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.