Researchers from the Universitat Autònoma de Barcelona andDurham University (UK) have discovered that a million years ago, globalclimate changes occurred due to changes in tropical circulation in thePacific similar to those caused by El Niño today. Changes inatmospheric circulation caused variations in heat fluxes and moisturetransport, triggering a large expansion of the polar ice sheets and areorganisation of the Earth's climate. The discovery, published inGeology, shows that local climate changes in the tropics can createmore global climate changes, and emphasises the hypothesis that thetropics play a more active role than was thought in controlling theEarth's climate.
The planet enters and leaves glacial periodsapproximately every 100,000 years. However, a million years ago thesecycles lasted only 40,000 years. Scientists have reconstructed thechain of climatic events that brought about a change in the frequencyof glacial periods and that occurred alongside changes in seatemperatures in the Pacific Ocean and alongside significant changes totropical climates. The researchers have worked mainly with dataobtained from the remains of marine organisms that have accumulatedover time in the tropical Pacific. These fossil records show thatapproximately 1.2 million years ago, the difference in sea temperaturesbetween the East and West Pacific began changing gradually over thecourse of 400,000 years. In the equatorial regions surrounding CentralAmerica, the sea cooled; while around Indonesia, sea temperaturesbarely changed. This caused changes in atmospheric circulation,creating what is now known as the Walker circulation.
Accordingto the researchers, these changes to tropical atmospheric circulationcaused a change in heat fluxes and moisture transport to the polarregions. This brought about an increase in snowfall, enabling the icesheets, particularly in the northern hemisphere, to expand and changein the frequency of glacial periods from 40,000 to 100,000 years. Untilnow this expansion was thought to have been influenced only by the icesheets themselves and by the ocean currents and the atmosphericcirculation at high altitude in the northern hemisphere, as well as byCO2 levels in the atmosphere. "Our results show that local climaticchanges in the tropics can produce global changes," stated AntoniRosell of the UAB, one of the authors of the research. "We are seeingthat the tropics play a more active role than was thought incontrolling the Earth's climate".
The two researchers, AntoniRosell, a researcher of the Catalan Institute for Research and AdvancedStudies (ICREA) for the UAB Institute of Environmental Science andTechnology, and Erin L. McClymont, of Durham University (UK), currentlyat the University of Bristol, have published these results in Geology,the most important scientific journal in this field.
The uneven rhythm of the Earth's cooling process
TheEarth has been passing through a cooling period for several millionyears. The process is not one of gradual, continuous cooling, butrather one of sporadic stops and starts. Professor Rosell's previousarticle, published in Nature, looked at one of these transitions. Thistransition was significant because it resulted in the cooling of largeparts of the northern hemisphere, especially North America. The latestarticle looks at another one of these transitions, this time in themore recent past and on a global scale. This transition is veryimportant in climatology, as it coincides with a change in thefrequency of glacial periods, the reasons for which are not fullyunderstood. Although it was a change in the North Pacific that causedthe northern hemisphere permafrost 2.7 million years ago, in the morerecent case 1 million years ago, the origin of the permafrost was atthe tropics.
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