Proxy Measures of Past Climates

Current Weather Finish Cryosphere Significance of Climate Proxies Types of Proxies Role of Proxies in Reconstructing Past Climate Limitations and StrengthsFor Next Class: Read Thompson 2000

Selected slides from: www.mun.ca/geog/courses/rwhite/Lec4.2_MeasuringPastClimate.ppt

Climate News

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Driving Question

How and why do scientists reconstruct the climate record prior to the instrument era?

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Reconstructing Past Climates: Why and How?

Climate reconstruction Improves our understanding of environmental

response to climate variability and climate change

Provides perspective on current climate Requires identification of a link between

quantitative climate

forcing and

environmental response

The Earth warmed by 0.6°C (1°F) during the 20th century.

Paleoclimatology (the study of past climates), can help place this warming in the context of natural climate variability.

Until recently, most reconstructions of climate variations over the last ~1000 years focused on specific locations or regions.

Because widespread, reliable instrumental records are available only for the last ~100-150 years, scientists estimate climatic conditions in the more distant past by analyzing proxy evidence.

What kinds of climate proxies can be used to estimate surface temperatures (last 2000+

years)?

What kinds of climate proxies can be used to estimate surface temperatures (last 2000+

years)? Tree rings Corals Ocean and lake sediments (varves) Cave deposits (speleothems) Ice cores Pollen deposits Packrat middens Glaciers Documentary evidence (historical records, paintings,

etc.)

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Reconstructing Past Climates: Why and How?

Tree Growth Rings The study of tree growth rings for climate data

is known as dendroclimatology A year’s growth of spring wood plus summer

wood constitutes an annual growth ring• Counting the number of growth rings gives the age

of the tree in years

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Reconstructing Past Climates: Why and How?

Tree Growth Rings Width of growth rings normally

decreases as the tree ages• Widths are usually expressed in terms

of a tree-growth index Cross-dating is used to match tree

growth ring records from living trees with those from timbers in prehistoric dwellings• Allows detailed tree ring chronologies

to be extended back in time thousands of years

Narrow ring, thin latewood

Wide ring, thick latewood

Variation is key!

Principle of Limiting Factors

Principle of Limiting Factors

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Reconstructing Past Climates: Why and How?

Pollen Profiles Ponds, peat bogs, marshes, and swamps are

favorable sites for the accumulation and preservation of wind-borne pollen

Types of pollen delivered to depositional sites change as climate and vegetation change• Changes in the abundance of pollen of different

species at various depths within accumulated sediment may provide a record of past climatic regimes and climate change

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Reconstructing Past Climates: Why and How?

Varves Thin layer of sediment deposited annually in a

body of still water, usually a lake fed by a stream• A sequence of varves may provide a high-

resolution record of variations in the annual mass budget of a glacier

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Reconstructing Past Climates: Why and How?

Speleothems Calcite deposit in a limestone cave or cavern that can

yield high-resolution records of past temperature and rainfall

Forms when calcite precipitates from groundwater that seeps into a cave• Can build downward from the roof of the cave creating a

stalactite, or grow upward from the floor of the cave to form a stalagmite

Climate is reconstructed using oxygen isotope analysis from samples of calcite extracted from a speleothem

Climate Proxies

Which of the climate proxies excites you the most? Why?

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How are proxy data used to reconstruct climate?

For most proxies, statistical techniques are used to define the relationship between the proxy measurements and the concurrent instrumental records.

Then, this relationship is used to reconstruct the past climate from the remaining proxy data.

Steps involved Collect proxy data Dating the proxy data (e.g., matching growth

rings of trees to calendar year) Calibration – usually, this involves using

regression to relate the proxy measurement to know climatic conditions

Validation – basically tests the skill of the calibration

Reconstruction – once the proxy/climate relationship is established, we again use regression to predict what past climate was like

What are the limitations and strengths of large-scale climate reconstructions?

1. The instrumental record is short (~100-150 years at most)

This provides somewhat limited information needed to calibrate and validate the models used to reconstruct

2. The relationship between the proxy data and the climate variable being reconstructed (e.g., temperature) may have varied over time.

limitations

3. There is no consensus among scientists as to which statistical methods/formulae are most appropriate for calibrating and validating the models.

Thus, different teams of researchers could reach different conclusions when dealing with the same proxy data (e.g., the “hockey stick” controversy)

limitations

4. Spatial limitations – collecting proxy data is both time consuming and expensive. This limits the amount of data available in terms of spatial coverage.

Spatial pattern of ice core sampling in Antarctica and Greenland

Key strengths of proxy measures

1. Proxy records are meaningful recorders of environmental variables. The science is sound as the connections between the proxy-derived data and climate variability are well established.

Strengths

2. Tree ring-derived data are especially valuable in climate reconstruction as they often represent regions (i.e., multiple sites within a region are sampled and replication is a key element of the scientific method).

Strengths

3. Most surface temperature reconstructions incorporate proxy data from a variety of sources over wide geographic areas. Thus, even if we start to remove individual records, the overall results remain robust.

Strengths

4. The same general trends (e.g., temperature trends) emerge from multiple reconstructions.

From about 1600 till the present, the confidence in the general character of climate reconstructions is high. Why?

Different reconstructions based on different types of proxy evidence, different selections of proxy data of a given type, and different methodologies yield similar results.

------ replication, variety, similarity