#2. Climatologists are not interested in average climate conditions because climate varies continuously. Instead, climatologists look for trends in the climate conditions.
#3. Proxy data is data that cannot be obtained by direct measurement but can be inferred from other evidence. Proxy data include climate data derived from pollen, microfossils, and tree rings. Proxy data of past temperatures and glaciation events may also be obtained from the composition, structure, and pattern of coastal and oceanic sediments. Atmospheric and climate reconstructions derived from ice cores are proxy data. Note palynology (the study of pollen and organic microfossils) and dendrochronology (study of tree rings) are both methods of climate reconstruction using proxy data but are not consider proxy data.
#4. The last glaciation ended around 15,000 years ago, as the earth started warming (which marks the beginning of the Holocene). The Younger Dryas, a rapid climatic reversal to this warming trend, occurred 12,500 years ago resulting in a temporary cold period that lasted until 11,600 (See Figure 12-2 and Figure 12-1 (b)). Evidence of climate change at this time is seen in various parts of the world but the main effects of the Younger Dryas event appear to be centered in the North Atlantic region. The Younger Dryas ended abruptly marking the beginning of The Holocene Climatic Optimum which lasted for several thousand years and reached a peak (for the entire Holocene) 5000-6000 years ago (Figure 12-1 (b)). This period was slightly warmer than today. Temperatures fell after the Holocene Climatic Optimum, reaching a minimum about 3000 years ago. Temperatures rose and then declined again. The European Medieval Warm Period then occurred with relatively warm temperatures from 400AD to 1290 AD (Figure 12-1 (c)) It was warm enough in this period to support a large community of Vikings and even their farms on the southwest coast of Greenland. The Little Ice Age occurred not long after this. This interval of colder temperatures from the 15th to 19th centuries, interrupted by a warmer interval in the 17th century is shown on Figure 12-1 C.
#5. Toward the end of the last glaciation, temperatures rose and the glaciers began to melt. Much of the melt water over North America flowed southward forming what is now the Mississippi River Basin. It has been suggested that a large portion of the glacier remained in the southern section blocking the normal southern flow. This caused a large amount of meltwater to be diverted eastward through the Gulf of St. Lawrence (Great Lakes region and St. Lawrence waterway today) to the North Atlantic. Because fresh water is less dense than oceanic saline water, the meltwater would have floated on the surface. Because fresh water freezes at higher temperatures (0 C) than salt water (-4C), it would have formed a large ice mass rather easily. This would have extended the northern ice cap and thus prevented deep water from forming where it normally would have. [Recall the formation of deep water: As oceanic saline waters freeze, the salts are excluded from the ice which causes the surrounding water to increase in density and sink resulting in thermohaline circulation and deep water formation.] Once NADW quit forming, the Gulf Stream and the North Atlantic Drift would have been disrupted changing climate conditions for this entire region.
#6. Volcanoes affect climate by releasing gasses to the atmosphere. (Flyash and particles impact weather conditions but not climate because they have a very short residence time in the atmosphere). The sulfur dioxide injected into the stratosphere by an eruption oxidizes, forming sulfuric acid droplets (sulfate aerosols). The aerosol scatters and reflects solar radiation reducing the radiation reaching the surface of the earth. The aerosol also absorbs some of the long-wave radiation emitted from the troposphere. The residence time of the aerosol is only about a year over which time it decreases, so the climate effect of lower-than-normal surface air temperatures last only 1-2 years after an volcanic explosion. The combined effect of an increased albedo and increased long-wave absorption is to heat the stratosphere and cool the troposphere. The degree to which it will affect climate depends on location of the eruption, extent that the aerosol is transported, and the amount of gases that are emitted.
Also remember that volcanoes emit CO2 and H2O to atmosphere. These are both greenhouse gasses and may have a slight warming effect in the longer term.
#11. There are three major feedback processes between sea ice and climate.
Sea Ice – Albedo Positive feedback (effect may be seasonal for certain regions)
Ice Cover (Increase) -> Albedo (Increase) -o Temp. (Decrease) –o Ice Cover (Increase)
Sea Ice – Heat Flux Positive feedback
Ice Cover (In) –o Heat flux ocean to atm (De) -> Temp (De) –o Ice Cover (Increase)
Sea Ice – Thermohaline circulation
This relationship is not as easy to diagram because it is more complicated.
However, the build of sea ice may prevent deep water formation and thus slow or stop thermohaline circulation.