Week 6.2 millennial scale climate change

MILLENNIAL SCALE CLIMATE CHANGE

Week 6.2: 29th August

Millennial Scale Climate Change

The 100,000 year problem.

Millenial Scale Climate Change

Climate at most recent Glacial period (21,000 years ago)

1. Northern Hemisphere

2. Southern Hemisphere

3. Tropics

Climate from 21,000 years ago to today

Millennial Oscillations in climate

Human induced climate change?

The 100,000 year problem

Remember from yesterday:

The Milankovitch cycles consist of cycles of obliquity (tilt), precession and eccentricity.

The Milankovitch cycles correlate well with Glacial cycles, with 41,000 yr and 23,000 yrobliquity and precession cycles offering the strongest influence on seasonal insolation levels.

There is also a 100,000 year eccentricity cycle, however it has a smaller effect on seasonal insolation levels.

This occurs up until about 0.9 mya.

After this, a 100,000 year cycle overarches the other two.

WHY?



Abe-Ouchi et. Al. studied the 100,000 year problem using climate and ice sheet models, taking into account the orbital oscillations, and internal feedback influences including ice-sheet size and shape, atmospheric CO2 levels and lithospheric movements/reactions.

They came to the conclusion that the 100,000 year glacial cycles that have dominated patterns in the last 0.9 million years are in fact a reaction to the 100,000 year cycle of eccentricity.


They concluded that the mass balance (formation-ablation) of the North Atlantic ice sheet

remains positive through several series of glacial cycles, and grows larger and extends

further south.

When the N Atlantic ice sheet is at a large size, and extends farther south, it will require a

smaller increase in summer insolation levels in order to tip the mass balance towards the

negative.

Thus, once the ice sheet reaches a certain size, the influence of the 100,000 year

eccentricity cycle on summer insolation levels is enough to begin the melting of the ice sheet

and trigger an interglacial period.


Once the ice sheet starts melting, it continues to melt so rapidly that the effect of the shorter orbital cycles is not strong enough to trigger another ice age. WHY?

Ice is dense, and large ice sheets actually weigh down the lithosphere, causing it to sink into the asthenosphere!

This means that the ice sheet is now sitting at lower elevation, so will melt faster.

The larger the ice sheet, the stronger the effect.

As the ice sheet melts, the lithosphere will rebound again, and “bob” back up. This process is slow.

This process is called Isostatic depression.


http://www.gfz-potsdam.de/en/section/earth-system-modelling/topics/ice-sheet-and-

solid-earth-dynamics/glacial-isostatic-adjustment-gia/

How do we study Millennial Scale Climate Change:?

There are many techniques to study climate change in this period.

Time period lies within the range C14 dating.

Coral reefs offer a record of sea level shifts.

Bedrock lifting

Pollen

Dust analysis

Millennial Scale Climate Change: major climate drivers

Size and shape of ice sheets

Seasonal insolation (from orbital movements)

Levels of Greenhouse gases in the atmosphere

The last Glacial Maximum 21,000 years ago

Ice, 2 or more km high, covered Canada, Northern USA, Northern Europe and parts of Eurasia.

Global sea level was up to 110 meters below todays!

Mid latitudes were dry, cold and windy and sparsely vegetated.

Large mammals that are now extinct roamed the land.


https://news.uchicago.edu/article/2015/07/02/landmark-genetic-

analysis-identifies-how-woolly-mammoth-adapted-arctic-life


The climate 21000 years ago was very different from that of today

However, seasonal insolation levels were pretty much THE SAME as they are today.

What does this mean about the influence of orbital oscillations at this time?

Ice Formation lags Behind Changes in Insolation

Recall from last lecture that ocean sediment analysis can show us when we have more ice on Earth, and that glaciation follows a pattern that corresponds to orbital oscillation patterns.

Ice Formation lags Behind Changes in Insolation

In fact, ice formation and ablation follows the pattern in a lagged way.

This is because large ice sheets only begin melting once insolation levels have increased near their maxima. And ice only begins to build when insolation levels are low, near a minima.


Insolation had forced glacial build-up many thousands of years earlier, and they had sluggishly grown to a maximum at 21,000 years ago.

By the time the ice had reached maximum size, summer insolation had reached todays levels, and was headed towards higher levels that would begin to melt the ice.

This is why large glacial ice sheets were present 21,000 years ago, that are not present today.

The climate 21,000 years ago was influenced by the presence of these ice sheets, as well as CO2 and other greenhouse gas levels in the atmosphere.

The last Glacial Maximum: Northern Hemisphere

Large glaciers across the Northern Hemisphere significantly influenced the climate 21,000 years ago.

The world was about 4 degrees colder than today.

The Oceans in North Atlantic and North Pacific up to 8 degrees cooler.

Ice sheets reached as far south as 48° N (Scandanavia) and 37° N (North America)!


Laurentide ice sheet

Cordilleran ice sheet

Scandanavian ice sheet

Barents ice sheet

Greenland


https://contemplativemammoth.com/2014/07/07/what-causes-an-ice-age-the-many-scales-of-climate-change-part-2-

orbital-cycles/


Laurentide ice sheet: Over Canada. Thick over areas of hard bedrock, thinner over sediment.

Cordilleran ice sheet: Over the American West (Rocky mountains)

Scandanavian ice sheet: Northern Europe

Barents ice sheet: Over the Northern Eurasian continental shelf

Greenland: extended over continental shelf that was uncovered because of the lower sea level.

Mountain glaciers around the world were generally larger, and the appearance of high terrain would have been very different than today.


Ice sheets grind across the landscape, and create huge amounts of debris and dust.

Glacial erosion and abrasion pulls debris of all sizes and deposits it in moraines at the edge of the glacier.

Melting and other water processes sorts the moraine deposits into particles of different sizes.

Smaller particles are exposed and can be transported around the Earth via water and wind.

Glacial time periods are much dustier than interglacial periods.


http://www.landforms.eu/cairngorms/moraine.htm

http://www.keyword-

suggestions.com/bW9yYWluZSBhbmQgdGlsbA/


Ice sheets protruded upwards as icy plateaus, redirecting air and wind flow.

Winds in the lower atmosphere are disrupted, causing further disruption in the higher jet stream flow.

The large ice sheet over North America (Laurentide ice sheet) likely spilt the winter jet stream into two flows.

This would have created a clockwise spiral of cold wind moving down off the ice sheets, chilling the surface of the North Atlantic and blowing cold wind westward across the Northern USA, opposite to what flows today.

Similar winds over the Scandinavian ice sheet blew cold dry air over Europe.


Europe had a much colder, more arid climate 21,000 years ago.

This was due to cold winds coming off the Scandinavian ice sheet, and a chilling of the North Atlantic Ocean.

The North Atlantic was chilled by winds off the ice sheets, and ice melts.

The moderating influence of the ocean on modern day winters in Europe is greatly reduced when Atlantic temperatures are cooler.

Colder, harsher winter conditions also extended through Asia, towards South East Asia.


https://commons.wikimedia.org/wiki/File:Mammoth_House_(Replica).JPG

The last Glacial Maximum: Southern Hemisphere

Far away from the Northern Ice sheets, differences in climate between 21,000 years ago and today were less dramatic.

Greatest influence was probably atmospheric CO2 levels. Lower atmospheric levels lead to cooler Southern Hemisphere climate.

Antarctica sea ice reach was larger.



Australia was more arid.

Expanded desert sand dunes.

Possibly due to the withdrawal of the ocean north of Australia, which exposed the continental shelf between Australia and PNG.

Lower atmospheric CO2 also cooled the atmosphere and contributed to a drier climate over Australia.

The last Glacial Maximum: The Tropics

The tropics of 21,000 years ago where cooler than today, but scientists argue over how much.

The tropics are far from the Northern and Southern ice sheets, and not largely effected by winds and ocean currents from the ice sheets.

Insolation would have an influence, but remember seasonal insolation levels where much the same then as today.

The effect must be from greenhouse gases: CO2 and, to a smaller effect, methane (CH4).

This provides a good insight into the sensitivity of tropical climate to greenhouse gas levels.

The last Glacial Maximum: The Tropics

CO2 was 190ppm (today up to 400!)

Methane was 50% less than today.

Tropical temperatures were cooled somewhere between 2-6 degrees Celsius

The Interglacial Period 21,000 years to today

Climate Drivers:

Summer insolation maximum.

Melting of huge northern ice sheets.

Increase of CO2 and methane gases in atmosphere.


Summer Insolation increased during this period.

By 10,000 years ago, the angle of Earth’s axial tilt reached a maximum.

At the same time, Earth’s processional motion moved it closest to the sun on June 21 (Northern Hemisphere summer).

This rise in summer insolation on the Northern Ice sheets triggered melting.

Remember that at 21000 years ago, the biggest climate driver was these Northern ice sheets.

When these sheets started melting, the influence of them on climate became less significant.


The Northern ice sheets began melting just before the summer insolation maximum around 10,000 years ago.

We can study the speed that the ice melted at by looking at evidence for past sea level rise.

Coral reefs form near the water’s surface, so by C14 dating coral from different depths, we can work out sea level change.


Dating of old coral reefs has indicated that the sea level rose (and therefore the glaciers melted) in two steps.

This is called the de-glacial two step.

This shows that around 12,000 years ago, the melting of the ice sheets slowed significantly.

Why? Likely due to an event called the Younger Dryas which we will talk about soon.

http://www.geo.utexas.edu/courses/302c/L23-N.pdf


The level of greenhouse gases CO2 and Methane changed significantly in the period from 21,000 years ago to today.

CO2 rose from 190 ppm to 290 ppm (pre-industrial levels)

Methane doubled.

These increases more or less coincided with the melting of the ice.


Why did atmospheric CO2 increase over this period?

Large ice sheets increase the reflectivity of the land, reflecting more heat energy back into space. When these ice sheets melt, more heat energy is absorbed by the oceans.

Oceans release CO2 when heated, so the increased summer insolation plus increased absorption releases more CO2.

The higher levels of CO2 in the atmosphere create increasing greenhouse effect (next lecture) which in turn increases heat retention and melting.

This is a feedback effect between melting glaciers and increasing CO2 levels.

Oscillations in Millennial Climate

So why did the melting of the last glacial ice sheets occur in two steps?


The Younger Dryas is the name given to the event 12,000 years ago, where glacial melting slowed.

During the Younger Dryas, temperatures show a return to cooler temperatures, more like those at the glacial maximum.

Ice sheets in some areas stopped retreating, and even advanced in some places.

This event is due to shorter term oscillations than the 100,000 yr and 41,000 year orbital cycles.

These are called the millennial oscillations.


The millennial oscillations range from about 1500 to 4000 years long.

δ18O analysis of ice cores from several ice sheets in the northern hemisphere have suggested that small scale glaciations and deglaciations have occurred via these oscillations.

Millennial oscillations were first in the Greenland-North Atlantic region and they involve changes in air and surface-ocean temperature and ice sheet margins.

Similar oscillations have been detected in other parts of the world, including the southern hemisphere.

Note: These oscillations are not at regular, cyclic intervals, so are not cycles.


http://www.ltrr.arizona.edu/webhome/aprilc/data/my%20stuff/antarctica%20website/deglacialcriticalissues.htm

Oscillations in Millennial Climate: WHY?

Scientists have contested over the cause of the millennial oscillations.

Dating these oscillations is not straightforward, and dates can be inaccurate.

For example: some evidence suggests that the millennial oscillations experienced in the Southern Hemisphere are at an opposite timing to those experienced by the Northern Hemisphere, but this has not yet been conclusively agreed on.

Because of issues with dating and finding good climate proxies, it has been difficult for scientists to agree on why these millennial oscillations occur.


The true reason for millennial oscillations would need to take into account:

1. The behaviour of ice sheets, and feedback effects of melting and advancing sheets

2. Internal interactions among several parts of the climate system (large ocean currents, wind circulation etc.)

3. Solar variations that may occur externally to the climate system.


1. The behaviour of ice sheets, and feedback effects of melting and advancing sheets.

Ice sheets show very slow response times over thousands of years, but the margins of ice sheets tend to be thinner and can have much more rapid interactions with surroundings.

Ice sheet margins can get stuck, if they are moving forward across hard bedrock, but can move much faster across water.

Interactions with the underlying surface can cause ice margins to break off into large chunks and float away before melting.

This is called ice rafting


1. The behaviour of ice sheets, and feedback effects of melting and advancing sheets.

Ice rafting tends to occur in intervals, melting chunks of ice sheets at a time.

One theory of millennial oscillations suggests that the large North American ice sheet would melt by ice rafting at intervals, and the ice rafts would increase sea level enough to trigger rafting from other large northern ice sheets.

This theory does not currently stack up well against dating techniques, as they suggest that smaller ice sheets had already “ice rafted” before being triggered by the North American ice sheet.



2. Internal interactions among several parts of the climate system (large ocean currents, wind circulation etc.)

Some theories suggest that the millennial scale oscillations are caused by internal interactions within the climate system.

These theories take into account the carbon cycle or salinity, as these are both linked with deep water current flow. Theories suggest that movement of these minerals due to the ice melting, changes the deep water conveyer system and alters the climate.

These theories highlight the sensitivity of the deep water conveyer system to small changes in temperature and mineral concentration, and the power that this system has over global climate.


3. Solar variations that may occur externally to the climate system.

Scientists have suggested that the sun has increased and decreased in power over intervals of several hundred years. Some have suggested that the changes in the power of the sun may be what is causing the millennial oscillations.

So far, scientists have not linked the two.


There has not yet been scientific consensus on cause of the millennial scale oscillations in climate.

This is concerning, because we cannot predict how they might effect our climate in future, and they may do so within relatively short time scales (500-1000 years)

Climate change in human history

Climate change in the last several thousand years has been on a much smaller scale than tectonic, orbital and millennial scale changes.

Temperatures have really only varied around 1 degree Celsius, and have been highly variable from region to region.

From 1000-1300 years, the climate in the high latitudes of the Northern Hemisphere was slightly warmer.

From 1300-1900, climate has been slightly cooler.

Since 1900, the climate has been warming. Is this increase in temperatures because of human activity? Or a result of some other change, eg the millennial oscilations?


The cooling from 1300 to 1900 can be explained with orbital and millennial predictions, but the warming from 1900 cannot.

The last few decades have shown temperatures that rise above the highest temperatures possible, as predicted by orbital and millennial oscillations.

Something else must be going on!


https://www.e-education.psu.edu/meteo469/?q=book/export/html/134

Summary

Feedback effects have shown themselves to us by influencing the climate in the last millennium.

There are many feedback effects of glaciers, atmospheric gases, solar radiation, ocean and atmospheric currents.

Studying past climate is complicated, we use climate proxies, dating techniques, and these don’t always offer us exact answers.

Because feedback effects can be difficult to predict and understand, and climate studies over long time periods are difficult, it is also difficult to predict FUTURE CLIMATE.

Note however, that THERE IS overall scientific consensus about the reason for the rise in temperature in the last 100 years.

See you in Week 10!

Next week

Dr Kristen Anderson will present….

FUTURE CLIMATE

References

Ruddiman, W. F. (2008) Earth’s Climate: past, present and future.

http://download.springer.com/static/pdf/870/art%253A10.1007%252FBF00223498.pdf?originUrl=http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2FBF00223498&token2=exp=1467249829~acl=%2Fstatic%2Fpdf%2F870%2Fart%25253A10.1007%25252FBF00223498.pdf%3ForiginUrl%3Dhttp%253A%252F%252Flink.springer.com%252Farticle%252F10.1007%252FBF00223498*~hmac=bbc2118ace4231c9d59d445888bc7d4aef220aa273714ad623f75f1ef5c29d9b


https://scripps.ucsd.edu/programs/keelingcurve/2014/06/20/how-do-co2-levels-relate-to-ice-ages-and-sea-level/

file:///C:/Users/emmac_000/Dropbox/EA2404%205404/Lectures/nature12374.pdf


Science

Week 6.2 millennial scale climate change