it is perfectly feasible that sea level rise from ice-cap melting could occur on a grand-scale and zones of drought could travel northwards.
This blog has successfully detailed the abrupt climate events in the Holocene and Late Glacial. From this research, the possibility of abrupt climate events has been brought into the context of the present and indeed the near future. I would encourage readers to continue to contact me for more information about any of my research. Alternatively, papers on abrupt climate change are common in popular journals such as Science and Nature, as well as the Journal of Climate and Oceanography.
Although major changes are unlikely in the near future, there is a real possibility of tangible change within centuries. This blog, therefore, should add further weight to the need for energy sustainability and greener practices to reduce anthropogenic warming and to allow for natural rather than anthropogenic climate variability.
