CLIMATE CHANGE

THE GREAT DEBATESession 5

PROXY CLIMATE DATAFROM ICE CORES

Much information about Quaternary Ice Age climates can be obtained from cores taken from Greenland and Antarctica

Ice sheets preserve a continuous record of climate change that may extent for hundreds of thousands of years. Many characteristics that relate to environmental conditions can be measured

ICE ACCUMULATION RATES

The Greenland Ice Sheet experiences much greater rates of accumulation than the Antarctic one because it receives much larger amounts of snowfall

Greenland ice cores provide much greater time resolution than Antarctic ones but they do so over much shorter time intervals

The Antarctic records go back 800,000 years while the Greenland ones go back only 200,000 years

ICE PROPERTIES Many properties can be measured. They include the following:

Annual layering

Stable isotope analysis of oxygen and hydrogen

Concentrations of trace elements

Concentrations of microparticles, eg dust

Air inclusions

ANNUAL LAYERING

Annual layering can be identified in Greenland ice cores over the last 30,000 years. Variations in the amount of snowfall and ice accumulation can be measured for individual years. It means that when climate changes, we can measure with a high degree of accuracy how fast such changes are taking place

STABLE ISOTOPE RATIOS

The stable isotope ratios of oxygen and hydrogen (18O/16O and D/H) provide a measure for temperature conditions at the time when the snow fell. They provide us with palaeothermometers. Annual variations can be seen in the Greenland ice cores

The curves show marked similarities to the stable isotope curves for oxygen derived from the deep sea cores even though they record changes in ice volume rather than temperature

Oxygen isotope and carbon dioxide variationsover the last 500,000 years in Vostok ice core from Antarctica

TRACE ELEMENTS They are chemical elements that are present in small but varying quantities. For example:

Nitrogen. Variations may reflect variations in sunspot activity

Sodium. Variations may relate to the amount of sea salt present in the precipitation

Calcium. Atmospheric dust is usually rich in this element

Sulphur. Its concentration may indicate the level of volcanic activity

MICROPARTICLES

Small particles in the atmosphere are derived mainly from dust storms and volcanic eruptions. Strong winds can carry them to the ice sheets where they are deposited with the precipitation and incorporated into the ice.

When dust particle concentrations are analysed throughout the ice core, they are found to be much more abundant during glacial times because winds were stronger and there was less vegetation cover

Variations in particleconcentrations in ice cores over time

INCLUSIONS OF AIR

As snow is compacted to form glacial ice, small amounts of air are trapped in inclusions that have become isolated from the atmosphere. With modern techniques it is possible to measure concentrations of the greenhouse gases carbon dioxide and methane. A surprise discovery is that their concentrations are closely related to climate. They are much lower during glacials than they are during interglacials. It also shows that they have never been as high during the last 800,000 years as they are today

ABRUPT CLIMATE CHANGE

Greenland ice cores provide a unique opportunity to discover how quickly climate change can occur. After the end of the Last Glacial there was a brief return to full glacial conditions in Western Europe. This event is known as the “Younger Dryas” and is well documented from lake sediment cores. The name “Dryas” comes from a genus of Arctic daisies. Its pollen was dominant in the lake sediment cores at that time

Climate changes recorded in GISP and GRIPice cores from 18,000 to 10,000 years ago. YD indicates the “Younger Dryas”

ThermohalineCirculation of Atlantic OceanIn NorthernHemisphere

Information obtained from marine cores off the coast of Venezuela indicates that tropical climates were also affected

DANSGAARD-OESCHER EVENTS

They are rapid climate fluctuations that occurred 25 times during the Last Glacial Period from approximately 120,000 to 15,000 years ago. They are initiated by abrupt warming followed by rapid cooling

They have occurred at somewhat irregular intervals and appear to be confined to the Northern Hemisphere

None have occurred during the present interglacial (Holocene) that is during the last 12,000 years

Stable isotope variations in ice cores from Antarctica and Greenland from 140 ka to the presentSource: Wikipedia

Details of Dansgaard-Oescher Events from two Greenland ice cores between 50 and 30 ka. Note very abrupt warming followed by rapid cooling

SPELEOTHEMS

They can also provide proxy climate data:

Oxygen isotope data provide information about past temperatures

Carbon isotope data provide information about plant productivity

Trace element analysis also provides useful information about past environments

Figure 1: Oxygen isotope changesin FT stalagmite

Figure 2: Estimates of temperature change obtained from Figure 1

Figure 3: Estimates of temperature change obtained from the Vostok Ice Core over the same time period

Changes in vegetation activity compared with changes in summer solar radiation