E. Christa Farmer
Assistant Professor, Department of Geology
There’s an old saying that goes, “Climate is what you expect, weather is what you get.” This past February, a large international group of scientists associated with the United Nations and the World Meteorological Organization issued their consensus report on the current state of knowledge about the climate system. Called Climate Change 2007: The Physical Science Basis for Policymakers, the report is the fourth in a series issued by the Intergovernmental Panel on Climate Change (IPCC). The sobering conclusion by hundreds of scientists from dozens of countries with “very high confidence” was that “the globally averaged net effect of human activities since 1750 has been one of warming” (Alley et al., 2007). More specifically, the scientists said, “Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”
Since the industrial era began at the end of the 19th century, humans have increasingly relied on fossil fuels such as coal, oil and natural gas. Burning all of these for energy releases carbon dioxide, which has now reached levels in the Earth’s atmosphere that are unprecedented in the last 650,000 years. These excessively high levels of carbon dioxide, among other so-called “greenhouse gases,” are being held responsible for several changes in the Earth’s climate system over the last century or so: an increase in global average temperatures, an increase in the incidence of heat spells, an increase in the extent of drought, an increase in the percentage of rainfall in a given area that comes in “heavy events,” and an increase in incidents of extreme high sea level. All of these effects are expected to intensify in the coming century. Even if we drastically lower our emissions of greenhouse gases, the planet will still likely see more of these effects in the coming century. This is because of inertia in the climate system and the long life of carbon dioxide in particular in the atmosphere.
Therefore, unless we undertake the enormous engineering challenge of removing carbon dioxide and other greenhouse gases from the atmosphere, we are likely to face a future with very different weather than we are currently used to. In order to prepare for this future, we need to understand how the Earth’s climate system operates. We need to know the mechanisms by which the oceans, the atmosphere, and the land surface processes interact to create the long-term patterns in the daily weather we experience. Only by fully understanding these mechanisms can we make accurate predictions about the results of our perturbations of the atmosphere and more comfortably adapt to the changes.
The geologic record cannot only be very helpful in the endeavor to understand our planet’s climate system, but it’s also really the only chance we’ve got to “experiment” with the actual planet. Computer models can do a fairly good job of simulating the atmosphere, and have now successfully been linked to other computer models of ocean and land surface process components. But the resolution of these models is limited by available computing power, and some small-scale processes can only be approximated. Unique clues exist in the geologic record about how the climate system operated in the past under very different conditions, and how the system shifted from one state to another. These clues can help us understand how the system might shift in the future.
The Geologic Record of Climate Change
A compelling history of the Earth’s climate system can be extracted from sediments on the floor of the oceans; this particular corner of geology is called paleoceanography. Like many geologists, paleoceanographers rely on two longstanding geologic principles. One is known as the Principle of Original Horizontality, and can be readily observed in lakes or streams or deltas: sediments being deposited tend to be spread out in layers that are roughly horizontal. This is especially true of the relatively quiet waters of the deep ocean. The other is known as the Principle of Superposition, and is inferred from the first principle: layers of undisturbed sediments get older from the top to the bottom.
Imagine snow quietly accumulating on the ground, and you can visualize how the best locations for paleoceanographic research accumulate sediments. Fine clay particles from continental erosion are carried out in the relatively slow-moving ocean currents and gradually settle to the sea floor. Like DNA left at a crime scene, these sediments contain evidence for environmental conditions at the time the sediments were accumulated. Some of our best paleoceanographic clues come from fossils of little organisms known as foraminifera (Figure 1). These one-celled creatures create a calcium carbonate skeleton, just like humans have, when they grow in the surface layers of the ocean. After a lifetime that passes in about a month for these organisms, the organisms die and sink to the bottom of the ocean. Some of their skeletons, known as tests, are preserved in the sediments and buried by subsequent deposition.
Since the foraminifera have specific habitat preferences, the ratio of the abundance of one species to the abundance of another can tell us something about the environmental conditions prevailing when those organisms lived. If one foraminifer prefers warm conditions, and another prefers cold conditions, an increase in abundance of the first through time tells us that conditions warmed. The chemistry of the skeleton also gives us terrific clues. Experiments with certain species of foraminifera grown under known conditions have shown that as temperatures increase, the ratio of magnesium (Mg) to calcium (Ca) in the skeleton increases as well (Figure 2) (Mashiotta et al., 1999). There are thermodynamic reasons why it is more favorable to substitute Mg for Ca in the calcium carbonate at higher temperatures, although the biological processes in the living organism also affect the ratio somewhat.
So if we collect deep-ocean sediments of different ages, track the number of individuals of different foraminiferal species with known ecological preferences, and measure the Mg/Ca of their skeletons, we can construct a detailed history of the temperature of the surface ocean above that spot. Oceanographic research vessels have collected deep-ocean sediments all over the world in the last 60 years or so for just this purpose. These ships have equipment that can be lowered several kilometers to the bottom of the ocean in order to extract sediment “cores” from the seafloor below. This process is like pushing a straw into a layer cake, and removing it to see the layers inside the cake. We just need a “clock” for these sediments, a way of linking the sedimentary record in one place with records in other places.
As it turns out, radiocarbon dating is extremely useful for this purpose over the last 50,000 years or so. Like all living organisms, foraminifera incorporate the radioactive isotope of carbon into their tissues, including their skeletons, while they are living. This radioactive isotope decays with a half-life of 5,730 years. Since the organism no longer accumulates new radioactive carbon after it dies, the ratio of radioactive to stable isotopes gives an age for the fossilized material.
Finding the “Younger Dryas” in the Southern Hemisphere
Around 20,000 years ago, during the period commonly known as the ice age, New York City was buried under about a mile of ice. Huge continental ice sheets covered most of the northern edges of the continents in the Northern Hemisphere. Isotopes in water and air bubbles trapped in some of the remaining continental ice sheets, such as those covering Greenland, indicate that when this last glacial period ended, it did not end smoothly. Instead of simply a gradual warming, climate abruptly warmed to temperature values about as high as today, and then abruptly cooled again to glacial temperature values. This resurgence of glacial temperatures (known as the “Younger Dryas”) began about 13,000 years ago and lasted about 1,500 years before climate abruptly warmed again to modern temperature values (Stuiver and Grootes, 2000). Scientists believe that this cold snap was triggered by the sudden input of glacial meltwater to the North Atlantic. This sensitive region is the site of most of the world’s deep water formation, meaning that surface waters sink in the North Atlantic and spread throughout the bottom of the world’s oceans. This circulation, known as the “conveyor belt,” was probably disrupted by the input of melting ice sheets, with apparently global implications for climate.
Understanding the mechanism of the Younger Dryas climate shift is important for understanding future climate changes. Changes in oceanic circulation have been implicated in the Younger Dryas climate shift; changes in oceanic circulation have also been predicted as one possible side effect of anthropogenic (or, caused by humans) global warming (Clark et al., 2002). Computer models suggest that as the planet warms and land-locked glaciers melt, freshwater input to the North Atlantic would increase. Because the formation of deep waters depends in part on the great density of salty North Atlantic waters, decreasing this density by adding fresh water disturbs the “conveyor” circulation. The most recent IPCC report rates the chances of slowing the Atlantic limb of the conveyor circulation during the 21st century as “very likely,” although the report indicates that it is “very unlikely” that any transition this century would be large and abrupt.
Geologic records documenting the global climate changes associated with the Younger Dryas are sparse in the Southern Hemisphere, however. A compilation of high-resolution paleoclimate records of the Younger Dryas published in 2003 listed nine locations in the Northern Hemisphere, but only one in the Southern Hemisphere (Broecker, 2003). My doctoral research at the Lamont-Doherty Earth Observatory of Columbia University successfully located another high-resolution record of the Younger Dryas in the Southern Hemisphere (Farmer et al., 2005).
Working with my doctoral adviser, Peter deMenocal, and a postdoctoral researcher in his lab at the time, Tom Marchitto, I set out to determine a timescale for the last 20,000 years for a core from the Benguela upwelling system. Collected by the Ocean Drilling Program in 1997, the core known as “ODP1084B” is located about 300 km off the coast of Namibia. We chose to work with this core because it is located in a strong coastal upwelling system: prevailing winds blow surface ocean waters away from the coastline, bringing deeper cold ocean waters to the surface. This means that any paleoceanographic climate signal found here would probably involve atmospheric circulation as well. As Figure 3 indicates, the trade winds blow predominantly parallel to the coastline here, pushing surface waters away from the continent and bringing deeper, cooler waters to the surface.
We used radiocarbon to date the sediments in the upper 360 cm of core ODP1084B, and found that the sedimentary layers record the last 22,000 years smoothly with few disturbances (Figure 4a). The arduous part of the research included counting the numbers of different fossil species in each layer of sediment. Fortunately, we had help here from Martha Dees, a very experienced technician. A species known as Neogloboquadrina pachyderma (left-coiling) is associated with the core of the upwelling zone, and increases in its relative abundance are inferred to represent an increase in the intensity of the upwelling (Figure 5). We also measured the Mg/Ca in the fossils of another species, Globigerina bulloides. There is a sharp decrease in the Mg/Ca, which represents a decrease in sea surface temperature (SST), between 11,500 and 13,000 years ago (Figure 4b). Relative abundance of N. pachyderma (left-coiling) also increases slightly during this time (Figure 4c). Both of these signals are consistent with an increase in the coastal upwelling during the Younger Dryas; this implies involvement of Southern Hemisphere wind patterns in this climate shift.
Implications for Future Climate Change
The evidence we have collected from ODP1084B shows that the Younger Dryas climate shift, long known to involve changes in the North Atlantic, also involves significant changes in the tropical South Atlantic. This global effect of the Younger Dryas emphasizes the far-reaching impact that reorganizations of ocean circulation can have on the planet’s climate system. The likely mechanism for such impact seems to be that changes in the deep ocean “conveyor” circulation alter tropical sea surface temperatures (SST), which alters African monsoon circulation (Weldeab et al., 2005). Since predictions of future anthropogenic climate change include alterations of the deep-ocean circulation, understanding the precise interactions contributing to past events like the Younger Dryas will help us anticipate changes we may see in the future. My future laboratory research, along with that of my colleagues, will include extending the paleoceanographic record of ODP1084B back in time several thousand years. In addition to evidence of the Younger Dryas, there is some evidence to suggest that other significant climate events had an impact on the South Atlantic as well. Known as “Heinrich Events,” these climate events were also first recognized in the North Atlantic. Various layers of deep ocean sediments in the North Atlantic contain pebbles that are too large to have been carried by ocean currents – the most likely explanation for their presence is an increase in the amount of ice drifting out to sea from the glaciers that ring the North Atlantic. Pebbles frozen into the ice get dropped to the sea floor when the ice melts. Studying natural climate cycles such as Heinrich Events is important because we need to understand how the climate system operates in order to predict future changes.
Heinrich Events have been proposed to have a tropical trigger: two paleoceanographers proposed that the African monsoon system is strengthened every several thousand years by redistribution of solar energy due to variations in the Earth’s orbit, and that this strengthening of the atmospheric circulation alters the upwelling along the equator and the export of heat to the high latitudes, ultimately causing Heinrich Events (McIntyre and Molfino, 1996). If it’s true that Heinrich Events are triggered by changes in tropical climate, then we would expect to find evidence of these changes outside the North Atlantic. Finding more evidence for these events in the Benguela upwelling system in the Southern Hemisphere would build support for McIntyre and Molfino’s hypothesis.
The lesson of the Younger Dryas climate event warns us that changes in climate in a small region such as the North Atlantic can have global consequences through alterations in the deep-ocean circulation. The lesson of Heinrich Events may be that small changes in tropical climate can have global consequences through alterations in atmospheric circulation. Gradually, as we learn more about the climate ì system, we can prepare for the changes we are likely to see in the future. Otherwise, the weather we get will definitely not be what we expect.
This work would not have been possible without generous assistance from many others. Walter Hale of the Ocean Drilling Program core repository in Bremen supplied samples; Martha Dees helped with picking and sample preparation; Dan Schrag and Ethan Goddard provided initial training on their Inductively Coupled Plasma Atomic Emission Spectrophotometer; and Tom Guilderson performed radiocarbon analyses. Emma Christa Farmer was supported by a Graduate Research Environmental Fellowship from the Global Change Education Program, which is administered by the Oak Ridge Institute for Science and Education for the U.S. Department of Energy’s Office of Biological and Environmental Research. The Lamont-Doherty Earth Observatory of Columbia University Climate Center, the National Science Foundation Marine Geology and Geophysics program, and the Hofstra University Faculty Development and Research Grant program also provided support.
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