Natural Climate Variability: Longer Term
Natural Climate Change Today: Natural Climate Change-2: Ice Ages, and Deep Time
Geologic Time Scale background: Need a system for talking about unimaginable lengths of time
Typical version Geologic Time Scale Earth: ~ 4.6 billion years old The oldest known fossils: in rocks ~ 3.5 billion yrs old The first abundant shelly (multi-cellular) fossils : ~570 million years old. The last ice age ended about 10,000 ago?
Clock version of Geologic Time Scale 4 Last time, talked about climate fluctuations in Holocene epoch (=last ~20k yrs) Context: most recent tiny part of quaternary period- =relatively stable climate period, within recent cold/ warm cycles.
recent times KT boundary
Side fact: no dinosaur fossils found above KT boundary- Very rapid extinction event. (Impact remains major theory)
Late Tertiary period: Last ~ 500 YRS, marked by distinct glaciation cycles Earth appears to have entered period near of relative non-stability
For approximately the last 500,000 + yrs, major climate cycles have been very regular (before that, they seem less regular) Major feature: alternation between glacial and interglacial Suggests that some very regular forcing seems to be at work.. Interglacial periods (T~15C, 280ppm CO 2 ) Current interglacial called the Holocene Glacial periods (T~9-10C, 200ppm CO 2 )
Cold periods are called Glacial Periods (or Ice Ages). Warm periods are called Interglacial Periods. Time interval separating a glacial and interglacial period is ~100,000 yrs. LGM - Last glacial maximum ~ 20k yrs ago
Change in N. American ice cover since LGM
Note the lower sea level Also! Northern Hemisphere Ice Cover During the Height of the Last Glacial Period
Avg. sea level ~ 360 feet lower Sea Level change from LGM
Where are we now? currently in an interglacial period, began ~ 10kyrs bp (ordinarily should last 40-65 kyrs). However, increase of greenhouse gases could delay the onset of next glacial period! (silver lining in another 40k yrs..)
So what is underneath these cycles?
Milankovitch Cycles The best explanation was provided by Milutin Milankovitch (Serbian astrophysicist, 1879-1958). showed that cycles could be explained by orbital variations of the Earth.
Orbital variations: Eccentricity, Tilt and Precession Three separate orbital variations are well known to astronomers: Eccentricity : Variations in the shape of the orbit around the sun Obliquity (or Tilt ): changes in the angle that Earth's axis makes with the plane of Earth's orbit. Precession:change in the direction of the Earth's axis of rotation (i.e., the axis of rotation behaves like the spin axis of a top that is winding down; hence it traces a circle on the celestial sphere over a period of time. )
Each Orbital variations has different period Precession: 23k yrs Eccentricity: 100k yrs tilt: 41k yrs Each orbital variation causes a change in the surface and seasonal distribution of incident solar radiation which can leads to a net accumulation or loss of snow and ice
Example: Earth Axis Tilt (Obliquity is another name for tilt of earth axis) Less contrast in insolation between seasons. Changes in the seasonal contrast of isolation leads to accumulation or loss of snow and ice between winter and summer. Greater contrast in insolation between seasons.
Can work this out mathematicallypredictions fit VERY well Forcing components Mathematical summation: matches observed ice core and other proxy data!
Equilibria of the Pleistocene Climate The Milankovitch orbital cycles- the periodic thing cause the Earth to move from one equilibrium state to another.
However: the catch Variations in solar forcing are relatively small (even if timing is perfect..) But response of the Earth climate : very large. This means there must be AMPLIFIERS of the signal. These amplifiers are positive feedbacks.
Small Forcing vs. Large Effect Total solar forcing Forcing components Small forcing but large response Response to each forcing component Total response
What feedbacks might be important in triggering a glacial period? To transition interglacial to glacial period, there must be an accumulation of snow and ice. summers must be cooler (so snow and ice from preceding winters survives). How does this happen?
Not Fully Known Its not fully known which feedbacks are key to amplification over the tipping point Three examples that are thought to be key players
1) The Ice-Albedo Feedback AMPLIFIER TRIGGER
Other key Feedbacks Involve Greenhouse Effect Many known feedbacks involve greenhouse gasses- especially CO2 boil down to the carbon balance between ocean/land and Atm. Some examples of known feedbacks include * Ocean production rates due to: dust inputs, upwelling, etc Shelf erosion (puts nutrients back in) *Coral reef formation/ erosion * Terrestrial Biomass sequestering Carbon
2) CO2/ Ocean production Rise in temperature due to orbital variations What causes the corresponding rise in CO 2? 400K BP
Dust.and Iron?
Turns out Fe (in dust) is key ocean nutrient.. When you add more dust (Fe) to certain surface ocean regions, You produce more plankton- removes CO2 from atmosphere!
Intensification of the Oceanic Biological Pump by Iron Fertilization An important source of iron is dust form arid areas. During ice ages, there is actually more dust (more arid areas)- partly b/c planet is colder- partly because less water in hydrologic cycle *
2) Coral Reefs?..oddly..yes
Coral reef structure are extensive in warm periods but Erode during cold ones
Coral Reefs form Calcium Carbonate Skeletons adds CO 2 to atmosphere
Warm times: Coral Reefs form Calcium Carbonate Skeletons adds CO 2 to atmosphere Glacial times Coral Reefs erode take CO2 back out
WHY? (does this seem odd? ) Has to do with the Calcium (Ca) in coral skeletons Adding or taking away Ca in ocean, changes overall chemistry of seawater, to either take up or release CO2 Long term carbon cycle: all about carbon being stored in rocks / weathered via these reactions.
The Coral Reef Hypothesis During a glacial period, sea-level falls ~100m exposing coral reefs. Subsequent weathering of coral reef skeletons REMOVES CO2 from atmos. * Increased weathering
3) Changes in Terrestrial Biomass? Last Glacial Minimum Today
Terrestrial Biomass Feedbacks A decrease in terrestrial biomass leads to an increase in atmos CO 2 concentration due to a reduction in the rate of photosynthesis. *
Recent ice ages: The pleistocene glaciations reveal the importance of feedbacks within the climate system. They also reveal the complexity of the climate system.
What about farther back?
The Earth has undergone many glaciations during its 4.6 billion year history. The largest, longest, and least understood are 250 million to a billion years ago- Some of these are thought to be so intense, they were referred to as snowball Earth
Snowball Earth periods: Earth totally (or nearly) frozen Two major snowball periods in last billion years ( ~ 500my, and 700my)- lasting millions of years each.. long time implies totally different mechanism
Why? Geologists think that these were due to a combination of 1) faint sun (30% fainter than it is now), and 2) very low levels of CO 2 caused by tectonics/ weathering * of rocks (all the continents were at low latitudes and able to remain icefree for longer) Possible: Impacts by large asteroids and comets are also a possible cause. * Weathering removes CO 2 from atmosphere- like weathering coral reefs
How did the earth recover? The greenhouse effect, due to the slow rise of Carbon Dioxide, is thought to have been critical for terminating these glaciations. The slow recycling of carbon and volcanic activity are thought to have been the key processes at work.
One example Scenario:
The normal biological + Carbonate-Silicate Geochemical Cycle Biological pump
Catastrophe!
Snowball Earth ` Combination of fainter sun, plus dust from meteorplunge earth into giant glaciation- most surface plant life gone. ICE ICE
` Slow recovery.. Leads to slow accumulation of atmos CO 2 due to volcanism Land ice inhibits draw down of CO 2 by weathering Sea-ice inhibits draw down of atmos CO 2 by biological pump ICE ICE
Overall: Lessons of the long Past Natural sources of large climate change exist. Potent and complex climate feedbacks exist. Some large climate changes are accompanied by GHG changes (some are not). Today s temperatures are higher than last 1000 years, and possibly the last 100,000 years. Future GHGs could be as high as ~50 million years ago How will climate change in the future?
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