Thursday, August 23, 2012

New paper finds tropical Atlantic temperatures were up to 2.5C warmer 500-1000 years ago

A new paper published in PNAS finds from 2 sediment temperature proxies that the upper 1,500 feet of the tropical Atlantic Ocean experienced abrupt warming of about 3-4C during each of at least two periods over the past 22,000 years due to a natural change in ocean oscillations [i.e. not greenhouse gases]. The paper also shows "the modern mean annual temperature" is about 0.5 to 2.5C colder than the end of the proxy records around 500 years before the present. 

Graph and legend from the paper's supplemental information. The final segments of both temperature proxies in blue and red were added to connect the proxy temperatures to "the modern mean annual temperatures" The Younger Dryas [YD] was a period of natural global warming at a much faster rate than over the 20th century. Horizontal x axis is thousands of years before the present [ky BP].

Related: New Paper finds significant cooling of Atlantic Ocean over past millennium

Impact of abrupt deglacial climate change on tropical Atlantic subsurface temperatures

  1. Bette L. Otto-Bliesnerc
+Author Affiliations
  1. aDepartment of Oceanography, Texas A&M University, College Station, TX 77843;
  2. bPhysical Oceanography Laboratory, Ocean University of China, Qingdao 266003, People’s Republic of China; and
  3. cNational Center for Atmospheric Research, Boulder, CO 80307
  1. Edited by James C. McWilliams, UCLA, Los Angeles, CA, and approved August 1, 2012 (received for review May 8, 2012)


Both instrumental data analyses and coupled ocean-atmosphere models indicate that Atlantic meridional overturning circulation (AMOC) variability is tightly linked to abrupt tropical North Atlantic (TNA) climate change through both atmospheric and oceanic processes. Although a slowdown of AMOC results in an atmospheric-induced surface cooling in the entire TNA, the subsurface experiences an even larger warming because of rapid reorganizations of ocean circulation patterns at intermediate water depths. Here, we reconstruct high-resolution temperature records using oxygen isotope values and Mg/Ca ratios in both surface- and subthermocline-dwelling planktonic foraminifera from a sediment core located in the TNA over the last 22 ky. Our results show significant changes in the vertical thermal gradient of the upper water column, with the warmest subsurface temperatures of the last deglacial transition corresponding to the onset of the Younger Dryas. Furthermore, we present new analyses of a climate model simulation forced with freshwater discharge into the North Atlantic under Last Glacial Maximum forcings and boundary conditions that reveal a maximum subsurface warming in the vicinity of the core site and a vertical thermal gradient change at the onset of AMOC weakening, consistent with the reconstructed record. Together, our proxy reconstructions and modeling results provide convincing evidence for a subsurface oceanic teleconnection linking high-latitude North Atlantic climate to the tropical Atlantic during periods of reduced AMOC across the last deglacial transition.

article from Science Daily on this paper:

Past Tropical Climate Change Linked to Ocean Circulation

ScienceDaily (Aug. 23, 2012) — A new record of past temperature change in the tropical Atlantic Ocean's subsurface provides clues as to why Earth's climate is so sensitive to ocean circulation patterns, according to climate scientists at Texas A&M University.

Geological oceanographer Matthew Schmidt and two of his graduate students teamed up with Ping Chang, a physical oceanographer and climate modeler, to help uncover an important climate connection between the tropics and the high latitude North Atlantic. Their new findings are in the current issue ofPNAS (Proceedings of the National Academy of Sciences).
The researchers used geochemical clues in fossils called foraminifera, tiny sea creatures with a hard shell, collected from a sediment core located off the northern coast of Venezuela, to generate a 22,000-year record of past ocean temperature and salinity changes in the upper 1,500 feet of water in the western tropical Atlantic. They also conducted global climate model simulations under the past climate condition to interpret this new observational record in the context of changes in the strength of the global ocean conveyor-belt circulation.
"What we found was that subsurface temperatures in the western tropical Atlantic rapidly warmed during cold periods in Earth's past," Schmidt explains.
"Together with our new modeling experiments, we think this is evidence that when the global conveyor slowed down during cold periods in the past, warm subsurface waters that are normally trapped in the subtropical North Atlantic flowed southward and rapidly warmed the deep tropics. When the tropics warmed, it altered climate patterns around the globe."
He notes that as an example, if ocean temperatures were to warm along the west coast of Africa, the monsoon rainfall in that region would be dramatically reduced, affecting millions of people living in sub-Saharan Africa. The researchers also point out that the southward flow of ocean heat during cold periods in the North Atlantic also causes the band of rainfall in the tropics known as the Intertropical Convergence Zone to migrate southward, resulting in much drier conditions in northern South American countries and a wetter South Atlantic.
"Evidence is mounting that the Earth's climate system has sensitive triggers that can cause abrupt and dramatic shifts in global climate," Schmidt said.
"What we found in our subsurface reconstruction was that the onset of warmer temperatures, thought to reflect the opening of this 'gateway' mechanism, occurred in less than a few centuries. It also tells us that it might be a good idea to monitor subsurface temperatures in the western tropical Atlantic to assess how the strength of the ocean conveyor might be changing over the next few decades as Earth's climate continues to warm."
"One way to prepare for future climate change is to increase our understanding of how it has operated in the recent past."


  1. I am the author of this paper any you completely misunderstood the point of this study! The warming events we discuss take place during a transition out of the last ice age and yes, CO2 was on the rise during this time too. It is terrible how you take the science out of its original context and twist it to what you want to say. In addition, you completely miss interpret the point of that figure from the SI section you show. If anything, the study should make us MORE concerned about climate changes in the near future!

    1. Thanks for your reply.

      1. The rise in CO2 follows warming by ~800-1000 years and thus is not the cause. The relatively small changes in CO2 forcing during the period of your study do not explain the temperature changes or the changes in AMOC.

      2. IR radiation from greenhouse gases cannot warm the oceans due to a penetration depth of a few microns, which only results in evaporative cooling

      3. While your graph above had a different "point" I am simply pointing out the differences between the proxy temps and the "the modern mean annual temperatures" on your graph. Are you claiming that your graph does not show the "modern mean annual temperatures" were colder than the 2 proxies ~500 years ago?

  2. You don't understand that figure. I argue that the blue line doesn't make sense. I argue the red line reconstruction is best, and NO, that one doesn't show a cooling over the last 500 years.

    1. Read the graph legend:

      Red is a proxy for 30m depth and blue is a proxy for SST. They are not proxies for the same depth, so one is not "better than" the other and BOTH show a cooling, albeit smaller at depth than the surface, over the past ~500 years.

  3. Those are referring to the stars on the Y-axis only, NOT the plot lines. You have mistakenly drawn your own lines in connecting the plot lines to these stars. The actual lines plotted are both for mixed-layer temperatures at around 30m depth, but using DIFFERENT calibration equations. The authors argue that the equation used to plot the blue line is entirely unrealistic, and the stars showing the modern temperatures at both the surface and 30m depth illustrate that the blue line is not correct. If you read in the supplemental text, you will read additional reasons why that equation used for the blue line is incorrect. Look at the red line ONLY (the one the authors say is correct), you cannot say that it was warmer 500 years ago. The species used for the reconstruction can live anywhere between the surface and 30m (the blue star and red star, respectively), so you cannot say with any certainty what the modern temperature for that species should be. If the species is mainly living at around 15m depth, the modern temperature would be somewhere between the two stars, indicating a WARMING over the last 500 years.

  4. the pertinent supplemental text:

    "The resulting multivariate equation results in a calculated
    core-top temperature of 29 °C for the upper mixed layer at
    our study site. This is 3 °C warmer than the modern average annual upper mixed layer (30 m depth) temperature for the Bonaire
    Basin, and 1.4 °C warmer than the maximum seasonal sea surface
    temperature of 27.6 °C during the fall. For comparison, the calculated upper mixed layer temperature using the Sargasso Sea
    Mg/Ca:temperature calibration (7) is 25.7 °C, in excellent agreement with the modern annual mean mixed layer temperature of
    25.6 °C at 30 m depth and with the annual mean sea surface temperature of 26.7 °C (19). The coolest temperatures calculated
    using the multivariate equation are 21.3 °C at approximately
    16 ky and 22.2 °C during the Last Glacial Maximum (LGM)
    (Fig. S5), suggesting a maximum glacial-interglacial temperature
    gradient of almost 7 °C at our site. This amount of cooling in the
    western tropical Atlantic is much larger than the estimated cooling of only approximately 3 °C in the western tropical Atlantic at
    the LGM based on the results of the MARGO project (20).
    Although the Bonaire Basin may have cooled more than the average western tropical Atlantic because of the influence of upwelling, a cooling of 7 °C at the LGM is too large to be considered
    realistic. Given that neither the calculated core-top temperature
    nor the estimated glacial-interglacial temperature gradient seems
    reasonable using the multivariate temperature equation, we
    chose to use the Sargasso Sea sediment trap calibration equation
    when calculating upper mixed layer temperatures for this study"

    indicates modern mixed layer temps with their calibration of "choice" is 0.1C colder than ~500 years ago. The graph shows the mixed layer with their method of "choice" was warmer 3000 years ago during the Minoan warming period than either 500 years ago during the Little Ice Age or modern temps. There is no indication from either method that modern temps are unprecedented, accelerated, or unnatural.