Wiedererwärmung nach den Eiszeiten bringt CO2-Anstieg – mit 800 Jahren Verzögerung

Alle 100.000 Jahre ereignet sich eine Eiszeit. Die dauert dann ca. 85.000 Jahre und wird dann von einer Warmzeit (einem sogenannten Interglazial) abgelöst, die etwa 10.000-15.000 Jahre dauert. Momentan leben wir in einem solchen Interglazial. Aber die Uhr tickt und schon bald ist die Zeit um…

In den Eiszeiten liegt der CO2-Gehalt der Atmosphäre bei etwa 180 ppm, in den Warmzeiten bei 300 ppm. Da warmes Wasser weniger CO2 lösen kann, gast das CO2 im Übergang von Eiszeit zu Warmzeit aus den Weltozeanen aus. Das geht aber nicht von heute auf morgen, sondern läuft über längere Zeit ab. Insofern folgt die CO2-Kurve der Temperaturkurve in den letzten 800.000 Jahren mit einer Verzögerung von 800 Jahren. Immer wieder gab es von Aktivistenseite Bestrebungen, diese 800 Jahre aus der Welt zu schaffen. Würde es nicht viel besser ins Narrativ passen, wenn die Temperaturen eine Folge der CO2-Veränderungen wären? Dann passt aber die Zeitlichkeit nicht mehr. Es wurde mit allen erdenklichen statistischen und graphischen Tricks gearbeitet. An den Fakten änderte dies aber nichts.

Wir schauen heute, was es Neues zum Thema gibt.

Im Mai 2019 erschien ein Paper von Jai Chowdhry Beeman und Kollegen, die sich die Zeitlichkeiten von Temperatur und CO2 während des Übergangs der letzten Eiszeit zur aktuellen Warmzeit anhand von antarktischen Eiskernen anschauten. Dabei fanden sie eine Verzögerung der CO2-Entwicklung von durchschnittlich 550 Jahren. Auszug aus dem Abstract:

During the onset of the last deglaciation at 18 ka and the deglaciation end at 11.5 ka, Antarctic temperature most likely led CO2 by several centuries (by 570 years, within a range of 127 to 751 years, 68 % probability, at the T1 onset; and by 532 years, within a range of 337 to 629 years, 68 % probability, at the deglaciation end).

Vor 14.400 Jahren gab es eine kürzere Kältephase, die aber keinen systematischen Vorlauf/Nachlauf (Lead/Lag) aufweist.

Uemura et al. 2018 untersuchten gleich mehrere Eiszeitzyklen der letzten 720.000 Jahre und fassten den Wissensstand in der Einleitung zusammen. δD steht dabei für Änderungen im Deuterium, das als Temperaturproxy fungiert. Es wurden je nach Eiskern und betrachtetem Zeitraum Verzögerungen des CO2 gegenüber der Temperatur von 800-2000 Jahren berichtet. Aus der Introduction:

The values of a temperature proxy, the hydrogen isotopic composition (δD), in the Antarctic EDC ice core1,2 have varied in parallel with CO2 concentrations over the past 800 thousand years (kyr; r2 = 0.82)3. However, δD apparently leads CO2 variations. For example, during the last termination (TI), the start of Antarctic warming has been estimated to be synchronous with CO2 increase4 or to lead CO2 increases by 800 ± 600 years5 on the East Antarctic Plateau. The lead is ca. 2000 years at a West Antarctic site6. Over the past 420 kyr, the Vostok ice core shows that the Antarctic δD temperatures lead the CO2 variations by 1.3 ± 1.0 kyr7. During the lukewarm interglacials (430–650 kyr BP), Antarctic δD leads CO2 by 1900 years, and the correlation between CO2 and δD is weaker (r2 = 0.57), as determined from the EDC core8.

Siehe auch Beiträge auf Notrickszone und WUWT mit weiteren Literaturzitaten.

Eine Studie aus dem Pazifik berichtet 1000 Jahre Verzögerung des CO2 gegenüber der Temperatur und erläutert, wie das in der pazifischen Tiefsee gespeicherte CO2 im Übergang zur Warmzeit allmählich in die Atmosphäre überführt wird. Die Pressemitteilung der Oregon State University von 2018 ist launisch-kurzweilig geschrieben. Ganz am Ende kommt dann noch ein bisschen Klimaalarm, über den wir großzügig hinwegsehen:

Scientists trace atmospheric rise in CO2 during deglaciation to deep Pacific Ocean

Long before humans started injecting carbon dioxide into the atmosphere by burning fossil fuels like oil, gas, and coal, the level of atmospheric CO2 rose significantly as the Earth came out of its last ice age. Many scientists have long suspected that the source of that carbon was from the deep sea. But researchers haven’t been able to document just how the carbon made it out of the ocean and into the atmosphere. It has remained one of the most important mysteries of science. A new study, published today in the journal Nature Geoscience, provides some of the most compelling evidence for how it happened – a “flushing” of the deep Pacific Ocean caused by the acceleration of water circulation patterns that begin around Antarctica. The concern, researchers say, is that it could happen again, potentially magnifying and accelerating human-caused climate change.

“The Pacific Ocean is big and you can store a lot of stuff down there – it’s kind of like Grandma’s root cellar – stuff accumulates there and sometimes doesn’t get cleaned out,” said Alan Mix, an Oregon State University oceanographer and co-author on the study. “We’ve known that CO2 in the atmosphere went up and down in the past, we know that it was part of big climate changes, and we thought it came out of the deep ocean. “But it has not been clear how the carbon actually got out of the ocean to cause the CO2 rise.”

Lead author Jianghui Du, a doctoral student in oceanography at Oregon State, said there is a circulation pattern in the Pacific that begins with water around Antarctica sinking and moving northward at great depth a few miles below the surface. It continues all the way to Alaska, where it rises, turns back southward, and flows back to Antarctica where it mixes back up to the sea surface.

It takes a long time for the water’s round trip journey in the abyss – almost 1,000 years, Du said.  Along with the rest of the OSU team, Du found that flow slowed down during glacial maximums but sped up during deglaciation, as the Earth warmed. This faster flow flushed the carbon from the deep Pacific Ocean – “cleaning out Grandma’s root cellar” – and brought the CO2 to the surface near Antarctica. There it was released into the atmosphere.

“It happened roughly in two steps during the last deglaciation – an initial phase from 18,000 to 15,000 years ago, when CO2 rose by about 50 parts per million, and a second pulse later added another 30 parts per million,” Du said. That total is just a bit less than the amount CO2 has risen since the industrial revolution. So the ocean can be a powerful source of carbon.

Brian Haley, also an Oregon State University oceanographer and co-author on the study, noted that carbon is always falling down into the deep ocean. Up near the surface, plankton grow, but when they die they sink and decompose. That is a biological pump that is always sending carbon to the bottom. “The slower the circulation,” Haley said, “the more time the water spends down there, and carbon can build up.”

Du said that during a glacial maximum, the water slows down and accumulates lots of carbon. “When the Earth began warming, the water movement sped up by about a factor of three,” he noted, “and that carbon came back to the surface.” The key to the researchers’ discovery is the analysis of neodymium isotopes in North Pacific sediment cores. Haley noted that the isotopes are “like a return address label on a letter from the deep ocean.”  When the ratio of isotope 143 to 144 is higher in the sediments, the water movement during that period was slower. When water movement speeds up during warming events, the ratio of neodymium isotopes reflects that too. “This finding that the deep circulation sped up is the smoking gun in this mystery story about how CO2 got out to the deep sea,” Mix said. “We now know how it happened, and the deep Pacific is the culprit – a partner in crime with Antarctica.”

What concerns the researchers is that it could happen again as the climate continues to warm. “We don’t know that the circulation will speed up and bring that carbon to the surface, but it seems like a reasonable thing to think about,” Du said. “Our evidence that this actually happened in the past will help the people who run climate models figure out whether it is a real risk for the future.” The researchers say their findings should be considered from a policy perspective.

“So far the ocean has absorbed about a third of the total carbon emitted from fossil fuels,” Mix said. “That has helped slow down warming. The Paris Climate Agreement has set goals of containing warming to 1.5 to 2 degrees (Celsius) and we know pretty well how much carbon can be released to the atmosphere while keeping to that level.

“But if the ocean stops absorbing the excess CO2, and instead releases more from the deep sea, that spells trouble.  Ocean release would subtract from our remaining emissions budget and that means we’re going to have to get our emissions down a heck of a lot faster. We need to figure out how much.” The authors are from College of Earth, Ocean, and Atmospheric Sciences at Oregon State, and from United States Geological Survey. The study was supported by the National Science Foundation.

Das dazugehörige Paper von Du et al. 2018:

Flushing of the deep Pacific Ocean and the deglacial rise of atmospheric CO2 concentrations

During the last deglaciation (19,000–9,000 years ago), atmospheric CO2 increased by about 80 ppm. Understanding the mechanisms responsible for this change is a central theme of palaeoclimatology, relevant for predicting future CO2 transfers in a warming world. Deglacial CO2 rise hypothetically tapped an accumulated deep Pacific carbon reservoir, but the processes remain elusive as they are underconstrained by existing tracers. Here we report high-resolution authigenic neodymium isotope data in North Pacific sediment cores and infer abyssal Pacific overturning weaker than today during the Last Glacial Maximum but intermittently stronger during steps of deglacial CO2 rise. Radiocarbon evidence suggestive of relatively ‘old’ deglacial deep Pacific water is reinterpreted here as an increase in preformed 14C age of subsurface waters sourced near Antarctica, consistent with movement of aged carbon out of the deep ocean and release of CO2 to the atmosphere during the abyssal flushing events. The timing of neodymium isotope changes suggests that deglacial acceleration of Pacific abyssal circulation tracked Southern Hemisphere warming, sea-ice retreat and increase of mean ocean temperature. The inferred magnitude of circulation changes is consistent with deep Pacific flushing as a significant, and perhaps dominant, control of the deglacial rise of atmospheric CO2.

Ferreira et al. 2018 nehmen an, dass Eiszeit und Warmzeit zwei stabile Zustände sind und beschreiben, wie sie abwechseln. Dabei spielt auch das Ausgasen von CO2 aus den Ozeanen eine Rolle:

Linking Glacial‐Interglacial States to Multiple Equilibria of Climate

Glacial‐interglacial cycles are often described as an amplified global response of the climate to perturbations in solar radiation caused by oscillations of Earth’s orbit. However, it remains unclear whether internal feedbacks are large enough to account for the radically different glacial and interglacial states. Here we provide support for an alternative view: Glacial‐interglacial states are multiple equilibria of the climate system that exist for the same external forcing. We show that such multiple equilibria resembling glacial and interglacial states can be found in a complex coupled general circulation model of the ocean‐atmosphere‐sea ice system. The multiple states are sustained by ice‐albedo feedback modified by ocean heat transport and are not caused by the bistability of the ocean’s overturning circulation. In addition, expansion/contraction of the Southern Hemisphere ice pack over regions of upwelling, regulating outgassing of CO2 to the atmosphere, is the primary mechanism behind a large pCO2 change between states.

Kobayashi & Oka 2018 weisen auf eine stärkere Schichtung des Pazifikwassers während der Eiszeiten hin, die über eine verbesserte Kalklöslichkeit das CO2-Speichervermögen erhöht:

Response of Atmospheric pCO2 to Glacial Changes in the Southern Ocean Amplified by Carbonate Compensation

Atmospheric carbon dioxide concentration (pCO2) varies by about 100ppm during glacial‐interglacial cycles. Previous studies suggest that the strongly stratified Southern Ocean at the Last Glacial Maximum increases the oceanic storage of carbon, but the glacial reduction of atmospheric pCO2 simulated by ocean general circulation models (OGCMs) does not reach 100ppm. One candidate for the underestimation is that carbonate compensation is not explicitly incorporated in the previous OGCM simulations. Therefore, we quantitatively evaluate the impact of carbonate compensation on the glacial atmospheric pCO2 by using an OGCM coupled with an ocean sediment model. As suggested by previous box model studies, our OGCM simulations show that the enhanced Southern Ocean stratification amplifies the decrease in atmospheric pCO2 due to carbonate compensation. Considering the enhanced stratification in the Southern Ocean, we obtain a 26‐ppm drawdown of atmospheric pCO2 by carbonate compensation, and the full reduction from our pre‐industrial simulation reaches 73ppm. Both the increase in ventilation ages in the deep Atlantic and Southern Oceans and the growth of export production in the subantarctic region reduce the bottom‐water carbonate ion and promote deposited carbonate dissolution. Consequently, a greater imbalance between the river inflow and burial loss of carbonate rises ocean alkalinity, lowering atmospheric pCO2. We suggest that the reproducibility of the Southern Ocean process is essential for controlling the magnitude of atmospheric pCO2 decline due to carbonate compensation.