Ice Age Terminations: Orbital Cycles, Ocean Circulation and Shifting Monsoons
A new study has confirmed the astronomical theory of the ice ages, but with a new twist: The shutoff of the meridional ocean circulation, or MOC, and an associated southward shift of tropical monsoon rain belts seems to play an integral role in the melting of glacial period ice sheets. These changes cause warming of the Southern Hemisphere and a rise in atmospheric CO2 levels, which in turn provides a positive feedback loop that helps drive glacial termination. This is why, every 100,000 years or so, the great Northern Hemisphere ice sheets collapse and glacial conditions give way to a warm interglacial period, such as the Holocene warming humanity is currently enjoying. This, however, does not support recent claims that global warming is causing the Southeast Asian monsoon to fail.
There were two related articles in the October 9, 2009, issue of Science: “Ice Age Terminations” by Hai Cheng et al. and “Monsoons and Meltdowns” by Jeffrey P. Severinghaus, a perspective on the first article. What both articles report is that “the last four meltdowns began when northern sunshine was intensifying, in accordance with the classical Milankovitch or astronomical theory of the ice ages.” Using monsoon cycles to improve dating precision for other sources of historical climate data, Hai Cheng et al. help explain the climate mechanisms that control glacial terminations and the underlying causes of ice age cycles. According to their study, most of the meltdown and sea-level rise occurs during periods of weak monsoons, when the MOC is shut down and CO2 levels are rising.
The ice age cycle, with its gradual buildup and rapid collapse of ice sheets, has been known to science for more than a half century. As previously reported in this blog, evidence linking Earth's orbital variations seems stronger than ever (see “Confirmed! Orbital Cycles Control Ice Ages”), but the detailed mechanisms at work have remained a mystery. According to Cheng et al.:
Explanations of the rapid collapses, dubbed “terminations,” have long been sought. The ice-age cycles have been linked to changes in Earth’s orbital geometry (the Milankovitch or Astronomical theory) through spectral analysis of marine oxygen-isotope records, which demonstrate power in the ice-age record at the same three spectral periods as orbitally driven changes in insolation. However, explaining the 100 thousand-year (ky)–recurrence period of ice ages has proved to be problematic because although the 100-ky cycle dominates the ice-volume power spectrum, it is small in the insolation spectrum. In order to understand what factors control ice age cycles, we must know the extent to which terminations are systematically linked to insolation and how any such linkage can produce a nonlinear response by the climate system at the end of ice ages.
Correlating data from a wide varity of sources, including Chinese cave deposits and benthic oxygen isotope ratios, Cheng et al. have produced a detailed history of various paleoclimate factors for the last four glacial terminations. In the figure below (Figure 4 from the article) the light green and yellow bars highlight similar events.
(A) Obliquity and (B) 21 July insolation at 65°N (29). Black bars highlight the highest and lowest insolation value bounding each major termination. (C) Rate of change of 21 July insolation at 65°N. Red shading indicates the timing of the WMIs. The yellow dashed line indicates the lowest maximum for the four terminations. (D) δ18O from Hulu (purple), Dongge Caves (dark blue), Sanbao Cave [light green (11), dark green (this study)], and Linzhu Cave [yellow-green (this study)]. (E) Vostok CO2 record. (F) Benthic δ18O values.
As can be seen from the figure, when an interglacial starts CO2 levels do increase from the lower levels of the previous glacial. This increase is part of a feedback loop that amplifies the warming trend. None of this is news, the exciting part of the Cheng paper is the link between melting northern ice sheets and weakened monsoons. The link to Heinrich events, brief periods of sudden warming marked by large amounts of ice rafted debris, suggests that the monsoon responds to the breakup of the Laurentide Ice Sheet. According to cave records spanning the last two glacial terminations (T-I and T-II), the monsoon generally follows summer insolation except in times of weak monsoon activity. These distinct “gouges” correlate broadly to Heinrich stadial I (H-I) and to the Younger Dryas (during T-I) and to H-11 (during T-II). For more background information on Heinrich events see “Modeling Ice Age's End Lessens Climate Change Worries” and for more on the Younger Dryas see our book, The Resilient Earth.
According to the perspective by Jeffrey P. Severinghaus, a scientist at the Scripps Institution of Oceanography, Cheng et al.'s timing data provide support for the hypothesis that a reduced MOC forces CO2 out of the Southern Ocean, warming the globe by its greenhouse effect, which in turn causes more melting of the ice sheets, ensuring that the MOC stays in its “off” position—an environmental positive-feedback loop. “The melting ice sheets inject so much low-density fresh water into the North Atlantic that they weaken or entirely shut down the normal sinking of dense water that fuels the ocean circulation,” says Severinghaus. “The loss of this circulation allows sea ice to cover the North Atlantic in winter, preventing ocean heat from warming the air and leading to extremely cold winters in Europe and Eurasia, which seem to weaken the following summer's monsoon in Asia.”
Annotations by J. Severinghaus, photo credit NASA.
The scenario goes something like this: because fresh water has a lower density than salt water, meltwater runoff into the North Atlantic prevents sinking of water around Greenland. This causes the MOC to weaken and collapse. Without the northward transport of salty tropical water by the MOC the North Atlantic surface waters freshen even more. This fresh surface layer prevents deep convection which enhances winter sea ice formation. Increased sea ice cover causes extremely cold winter air temperatures over the North Atlantic and a southward-shifted atmospheric jet carries the cold air to the Mideast and Indian Ocean regions. Finally, cooling of the North Indian Ocean and the Asian landmass during the winter season weakens and delays the onset of the following summer's monsoon.
What are the possible impacts of these new new hypotheses on global warming driven by human generated CO2? First off, the data presented here show that nature if fully capable of rapidly transitioning from frigid glacial conditions to more temperate interglacial climes, and it has done so repeatedly without human prodding. Second, regardless of what some have said, there is nothing particularly anomalous about the Holocene warming when compared with the glacial terminations in the past. Cheng et al. do present a number of interesting hypothetical links between the end of the glacial and the rise of CO2 levels:
A number of mechanistic ties between this set of events and CO2 rise seem plausible. First, simple southward movement of climatic zones [observed for ITCZ and southern Brazil] could include a southward shift in the westerlies, resulting in enhanced wind-driven upwelling in the ocean around Antarctica, promoting ventilation of respired CO2, atmospheric CO2 rise, and observed productivity peaks. Second, warming from the bipolar seesaw mechanism could melt sea ice in the Southern Ocean, also promoting CO2 ventilation. Third, warming associated with southerly shifts in climate zones could reduce Patagonian glaciation, lowering the flux of dust and iron from Patagonia to the Southern Ocean, reducing the efficiency of the biological pump.
These relationships reinforce the well accepted theory that CO2 is driven by the change in temperature at the end of a glacial period, not the other way around. Indeed, other scientists have recently reported similar observations going back as far as 1.2 million years (see “Change In Ice Ages Not Caused By CO2”). In fact, the association between cyclically melting ice and the ocean “carbon pump” is well established. While others have stated that no single mechanism could explain the full glacial-interglacial range in CO2, this report reaches a different conclusion: “Here, we present a scenario in which CO2 rise could be caused by a set of mechanisms all ultimately linked to the rise in boreal summer insolation. Both rising insolation and rising CO2, generated with multiple positive feedbacks, drove the termination.”
The addition of CO2 to the atmosphere would have the biggest impact when levels are lowest, with subsequent temperature increases trailing off in time as concentrations rise. In this sense greenhouse gas warming is a positive feedback but self limiting, if it wasn't Earth's climate would runaway in an upward spiral of increasing temperatures and GHG release. Scientists are just coming to realize that there are massive reserves of GHG in Arctic tundra and in ocean methane clathrate deposits that could drive atmospheric concentrations of CO2 and CH4. One recent paper in Nature claims that tundra doesn't even need to fully defrost to emit significant volumes of greenhouse gas. Natural mechanisms have triggered sudden increases in GHG levels in the past, particularly releases of methane, which is a considerably more potent GHG than carbon dioxide.
Warming tundra releases greenhouse gases. National Science Foundation.
One of the most spectacular of these events was the PETM some 55 million years ago (see “Could Human CO2 Emissions Cause Another PETM?”). Earth's climate not only recovered from that warming spike, it eventually entered a cooling cycle 30 million years ago that let to the formation of permanent ice caps on Antarctica and Northern Hemisphere land masses. Eventually this cooling trend resulted in the Pleistocene Ice Age, which dominates our planet's climate to this day. If Earth's climate was predisposed to runaway global warming, and the effects of atmospheric GHGs are potent enough to drive warming on their own, temperatures would have continued to climb since the last glacial termination. Clearly that hasn't happened. Instead, the Holocene climate has been quite stable when compared with glacial period environments, though it has exhibited periods of rising and falling temperatures.
One reader of the Resilient Earth blog asked if the rising in CO2 levels during glacial terminations contradicted my statement that there have been ice ages when the level of CO2 in the atmosphere was much higher than today. That statement was not a reference to the conditions that have prevailed during the Pleistocene Ice Age, which has been going on for the past 3 million years or so. It was a reference to earlier ice ages, of which there have been many. For details see my article, “The Grand View: 4 Billion Years Of Climate Change.” There have indeed been ice ages when CO2 levels have been several times higher than the “unprecedented” levels so alarming to the climate catastrophists.
Monsoons are affected by glacial terminations.
Cheng et al. have reaffirmed the astronomical theory of the ice ages by using monsoons to improve dating precision across the whole suite of paleodata. The shutoff of the MOC and the resulting southward shift of tropical rain belts holds important lessons for those climate catastrophists who have been pointing to global warming as the cause for the recently diminished monsoon—it is colder weather in the northern hemisphere that stymies the monsoon's arrival. True, the episodes dated by the researchers were each the result of a warming trend, which triggered a cooling backlash to the widespread melting of glacial ice. But those impacts on the monsoon, much more dramatic than the variations seen recently, were triggered by the melting of mile thick glacial ice from North America and Eurasia. As Severinghaus states, “terminations require an existing massive ice sheet, and that Earth's orbit becomes nearly circular every ~100,000 years, eliminating periods of intense sunshine and thereby permitting the gradual accumulation of a massive ice sheet.” The lack of ice sheets covering all of Canada and Northern Europe seems to have escaped the alarmists' notice.
What about all those recent pronouncements that global warming is going to severely impact the normal Southeast Asian monsoon cycle? It must be noted that the changes experienced by Earth's climate during a glacial termination are far more radical than anything projected for global warming, even in the feverish dreams of Al Gore and the IPCC. The volume of freshwater needed to shut down the MOC is more than the output of all the rivers on Earth and the temperature swings involved can be as great as 12°C (22°F). Unfortunately for the sky-is-falling crowd, the recent variations seen in the monsoon are nothing out of the ordinary.
A monsoon sunset.
According to a government report cited by the Times of India, climate model studies have shown no significant impact on change in the mean onset of monsoon in the country. “The long-term mean onset date of monsoon in India is 1st June, with a standard deviation of about 8 days,” stated Environment minister Jairam Ramesh in the article dated July 13, 2009. “However, year to year variations in the onset or the propagation are part of the natural variability and cannot be attributed to climate change,” he concluded. No, today's conditions are not at all like previous glacial terminations with their 100,000 year cycle.
One last observation: an interesting exception cited by Cheng et al. is a termination that does not fit neatly into the 100,000-year paradigm. Anomalously weak sunshine 229,000 years ago apparently allowed the accumulation of a massive ice sheet within a short time, causing an exception to the normal glacial-interglacial rhythm. So we see it is not just the Milankovitch Cycles on their own that drives the ice ages, they require a collaboration of orbital dynamics, solar activity and Earth's own climate engine to effect such changes. Yet the supporters of catastrophic climate change insist that humanity will cause unprecedented and irreversible change through the release of CO2. The climate catastrophists are unable to comprehend the truth—the interaction of our planet and its star is what drives climate change.
Be safe, enjoy the interglacial and stay skeptical.