Past and Current Climate Change: Implications for the
Future
Lonnie Thompson
(Department of Geological Sciences &
Byrd Polar Research Center, Ohio State University,
Columbus, Ohio, USA)
University Distinguished Lecture, Ohio State
University, Columbus, Ohio, USA
2 November 1998
What
physically causes climate change? This is what we’re trying to
determine. We’ve been taking ice cores from around the world - Greenland,
Antarctica, plus high-elevation mountain sites at lower latitudes.
Can
get lots of information from ice cores: looking at temperature curves
reconstructed from stable isotopes of O2 and H2;
atmospheric chemistry (looking at soluble material archived in the layers); net
accumulation/precipitation (from separation/thickness of layers), dustiness of
atmosphere (how strengths of winds and source areas for dust have changed
through time). New information from lower-latitude ice cores includes
vegetation changes from pollen (400-4000 pollen grains per liter of
water). Can get a handle on volcanic history on Earth - the volcanic
history from historical records is only good to ~200 years ago - need to look
at how volcanoes have changed climates, plus what the frequency and magnitude
of the volcanically induced changes have been. Also looking at
anthropogenic emissions - gas trapped in bubbles (CO2, CH4,
NOx variations through time). Another new area of information
from ice cores is looking at trapped organisms in ice cores.
High-latitude ice cores from tropical areas give monsoon records.
The
temperature on Earth is increasing - the record goes back 150 years, which is
geologically very short. Eight of the warmest years on record have
occurred since 1988. Not just greenhouse gases are doing this - several
interacting parameters affect Earth’s climate. We want to know how much
of this recent change is due to anthropogenic causes.
A
cross-section from an ice core shows ice crystals and trapped air bubbles.
Ice
core data from the year 1720 onward shows an exponential increase of CO2
in the Earth’s atmosphere. This increase is due to industrialization and
an increase in the global human population.
Antarctic
ice cores over the last 170,000 years - temperature tracks CO2
levels in the Earth’s atmosphere. When we have higher CO2
levels, we have warmer temperatures. Today’s concentration of CO2
in the atmosphere is 360 ppb by volume, and is increasing by 1.2 to 1.5 ppb by
volume per year. Projecting this into the future by 60 years, we find
that implementing the Kyoto Protocol pushes back reaching 600 ppb by only 20
years. So, the Kyoto Protocol is only a first step in tackling the
problem.
This
year (1998): every month this year has broken the warmest average temperature
record for that month. 1998 probably will be the warmest year on record,
and will probably be the warmest year in the last 600 years. This doesn’t
prove global warming is taking place, but this is what we’d expect if it was
occurring.
Ice
core records from low-latitude sites: first, we’ll look at the Tibetan Plateau,
the largest plateau on Earth. The heating of this plateau drives
monsoons, which strongly affect crop production and farming in this part of the
world.
We
have ice cores from the Dunde Ice Cap, in the north-central part of the Tibetan
Plateau, and from the Guliya Ice Cap, in the far western part of the TP.
In 1997, ice cores were recovered from the top of the Himalayas at the far
southern end of the TP. Elevations of these sites are all 400-500 mb atmospheric
pressure. These ice caps are quite large. The Dunde Ice Cap is 55
km2.
The
length of ice record attainable at these high-elevation sites depends on 3
factors: 1) how much snow falls each year on the summit of the ice cap; 2) how
thick the glacier is - in the case of the Dunde Ice Cap, it is 140 meters
thick; and the most important factor controlling the length of the time record,
3) temperature at the ice-bedrock contact. If the glacier is frozen to
the bed, and has remained frozen to the bed through time, then time cannot be
removed, but it can be very much compressed.
We
recover 2-3 records so we can look at duplication of signal in these various
archives.
The
Dunde Ice Cap is the first ice core record of the most recent glaciation from
outside the polar regions. Oxygen isotope values during the glacial
period are more negative, and dust content is increased, and solubles are
decreased, indicating more water in that part of the world at that time.
In this ice core, as one proceeds upward to the present, the oxygen isotope
profile during the glacial period is becomes enriched. If 50 year
averages are taken over the last 12,000 years in this part of the world, can
see warmer and colder 50 year period excursions. The most recent 50 year
period (this particular ice core was drilled in 1987, so from 1937 to 1987) is
the warmest in the preserved ice core record.
Guliya
Ice Cap (western part of Tibetan Plateau) - summit elevation is 6200
meters. The ice cap covers ~200 km2, but it is embedded in an
ice mass that covers >8000 km2. The ice mass ends in lobes
and vertical walls. On the summit, the area is flat. This
particular summit is higher than Mt. McKinley. This record has provided
us the first look of the entire last glacial episode from outside the polar
regions. Comparing 3 profiles (from Greenland, Guliya, and Vostok,
Antarctica), we see that in the polar regions (both north and south), there is
little response, temperature-wise, to the interstadials (warmer periods).
But, looking at the methane in these cores, see very large peaks during
interstadials (from methane emissions from wetlands in the tropical
regions). Low-latitude cores show rather large changes in temperature
from stadials to interstadials compared with the polar records. In the
last 10,000 years in the polar regions, there is very little change in
temperature, but methane dips correspond to temperature changes seen in lower
latitude ice cores.
Looking
also at chlorine-36 in ice cores. Cl-36 has a half-life of ~300,000 years.
The mean value over the last 100,000 years allows us to get a production rate
of Cl-36, which allows us to date the bottom of the core. Lower dates are
~750,000 years BP - one of the oldest ice cores recovered to date on
Earth. In the last 100,000 years, a very large event occurs at ~40,000
years ago - it shows up in all polar cores. Not sure what caused it.
Now,
moving to the Himalayas in the southern end of the Tibetan Plateau - a new site
was drilled in 1997 - the first drill site was at an elevation of 7000
meters. In this part of the world, moisture comes from monsoons. At
the ice divide, drilled 2 cores down to bedrock - each is ~167 meters
long. The drill site was near the border between China and Nepal.
Have a beautiful annual signal in these cores. Get 20 per mil variation
in isotopes in each annual seasonal cycle. Also see annual variation is
dust and nitrates. Can tell how monsoons have varied through time.
This record will cover at least 20,000 years for this part of the world.
El
Niño record - impacts not only North America. El Niños are accompanied by
droughts and monsoon disruptions. Warm phase of El Niño results in
reduced rainfall in India and reduced rice yields. During the La Niña
phase (opposite cycle), get increased rice production.
Ice
core records from atop the Andes can give us a picture of this phenomenon
through time. Three sites have been drilled in the high Andes. Ice
is a viscous fluid, and as it moves off the higher slopes of the mountains,
crevasses form. Get a detailed ice core record from the Andes, especially
in the last 100 years. Have a record going back ~20,000 years. See
a very strong cooling of 6.3 per mil from the early Holocene. Shows
cooling periods throughout the Holocene, including the Little Ice Age, and a
very strong warming over the last couple 100 years. At 6000 years ago at
this site, we see that it was as warm as it is today and even earlier, we see
warmer values than today, due to natural variation in climate. Nitrates
come from the Amazon rain forest to the east. See lower forest cover
during glacial periods when it was colder and drier and increased forest cover
during warmer intervals, but there is a lag between increasing temperature and
a peak in nitrates. Dust - see a 200 fold increase during the glacial
maximum, and very little dust activity since then, except a very large event at
4500 years ago (a peak in dust content since the last glacial period), which
represents a very strong, 300 year drought.
Another
ice cap core in the Andes was drilled to the south of this - it was the first
tropical ice cap ever drilled (done in 1983). Information from this core
has already been used by archaeologists and anthropologists to look at what was
going on in this part of the world before the Spanish arrived in 1531.
The native cultures were advanced but had no written documentation. We do
know that they were agrarian. Where these archaeological sites exist in
Peru - coastal deserts, where they depended on water coming from the metling
glaicers high up in the Andes, or they are up on the plateau in southern
Peru. A precipitation history from the ice core goes back to 470
A.D. We see wet periods and droughts. The rise and fall of
cultures: when it is wet on the plateau, highland cultures fluorished; when it
was dry on the highlands, get coastal cultures developing along the coasts of
Peru and Ecuador.
1998
El Niño in Peru - had heavy rainfall in desert areas to the north, where you
got formation of very large lakes out in the sand dunes, and we have drought in
the southern parts, resulting in a drop in water levels in Lake Titicaca.
So, we see high-frequency oscillations in precipitation today, associated with
El Niño, but on the longer term, we see lower frequency oscillations due to El
Niño.
Compared
precipitation records from Peru and the Guliya Ice Cap in Tibet - see long
periods of drought and wet periods throughout the last 1000 years.
The
most recent site in South America - 6542 meter elevation. The Sajama
site. This is a volcano with an ice cap, the last ice cap in the tropics
until you get south to Chile or Argentina. Recovered 2 cores to bedrock
from the summit. This is a unique core - contains insects. C-14
dates age a 100 meter-down insect specimen at 6000 years BP. A number of
these organics occur in the core, allowing for a series of C-14 dates in the
core - the first C-14 dates for any ice core. The 2 cores show good
reproducibility in the record. Isotope depletion occurs in the lower 28
meters, accompanied by a decrease in dust. This part of the world has
seen tremendous climate changes due to natural variation. We can relate
the climate change record in this ice core with a time scale based on tritium
(atomic bomb) horizons, ash horizons from known volcanic eruptions, C-14
dating, and layer counting. All these methods have resulted in a detailed
time scale on this core. When it is colder, get an increase in
accumulation in this area. When that occurs, we get an decrease in
dust. This ice core will be written up in an article in Science in
the very near future.
CLIMAP
in the 1970s was a project that suggested that the tropics were a pretty
mundane place, even during glacial maxima, and that temperatures there wouldn’t
change, or would even increase in some areas. Our ice core records now
show that changes in isotopic composition from the warm period we’re in now to
the last glacial maximum from Greenland to the Andes to the Tibetan Plateau to
Antarctica were all about the same. Those changes from the late glacial
maximum to the Early Holocene shows very little difference between high
latitudes and low latitudes. This suggests global cooling, including
tropical areas. This cooling in the tropics is supported from evidence
from corals and from noble gases in groundwater from Brazil.
The present: we’ve
been struck by how rapidly things are changing in these tropical
glaciers. Annual cycles now are being melted at the peripheries of these
ice caps, as well as at the summits, even since the 1970s. The ice masses
themselves - the rate of retreat is increasing exponentially on some monitored
glaicers/ice caps. A rapid retreat of ice margins is going on in these
tropical glaciers.
We are seeing a global
retreat in glaciers, except some glaciers in Norway and Sweden, which are
growing because the precipitation that used to feed the Alps is being deflected
northward. Projections: 50% of expected sealevel rise is coming from the
melting of mountain glaciers and 50% from volume expansion of warmer ocean water.