ESS Glaciers and Global Change

Transcription

1 ESS Glaciers and Global Change Wednesday February 14, Outline for today Volunteer for today s highlights on Friday Highlights of last Monday s class Gabe Greene Ice cores and climate history

2 Assignment for Friday Feb 16 By Friday, turn in a 1-page report from your group including A paragraph outlining of the topic for your group paper. Reference citations to your popular press source or sources. Reference citations for at least 2 reliable sources that you have identified (e.g. peer-reviewed articles or government reports) that served as sources, or that cover the same topic closely.

3 Copenhagen Climate Summit 2009

4 Where are we now? We know what a glacier is We know how scientists communicate results Now we will explore how glaciers and ice sheets tell us about past climate Environmental clues embedded in ice (ice cores) Historical changes in the size of glaciers

5 ??? Crew processing an Antarctic ice core at NSF Ice Core Facility (NSF-ICF) in Denver T.J. Spruce Sean 1751-m core

6 SPICEcore drill operation

7 NSF Ice Core Facility - Denver a facility for storing, curating, and studying meteoric ice cores recovered from the glaciated regions of the world. NSF-ICF holds over 17,000 meters of ice core from various locations in Antarctica, Greenland, and North America.

8 Ice Cores as Paleo-Weather Stations What features of today s weather might you want to know about in a weather report? Temperature Precipitation Wind Pollution or air quality Others? What features of past weather and climate might you want to know about?

9 We can t measure the past climate as directly as today s weather Temperature of the ice today is not the same as the temperature of the snow when it fell long ago. Snow can warm up or cool down as it is buried and converted to ice (can t just stick a thermometer on an ice core, to measure temperature in the ancient past J). The direct memory of past air temperature has been overwritten by more recent air temperatures, i.e. the surface of the world knows only its present (or very recent) temperature. How can we learn what the temperature was back then?

10 We have to use proxies. What is a "proxy"? Proxies A measurable quantity that is used as a substitute for another (perhaps unmeasurable) quantity. (How does this relate to a Microsoft Inc. shareholders Annual Meeting?)

11 A Proxy for Past Temperature If we want to know what the temperature was in the past: We need to identify "something" that collected on the polar ice sheets in the past. Some characteristic about the "thing" deposited had to be different when it was deposited at different temperatures (so that we have a "handle" to go back and deduce what the temperature was back then). Obviously the characteristic that we are interested in should always have the same value for the same temperature.

12 Groups of Curious Scientists seek a "Proxy" Temperature Record Suppose that in the past, somebody threw a handful of colored beads onto a polar ice sheet every day. On warm days, the beads were red. On cold days, the beads were blue. On average days the beads were white. Many years later, we drill through the ice sheet, to discover the temperature history here. What is the "thing" that we collect as a temperature proxy? What is the characteristic of the "thing" that we want to convert to a temperature record? Suppose bead throwers were fickle, and changed their minds day by day, on what temperature constituted a "cold" day. Maybe one day they thought 30 o C was cold, but the next day they thought -30 o C was "average", and -50 o C was cold. Would you still like this proxy record?

13 Composition of Atoms Remember that the nucleus of any atom is made up of protons (which have a + charge), and neutrons (which have no charge). The nucleus is surrounded by a cloud of electrons (with - charge) in orbits specified by rules from quantum mechanics (which will not be on the next test!). The number of electrons is equal to the number of protons in the nucleus, in order to preserve the charge balance.

14 The Periodic Table The number of electrons in an atom determines which element it is in the periodic table. Why is that? An atom uses its electrons to form chemical bonds with other atoms. Its neutrons don t matter (much).

15 Stable isotopes of Oxygen OXYGEN atoms in the natural environment come in 2 forms, or "isotopes". This is also true for oxygen in water molecules (H 2 O). The 2 isotopes, oxygen-18 ( 18 O) and oxygen-16 ( 16 O) are both stable atoms (i.e. they not radioactive). The only difference is that 18 O has 2 extra neutrons in its nucleus, making each 18 O atom a little bit heavier than an 16 O atom. Approximately 1 in every 500 oxygen atoms is an 18 O atom. There are also a few oxygen molecules with 9 neutrons, but oxygen-17 ( 17 O) is very rare.

16 Stable isotopes of Oxygen 16 O and 18 O

17 Stable Isotopes and Environmental Research An H 2 O molecule can contain either 18 O or 16 O. In the 1950 s scientists knew that: When it rains, the molecules with 18 O tend to fall out preferentially in the rain. They asked the questions: Where does rain water come from? Can meteorologists track water back to its source using 18 O?

18 Willi Dansgaard s Mystery A young Danish physicist was interested in isotopes as a tracer in rainwater in the 1950 s. Here is one of his early, very sophisticated experiments on rainwater in Copenhagen in Although Carlsberg has often funded ice-core science in Denmark, the 18 O content, and ultimate fate of initial contents of this high-tech laboratory research vessel have been lost to the history of science J W. Dansgaard Frozen Annals Greenland Ice Cap Research. Niels Bohr Institute, Copenhagen.

19 On a research trip to Greenland to study precipitation, Willi Dansgaard took samples of ice from several icebergs. He found large differences in the ratio of 18 O to 16 O among icebergs. Some values were like rain and snow that Dansgaard collected along the coast. Others had much less 18 O. What was going on here?

20 Willi Dansgaard s Icebergs The icebergs came from glaciers that drain the center of the Greenland Ice Sheet. The icebergs with lower amounts of 18 O came from deep in the glaciers. The icebergs with higher amounts of 18 O came from shallower in the glaciers. Which ice had traveled farther? Which ice was older? Is this a climate-change story? ELA Maybe

21 δ 18 O - a way to report isotope ratios = 18 R 16 O R is the ratio of 18 O atoms to 16 O atoms in a sample. O Geochemists find it is easier to measure the difference between the ratio R sample of 18 O to 16 O in a sample and the ratio R standard of 18 O to 16 O in a reference standard, than it is to measure the absolute amount of 18 O in a sample. Definition of δ 18 O δ 18 O = R sample R R standard standard 1000 The standard is called Standard Mean Ocean Water, or SMOW.

22 TCS (Team Curious Scientists) figures out δ 18 O = 18 R 16 O O δ 18 O R is the ratio of 18 O atoms to 16 O atoms in a sample. = R sample R R standard standard 1000 The standard is called Standard Mean Ocean Water, or SMOW. You can buy it from IAEA (Intl Atomic Energy Agency). What is the δ-value of the water in your bottle of SMOW (from Trader SMOW s J)? Why do you think scientists might define δ this way?

23 It s a Nutty World You are watching the Super Bowl along with a Bowl of Super-Duper Mixed Nuts. Every 15 minutes, you reach in, stir up the nuts, and then you take out a handful. You then also reach back into the bowl and pick out a few extra cashews, and add them to your handful. You eat your handful of nuts. What has just happened to the cashews as a fraction of the remaining nut mix? The remaining nut mixture is progressively more depleted in cashews after every handful. The decreasing fraction of cashews in the bowl can be related directly to the number of handfuls that you have extracted, or to the time into the game.

24 δ 18 O and Cloud Temperature Water vapor evaporates from a tropical or temperate ocean. Vapor moves toward the polar regions in storm systems. Whenever an air mass cools by 1 o C, a predictable fraction of the vapor condenses, and falls out as rain or snow. The heavy isotope preferentially falls out in the rain or snow in a predictable way. [like your extra cashews]. The remaining vapor in the cloud is progressively more depleted in the heavy isotope as the air mass cools. [like the cashews remaining in your nut bowl] The isotopic composition δ 18 O of the precipitation can be related directly to the amount of rain or snow that has fallen from the clouds, or to the temperature in the clouds where the droplets condensed from water vapor.

25 δ 18 O as a Temperature Proxy

26 Choosing an Ice-Core Site Near the summit is often best. Climate signal relates to the same place for ice of all ages. Layers get thinner over time due to flow. When are layers too thin to resolve annual signal?

27 Willi Dansgaard s Legacy. egrip NEGIS South Dome core Dye 3 (Cold War radar site). Core went back into last ice age. Summit - GRIP 1992/GISP cores, 30 km apart. 110 ka ice. Fast climate changes are not iceflow artifacts. NGRIP Recovered ice from end of previous warm interglacial period (>110 ka BP). NEEM drilling finished in 2012 Found ice from entire previous warm interglacial period. NGRIP Currently underway Studying NEGIS(ice-stream) onset.

28 Dating an Ice Core GISP2 ice core, ~20,000 years old

29 Age-Depth relation for central Greenland From an ice-flow model. How many years of ice can we recover in 1 meter of core at each depth? How old is the oldest recoverable ice?

30 Going backwards in the Time Machine Holocene 0-10ka BP Climate stable and warm. Human civilizations and agriculture are possible. Younger Dryas ka Last gasp of the Ice Age. δ 18 O a proxy for past temperatures in Greenland 20 o C Wisconsinan Ice Age, ka Huge and fast transitions in climate in just a few years (sometimes in less than a decade). All humans are hunter-gatherers (Why would anyone want to be a farmer in a world like this?)