Overview (Scientific Content below)
There have been a variety of discussions in the past week about what ice core data tells us. There are several misconceptions about what information the ice core data contains. Some people believe that it is the air bubbles trapped in the ice that tell the temperature history of the Earth and others believe that the ice cores tell the temperature history of the location that the ice core came from.
Both of these believes are in fact incorrect. The reason is simple. It is the water itself that tells the temperature history in the ice cores. Since ice cores are a measure of the water in the ice, what the ice core is actually measuring are the conditions of the oceans that the water originally evaporated from.
There are two methods of determining the past temperature. One is to measure the ratio of heavy and light oxygen atoms in the ice and the other is to measure the ratios of heavy and light hydrogen. Since these are the two components of water there are plenty of both to measure in ice. The ratio of heavy and light oxygen is the standard measurement.
Water that is made of the light oxygen evaporates more easily. Water made of the heavy oxygen condenses more easily. This means that the warmer the oceans are near the location of the glaciers or ice sheets, the more heavy oxygen there is in the ice core.
Another way to explain it is how warm the ocean is near the location where the ice core is from. The less heavy oxygen there is, the colder the water is near the glacier. The warmer the water is near the location of the ice core, the higher the content of heavy water there will be in the core.
The water that evaporates and falls as snow on a glacier (or ice sheet, but I will only use glacier now) records how warm or cold the water near it was. Then the layer the year after that tells the story of that year. Each layer is recorded one on top of the other. Scientists then measure each layer and count down how many layers they are from today. Then they can know the story told for an exact year. It is like counting the rings on a tree, but it is actually much more accurate. It also tells a much larger story than a tree (or forest) possibly could.
Since Greenland is near the end of the path for the Gulf Stream and most of the water vapor in the atmosphere in that region is from the Gulf Stream, ice cores from Greenland are a good indicator of the ocean temperatures in the north Atlantic Ocean. Ice cores in Alaska tell the story of the north Pacific Ocean. Different regions of Antarctica tell different stories based on the weather patterns of that region.
The simplest reason that it works this way is that cold water does not evaporate very much. Water that is 25 °C (77°F) evaporates about twice as much water vapor as water that is 15 °C (59°F). Since there is much more light oxygen than heavy oxygen, cold water evaporates very little heavy oxygen. The warmer the water, the more heavy oxygen is released.
So ice cores are really telling a larger story than they are often given credit for. That is why they are so useful in understanding the global climate. A coral reef can tell a story, but only at the exact location that the coral exists. Since many of the coral reefs are near the equator, they see less of the change in the Earth’s climate and so they tell a very small portion of the story. This is especially true for the past couple of million years as most of the climate changes have been far stronger in the Northern Hemisphere than they have in any other place on Earth. There are very few corals in that part of the world.
This is also why ice cores in Antarctica can measure changes that are mostly happening in the Northern Hemisphere. Temperatures in Antarctica do not change as much as the ice cores indicate. What does change is how much warm water is close to Antarctica. During a glacial period (ice age) the oceans near both poles are much colder so the amount of heavy oxygen is very small. When the the northern hemisphere is warmer (like now) the oceans have a higher sea level and warmer water is closer to the both poles.
Even if a location in Antarctica stayed exactly the same temperature for 100,000 years, the ice core at that location would tell the temperature record of the ocean that evaporated the water that fell as snow at that location. In this way ice cores do not reflect the temperature of the location they are drilled. Ice cores primarily tell the record of the ocean the snow evaporated from and how far that water vapor traveled.
Any type of record that involves the ratio of oxygen that has fallen as rain can also tell the same story. Water that drips in caves to form stalagmites and stalactites can also be used to determine information like this. In places where there are no glaciers this type of thing can be done, but it is more complicated.
Ice cores give the broadest temperature reconstruction because of how the record accumulates. Each layer is distinct and can provide a wide view of the climate for the region for that specific year. This information is recorded for the period that matches the age of the glacier. The bottom and older layers do get squeezed by the weight above, but reliable ice cores that are hundreds of thousands of year old have been recovered.
The specific oxygen isotopes are 16O and 18O. The hydrogen isotopes are hydrogen and deuterium. These are often called the stable isotopes. They are used precisely for the reason that they are stable over time. There can also be combinations of the the different isotopes in a water molecule.. The heavier the overall molecule is, the less it will evaporate and the quicker it will condense. Anything that triggers condensation will drop the amount of 18O that is present. Altitude is another factor that makes a difference as it also decreases the 18O content of the ice.
What this does is complicate the comparison of ice cores. One location will have a different general path that the water molecules take. An ice core from a lower location will generally have a higher ratio of 18O. This makes comparing ice cores difficult. Each one must be separately calibrated to temperature. For this reason ice core data is not often converted to temperature, but left as isotope ratios. Plotting the isotope ratio’s will show the temperature history, but not calibrated to temperature. So scale is a factor that can be ignored or calibrated. Precise scale calibration is usually not needed though as the relative changes are sufficient.
This is also why different ice cores have different ranges in the isotope record. The farther they are from the ocean source, the less the range will be. Some of the Greenland ice cores have a very large range as the Gulf Stream can get warm water close to Greenland. The Taylor ice core from Antarctica has about half the oxygen isotope range that those in Greenland do.
The Vostok or EPICA ice cores deep in Antarctica use the deuterium ratio’s because those will resolve at that distance since so little heavy oxygen makes it that far. That makes the oxygen isotopes less useful in those locations. The light oxygen with the deuterium can make it that far though and those ratios can be used instead of oxygen.
This should make it clear that ice cores do not tell the local temperature. There is no mechanism for the temperature of a location on an ice sheet to dictate the stable isotope ratios in the snow that falls there. It is truly a function of the distance and path from the location of evaporation to the location that the water molecules became part of the ice sheet.
No single type of record carries as much of a global temperature signal as the ice cores do. That is why they are so often used in paleoclimatology. A single ice core reveals a broader picture than any other method. The main limiting factor for ice cores is of course location. They only exist where there is permanent ice sheets. Glaciers that have a flow are also useless as the record from a core is not a time series. That is why only certain glaciers or parts of glaciers can be cored.
Another problem is glaciers on locations that have warm rock (ie on a volcano). The bottom of the glacier is often melting and the age of the glacier is difficult to determine. That makes the temperature of the bottom ice of a core important. Cold ice at the bottom is an indication that the bottom ice was the forming layer of the glacier. That information can also be very, very useful.
Tags: ice core