CHAPTER 12
Fast vs. Slow Response Times
- With insolation as the driving force, air temperature follows a fast response time curve while the size of ice sheets follows a slow response time curve. Fig. 12-1A.
- Air temperature also follows a fast-response time curve when ice sheets are the forcing mechanism. Fig. 12-1B. Therefore, air temperature climate changes will closely follow ice sheet size changes and not insolation changes.
Ice-Driven Climate Responses
- Ice sheets influence the climate system due to:
- Height: Ice can protrude thousands of meters into the air which may be an obstacle to wind circulation in the lower atmosphere
- High Albedo: Ice is very reflective so insolation is much less than received on bare ground
- Calving: Icebergs can be calved which delivers fresh water to nearby oceans
- Ice-driven climate responses should therefore follow the same orbital cycles as ice sheets: 41,000 year cycles before .9 Myr ago and 100,000 year cycles since that time. They should also track the ice sheet cycles without an obvious lag. Fig. 12-2.
- The North Atlantic Ocean is almost entirely surrounded by ice so sea surface temperature (SST) changes should closely match changes in the size of ice sheets. SSTs should be cool when ice sheets were larger and SSTs should be warmer when ice sheets were smaller.
- Fig. 12-4 shows that between 1.2 Myr and 1.5 Myr ago, SSTs closely followed the changes in ice sheets along a 41,000 year cycle. After .9 Myr, the size of ice sheets and the SSTs follow the expected 100,000 year cycle. (No figure.)
- There are two ways that ice sheets can transfer their climate signal to the ocean:
- Icebergs calve off and melt which cools SSTs.
- Fig. 12-5. Large ice sheets can cause a clockwise air circulation over their centers which transports colder air southward over the nearby ocean.
- Fig. 12-6 also shows the relationship between climate and ice sheet volume using pollen type. Cold, dry conditions would yield more herbs and grasses while warmer conditions would yield more trees.
- At present, Europe has a moist, temperate climate which is mainly due to a warm, moist southwest flow off the Atlantic. Climate models have shown that a large cooling of the North Atlantic Ocean would result in a dramatically cooler climate over Europe. Fig. 12-7. Therefore, changes in the large ice sheets over North America had a direct influence on climate changes in Europe.
- Farther east there is evidence for climate change as a result of ice-driven North Atlantic Ocean changes.
- Fig. 12-8 shows that tracking loess vs. clay type soils can indicate climate changes. Loess is common in cool, dry climates while clay-rich soils are typical of warm, moist climates. Peaks in loess closely follow peaks in ice sheet size.
- Surprisingly, the changes in northern hemisphere ice sheets can be seen in the Arabian Desert (Fig. 12-9), South America (Fig. 12-10), New Zealand (12-11), and the Southern Indian Ocean (Fig. 12-12). Therefore, except for regional summer tropical monsoons, the northern ice sheet cycle shows up everywhere else. How is this possible?
- Changes in sea levels occur when ice sheets change volume. These changes can cause major climate shifts in coastal areas due to alternating periods of flooded vs. dry regions.
- Changes in deep-water circulation can influence climate in the southern oceans. Deep water formation is decreased during glaciations (Fig. 11-20.)
- However, changing sea levels and changing deep water production cannot explain how the climate signals from high elevation interior locations such as the eastern Andes of Colombia show a close relationship with ice sheet volume (Fig. 12-10).
- CO2 levels also closely follow ice sheet volume and can easily influence all parts of Earth's climate.
CO2 Level & Ice Volume: Which Drives Which?
- Fig. 12-15 shows that CO2 levels and ice sheet volume closely follow orbital-scale cycles. Ice sheets must be driving CO2 levels because ice sheet size is a slow response time phenomenon. If CO2 were driving ice sheet size, there would be a time lag which is not seen in Fig. 12-15.
- Even though ice sheets are the driving force, CO2 levels provide a positive feedback effect on ice volume. Growing ice sheets causes decreased CO2 levels which causes cooler climates. Cooler climates encourage more growth of ice sheets. Conversely, if ice sheets are shrinking, more CO2 would cause warmer climates which then causes more melting of ice sheets.
- Therefore, insolation drives the ice sheets, the ice sheets largely determine CO2 levels, and the CO2 then amplifies the size of the ice sheet change.
The Mystery of the 100,000 Year Cycle
- There are two major trends in the long-term δ18O records. More ice accumulated after .9 Myr ago and these larger ice sheets melted rapidly every 100,000 years.
- Fig. 12-16 shows a gradual tectonic-scale cooling of climate in the last 4.5 Myr. During much of that time, ice sheets would grow and melt along the 41,000 and 23,000 year cycles.
- However, as the cooling continued, ice might have been able to "survive" weaker insolation maximum periods. This ice would then be a "seed" to grow larger ice sheets during insolation minima. See the large glaciation phase in Fig. 10-14C.
- Fig. 12-17 shows how ice slipping may influence ice sheet size. Softer soils underneath ice can cause ice sheets to slide more south toward warmer latitudes. As the soil underneath was slowly eroded over time, much harder bedrock would have inhibited sliding and caused less ice ablation.
- Tectonic-scale cooling and ice slipping can explain why there was more ice accumulation after .9 Myr ago, but why the 100,000 year cycle?
- Fig. 12-18 shows that the 23,000 year precession cycle is amplified every 100,000 year due to the Milankovitch Cycle (orbital eccentricity.) Rapid deglaciations occur during the FIRST high-insolation peak or each cluster of two or three peaks.
- There is still much debate about why the first peak would cause the rapid deglaciation of large ice sheets but 100,000 year cycle mystery has been solved. Before ice sheets reached a critical size (before .9 Myr ago,) their size closely followed the 41,000 year cycle. Since .9 Myr ago, the gradual tectonic-scale cooling of earth's climate allowed much larger ice sheets to form which were not controlled by the 41,000 year cycle. Instead the Milankovitch Cycle amplified the maximum solar insolation on a 100,000 year cycle. This amplified warm period was enough to completely melt ice sheets.
Helpful Links:
Geology 150 - Milankovitch Theory and Ice Sheets
Glaciation
Ice Sheets Play Important Role in Climate Change
Special Online Collection: Climate Change -- Breaking the Ice
Ice Sheets Drive Atmospheric Carbon Dioxide Levels, Inverting Previous Ice-age Theory
