Transcript Document

USNC INQUA
AMQUA
NAGT/DLESE “On
the Cutting Edge”
Teaching Climate
Change with Ice
Core Data,
celebrating IPY,
June 2, 2008,
State College, PA
G. Comer
Foundation
Please note:
I work for
Pennsylvania State
University,
And help UN IPCC,
NRC, etc.,
But I am not
representing them,
Just me.
Richard B. Alley
Penn State
Ice Cores,
Climate
Change and
Polar Seesaws: What
Nature Does
And Doesn't
Do To Our
Climate
Ice core on light table, central Greenland. Depths of 1411.8 m and 1411.9 m
(slightly less than a mile) are noted. The mass-loss summer layers have made
bigger-bubbled layers that appear dark here (blue bars), and compaction and ice
flow have moved the layers closer together. These are about 8400 years old.
+ Ice cores give wonderful climate records
• Age from counting annual layers, etc.
(checked many ways!);
• Snow accumulation from layer thickness
with flow correction, other ways;
• Temperature at site in several ways;
• Wind-blown dust, sea-salt, etc. from
elsewhere;
• Trapped bubbles of old air with swampgas methane, etc.;
• All on common time scale.
+ Dating (ice-core age)
• Count annual layers;
Count more than once (I re-did half a mile of
Greenland ice);
In more than one way (visible, electrical,
chemical, isotopic indicators);
By more than one person (blind
intercomparisons--no cheating!);
Check vs. independent ages (chemically
fingerprinted fallout of historically dated
volcanoes; radiometric, tree-ring & varve ages
of abrupt climate changes);
• Other correlations (esp. all ice cores share
same gas record), flow models, etc.
+ Snow accumulation rate
• Annual-layer thickness, after flow correction;
Ice sheet spreads and thins, melting at edges
or making icebergs;
Snowfall more-or-less fills space from thinning;
Layers moving halfway to bed thinned by ~1/2;
Mathematically un-thin (also correct for air in
bubbles) to get original thickness;
• Others (change of snow to ice controlled by
temperature and accumulation rate, and
recorded in gas isotopes and bubble number
density, so measure these and estimate past
temperature to learn past accumulation).
+ Others
• If it blows through the air, it lands on the ice;
Dust, sea salt, pollen, micrometeorites, cosmicray-produced nuclides, volcanic ash, pollutant;
Once you know snow accumulation rate, you can
tell whether an ice layer is “dirtier” because it
received more dirt, or less snow to dilute dirt;
Thus, the ice yields histories of cosmic-ray
intensity (modulated by sun and magnetic
field), space-dust infall, dust production and
transport, pollution, volcanic eruptions, etc;
• Can fingerprint things--e.g., central Greenland
dust from Asia, Antarctic dust from Patagonia.
+ Temperature--Isotopic Ratios
• ~1 molecule in 500 in ocean has 1 or 2
extra protons in one or more atoms: 2H
(also called deuterium) instead of 1H, or
17O or 18O instead of 16O;
• The heavier molecules evaporate with
greater difficulty, condense more easily;
• As air mass moves over ice sheet, heavy
preferentially condenses to fall as snow;
• Colder-->more water removed from air-->
more heavy removed-->less heavy left-->
isotopically ”lighter” snow.
+ Temperature--Borehole Temperature
• In Greenland, ice ~1 mile down colder
than surface and colder than bed,
because not done warming from ice age
(works in Antarctica, and in ground, too);
• Consider cooking a turkey vs. hot-dog vs.
your skin if you touch the stove burner-twice as big takes four times as long;
• Temperature of ice today “remembers”
how cold the ice-age was;
• Fuzzy memory--can learn temperature
last night, last winter, last century, last
millennium, and last ice age.
+ Temperature--Gas isotopes
• After abrupt warming, takes ~100 years
to warm ice 70 m down where bubbles are
trapped from the air spaces in the snow;
• Takes ~5 years for gas to diffuse
through spaces to bubble-trapping depth;
• If not wind-mixed, heavy gases go to cold
end (well-known physics; even tried for
isolating uranium isotopes for bombs);
• Causes slight anomaly in trapped gases;
• Anomaly size shows abrupt-warming size.
+ Temperature--More
• Count years between ice-phase and gas-phase
record of abrupt warming;
• Measure gas-isotopic anomaly caused by
gravitational separation to bubble-trapping
depth in non-wind-mixed firn=old snow (Todd
Sowers discovered);
• Measure bubble number-density, controlled by
time and temperature to trapping depth;
• All three depend on snowfall and temperature
over time to bury snow to trapping depth;
• So allow check on temperature, snowfall history
estimates.
+ Gases
• Snow turning to ice traps old air;
• Only reliable samples of old air known;
• Reliable? Yes.
Good agreement with instrumental record;
Good agreement between records from
different cores from different places with
different snowfall, temperature and impurities;
Records of abrupt changes offset by firn
depth, not where chemistry changed;
“Breaking points” fairly well known (too warm or
too dirty and get poor agreement among cores
or with instrumental record, and gas changes
where chemistry changes).
+ So, ice cores show:
• Sun matters to climate, but:
Total output changes little;
Redistribution by orbits slow;
Volcanoes matter to climate, but mostly
noisemakers rather than organized;
• Magnetic field, cosmic rays don’t matter
to climate (at least not much);
• Space dust doesn’t change much, so
doesn’t matter to climate changes;
• CO2 matters to climate;
• Ocean circulation, sea ice matter to
climate.
1700
Year
1800
1900
Temperature
Sunspots
We can see solar effect on climate--real but small (and sun
hasn’t done anything while temperatures rose last decades).
Greenland temperatures do correlate significantly with sunspot numbers,
but most Greenland temperature changes are not explained by sunspots.
(GISP2 ice-core 18O, 5-20 yr bandpass, data from 1700-1930, shifted for optimal
correlation; correlation is highly significant, but accounts for only 13% of the power in the
passband. Other solar frequencies are also present, but again with small power.
Stuiver et al., 1995)
Volcano
erupts Acid falls on
Greenland
Cooling from
volcanoes
Big volcanoes cool (1-2oC for 2-3 years). But, big volcanoes
can’t get organized, so they haven’t & won’t control climate.
(Stack of GISP2, Greenland 18O records from 7 VEI 6-7 eruptions; Stuiver et
al. 1995.)
Climate didn’t
change
Warmer
Today
Climate didn’t change
When more cosmic
rays reached Earth
Cosmic rays, magnetic field don’t matter much to climate.
From Musch ler et al., 2005, QSR. 18O (proxy for temperature) from GRIP
core (top), the concentration of 10Be (middle), and the flux of 10Be (bottom).
The Laschamp event of near-zero magnetic field (red arrow) allowed increased
cosmic-ray flux producing more 10Be, but with no apparent effect on climate.
Changes in space dust have
been small, and haven’t
affected climate much.
Helium-3 is mostly from space dust.
If space dust changed a lot, that
might affect climate some.
But there has been little change in
space dust over last 30,000 years
(ice-core data shown here) and
beyond (other data not shown).
(Very rarely, a big meteorite does
matter, such as the one that killed
the dinosaurs 65 million years ago.)
Winckler & Fischer, 2006, Science
Petit et al., 1999
+ So, ice age cycle:
• Orbits move sun around on planet (big changes
locally, tiny globally);
• Global climate mostly has followed northern sun;
• Sun affects snow, ice, dust, sea level, ocean
circulation, plants… which affect temperature;
• These other changes affect CO2, which affects
temperature;
• If CO2 warms, temperature history makes sense;
• If CO2 doesn’t warm, size of temperature
changes is unexplained.
•
•
•
•
+ An ice-age analogy… Suppose
I overspend my credit card and go into debt;
Interest charges increase my debt;
The interest charges clearly lag the debt;
But, an accountant must consider interest as well
as overspending to explain my debt;
• For the ice ages:
 Orbits force temperature as overspending forces debt;
 Temperature, others force CO2 as debt forces interest;
 CO2 amplifies temperature as interest amplifies debt;
• Lots of apparently rational people say “CO2 lags
warming, so CO2 cannot contribute to warming”.
They also must believe that interest charges
cannot contribute to debt.
Without change in human behavior,
our students will see the CO2 leave
the page, and we may see this, too.
Independent dating;
Fairly clearly
same record.
Cold Greenland
=dry China
Ice-dammed floods
Ice-sheet surges
Image courtesy
W. Curry, WHOI
DRY
+ North Atlantic abrupt changes
• “Switch”=freeze vs. sink in winter;
• Difference>20oC in winter locally;
• Big deal is probably rain in the monsoons--fresh
and winter-frozen North Atlantic has gone with
dry monsoon (few billion people…);
• >90% confidence that Greenland will melt slowly
enough to avoid this;
• But >90% is not 100%;
• Like a lot of things in the climate-change game-even if unlikely, there is a possibility of really
bad things.
+ So where are we???
• Ice cores outstanding--an immense amount
learned, more to come, highly reliable;
• This deeper-time perspective supports the
shallow-time perspective of climate science:
Changes in sun matter, but are slow (orbital) or
small (faster);
Volcanoes unorganized noisemakers;
Cosmic rays, magnetic field, space dust, others
you might think of really don’t matter much;
Greenhouse gases (esp. CO2) matter a lot;
There may be surprises in the greenhouse…