Proxy Measures of Past Climates Current News & Weather Significance of Climate Proxies Role of Proxies in Reconstructing Past Climate Limitations and Strengths For Next Class: Read Thompson 2000 Dr. Paul Mayewski on Friday Homework for Monday on AsUlearn Selected slides from: www.mun.ca/geog/courses/rwhite/Lec4.2_MeasuringPastClimate.ppt

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

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 © AMS

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.)

Climate Proxies Which of the climate proxies excites you the most? Why? © AMS

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) 2. The relationship between the proxy data and the climate variable being reconstructed (e.g., temperature) may have varied over time. 3. There is no consensus among scientists as to which statistical methods/formulae are most appropriate for calibrating and validating the models. 4. Spatial limitations – collecting proxy data is both time consuming and expensive.

Key strengths of proxy measures 1. Proxy records are meaningful recorders of environmental variables. 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). 3. Most surface temperature reconstructions incorporate proxy data from a variety of sources over wide geographic areas. 4. The same general trends (e.g., temperature trends) emerge from multiple reconstructions.

Questions? Take out a sheet of paper and write down any questions about the material we covered in lecture this week.