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Climate Records


Google the word “hockey stick” and in addition to advertisements for sporting equipment, you will find links to one of the most chronicled and scrutinized datasets in modern science, the climate record of the last millennium. The curve was originally documented in a paper authored by Penn State Meteorology and Geoscience Professor Michael Mann and his colleagues. As shown in the figure below, the curve shows fairly constant temperatures or even a modest cooling from 1000 to about 1900 AD. About 1900, a sharp warming trend began, the base of the hockey stick. This warming has continued almost unabated to the present day. It so happens that the base of the stick corresponds to the beginning of the industrial revolution and the time when the release of greenhouse gases to the atmosphere spiked. The correlation of warming and the increase in levels of greenhouse gases, which are known to trap heat in the Earth’s atmosphere, is hard to argue with.

Data from thermometers and from tree rings, corals, ice cores, and historical records
The famous "Hockey Stick" curve showing the increase in global temperatures since about 1900.
Credit: Intergovernmental Panel on Climate Change 2001

Perhaps the most scrutinized aspect of the hockey stick graph, and where the scientific controversy has focused, is that a part of the data set derives from temperature measurements that have been made since about 1900. The remainder of the curve derives from historical records and "proxy" measurements that are made indirectly. "Proxies" are substitute measurements that are made to determine the conditions at times before humans could measure climate directly. Such proxies serve as the basis for our understanding of past climate. In this module, we will learn how proxy measurements are made.

Fortunately, the history of the earth, including the evolution of its physical environment and its life forms, is beautifully preserved by a variety of Earth processes. For example, as we discussed briefly on the last page, sediments deposited in the ocean basins accumulate in a layer-cake fashion and entrap the fossil remains of once-living organisms. In addition, when these organisms, including species of clams and corals, were living, they recorded Earth’s climate history in the chemistry of layer upon layer of their shell. Images of a layered clamshell and coral skeleton are shown below.

Samples of Climate Records

A core from the Ocean Drilling Program that contains evidence for a global warming event 55 million yrs ago The cross section of a clam shell showing individual layers that represent seasonal alternations. Section through a coral showing growth bands that represent seasonal layers.

Click on the images above to see a full-size image and complete sources information.

Video: Sedimentation (1:02)

Click for the video transcript.

There are three different sources of sediment to the deep sea sedimentary record. The first is terrigenous material that is delivered via rivers into the oceans. The second is dust that is blown in from the continents by winds. And the third is biogenic material, formed by organisms that lived on the surface of the ocean that died and then subsequently rain came down through the water column to the seafloor. Regardless of the source, these types of material are buried in the deep sea sedimentary record. The nature of the sedimentation process is such that the youngest material lies close to the sea floor and as the sediment becomes progressively deeper, it becomes progressively older. Thus, the deep sea sedimentary record provides a beautiful inventory of ancient climate from younger to older.

As we will detail below, chemical proxies allow the climate to be reconstructed from shell materials many hundreds of millions of years old. In fact, some of the best records of Earth’s climate are obtained from mile upon mile...., sorry kilometer upon kilometer, of cores removed from the ocean floor. The current ocean basins are less than 180 million years old, meaning that only the last 180 million years of Earth’s history are preserved in the current ocean. Fortunately, large-scale Earth tectonic processes have elevated vast piles of sedimentary rocks onto the land where they are preserved in mountains and elsewhere. These materials can be sampled in road cuts and cores and climate information gleaned from them, taking our understanding of climate history much further back.

Rocky coast on the South Island, New Zealand
Foreground shows sedimentary rocks uplifted on land on the South Island, New Zealand.
Credit: Tim Bralower

The major ice sheets have accumulated layer by layer over time. These layers record temperature, precipitation, and wind patterns at the time the ice formed. Tiny gas bubbles trapped within the ice preserve the composition of the atmosphere including levels of CO2 and methane (CH4). Kilometer upon kilometer of cored ice and the bubbles within it preserve an incredible record of Earth's climate variations for hundreds of thousands of years. See the images shown below of the GISP2 Ice Core and a sample of atmospheric gas bubbles in ice.

GISP2 Ice Core from Greenland
Photographs of changing properties of the GISP2 Ice Core from Greenland. From near the surface (upper photograph): unconsolidated ice, to 1800 meters depth (middle photograph): layered consolidated ice, to near the base at 3050 meters depth (lower photograph): showing ice with eroded sediment.
Credit: National Air and Space Administration

Video: Ice Accumulation (1:34)

Click for the video transcript.

Ice cores provide valuable inventories of ancient climate. This slide here shows various levels of the ice core, illustrating how the physical properties of ice change from the surface down to the bottom. Ice formed from snow that was precipitated on the top of the glacier. The uppermost panel shows the properties of snow, very loosely compacted, showing the few annual layers that we'll describe later on in the snow and very small track bubbles of air. In the middle panel, from deeper within the glacier, approximately 1838 meters down, the ice is more compacted and shows a very strong annual banding with darker layers indicating slow accumulation rates of ice during the warmer months and lighter layers showing the rapid accumulation of ice during the winter months. In this layer, you can see nicely, trapped bubbles of gas. Close to the bottom of the glacier where erosion of the underlying bedrock is common, the glacier has a dirty color, a brown color, because of the content of eroded minerals and rock particles in the ice. This here shows the glacier close to the base, about 3000 meters below the surface.

The accumulating ice also preserves dust, smoke, pollen, and ash from volcanic eruptions. The texture of summer snow and winter snow is very different and the resulting layers provide very accurate means to determine the age of ice cores as does C-14 dating of the CO2 trapped in ice bubbles. In this way, the composition of the atmosphere can be determined hundreds of thousands of years back in the past. Imagine this, the air that your great-great-great-great grandparents breathed has been collected in some lab around the world!

Atmospheric gas bubbles in ice core sample from the Taylor Dome core in Antarctica
Sample from the Taylor Dome core in Antarctica. The depths are indicated, and scale is shown.
Credit: Alley, R.B., and J.J. Fitzpatrick. Conditions for bubble elongation in cold ice-sheet ice. Journal of Glaciology 45(149), 147-154 (1999).

Check Your Understanding

Climate Question

Where is the base of the hockey stick?

A. 1700
B. 1800
C. 1900
D. 2000

Click for answer.

C. 1900 is when global temperatures started to rise rapidly as the result of anthropogenic activity.