Holocene
Climate The warming out of the last ice age was interupted by a return to very cold conditions for about 3,000 years. This is known as the Younger Dryas. Both temperature and accumulation on Greenland show clear evidence of the glacial re-advance. The YD signature is largest in coastal areas around the northern North Atalntic, although there is evidence of modest cooling and/or drying elsewhere too, see figure from National Academy report on Abrupt Climate Change. Check out the same link starting with page 24 for a compendium of evidence for the YD. One of the popular theories to explain the YD derives from the expectation that massive flooding events occurred as the ice sheet on North America deteriorated, dumping large doses of freshwater into the northern North Atlantic and lowering the surface salinity quite suddenly. This is thought to cause the thermohaline circulation to weaken or shut down, which reduces nothward heat transport and cools the high northern latitudes. Generally climate variability during the Holocene has been small compared to glacial times. Some scientists think this is because the thermohaline circulation is more stable during interglacial times. I'm not so convinced that the stability of the thermohaline circulation is the entire story. My money is on the role of sea ice, which may experience greater fluctuations during an ice age (models show this to be true). Perhaps sea ice fluctuations are driven by the thermohaline circulation or maybe winds. No one knows yet. This is an active area of research, so stay tuned. Back to the holocene variability though. The main sources of interglacial variability are
We do a good job of measuring the climatic influence of volcanoes. Satellites directly measure the change in earth's radiative budget and thermometers measure the temperature response. Scientists understand how volcanoes influence climate. This gives us a good opportunity to test the sensitivity of climate models, and models do a decent job of reproducing their behavior. The sunspot story is murkier. It is not immediately clear how to translate sunspot frequency into changes in the solar constant (radiative forcing). Furthermore we don't have direct evidence of their history for very long. People have observed them with telescopes since Galileo. All other records are based on proxies via auroras and isotopes of carbon and berilium captured in ice cores. Satellite measurements from the past few decades are accurate enough to give a relationship between sunspot frequency and radiative forcing. If we use this relationship to infer the radiative forcing for the longer records of sunspot frequency, we find that although sunspot frequency did increase nearly steadily during the 20th century, the radiative forcing is too small to explain the temperature trend. Some of the major climate anomalies (departures from mean) during the holocene are
Abrupt
Climate Change
Ice cores from Greenland tell us that the last ice age was punctuated by abrupt warming events.The end of the YD is the most recent event. The warmings were about 10 deg C and occurred with some regularity in 1500 yr intervals. The definition of an abrupt event from the National Academy report on Abrupt Climate Change is when a small unknown forcing causes a large abrupt response, hence a threshold-like mechanism. The figure on page 12 of the report shows the cute teeter-totter analogy that I showed in class. The study of abrupt climate change is in its infancy. Researchers are actively looking for a big positive feedback with a stronger amplitude during the ice age (I'm looking into the possible role of sea ice). The text book says the variability during the ice age is dominated by 1500 yr cycles, but this is simply not true. The 1500 year cycle is significant, but the ice ages had large variability on many timescales and most variability is not abrupt! Scientists have some ideas for how to explain abrupt climate change, but most of us admit we are stumped by the 1500 yr cycle. We know of other abrupt phenomena (volcanoes for one), but cycles are rare in climate. Other than the daily, annual, and orbital cycles - all of which come from astronomical sources - crisply repeating cycles are hard to imagine resulting from our messy complicated climate system. The text book says that stochastic resonants is an explanation that is growing in popularity. The idea is that the climate system chooses to amplify one prefered frequency from a whole host of noisy perturbations. Something like how bells resonate when struck. This idea flies in the face of my statement about the climate being messy. It seems too far fetched to me. We have independent data of abrupt events in ocean sediments where thick layers of rocks accumulated about 7 times during the ice age. These Heinrich layers are thought to derive from sudden surges in ice sheet on land that released a floatilla of icebergs, which carry coarse-grained sediments across the North Atlantic. These layers lend some support to the possibility that large doses of freshwater may have weakened the thermohaline circulation. However, theses layers rarely coincide with evidence for weakened thermohaline circulation and also are not cleanly related to any climatic change. |
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