Polar Sunlight Drives Climate Change
Around 3 million years ago, Earth's climate started growing colder. Glaciers began forming in high northern latitudes, while surface waters cooled in parts of the equatorial Atlantic and Pacific Oceans. At the same time, climate sensitivity to variation in the tilt of Earth's axis—called obliquity—increased substantially. Since that time, changes in sunlight associated with obliquity have caused variation in global ice volume and equatorial sea surface temperatures (SST). Inexplicably, variations at the equator occurred a few thousand years before those in high latitudes and thus could not have been a direct consequence of the waxing and waning of glaciers. Two new papers in the June 18, 2010, issue of Science attempt to explain the true causes of climate change.
The mystery of how slight, obliquity driven, changes in insolation, primarily in the polar regions, dictate the glacial-interglacial cycles that have dominated Earth's climate since the onset of the Pleistocene Ice Age, remains a challenge to modern science. One hypothesis is that oceanic links can turn variation in obliquity into SST response in low latitudes. The oceans gain large amounts of heat from sunlight in equatorial regions where surface waters are cold. Currents transport that heat to higher latitudes where it is lost to the atmosphere. The loss occurs mainly in winter, when cold continental air blows over the warmer ocean. Scientists propose that changes in the ocean thermocline, the boundry between warm surface water and colder deep water, can act as an amplifier for the small, obliquity induced changes in insolation at high latitudes. As explained by S. George Philander, in an accompanying perspective article, “Tilting at Connections, from Pole to Equator”:
Researchers have learned much about the factors that determine equatorial SST from studies of El Niño and La Niña. These events involve an adiabatic, horizontal redistribution of warm surface waters along the equator (see the figure, panel B). Because the thermocline, the interface between the warm surface water and the colder deeper water, is so shallow, a mere change in its slope affects SST. Surface temperatures, in turn, influence the winds that determine the thermocline's slope. The positive feedbacks implied by this circular argument affect the global climate, as is evident during an intense El Niño. The impact on climate would be even larger if extreme SST patterns were to persist for prolonged periods, not merely for a few months as in the case of El Niño. That is because SST patterns determine the extent of the stratus clouds that cover cold surface waters in low latitudes, and also the atmospheric concentration of water vapor, a powerful greenhouse gas.
Initially, the equatorial thermocline was deep, but as the globe and the deep ocean cooled over the past tens of millions of years, the thermocline has moved closer to the surface. This shallower thermocline is much more sensitive to change, perhaps providing the missing mechanism. Philander and Fedorov propose that a threshold was reached about 3 million years ago when cold water appeared at the surface in the tropical Pacific and Atlantic. This altered the ocean's heat budget and circulation, allowing conditions in high latitudes to influence tropical sea surface temperatures by changing the depth of the thermocline. In their 2003 paper in Paleoceanography, “Role of tropics in changing the response to Milankovich forcing some three million years ago,” Philander and Fedorov state:
Throughout the Cenozoic the Earth experienced global cooling that led to the appearance of continental glaciers in high northern latitudes around 3 Ma ago. At approximately the same time, cold surface waters first appeared in regions that today have intense oceanic upwelling: the eastern equatorial Pacific and the coastal zones of southwestern Africa and California. There was furthermore a significant change in the Earth's response to Milankovich forcing: obliquity signals became large, but those associated with precession and eccentricity remained the same.
At first, this mechanism caused the glacial-interglacial cycle to follow a period of 41,000 years. This was true up until about 1.2 million years ago, but since ~700,000 years ago ice-age cycles have lasted for around 100,000 years (see “Change In Ice Ages Not Caused By CO2”). It has also been noted that, since the Mid-Brunhes Event, around 430,000 years ago, interglacial periods have grown warmer and their CO2 levels higher. For more information on these changes, and a description of the Croll-Milankovitch cycles, see “Sun & Cycles Heat Up Ice Age Interglacials.”
Now, as reported in the June 18, 2010, issue of Science, Alfredo Martínez-Garcia et al. think they have found a link to the cold upwelling “tongue” of the eastern equatorial Pacific. In the equatorial Pacific Ocean, upwelling extends westward along the equator in a cold tongue of water from the coast of South America, eventually encountering a large pool of warmer water in the western Pacific (the cold tongue-warm pool system). The eastern cold tongue system is characterized by high levels of nutrients and hence growth in fish and other aquatic life. This area is shown in the map below.
The Pacific cold tongue.
It has long been known that variation in the cold tongue is closely related to the El Niño/La Niña oscillation. According to the article, “Subpolar Link to the Emergence of the Modern Equatorial Pacific Cold Tongue,” by Martínez-Garcia et al., the cold water tongue is a relatively recent development and has only been present since the start of the start of the Pleistocene. Development of the modern cold tongue during the Pliocene-Pleistocene transition has been explained as the result of extratropical cooling, perhaps due to the rerouting of ocean currents caused by the joining of North and South America over the course of the preceding Miocene epoch (23-5.3 mya). This, in turn, drove a shoaling of the thermocline.
The Americas were not joined at the start of the Miocene. Source Ron Blakey/NAU Geology
During the Pliocene epoch climate became cooler and drier, and seasonal, similar to modern climates. The global average temperature in the mid-Pliocene (3.3 mya - 3 mya) was 3.6-5.4°F (2-3°C) higher than today, and the global sea level 80 feet (25 m) higher. Northern hemisphere ice sheet coverage was fleeting before the onset of extensive glaciation in Greenland, which occurred in the late Pliocene around 3 Ma. Mid-latitude mountain glaciation was probably underway before the end of the epoch.
By the mid-Pleistocene the Ice Age was in full swing. Source Ron Blakey/NAU Geology
“We have found that the sub-Antarctic and sub-Arctic regions underwent substantial cooling nearly synchronous to the cold tongue development, thereby providing support for this hypothesis,” they report. “In addition, we show that sub-Antarctic climate changed in its response to Earth’s orbital variations, from a subtropical to a subpolar pattern, as expected if cooling shrank the warm-water sphere of the ocean and thus contracted the subtropical gyres.”
An ocean gyre is a large-scale feature made up of currents that spiral around a central point. Circulation is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, due to the Coriolis effect. There are five major subtropical gyres: the North and South Pacific Subtropical Gyres, the North and South Atlantic Subtropical Gyres, and the Indian Ocean Subtropical Gyre. The North Pacific Subtropical Gyre is notable because of its tendency to collect floating debris. It is difficult to measure the exact size of a gyre, but the North Pacific Subtropical Gyre is estimated to be between 7 to 9 million square miles—equivalent to three times the area of the continental US.
Over time the gyres have grown and shrunk. Driven by the long-term average winds in the subtropical highs, Ekman transport causes surface waters to move toward the central region of a subtropical gyre. This can have the side-effect of lowering coastal sea levels. In fact, due to variations in the ocean gyres, most of the long tide gauge records in the North Atlantic and North Pacific commonly used to estimate global sea level rise and acceleration show a marked difference in behavior a century ago, when compared to the latter half of the 20th century.
Ekman transport produces a broad mounding of water as high as 3 ft (1 m) above mean sea level near the center of the gyre. As more water is transported toward the center of the gyre, the surface slope of the mound becomes steeper. At the same time, the horizontal water pressure gradient produced under the sloping sea surface increases. In response to the horizontal gradient in water pressure, water moves from where the pressure is higher toward where the pressure is lower, that is, downhill. Surface water parcels flow outward and down slope from the center of the gyre.
According to Martínez-Garcia et al., the contraction of the subtropical gyres toward the equator caused the sub-Antarctic Atlantic climate to become less sensitive to variations in low-latitude, Southern Hemisphere insolation driven by precession and more sensitive to obliquity-driven variations in Antarctic climate around 1.6 million years ago. While this is a step forward in our understanding the long-term climate changes of the Pleistocene, questions remain:
Why the cold tongue expanded toward its present-day configuration around 1.8 to 1.2 Ma, during what would appear to be a time of relatively stable polar climate and glacial ice volume, has been a long-standing question. Our data show that the tropics were not alone in changing at this time, with a bipolar cooling and equatorward expansion of the subpolar water masses also occurring. Thus, our data appear to confirm the hypothesis that the expansion of the subpolar oceans shoaled the thermocline and thus led to the emergence of the modern cold tongue. The converse causation, with the equatorial Pacific driving subpolar changes, lacks the same grounding in dynamical expectations.
This brings us to the second new paper in Science, “Tropical Ocean Temperatures Over the Past 3.5 Million Years,”by Timothy D. Herbert et al.. They found that, over the past 3.5 million years, tropical SST records show coherent glacial-interglacial temperature changes of 1° to 3°C that align with, but slightly lead, global changes in ice volume and deep ocean temperature. Exactly why this should be is problematic and the authors describe what is known or suspected:
On the one hand, the tropical oceans should be shielded from processes that produce large temperature sensitivity in the high latitudes, including ice-albedo feedback on land, sea-ice feedbacks, and steering of wind fields by ice sheets. Modeling studies suggest that the direct effects of large continental ice sheets should extend equatorward on a length scale of ~2000 km from the (primarily Northern Hemisphere) edge of polar ice sheets and sea ice—a substantial radius, but not nearly enough to extend climate change throughout the tropics. However, the high latitudes may possess other potent influences on tropical ocean temperatures. High-latitude oceanographic processes determine the properties of subsurface waters that help to determine vertical density stability, mixing, and upwelling in the tropical oceans. Most ocean carbon cycle models also give the high-latitude oceans the dominant role in modulating the carbon dioxide (CO2) greenhouse effect that helps determine global surface temperatures.
While both Martínez-Garcia et al. and Herbert et al. agree that major, dramatic changes in climate are forced by orbital cycles, they do not fully agree about how the orbital changes are amplified. Herbert et al. think that rising CO2 levels, triggered by warming SST, provides significant feedback. “The observations of Martínez-Garcia et al. and of Herbert et al. corroborate the hypothesis that oceanic links can translate obliquity forcing in high latitudes into SST response in low latitudes, but several questions remain unanswered,” states Philander, in his summary of the two papers.
Ice age cycles and tropical SST varied coherently at the 41-ky time scale.
Science still cannot explain why, over the past 700,000 years, cycles of about 100,000 years became dominant. Moreover, though Herbert et al. suggest that CO2 played an important role in producing tropical SST variations, those arguments “are undermined by the striking differences between the eastern and western equatorial Pacific, between the regions north and south of the equator along 95°W, and between the equatorial Pacific and Atlantic.”
So while many questions remain, here is what science thinks it knows: more than 3 million years ago, the slowly drifting continents triggered changes in ocean circulation, making the oceans much more sensitive to small changes in insolation in higher latitudes. The ever changing cycles that affect Earth's orbit came to dominate climatic changes, even though the change in insolation engendered by the orbital cycles is relatively small.
Changes in high latitude insolation force changes in sea surface temperatures at lower latitudes, made possible by the shallowness of the ocean thermocline, bringing about global change. Greenhouse gases have a supporting role to play, both increased atmospheric levels of H2O and CO2. But consider this: as the orbital cycles progress, a equally small change in insolation—this time a reduction—will trigger the next glacial period, despite the high levels of CO2 in the atmosphere.
A small reduction in high latitude insolation can negate the effect of all the carbon dioxide the interglacial warming released from the ocean and frozen land. Advocates of CO2 driven anthropogenic global warming have refused to accept the role the Sun and changes in insolation play in climate change, preferring instead to exaggerate the potency of CO2. The changes in insolation are too small and affect the wrong regions, they claim. We now know better: it's the small changes in insolation that matter, not the CO2 levels.
Be safe, enjoy the interglacial and stay skeptical.