Here is one of the oldest tree ring records ever put together. For this type of tree (the Bristlecone Pine) thicker growth bands represent warmer summers. Note that are distinct times of warmer and cooler summers over the last several millenia:

Other Examples:

Assemblage Changes:

Problems with Biological Proxy Records:

One Record from only One Location (how representative is it)?

How good is the link between the Proxy record and the Climate record (is the proxy well calibrated)?

Physical Records  left by physical processes (sediment or ice accumulation)

Examples:

Problems with Physical Proxy Records:

One Record from only One Location (how representative is it)?

Limited scope of interpretation (regional runoff/precip.; recent temperature change)

Lack of sub-annual resolution

Chemical Records of changes in the chemistry and isotopic composition of important environmental fluids (air, water)

Isotopic Records

Isotopes are atoms of an element that have different atomic weights (sum of protons and neutrons)

Radioactive isotopes spontaneously alter themselves by emitting subatomic particles, gamma rays. Many of these isotopes can be used for dating pre-recent records (e.g. ¹⁴C).

Stable Isotopes do not spontaneously change; however, natural processes can fractionate them so that the lighter isotopes are preferentially selected, leaving the heavier isotopes behind.

Degree of Fractionation of stable isotopes:

Carbon: ¹²C selected over ¹³C when plants build cellular tissue.

RESULT:  As more organic carbon created and/or buried, the global pool of carbon has a higher proportion of ¹³C (is "heavier")

Oxygen: ¹⁶O selected over ¹⁸O during evaporation of H₂O.

RESULT 1:  the more water stored on land, the more ¹⁸O in ocean water

       ¹⁶O selected over ¹⁸O when marine organisms build CaCO3 shells. The selection depends both on:
1)¹⁸O/¹⁶O ratio in the water and
2) the temperature of the water. The warmer the temperature, the more ¹⁶O is selected.

RESULT 3:  ¹⁸O/¹⁶O Ratio measured in ice in the Greenland and Antarctic ice sheets tends to be rich in ¹⁶O and have strongly negative values relative to a standard (Standard Mean Ocean Water). The more negative values are taken to indicate colder climate.

On a shorter timescale we can look at the ice record from the Greenland ice sheet that accumulates ate a higher average rate than ice at the South Pole, and thus gives us a higher resolution record. Here we show not only the oxygen isotope proxy for temperature on the ice sheet as it warms from the most recent advance of glaciers, we also see a record of the concentration of methane (CH₄) in the atmosphere as preserved in tiny bubbles in the glacial ice. Air bubbles ice are an extremely accurate and valuable record of how the atmospheric composition changes with climate -- and at least to some degree such changes in the concentration of greenhouse gases may drive climate change.

When we grow ice sheets we remove water from the ocean to do so. The evaporation process tends to preferrentially remove (fractionate) the lighter isotope of Oxygen (¹⁶O) and leave behind the heavier isotope (¹⁸O). This fractionation has an effect on the oxygen isotopic composition of ocean waters. This effect is recorded by marine organisms in there carbonate shells.

As the shells are being built there is a second fractionation that takes place that is a function of the temperature of the waters in which the shell is being built. The warmer the temperature the greater the fractionation (the more ¹⁶O incorparated in the CaCO₃ shell)

¹⁸O/¹⁶O ~ Temp of precipitation of CaCO₃
               ~ Isotopic Composition of the Water

Cooler Waters = More ¹⁸O in Shells
Greater Ice Volume = More ¹⁸O in the Ocean
                      AND = More ¹⁸O in Shells

Thus, d¹⁸O Measured in Shells Back Through Time is a:

• Climatic Index (Cooler Climate, more ¹⁸O)
• Measure of Ice Volume (if you Correct for Temp. Change)
• Measure of Temp. (if you Correct for Ice Volume Change)

Oxygen isotopes as recorded in fossil marine shells can give us a record of past climates that stretches back tens of millions of years - back to times before there were ice sheets in Antarctica and Greenland when the Earth's climate was as warm or warmer than what we are predicted to achieve in the future.

Although this longer term record does not have annual resolution, it can give us a clear picture of climate oscillations that is comparable to the main features of the Ice Core records;

Problems with Chemical Proxy Records:

What were the conditions under which fractionation occurred?

For example: the ¹⁸O/¹⁶O ratio in a shell could be high

Which was it?? (probably both!)

The composite record taken from many individual proxy records:

This record was developed by statistically combining many different proxy indicators of past temperatures, including a hemispheric network of tree ring measurements, coral growth rings, and glacial ice core oxygen isotopes and accumulation rates.

Long term Estimated Northerm Hemisphere temperature record correlated with suspected factors that could change the climate of the Earth :

We would expect that higher Northern Hemisphere temperatures would be related to higher CO₂, higher solar intensity, and lower atmospheric dust.

The correlation of these factors with the proxy record of temperature seems to indicate that in the last century or so CO₂ concentration in the atmosphere is the main driver of increased Northern Hemisphere temperatures.

Between 1700 and about 1800, variation in solar intensity was the most important variable related to Northern Hemisphere temperature, and from the early to the late 1800s, volcanic dust in the atmosphere had the highest (negative) correlation with Northern Hemisphere temperature.