Tropical ‘thermostat’ theory challenged by climate model

Posted: March 4, 2017 by oldbrew in atmosphere, climate, Geology, modelling, research
Tags:

Tropical beach

Tropical beach


Can the tropics ever get too hot for life on Earth, or not? That’s the question posed by this research. As the report notes: ‘these theories are controversial’.

New research findings show that as the world warmed millions of years ago, conditions in the tropics may have made it so hot some organisms couldn’t survive, reports Phys.org.

Longstanding theories dating to the 1980s suggest that as the rest of the earth warms, the tropical temperatures would be strictly limited, or regulated by an internal ‘thermostat.’

These theories are controversial, but the debate is of great importance because the tropics and subtropics comprise half of the earth’s surface area, greater than half of the earth’s biodiversity, as well as over half the earth’s human population.

But new geological and climate-based research indicates the tropics may have reached a temperature 56 million years ago that was, indeed, too hot for living organisms to survive in parts of the tropics.

That conclusion is detailed in the article “Extreme Warmth and Heat-Stressed Plankton in the Tropics during the Paleocene-Eocene Thermal Maximum,” published by the online journal Science Advances and co-authored by Matthew Huber, professor in the Earth, Atmospheric, and Planetary Sciences Department at Purdue University and member of the Purdue Climate Change Research Center.

Huber’s contribution focused on climate modeling and interpreting paleoclimate data within the context of modern theoretical understanding. Part of this work was performed while Huber was also at the University of New Hampshire.

The Paleocene-Eocene Thermal Maximum (PETM) period occurred 56 million years ago and is considered the warmest period during the past 100 million years. Global temperatures rapidly warmed by about 5 degrees Celsius (9 F), from an already steamy baseline temperature, and this study provides the first convincing evidence that the tropics also warmed by about 3 degrees Celsius (5 F) during that time.

“The records produced in this study indicate that when the tropics warmed that last little bit, a threshold was passed and parts of the tropical biosphere seems to have died,” Huber said. “This is the first time that we’ve found really good information, in a very detailed way, where we saw major changes in the tropics directly associated with warming past a key threshold in the past 60 million years.”

The study is unique because of the quality of the geological records utilized. Geological records from the PETM are difficult to find, especially from an area of the tropics, Huber said. The research was based on a shallow marine sedimentary section deposited in Nigeria.

“We don’t find 50-million-year-old thermometers at the bottom of the ocean,” Huber said. “What we do find are shells, and we use the isotopes of carbon and oxygen within the shells, complemented by temperature proxies from organic material, to say something about the carbon cycle and about the temperature in the past.”

The report continues here.

Comments
  1. daveburton says:

    I know of three of these “thermostat” theories, dating back to 1991:
    http://www.sealevel.info/feedbacks.html#tropicalsst

    Did I miss any?

  2. daveburton says:

    Also: if global temperatures really rose 5°C, but temperatures in the tropics rose only 3°C (both of which are by no means certain), then there probably was some sort of “thermostat-like” mechanism at work, which reduced the amount of warming n in the tropics.

  3. A C Osborn says:

    I believe in the theory as the more heat applied to the Ocean surface the more water vapour is produced creating more cloud cover.
    The tropic temperatures are far more stable than the rest of the world.

  4. Bloke down the pub says:

    ‘The research was based on a shallow marine sedimentary section deposited in Nigeria.’

    I’m presuming that a large part of the ‘thermostat’ that controls temperatures in the tropics is the ocean currents that transport heat from the equator to the poles. If, as seems likely, this ‘research’ was based on deposits laid down in what at the time was a shallow sea, isolated from the rest of the oceans, then that part of the thermostat would not have worked. While this sea may have been quite large, the effect on temperatures would have been local in nature and not global.

  5. hunter says:

    Hmmm, was the deposition in an inland shallow “sea”, like Aral “sea” of today, or was it in openly connected to an ocean? Extrapolating an entire world from one proxy sounds like another set of claims made from a single proxy. And that didn’t hold up so well…..

  6. hunter says:

    daveburton, that is a great site you referenced. Having a comprehensive, referenced source on feedbacks is very useful. Thank you for posting it here.

  7. AlecM says:

    Yup: and it was because there was no thermohaline circulation, being as there were no polar icecaps. 36 deg C equatorial ocean temperature and little dissolved oxygen killed off most higher life forms (an ocean desert).

    That and 16% higher atmospheric pressure meant no CCN to nucleate clouds hence drought on Gondwana. Only when Pangea went to where Antarctica is now did [CO2] fall from 12 x to 1 x present level, about the middle of the Carboniferous. However, temperature only fell by ~3.5 K.

    However, the bacteria which consumed lignin evolved and its C was converted to CO2 which dissolved in the oceans. The result was that the giant dinosaurs couldn’t survive hence mammals evolved.

    Simples!

  8. jim says:

    Another part of the conundrum, the tropics were not in the same locations then. The surface scum of our planet moves in relation to the central portion of the planet. That may have been a pole or a mid latitude then. Plus our sun was warmer then, plus other factors, but it sounds as if there were plants, and other foodstuffs chasing each other around.

  9. Paul Vaughan says:

    “25. Ice Topography Feedback. Melting at the edges of the Greenland ice sheet and ice accumulation at the center changes the topography, which might change snowfall patterns, which might change the topography:
    http://blogs.plos.org/models/open-to-positive-feedback/
    This is almost certainly very minor. “

    http://www.sealevel.info/feedbacks.html#tropicalsst

    Ridiculous!!

    Pile up 2km of snow and the precipitation comes down as snow and accumulates rather than running off as rain.

    “This is almost certainly very minor. “

    TOTALLY clueless.

  10. Sphene says:

    From:
     https://en.m.wikipedia.org/wiki/Paleocene%E2%80%93Eocene_Thermal_Maximum

    “At the start of the PETM, the ocean circulation patterns changed radically in the course of under 5,000 years.  Global-scale current directions reversed due to a shift in overturning from the southern hemisphere to northern hemisphere overturning.”

    “There is no evidence of any increased extinction rate among the terrestrial biota… Many major mammalian orders – including the Artiodactyla, horses, and primates – appeared and spread around the globe 13,000 to 22,000 years after the initiation of the PETM.”

  11. gymnosperm says:

    ““There is no evidence of any increased extinction rate among the terrestrial biota…”

    Yet we are exhorted to get all worked up about baking bonefish flats in Nigeria.

  12. Curious George says:

    Climate Change researchers are always so surprised when faced with evolution. Temperature changes, some organisms migrate, some adapt, some go extinct. Now I want my tenured professorship.

  13. AlecM says:

    @Jim: in the Devonian/Carboniferous the Sun was cooler than now. As the [CO2] fell the temperature fell by ~3.5 K. The main temperature drop, another ~11.5 K was the result of the subsequent pressure drop at sea level – lapse rate being constant.

  14. Pablo says:

    It was an ice age in carboniferous times when CO2 levels were higher than 56 million years ago.

  15. Paul Vaughan says:

    It’s remarkable how politically combative the abstract is:

    “We identify the PETM in a shallow marine sedimentary section deposited in Nigeria. […] This confirms model predictions on the magnitude of polar amplification and refutes the tropical thermostat theory.” http://advances.sciencemag.org/content/3/3/e1600891

    It’s as comical as Major Fault agents (Bay area group infiltrating right wing groups to control the opposition) insisting the thermostat theory is theirs.

  16. oldbrew says:

    ‘This confirms model predictions’

    A climate model confirms other climate models 😐

  17. michael hart says:

    “Can the tropics ever get too hot for life on Earth, or not? ”

    As a generality, they’ve got a long way to go. Some thermophiles thrive at temperatures higher than 120 °C.

  18. Stephen Richards says:

    because the tropics and subtropics comprise half of the earth’s surface area, greater than half of the earth’s biodiversity, as well as over half the earth’s human population

    There is a reason for all that. Its called warmth.

    michael hart says:
    As a generality, they’ve got a long way to go. Some thermophiles thrive at temperatures higher than 120 °C

  19. Ned Nikolov says:

    Our extended model of planetary temperatures, which predicts meridional temperature gradients, describes quite accurately the observed increase in tropical temperatures documented in this new paper ….

    Yes, the tropics can get too hot for supporting life. This has happened in a big way during the Permian–Triassic mass extinction some 253 My ago. However, the mechanism that heats the tropics to such a high level is totally outside the current understanding of the Greenhouse theory!

  20. daveburton says:

    hunter wrote, “…great site.. Having a comprehensive, referenced source on feedbacks is very useful.”

    Thank you, hunter! If you notice any errors or omissions, please let me know. My contact info is on the site.

    Paul Vaughan wrote, “‘Ice Topography Feedback. Melting at the edges of the Greenland ice sheet and ice accumulation at the center changes the topography, which might change snowfall patterns, which might change the topography… This is almost certainly very minor.“ [is] Ridiculous!!”

    “Pile up 2km of snow and the precipitation comes down as snow and accumulates rather than running off as rain” [is not minor].

    Paul, you’ve misunderstood what is “very minor.” The accumulation of snow/ice on the ice sheets is VERY important, and every bit as important as melting & calving, as a determinant of the ice mass. In fact, elsewhere on that same page you’ll read the following:

    6. …
    Note that snow accumulation has a large effect on grounded ice mass, which in turn affects sea-level. The magnitude of ice accretion from snowfall on ice sheets was illustrated by the team which salvaged Glacier Girl from under 268 feet(!) of accumulated ice, 50 years after she landed on the Greenland ice sheet.
    http://wattsupwiththat.com/2012/03/29/negative-feedback-declining-sea-ice-to-lead-to-cloudier-arctic/

    http://p38assn.org/glacier-girl-recovery.htm

    But as a climate FEEDBACK mechanism, the topography of the ice sheet is almost certainly of very minor importance.

    As everyone who lives near the Sierra Nevadas knows, topography can affect precipitation patterns. But the modest changes in the profile of the Greenland ice sheet — e.g., a buildup in the center and retreat at the edges, as seems to happen during warmer periods — won’t have very much effect on the Earth’s climate, or Greenland snowfall.

  21. Pablo says:

    What? ..You deleted (moderated) my comment.
    I thought we were in general agreement that catastrophic CO2 warming is a nonsense.
    [mod] moderation delay – apologies

    All I said was:

    The carboniferous period was within an ice age. The Karoo ice age. Our last major ice age.

    To be more accurate ..Carboniferous age 360- 298 Mya.
    Karoo ice age 360-260 Mya.

    So it started to warm up AFTER most of the CO2 had been sucked out by plants.

    CO2 levels were higher during that ice age than the supposed super hot period 56 million years ago.

    So where is the link between CO2 and catastrophic warming?

    And thus its relevance to any Tropical thermostats?

    Water in all its forms is a moderator not an amplifier of surface temperatures.
    As I suspect is CO2 .

    CO2 is heavier than air.
    At night the air is still.
    Plants give off CO2 at night.
    CO2 sinks to the ground.
    CO2 absorbs outgoing-IR.
    The ground is warmer at night.

    Plants love warmer nights.

    In daytime water vapour intercepts incoming near-IR and stops the ground getting as hot.
    Evaporation when the ground is moist also cools the surface when it is too hot.

    Plants love warm but not too hot days.

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