Coral Records of Climate Change Kim M. Cobb Georgia Inst. of Technology Oceanography class, Oct 21, 2011

Research Goal: To reconstruct tropical Pacific climate change of the recent past, so that we might better predict future climate change, and its regional signatures. Fanning 2005-? Palmyra 1997-? Christmas 1998-? Research Funded by: NOAA NSF Fieldwork funded by: NCL The Nature Conservancy Prince Khaled Bin Sultan Bin Abdulaziz

El Niño-Southern Oscillation An ocean-atmosphere phenomenon that originates in the tropical Pacific but affects global climate patterns December 1997 ocean temperature anomalies Why study tropical Pacific climate? -ENSO extremes carry serious economic and social costs - improved ENSO forecasts minimize the costs - the impacts are not confined to the tropical Pacific +6° +3° 0° -3° Sea Surface Temperature Anomaly (ºC) El Niño impacts Palmyra, Fanning, Christmas Islands

El Niño La Niña 1997 El Niño 1982 El Niño The instrumental record of El Niño’s Are severe El Niño events becoming more frequent as global temperatures increase?

The instrumental record of El Niño’s is too short to answer some key questions: 1. Are late 20th century El Niño events more frequent and more severe than those of the recent past? 2. Is there a correlation between average global temperature and El Niño activity? 3. How much and how fast has ENSO changed in the past? A well-placed rope swing in the Palmyra lagoon

Corals: The geologic record of El Niño CORALS from the tropical Pacific record ENSO in the geochemistry of their skeletons Living corals provide records for the last 200 years Fossil corals enable us to extend the record (ex A.D.) COMMON RARE

The search for fossil corals leads to the Northern Line Islands Palmyra Atoll

Research Objective: To generate >100-yr-long, high-resolution, high-fidelity climate proxy records from the tropical Pacific Ocean; to extend the record of El Niño back in time Materials: Modern and Fossil Corals Methods: Dating: U-Th radioactive decay series Climate proxy: Coral skeletal oxygen isotopes December 1997 SST And Rainfall Anomalies Site A baby booby at Palmyra

Generating climate reconstructions from the Palmyra corals : 1)Recover the corals, both modern (~10) and fossil (~100). 2)Prove that the coral geochemistry tracks large-scale climate. ie. Calibrate the modern coral record against the instrumental record of climate. 3)Apply geochemistry to fossil corals and date them (U/Th dating). Aerial view of Palmyra

Palmyra 40 cores U/Th dated 28 cores undated Christmas 18 cores U/Th dated 63 cores undated Fanning 33 cores undated The Line Island Coral Collection: A work in progress…

Modern The Palmyra Island Coral Collection Little Ice Age (LIA) canals frozen in Europe Medieval Warm Period (MWP) Greenland green

SST (°C) Palmyra Coral  18 O (‰) Coral  18 O is primarily a function of sea-surface temperature BUT It also will record changes In the  18 O of seawater (i.e. salinity) Climate Proxy: Coral oxygen isotopes (  18 O) Drilled in May 1998 Sampling transect

How well do Palmyra corals record El Niños? Red = instrumental record of El Niños Black = modern coral  18 O Less smoothed More smoothed

Palmyra Island Coral Collection Turning to the fossil corals….

17 th century fossil coral-based climate reconstruction As number of overlapping corals increases splice 3 corals, 13 dates, 3,000  18 O measurements = 1 year of work

Palmyra Island Coral Collection

splice 14 th -15 th Century Splice 5 corals, 29 dates, 14,000  18 O measurements = 3 years of work

Single records 1°C Palmyra Coral  18 O Sequences warmer colder Date A.D. 5-coral splice 3-coral spliceModern Single records 1°C 2-coral splice Cobb et al., Nature, 2003 What does this coral reconstruction of tropical Pacific climate tell us?

What about that Late 20 th century Trend? Approach: use coral Sr/Ca ratios as an SST-only proxy Nurhati et al., 2009 combine Sr/Ca (SST) with δ 18 O (SST + δ 18 O sw ) to obtain δ 18 O sw (salinity)

Answer: the late 20 th century trend is mostly salinity! So climate change is affecting rainfall in this area. Nurhati et al., 2011

El Niño La Niña 1997 El Niño Are late 20 th century El Nino events unprecedented in the last millennium? Most frequent, intense El Niño events of reconstruction ENSO characteristics can change in less than a decade

Conclusions Climate change is changing precipitation patterns in the tropical Pacific  more rain (What are the implications for rest of globe?) Present-day El Nino events are not unusual. (What caused the strong El Nino events in the 17 th century, if anything?)

Food for Thought Coral reefs are disappearing at alarming rates worldwide, due to the combined influence of rising ocean temperatures and human disturbances (sediment runoff, over-fishing, dynamite fishing, etc). Reef ecosystems have weakened to the point that natural climate variations, such as a large El Niño event, may cause widespread bleaching and coral mortality (ex: 16% of world’s coral died during 1997 El Nino event (WMO report #1063)) Web Resources My homepage: General El Niño info: NOVA El Niño page: