Orbital-Scale Interactions in the Climate System. Speaker:

Transcription

1 Orbital-Scale Interactions in the Climate System Speaker:

2 Introduction First, many orbital-scale response are examined.then return to the problem of interactions between atmospheric CO 2 and the ice sheets and attempt to figure out which drives which. Finally, we look at a major mystery: What explains the 100,000- year ice sheet cycles during the last 900,000 yr?

3 Orbital-Scale Forcing and Response Revisited The fast response time, measured in months or year, and the slow response time, measured in many thousands of year behind. Long-term air temperature in a region tracks the size of the ice sheet fairly closely. Slow and fast responses to insolation and ice

4 Ice-Driven Climate Response The ice driven responses should show the same major orbital rhythms as the ice sheet 41,000-year cycles before 0.9 Myr ago and 100,000-year cycles since that time. They should track the ice sheets cycles without any obvious lag. Ice-driven responses

5 Ice-Driven Response in High Northern Latitudes (Ocean Surface Temperature) Scientists have traced evidence of ice-driven responses across the high northern from the North Atlantic Ocean to Europe to Asia and into the western North Pacific. The high-latitude North Atlantic is surrounded by the great ice sheets of North America and Eurasia. The surface ocean is a fast- response part of the climate system. Regions of ice-driven responses

6 The North Atlantic sea temperature signal track the ice volume signal peak for peak during 300,000-year interval. This match is exactly the kind of signal we expect to see if orbital-scale changes in sea surface temperature are driven by the ice sheets rather than by insolation. North Atlantic surface responses to ice

7 The ice sheets transfer climatic signal to ocean maybe by melting icebergs to cool the ocean surface. Another way for ice sheets to influence nearby regions is through the atmosphere. Ice sheet sensitivity test

8 Climate in Northern Europe and Asia European vegetation The record of European pollen fluctuations over the last 130,000 years correlates well with the ice volume changes recorded in ocean cores. This evidence tells us that central Europe was covered by cold, dry, tundralike vegetation during glaciations.

9 Ocean temperatures north of 20 0 N were reduced to their full glacial values in the experiment, while the rest of Earth s surface was left in exactly its present state, with no ice sheets present except the one on Greenland. Cold North Atlantic sea surface produces cold air temperature no just over the North Atlantic but downwind into the western maritime part of Europe, and even into the Eurasian continent. Surface-ocean sensitivity test

10 Over the last 500,000 year, alternation between soils and less have occurred at a 100,000-year cycle, which each loess layer deposited during a time of major glaciation. The western North Pacific Ocean sediment cores contain records of fine- grained, wind-transported silt from east-central Asia. Responses of windblown debris in East Asia to ice volume

11 In summary, the ice volume signal can be transferred far from the immediate proximity of the ice sheets by altered wind pattern and resulting changes in air and surface- ocean temperatures and precipitation over land.

12 Orbital Cycles in Regions Remote from Northern Hemisphere Ice In the western Indian Ocean, dust is blown out to sea from the Arabian Desert at a tempo that closely matches the ice sheets cycles. More dust arrives during the glacial intervals, and less dust interglacial intervals like today. Responses of windblown debris in the Arabian Desert to ice volume

13 Dating of this record indicates that the major cycle of pollen changes occurs at 100,000 years. Major shift between forest and grassland pollen that match 100,000-year glacial- interglacial ice volume changes in the northern hemisphere. Pollen responses in South America to ice volume

14 The dominant cycle of pollen changes on New Zealand matches the 100,000-year ice volume cycles recorded in the same core. Pollen responses in New Zealand to ice volume

15 Marine sediment cores from the Southern Ocean show changes in estimate sea surface temperature that match in general way the changes in ice volume recorded in the same core. Response of Southern Ocean temperature to ice volume

16 We already know that immediate driver of regional climatic responses must then be ice sheets, but which ice sheets? Northern or Southern ice sheet?

17 Because of the difference caused by precession, insolation changes at high latitudes of northern and southern hemisphere differ considerably. Out-of-phase summer insolation between the hemispheres

18 41,000 and 23,000 year components of ice volume Lag behind northern hemisphere insolation by by physically reasonable amount. 41,000 year southern hemisphere cycles has same lag 23,000 year ice volume leads southern hemisphere summer insolation forcing Unreasonable relationship Phasing of insolation vs. ice volume

19 This mismatch confirms that the south polar ice sheet was not the mean of transferring the ice-driven climatic signal to the rest of the world. A transfer from the northern ice sheets to the south polar ocean makes better sense.

20 Global Transfer of Signal from Northern Hemisphere Ice Sheet One way is by changes in sea level tied to storage and release of ocean water in ice sheets. A second way is by changes in deep- water circulation. Another way is atmospheric CO 2.

21 CO 2 Level and Ice Volume : Which Drives Which? There is no persistent lead or lag between CO 2 and ice volume signals. Relationship unknown Does CO 2 lead ice volume? Relative timing of ice volume and changes in CO 2

22 The Mystery of the 100,000-Year Cycle After 0.9 Myr ago, the 41,000-year and 23,000-year cycles of ice volume change were overridden by larger and more dominant fluctuations of ice sheets at a period of 100,000 years. The large 100,000-year cycle is not related to any cycle of orbital variations. Two questions : (1) Why did more ice accumulate after 0.9 Myr ago? (2) Why did these large ice sheets melt rapidly every 100,000 years?

23 Why Have Ice Sheets Grown Larger Since 0.9 Myr Ago? Explanation 1: A result of the gradual (tectonic-scale) cooling. Tectonic-scale cooling due to the decrease of CO 2. Cooling reach a threshold value 2.75 Myr ago. The falling CO2 levels continued. Cooling reached another threshold value 0.9 Myr ago that ice sheets never completely disappeared during weak summer insolation maxima. Ice sheets began to grow larger and persist longer.

24 Northern hemisphere ice sheet could have contributed significantly to trend toward increased value after 2.75 Myr ago. δ 18 Ο Changes in in the last 4.5 Myr

25 Why Have Ice Sheets Grown Larger Since 0.9 Myr Ago? Explanation 2: Ice slipping effect (nothing to do with climate changes) During earlier glaciations ( Myr ago), ice sheets may have been thin because they slid on water saturated soil toward lower elevations and warmer temperatures. During later glaciations (beginning from 0.9 Myr ago), after ice sheets stripped off most of the underlying soil, their central regions could grow higher because they no longer slid. Ice slipping may affect ice sheet volume

26 What Causes Abrupt Deglaciations? Strong summer insolation peaks pace rapid deglaciations Slow global cooling together with the less sliding beneath ice sheets helped to create thicker and more extensive ice sheets beginning 0.9 Myr ago. What cause the rapid melting every 100,000 years? Explanation: the 100,000-year eccentricity cycle The 100,000-year eccentricity orbital cycle only produces a trivial amount of direct insolation changes. It is the modulation of the 100,000-year cycle on the amplitude of the 23,000-year precession cycle that actually affect the rapid melting every 100,000 years.

27 These models succeed in capturing much of the observed ice sheets response over the last 900,000 years, including the 100,000-year cycles of gradual ice buildup and rapid ice melt. Numerical model: insolation control of ice volume

28 Conceptual (SPECMAP) Model The ice volume signals during this interval were quickly transferred to other locations in climate system. When ice sheets began to exceed a critical size threshold, the signals can create powerful feedback that produced a 100,000- year response. Conceptual (SPECMAP) model of icedriven climate changes

29 Roles of Internal Climate Interactions The pace of the 100,000-year variations in ice sheet size is set by the external forcing from the eccentricity orbital cycle. The amplitude of the 100,000-year cycle depends in part on the internal interactions of the climate system. The large ice sheets themselves produced internal interactions within the climate system that hastened their own destruction every 100,000 years. The possible positive feedbacks to accelerate ice sheet melting include : rising CO2 level during deglaciations,, the delayed in bedrock rebound provided warmer temperatures in the depressed land surface for fast ice melting, and other ice-controlled controlled processes in wind, sea level, and deep ocean circulation.

30 Summary Over the last 2.75 Myr,, the first two-thirds thirds of this interval, growth, and melting of northern hemisphere ice sheets were controlled by changes in summer insolation at rhythms of 41,000 and 23,000 years. Just as Milankovitch predicted. By 900,000 year ago, the dominant 100,000- year rhythm of change in ice sheets was paced by changes in summer insolation but ultimately governed by internal feedbacks produced by the ice sheets,, including changes in atmospheric CO 2.

31 Milankovitch Theory Milankovitch suggested that the critical factor for Northern Hemisphere continental glaciation was the amount of summertime insolation at high northern latitudes. Low summer insolation occurs during times when Earth s s orbital tilt is small. Low summer insolation also results from the fact that the northern hemisphere s s summer solstice occurs when Earth is farthest from the Sun and when the orbit is highly eccentric.