Wednesday, December 6, 2017 The Pleistocene Glaciations, Continued (Chapter 14) Reminders: Week 14 Assessment closes tonight Watch for Week 15 Assessment (will close Wednesday, Dec. 13) Homework 5 due today Exam 4 (our final exam) Sunday, December 17, 7:30-10:00 pm
Chapter 14 Glacial Climate Feedbacks (page 281-288) CO 2 /biological pump (p 281) Shelf-nutrient (p 281-282) Iron fertilization (p 282-283) Coral reef (p 283-285) Cloud/albedo (see text, p 285-287)
Last week we saw that the ice ages (at least for the last million years or so) have occurred at times when Earth s orbit favored the build up of ice in the northern hemisphere We noted that the climate signal at 100,000 years (the eccentricity mode) was far larger than expected based on changes in sunlight. We already know that CO 2 abundances and temperatures have correlated strongly during the current ice age epoch (the Pleistocene). How does CO 2 vary with temperature?
Shelf Nutrient Hypothesis Mechanism - Sea levels rise during a warm interglacial period, trapping nutrients underwater on the continental shelf. Photosynthesis is reduced, and less CO 2 is removed from the atmosphere. Figure 14-11
Shelf Nutrient Hypothesis When sea levels fall (as ice builds up again), nutrients in the continental shelves are exposed, and photosynthesis increases, once again removing CO 2 from the atmosphere Figure 14-11
The biological Pump recall that nutrients come from both land (river runoff) and sea (upwelling) Figure 14-10
Shelf Nutrient Hypothesis Overall positive Figure 14-13
Shelf Nutrient Hypothesis Nutrients in ocean balance between input by river and loss by sedimentation of organic matter If CO 2 in atmosphere was lower during a glaciation, either input from rivers was enhanced, or there was less loss by sedimentation Glaciers sea level weathering phosphate CO 2 Problem in seawater, abundances of another element (Cd = cadmium) track those of phosphate, but measurements of Cd in fossil shells show that it wasn t higher during glacials implies that phosphate wasn t higher either!
Iron Fertilization Hypothesis Iron abundances limit primary productivity (new growth) in certain regions of the ocean (where the other limiting nutrients are plentiful). Iron is supplied by wind-blown dust (Sarahan, Gobi deserts wait until next week or the week after!) Lower CO 2 during glaciations would imply more wind-blown dust, so either dryer climate or greater average winds. Surface temp temp gradient winds iron CO 2 Evidence supports this feedback windblown dust in ocean sediments is higher during glacials
Iron Fertilization Hypothesis Positive feedback Figure 14-15
Coral Reef Hypothesis Note the hard piece of this feedback cycle to remember is that when coral reefs grow, they increase the ocean acidity, which increases the diffusion of dissolved CO 2 back to the atmosphere see page 165. Positive feedback (if temperatures go down, glacial ice volume goes up)
Coral Reef Chemistry Although coral is made of carbonate, it uses bicarbonate ion in the ocean as the building block of its calcium carbonate skeleton. The assimilation of bicarbonate ion increases the acidity, thereby driving some bicarbonate back to dissolved CO 2 (H 2 CO 3 ) and increasing the release of CO 2 to the atmosphere. Coral forms ocean releases CO 2 Ca 2+ + 2HCO 3 - CaCO 3 + CO 2 + H 2 O Alternatively, recall that when CO 2 dissolves in the ocean, calcium carbonate (e.g., coral) dissolves: CaCO 3 + CO 2 + H 2 O Ca 2+ + 2HCO 3 - Coral dissolves ocean takes up CO 2
Coral Reef Hypothesis Therefore, when sea levels rise and coral reefs grow thicker (corals are formed by photosynthesis at the surface of the ocean), formation of the new coral draws calcium and bicarbonate out of solution, making calcium carbonate, releasing CO 2 to the atmosphere. Ca 2+ + 2HCO 3 - CaCO 3 + CO 2 + H 2 Problem explaining ice age cycles coral growth can keep up with changes in sea level (i.e., release of CO 2 in a warming climate), but dissolution of limestone is slow, so if this occurs, the process may be unbalanced CO 2 release may be faster than CO 2 uptake (we call this hysteresis when the rate in one direction is faster than in the other direction)
Cloud condensation nuclei, formed by gases produced by marine plants, are higher during cold periods than warm ones Increased nutrients to ocean during cold periods increases growth of algae which promote additional cloudiness, which further cools the earth. Methane sulfonic acid is a proxy for clouds Figure 14-18
One negative feedback: Changes in terrestrial biomass During interglacials (warm periods) compared to glacials, there is fractionally less CO 2 in the atmosphere relative to the biosphere. This acts as a negative feedback on climate. Figure 14-16
One negative feedback: Changes in terrestrial biomass Warm periods Smaller ice sheet, more land for plants and trees Less arid rainforests are larger, more biomass in tropics. Both result in additional uptake of CO 2 from the atmosphere Tropical rainforests expand during warm periods Figure 14-16
New ice-core evidence indicates that changes between glacials and interglacials are not gradual, but rather occur abruptly, shifting between two very different states except recently! Box Figure 15-1
What do the high-resolution ice core records tell us about climate stability? Hint Glacial periods tend to be unstable (relatively warm periods mixed in with extremely cold periods) (p 288-290) Figure 14-9
One of the other recent discoveries from ice core data taken in Greenland is that past climate changes have been very abrupt sometimes occurring within a few years. The Younger Dryas (p 288-291) The Younger Dryas saw a rapid return to glacial conditions in the higher latitudes of the Northern Hemisphere between 12,900 11,500 years before present (BP) in sharp contrast to the warming of the preceding interstadial deglaciation. The transitions each occurred over a period of a decade or so.thermally fractionated nitrogen and argon isotope data from Greenland ice core GISP2 indicates that the summit of Greenland was ~15 C colder than today during the Younger Dryas. In the UK, coleopteran (fossil beetle) evidence suggests mean annual temperature dropped to approximately -5 C, and periglacial conditions prevailed in lowland areas, while icefields and glaciers formed in upland areas. Nothing of the size, extent, or rapidity of this period of abrupt climate change has been experienced since.
Climate changes can be abrupt! The Younger Dryas (named after the Dryas flower that is found in arctic and alpine tundra) evidence suggests that this switch is due to ocean circulation shutting down of the thermo-haline circulation Accumulation of snow (similar record in dust) Warm to cold in just a few years! Figure 15-2
Chapter 15 Short term climate variability How can we document past climate change? What causes past climate change? What factors may cause change on decadal timescales?
Short-term Climate Variability and Global Warming Important material to read The temperature record over the past ~10, 100 and 1000 years (Chapter 1, and Chapter 15, p 295-297) Proxy Climate Data (p 296) Bore Hole records (not in textbook) Abrupt climate change (p 288-289) The Holocene Climatic Optimum (p 297-298)
The Holocene Let s look at where we are today (and for the past ~20,000 years or so The Holocene ) To observe a Holocene environment, simply look around you! The Holocene is the name given to the last ~10,000 years of the Earth's history -- the time since the end of the last major glacial epoch, or "ice age." Since then, there have been smallscale climate shifts -- notably the "Little Ice Age" between about 1200 and 1700 A.D. -- but in general, the Holocene has been a relatively warm period in between ice ages.
Short-term Climate Variability The temperature record over the past ~10, 100 and 1000 years (p 295-299) Factors that cause change on decadal timescales Volcanoes (p 299-300) Solar contributions (p 300-301) Present day variability (p 301-303) The inexorable rise in mean global temperatures due to anthropogenic greenhouse gases
Evidence Oxygen isotopes Pollen and alpine glaciers Historical documents Figure 15-1a,b,c
What are climate proxies? (p 296) Palynology is the science that studies contemporary and fossil palynomorphs, including pollen, spores, dinoflagellate cysts, acritarchs, chitinozoans and scolecodonts, together with particulate organic matter (POM) and kerogen found in sedimentary rocks and sediments.
Dendrochronology - Dendrochronology or tree-ring dating is the method of scientific dating based on the analysis of tree-ring growth patterns.
The Holocene To observe a Holocene environment, simply look around you! The Holocene is the name given to the last ~10,000 years of the Earth's history -- the time since the end of the last major glacial epoch, or "ice age." Since then, there have been smallscale climate shifts -- notably the "Little Ice Age" between about 1200 and 1700 A.D. -- but in general, the Holocene has been a relatively warm period in between ice ages.
Small temperature changes can result in large local or regional changes in climate Vikings in Greenland
What about volcanic eruptions? Produce SO 2 aerosols Increase albedo Average of five eruptions
Solar variability is more complicated to understand Changes in solar brightness are too small (less than a fraction of one percent) to explain climate changes that are observed (and certainly don t explain the Pleistocene ice ages, which are driven by the greenhouse effect (CO 2 and methane), amplified by the ice albedo feedback. It is thought that changes in the magnetic field of the sun change the number of cosmic rays that can enter the atmosphere, and these can be a source of condensation nuclei for clouds so solar variations, like sunspots, can affect cloudiness in a highly non-linear (but still very small) way.
Solar variability is more complicated to understand Changes in solar brightness are too small (less than a fraction of one percent) to explain climate changes that are observed (and certainly don t explain the Pleistocene ice ages, which are driven by the greenhouse effect (CO 2 and methane), amplified by the ice albedo feedback. It is thought that changes in the magnetic field of the sun change the number of cosmic rays that can enter the atmosphere, and these can be a source of condensation nuclei for clouds so solar variations, like sunspots, can affect cloudiness in a highly non-linear (but still very small) way.