October 23, 2006

Tropical Seas Sink Hockey Stick

Defending the “Hockey Stick” depiction of hemispheric or global temperature for the past 1,000 years just got a lot tougher. The “Hockey Stick” curiously wipes out the “Medieval Warm Period” of 1,000 years ago and the “Little Ice Age” that began 450 years ago and ended around 1900. We are supposed to look at the blade of the stick and conclude that the warming of the past 100 years is completely unlike anything seen for at least 1,000 years. It comes as no surprise that the “Hockey Stick” is prominently presented in many of the documents of the Intergovernmental Panel on Climate Change (IPCC). Defenders of the “Hockey Stick” make claims that the Medieval Warm Period and Little Ice Age were confined to the mid-to-high latitudes of the Northern Hemisphere and not felt throughout the rest of the world. This always seemed odd to us at World Climate Report given that variations of solar output seem to explain the higher temperatures 1,000 years ago and the colder temperatures of the Little Ice Age.


Figure 1. IPCC Northern Hemispheric temperature reconstruction for the past 1,000 years.

The latest headache for “stick” supporters is an article by Alicia Newton of the University of South Carolina and two associates published in Geophysical Research Letters. Attention is placed on the Indo-Pacific Warm Pool located in the waters of Indonesia. Unlike Las Vegas, what happens in the Indo-Pacific Warm Pool does not stay in the Indo-Pacific Warm Pool. The authors note that “The Indo-Pacific Warm Pool (IPWP) is one of the warmest regions in the modern oceans, and consequently, air-sea interactions in this region strongly influence global heat and water vapor exchange between the ocean and atmosphere.” What happens there substantially impacts the El Niño – Southern Oscillation (ENSO) cycle on time frames of 5 to 10 years and the Pacific Decadal Oscillation (PDO) on the time scale of decades. Teleconnections with ENSO and PDO ultimately mean that variations in Indo-Pacific Warm Pool impact variations in climate in many parts of the world.

Newton et al. analyzed a core collected aboard the Marion Dufrense in 1998 drilled at 5°12.07 S, 117°29.20 E from a water depth of 1185 m. They note that “the average Holocene sedimentation rate at this location is well over 100 cm per 1,000 years, making it an ideal core for high resolution studies of hydrographic changes in the Makassar Strait.” The core contains the fossil remains of planktonic foraminifera, and the fossil shells can be tested for levels of magnesium, calcium, and oxygen 18 isotope. These chemical properties of the shells are related to sea surface temperature and salinity of the water (which is related to rainfall levels). Incredibly, the fossil remains of the planktonic foraminifera preserve a history of climate in the Indo-Pacific Warm Pool area over the past 1,000 years.

Regarding sea surface temperature (SST) and sea surface salinity (SSS), Newton et al. report “The SST and SSS estimates for the southern Makassar Strait primarily range from 28 to 30°C and 33.5 to 36.0, respectively, over the entire time period. The average modern summer SST and SSS are 29°C and 35, respectively. The period from ~1000 to 1400 AD is marked by the warmest temperatures and highest salinities. During this time interval, the average SSTs were between 29–30°C (mean of 29.4°C) and salinities fluctuated between ~34–35.8 (mean of 34.9), and are very similar to present day conditions in this area. This 400-year long period of warm temperatures and high salinities is equivalent in age to the Medieval Warm Period (MWP), a time when radiative forcing was high.” In other words, the sea surface temperatures in the Makassar Strait were higher in the Medieval Warm Period than they are today. If nothing else, the Medieval Warm Period stands out loud and clear in the temperature reconstruction (see below).


Figure 2. Reconstructed sea surface temperatures from the Indo-Pacific Warm Pool

With respect to the Little Ice Age, Newton et al. report “Specifically, the SST record shows a distinct cooling trend beginning at ~1400 AD and lasting for several hundred years, a period equivalent in time to the Little Ice Age (LIA). In particular, the lowest temperatures (~28°C) occur around 1700 A.D., during the period of reduced solar intensity known as the Maunder Minimum.” Further, they write “These results clearly indicate a climatic cooling during the LIA that extended well outside the higher northern latitudes. In fact, the recognition that that tropical Pacific warm pool temperatures were as much as 1.5°C cooler during the LIA must be considered an important factor itself in establishing what caused the climate to cool as it did.”

Recall that this deep ocean core came from a tropical ocean 5° latitude south of the Equator. Yet, the planktonic foraminifera certainly noticed the Medieval Warm Period and the Little Ice Age and clearly recorded the signal in their fossil remains. It seems the only fossils not recording these two climate episodes are the fossils at the IPCC who continue defending the Hockey Stick.

Reference:

Newton, A., R. Thunell, and L. Stott (2006), Climate and hydrographic variability in the Indo-Pacific Warm Pool during the last millennium, Geophysical Research Letters, 33, L19710, doi:10.1029/2006GL027234.




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