Equivocal evidence for a thermostat and unusually low levels of coral bleaching in the Western Pacific Warm Pool and Snakes tell a torrid tale Ruben van Hooidonk and Mathew Huber Nathaniel Shutt Atmospheric Science Major

Coral Bleaching -Caused by an abnormal increase in Sea Surface Temperatures(SST) -Corals release symbiotic algae that results in a white or “bleached” appearance

Thermostat-bleaching hypothesis -A naturally occurring thermostat over the western pacific warm pool is limiting the sea surface temperature between degrees Celsius that is in turn reducing coral bleaching

Importance of Research -Global warming could have a detrimental impact on corals in the next 50 years due to coral bleaching -A significant loss of coral could result in a crippling of small fish who rely on corals to survive

The Data -Hadley Centre Sea Ice and Sea Surface Temperature data set (HadISST) - Extended Reconstructed Sea Surface Temperatures, version 3 (ERSST.v3)

Conditions 1.In regions where the SST is 29°C or warmer the trend in max SST should be zero or negative 2.Negative feedback on SST should be unique to warmest areas of the ocean 3.Anomalously low bleaching must be observed in the regions of the WPWP where the average max SST I greater than 30°C 4.A physical basis for the thermostat has to be identified 5.Evidence of the thermostat should be in geological archive

The Results Condition 1 (SST trend should be zero or negative) - satisfied over the small eastern region of the WPWP where the trend in max SST is close to zero or negative

The Results Condition 2 (Negative feedback unique to warmest areas of the ocean) - negative feedback is not unique to the warmest regions of the ocean - Implies another mechanism other than a “thermostat” may be at work

Condition 3 (Anomalously low bleaching) - Hooidonk and Huber discovered an alarming amount of unobserved coral reefs in the WPWP that could result in a bias for low bleaching -Had to estimate the amount of bleaching that occurs in the WPWP -Estimations based on observed monthly thermal thresholds for reefs nearby with observations of bleaching -Estimations on coral bleaching are not anomalously low

Condition 4 (A physical basis for the thermostat) - A physical basis for a true hard limit or thermostat has yet to be proven valid - Recent work using a coupled atmosphere-ocean model indicates that the observed limit of tropical SST will rise in a warming world - While a mechanism is definitely slowing the warming of the tropics there is little evidence for a mechanism that sets a hard limit on SSTs

Condition 5 (Evidence of the thermostat in geological archives) - Analysis of unaltered planktonic foraminifer shells showed that tropical SST did increase to 30 –35 ° C between 33 and 56 million years ago -Geological archives do not support evidence of a thermostat

Conclusion -Not enough conditions are meet to suggest a natural thermostat over the WPWP is the cause of a lower rate of coral bleaching in the WPWP - Likely cause for the low number of reports of coral bleaching in the WPWP is due to its remoteness -Coral bleaching will continue to occur with a possibility of a record setting event in the next 30 to 50 years - Other evidence for a potential “thermostat’?

SNAKES! Snake palaeothermometry -Scientists relate size to minimum annual temperature -Use empirical relationship between temperature and size derived from modern organisms

-Scientists used the fossil vertebrae of a giant snake discovered in Columbia aged between 58 million and 60 million years old -It was estimated that the giant snake required minimum temperatures of 32–33 °C, much warmer than values today -The snake observed was an estimated 13 meters(~43ft) long and weighed more than one tonne(2,204.6 pounds)

-Scientists compared values derived from fossil vertebrae and climate models from 58 million years ago and found several similar results for the average surface temperature

-Snake palaeothermometry provides evidence against a thermostat with much of the estimated values well above current mean surface temperatures -Snake palaeothermometry must be proven to be a valid way of estimating the surface temperatures before any results are taken as hard evidence against a potential thermostat

Summary -Very little evidence today for a “thermostat” limiting the sea surface temperatures over the tropics -Without a thermostat to protect coral reefs from bleaching we must turn to ourselves to help lessen the magnitude of the global warming trend -While snake palaeothermometry is good in theory more research is needed to prove the validity of the process -More observations are needed to conclude that there is not a abnormal low rate of bleaching in the WPWP. Is so what mechanism is responsible? -Continue to improve snake palaeothermometry through research in hopes of better understanding if a true “thermostat” exists