Proxy Measures of Past Climates Current Weather Current Weather Finish Cryosphere Finish Cryosphere Significance of Climate Proxies Significance of Climate Proxies Types of Proxies Types of Proxies Role of Proxies in Reconstructing Past Climate Role of Proxies in Reconstructing Past Climate Limitations and Strengths Limitations and Strengths For Next Class: Read Thompson 2000 Selected slides from:

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© AMS3 Driving Question  How and why do scientists reconstruct the climate record prior to the instrument era?

© AMS4 Reconstructing Past Climates: Why and How?  Climate reconstruction Improves our understanding of environmental response to climate variability and climate change Improves our understanding of environmental response to climate variability and climate change Provides perspective on current climate Provides perspective on current climate Requires identification of a link between quantitative climate Requires identification of a link between quantitative climate forcing and forcing and environmental response environmental response

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

© AMS9 Reconstructing Past Climates: Why and How?  Tree Growth Rings The study of tree growth rings for climate data is known as dendroclimatology 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 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 yearsCounting the number of growth rings gives the age of the tree in years

© AMS10 Reconstructing Past Climates: Why and How?  Tree Growth Rings Width of growth rings normally decreases as the tree ages Width of growth rings normally decreases as the tree ages Widths are usually expressed in terms of a tree-growth indexWidths 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 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 yearsAllows 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

© AMS13 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 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 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 changeChanges 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

© AMS14 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 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 glacierA sequence of varves may provide a high- resolution record of variations in the annual mass budget of a glacier

© AMS15 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 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 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 stalagmiteCan 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 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? © AMS16

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 (~ 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