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Transcript Introduction

MET 112 Global Climate Change -

Natural Climate Forcing

Professor Menglin Jin San Jose State University     Outline – Paleoclimate – temperature and CO2 Natural forcing for temperature change Features for Glacier and inter-glacier Activity MET 112 Global Climate Change 1

A lead to


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Earth geological time scale

Paleo : Greek root means “ancient” Modern age, ice age, last 2 million years Age of dinosaurs Animal explosion of diversity From the formation of earth to the evolution of macroscopic 3

Climate record resolution

(years) 1 ,000,000 100,000 10,000 1000 100 10 1 1mon 1day Satellite, in-situ observation Historical data Tree rings Lake core, pollen Ice core Glacial features Ocean sediment, isotopes Fossils, sedimentary rocks 4

Climate record distribution from 1000 to 1750

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C14 and O18 proxy

C14 dating proxy Cosmic rays produce C14 C14 has half-life of 5730 years and constitutes about one percent of the carbon in an organism.

When an organism dies, its C14 continues to decay.

The older the organism, the less C14 O18 temperature proxy O18 is heavier, harder to evaporate. As temperature decreases (in an ice age), snow deposits contains lessO18 while ocean water and marine organisms (CaCO3) contain more O18 The O18/ O16 ratio or δO18 in ice and marine deposits constitutes a proxy thermometer that indicates ice ages and interglacials.

Low O18 in ice indicates it was deposited during cold conditions worldwide, while low O18 in marine deposits indicates warmth MET 112 Global Climate Change 6

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Natural Climate Change

 External Forcing: –

The agent of change is outside of the Earth-atmosphere system

 Internal Forcing: –

The agent of change is within the Earth-atmosphere system itself

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Faint-sun paradox

According to solar models, solar luminosity is 30% stronger nowadays than 4.5 billion years ago due to thermonuclear H to He makes the sun denser and hotter Temperature should be 25K lower


There should be glaciations up to 2 billion years ago According to record, glaciations are absent from 2 to 3 billion years ago Possible reasons: Higher CO 2 concentration (Kuhn & Kasting 1983, Kasting 1993) or CH 4 causing greenhouse gas effect Less continent and faster rotation of earth increase temperature by 4k and 1.5K respectively (Jenckins 1993) Stronger solar wind stopped cosmic rays reaching earth leading to heating (harrison & Aplin 2001, Eichkorn et al.2002, Shaviv 2003) MET 112 Global Climate Change 10

Ice-covered earth Ice-free earth

700 million years ago due to very low CO2 concentration Hypothesis: plate tectonics and lack of weathering and photosynthesis left great amount of CO2 in the atmosphere (Kirshvink 1992) Support: thick layer of carbonate and banded iron formation on top of tropic glaciations Rapid transition from cold to warm climate would bring great changes in life on earth MET 112 Global Climate Change 11

Continental drift

In 1915, German scientist Alfred Wengener first proposed continental drift theory and published book On the Origin of Continents and Oceans Continental drift states: In the beginning, a supercontinent called Pangaea. During Jurrasic, Pangaea breaks up into two smaller supercontinents, Laurasia and Gondwanaland,. By the end of the Cretaceous period, the continents were separating into land masses that look like our MET 112 Global Climate Change 12 modern-day continents

Consequences of continental drift on climate

Polarward drifting of continents provides land area for ice formation  cold climate Antarctica separated from South America reduced oceanic heat transport  cold climate Joint of North and South America strengthens Gulf Stream and increased oceanic heat transport  warm climate Uplift of Tibetan Plateau  Indian monsoon MET 112 Global Climate Change 13

Warm during Cretaceous

High CO 2 may be responsible for the initiation of the warming Higher water vapor concentration leads to increased latent heat transport to high latitudes Decreased sensible heat transport to high latitudes results from decreased meridional temperature gradient Psulsen 2004, nature Thermal expansion of sea water increased oceanic heat transport to high latitudes The Arctic SST was 15 oC or higher in mid and last Cretaceous. Global models can only represent this feature by restoring high level of CO 2 14 MET 112 Global Climate Change

Cretaceous being the last period of the Mesozoic era characterized by continued dominance of reptiles, emergent dominance of angiosperms, diversification of mammals, and the extinction of many types of organisms at the close of the period

This hypothesis is proposed to 65 Million years ago for one possible reason that kills the dinosaurs

Asteroid impact initializes chain of forcing on climate

Short-term forcing: The kinetic energy of thebollide is transferred to the atmosphere sufficient to warm the global mean temperature near the surface by 30 K over the first 30 days The ejecta that are thrown up by the impact return to Earth over several days to weeks produce radiative heating. Long-term forcing: Over several weeks to months, a global cloud of dust obscures the Sun, cooling the Earth’s surface, effectively eliminating photosynthesis and stabilizing the atmosphere to the degree that the hydrologic cycle is cut off. The sum of these effects together could kill most flora. The latter results in a large increase in atmospheric CO2, enabling a large warming of the climate in the period after the dust cloud has settled back to Earth MET 112 Global Climate Change 16

External Forcing

Variations in solar output

Orbital variations


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  A meteor is a bright streak of light that appears briefly in the sky. Observers often call meteors shooting stars or falling stars because they look like stars falling from the sky

Meteor showers

– dbook.html

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Solar Variations

 Sunspots correlate with solar activity  More sunspots, more solar energy  Sunspots are the most familiar type of solar activity.

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 Sunspots are the most familiar type of solar activity.


    Sunspot numbers increase and decrease – over an 11-year cycle Observed for centuries.

Individual spots last from a few hours to months.

Studies show the Sun is in fact about – 0.1% brighter when solar activity is high.


 Solar activity appears to slightly change the Sun’s brightness and affect climate on the Earth...


    An absence of sunspots was well observed – from 1645 to 1715.

The so called “Maunder minimum” coincided with a cool climatic period in Europe and North America: – “Little Ice Age” The Maunder Minimum was not unique.

Increased medieval activity – correlated with climate change.

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Orbital forcing on climate change

Coupled orbital variation and snow-albedo feedback to explain and predict ice age He suggested that when orbital eccentricity is high, then winters will tend to be colder when earth is farther from the sun in that season. During the periods of high orbital eccentricity, ice ages occur on 22,000 year cycles in each hemisphere, and alternate between southern and northern hemispheres, lasting approximately 10,000 years each.

James Croll, 19 th century

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Scottish scientist


Further development of orbital forcing by Milutin Milankovitch

Mathematically calculated the timing and influence at different latitudes of changes in orbital eccentricity, precession of the equinoxes, and obliquity of the ecliptic. Deep Sea sediments in late 1970’s strengthen Milankovitch cycles theory. MET 112 Global Climate Change 27

Orbital changes

Milankovitch theory:

  Serbian astrophysicist in 1920’s who studied effects of solar radiation on the irregularity of ice ages Variations in the Earth’s orbit – Changes in shape of the earth’s orbit around sun:  E

ccentricity (100,000 years)

– Wobbling of the earth’s axis of rotation: 

Precession (22,000 years)

– Changes in the tilt of earth’s axis: 

Obliquity (41,000 years)

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Earth’s orbit: an ellipse

MET 112 Global Climate Change • Perihelion : place in the orbit closest to the Sun • Aphelion : place in the orbit farthest from the Sun 29

Eccentricity: period ~

100,000 years MET 112 Global Climate Change 31

Precession: period ~

22,000 years MET 112 Global Climate Change 34

Axis tilt: period ~

41,000 years MET 112 Global Climate Change 35

Eccentricity affects seasons

Small eccentricity --> 7% energy difference between summer and winter Large eccentricity --> 20% energy difference between summer and winter Large eccentricity also changes the length of the seasons MET 112 Global Climate Change 36

Obliquity explain seasonal variations

Ranges from 21.5 to 24.5 with current value of 23.439281

Small tilt = less seasonal variation cooler summers (less snow melt), warmer winters -> more snowfall because air can hold more moisture Source:

Precession of equinoxes

 Vernal equinox has 24 000 period around the orbit.

 Moon’s gravitational pull on Earth’s equatorial bulge causes wobling MET 112 Global Climate Change 38

Milankovitch cycles suggest changes in the mean temperatures of earth

Source: Whyte (1995)

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Temperature: the last 400,000 years

From the Vostok ice core (Antarctica)

Fig 4.5

High summer sunshine, lower ice volume

Formation of Glaciers

 Glaciers - composed of fallen snow that is compressed into a large, thickened mass of ice over many years   Glacier Growth: When over a year snowfall (winter) is larger than snowmelt (summer) Glacier Decay: When over a year snowfall (winter) is less than snowmelt (summer)  Glacier growth and decay largely influenced by summer temperatures.

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Internal Forcing

    Ocean changes Chemical changes in the atmosphere (i.e. CO 2 ) – Natural variations MET 112 Global Climate Change 44


Consider the fact that today, the perihelion of the Earth’s orbit around the sun occurs in the Northern Hemisphere winter. In 11,000 years, the perihelion will occur during Northern Hemisphere summer. A) Explain how the climate (i.e. temperature of summer compared to temperature of winter) of the Northern Hemisphere would change in 11,000 years just due to the precession. B) How would this affect the presence of Northern Hemisphere glaciers (growing or decaying)? Assume growth is largely controlled by summer temperature.

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If the earth’s tilt was to decrease, how would the summer temperature change at our latitude

1. Warmer summer 2. Cooler summer 3. Summer would stay the same 4. Impossible to tell

21% 79% 0% 0% W ar m er s um m er C oo le r S su um m m m er er w ou ld s ta y t..

Im po ss ib le to te ll

A: How would climate change

1. Warmer winters, cooler summers 2. Warmer winters, warmer summers


3. Cooler winters, warmer summers

7% 5%

4. Cooler winter, cooler summer

0% W ar m er w in te rs W , c ar oo m le er ...

w in te C rs , w oo le ar r m w ..

in te rs , w C ar oo le m r e.

w ..

in te r, c oo le r..


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B: How would glaciers change?

1. Glaciers would grow 2. Glaciers would decay 3. Glaciers would stay about constant

88% 10% 2% G la ci er s w ou ld g ro G w la ci er s w ou ld d G ec la ay ci er s w ou ld s ta y ...

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Major features of ice age

Minimum insolation could be explained by Milankovitch cycle followed by advancement of glaciers Polar front moves south Salinity increases Thermohaline circulation increases Lower sea surface temperatures and sea levels followed by reduced evaporation and precipitation Nutrients and biological productivity increase Deep water sequesters CO 2 from atmosphere Cooling due to expanding ice caps and decreased CO 2

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Last Glacial Maximum (


) 22 ~ 14 K year

3.5 –4 km thick 50-60 x 106 km3 water 120 m sea level reduction 700 –800 m geosyncline depression (still rebounding) Large changes in flora and fauna Most of planet equatorward of ice sheets: →colder and drier →wind speed 20 –50% higher →higher dust levels →lower CO2 concentration (~200ppm) and CH4 concentration feedback MET 112 Global Climate Change 50

Major features of interglacial (Honocene)

Glaciers retreat shows maximum insolation Milankovitch cycle Higher sea levels Higher sea surface temperatures Enhanced evaporation and precipitation Salinity decreases Polar front moves north Thermohaline circulation decreases Nutrients and biological productivity decrease Warming due to shrinking ice caps and increased CO2 Abrupt warming: one of most rapid transitions Interrupted by brief period of cold –YoungerDryas (~11 KABP) Continuation of warming beginning in ~10 KABP MET 112 Global Climate Change 51

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Glacial to interglacial cycle

Vostok data

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Temperature changes in LGM

AR4 figure 6.5

Left: Multi-model average SST change for LGM PMIP-2 simulations by five AOGCMs. North America and east europe were largely covered by ice sheet Right: LGM regional cooling compared to LGM global cooling as simulatedin PMIP-2, with AOGCM results shown as red circles and EMIC (ECBilt-CLIO) results shown as blue circles. Grey shading indicates the range of observed 54

Temperature changes in Last Interglacial

AR4 figure 6.5

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