The long hot tail of global warming - new thinking on the Eocene greenhouse climate

Past climate changes like the Eocene Hyperthermals left many traces in the geological record. These tell scientists a great deal about what the Earth looked like in these hothouse eras, the changes they made to rainfall, drought, landscape, oceans, ecosystems and life. Ultimately those records contain clues to the causes of the climate changes, and are signposts to the effects we can expect from modern climate change.

A trio of new studies show that the Eocene Hyperthermals were the result of, not the cause of, global warming in the Eocene. This refocuses attention on abrupt global warming episodes like the PETM, and their role in converting the cooling Paleocene climate into the long-lived Eocene hothouse.

Modern climate change is even more abrupt, and is likely to have a similarly long, hot tail.

Hot flashes

The Paleocene and Eocene Hyperthermals were numerous recurring periods of hot global climate (a bit like human “hot flashes”) between around 59 and 34 million years ago. They were variations on a climate that was generally some 15°C (27°F) warmer than today, when the poles were free of ice even in winter and sea levels were about as high as they have ever been. The land was largely covered in jungle and even polar areas were lushly vegetated.

A trio of recent studies together represent a tipping-point in our understanding of this fluctuating greenhouse world. Littler et al present a detailed 8 million year record from a borehole drilled the South Atlantic, and compare that with another borehole in the Pacific. Turner et al integrate a 4.25 million year record from the equatorial Atlantic with published data to create a 10 million year record in the Eocene. Smith et al studied 1.8 million years of terrestrial sediments in Wyoming, cross-referenced to the same Atlantic marine sediment record as used by Turner et al. The focus of all 3 studies was the links between the Paleocene-Eocene climate and variations in solar energy distribution (insolation) caused by Earth’s orbital wobbles.

 Eocene Hyperthermal data from the Atlantic and Pacific Oceans (Littler et al)

Paleocene and Early Eocene Hyperthermals recorded in ocean sediments. Compilation of Atlantic and Pacific data from Littler et al EPSL 2014, © Elsevier, with annotation added for this article.

Cooling Paleocene

The data show that the Earth’s climate was on a long cooling trend in the Paleocene, associated with removal of CO2 from the atmosphere and burying it in sediments, deposits of peat, permafrost and methane hydrates. Those carbon reserves were like deposits in a bank account – available for withdrawal later. The first signs of warming began around 58.9 million years ago associated with a sharp jump in ocean bottom water temperatures (known as the “ELPE” - Early Late Paleocene Event), followed 1.2 million years later by a more steady warming trend that underpinned regular cycles of hotter and cooler climate (those hot flashes that we call Hyperthermals).

But what could cause those fluctuations?

Orbital wobbles

The clue is in the regularity of the fluctuations.

Scientists combined radiometric dates on volcanic ash layers with the record of Earth’s magnetic reversals and the known cycles in Earth’s orbit, to correlate the sedimentary records at the different locations. This also allowed a mathematical assessment of the correlation between the climate cycles recorded in sediments and Earth’s orbital cycles. There are 3 different kinds of orbital cycle that control the intensity of sunlight on Earth (solar insolation): eccentricity, precession, and obliquity – for more about them see this post.

The 3 papers show that the Hyperthermals and their effects were global, and were paced by orbital eccentricity on cycles of 405,000 and 100,000 years, and also by orbital precession (21,000 year cycles). The longer eccentricity cycles are associated with ocean bottom water temperature swings of up to 2-4°C (4-7°F), while the 100,000 year cycles correspond with 1.5°C (3°F) swings in bottom temperature. There’s very little contribution from the obliquity (41,000 year cycle) probably because there was very little ice at the poles at the time (unlike during the Pleistocene ice age, when obliquity was much more prevalent).

In the oceans these cycles show up as regular variations in carbon isotope ratios (reflecting carbon cycle changes) and oxygen isotope variations (reflecting ocean temperature) and the proportion of “Coarse Fraction” or Iron intensity in sediments (reflecting carbonate dissolution and sediment supply).

Fluctuating landscapes

On land, times of high rainfall with high lake levels and increased vegetation cover alternated with times when lakes diminished to salt pans and mudflats with loss of vegetation cover, on the 21,000  year precession pacing. In between those was a middle-ground climate with intermediate lake levels and vegetation cover, dominated by rivers. Every 100,000 and 405,000 years, orbital eccentricity suppressed the regular 21,000 year cycle, and the climate got “stuck” in the middle, river-dominated mode.

 Fluctuating Eocene landscapes

Orbital cycles of climate and landscape in Wyoming during the Eocene. Simplified from Smith et al EPSL 2014. Currently it isn’t clear if either A or C coincide with insolation maxima: salt pans may have coincided with the higher evaporation associated with summer insolation maxima, or alternatively enhancement of the monsoon over the western US due to insolation maxima could have led to high lake levels.

New thinking

Previous studies suggested that the strength of the Hyperthermals diminished over time and their frequency increased, implying that pulsed releases of carbon from methane hydrates or permafrost kept the climate hot throughout the Eocene. But Turner et al refute this, showing that the Hyperthermal beat continued long after reserves of methane hydrates and permafrost must have been emptied as the Eocene hothouse persisted through the extended “Early Eocene Climate Optimum” (53-50 million years ago). They also find that the:

“...carbon cycle processes behind these events, excluding the largest event, the Palaeocene–Eocene Thermal Maximum (about 56 million years ago), were not exceptional.”

They show that the regular orbitally-paced heartbeat of Hyperthermals continued over the whole 10 million years of their study, just like they did through many other eras of Earth’s past, including in the Devonian, Carboniferous to Permian, Cretaceous, Oligocene, Miocene and the Pleistocene ice age. In other words, orbitally-paced climate oscillations are not a uniquely Eocene phenomenon, so require an explanation not unique to the Eocene.

Oceans - a giant carbon reservoir

The jury is still out on the exact combination of changes in carbon storage drive these cycles (marine biological pump, carbon burial on land, terrestrial weathering), but it appears that the oceans are, once again, key to this.

The oscillations observed are faster than carbon can be exhumed or buried in the global sedimentary reservoir, so a faster-responding carbon source must have been in play that was able to equilibrate with the atmosphere on millennial timescales – the oceans. This reinforces earlier ideas which indicated that the Hyperthermals were accompanied by repeated, large-scale releases of dissolved organic carbon from the ocean by ventilation (strengthened oxidation) of the ocean interior. Orbital wobbles seem to modulate deep ocean acidity as well as the production and burial of global biomass, and the reason the 405,000-year eccentricity cycle is so prominent is because of the centuries-long residence time of carbon in the oceans. Temperature-enhanced metabolic processes and remineralization of organic carbon in deep ocean sediments, operating on timescales of tens of thousands of years, would also modulate the release of carbon into the oceans.

This new thinking is supported by Smith et al, who concluded that the regular carbon releases in the Eocene Hyperthermals:

“...were the effect rather than the cause of global paleoclimatic and geomorphic changes during the EECO.” (my emphasis)

Littler et al’s study would seem to concur. They found that ocean bottom temperatures lead the carbon cycle by around 6 millennia for the 405,000 year cycles (less in the Paleocene, more in the mid Eocene) and about 3,000 years for the 100,000 year cycles. This indicates that the ocean carbon signal in orbitally-controlled climate change is a feedback response to orbitally-driven temperatures.

In contrast, today’s climate change is not orbitally-driven – if it were we would be experiencing global cooling, not warming. It has also taken place in a time frame just 3% of even the fastest orbital cycle.

So if the Hyperthermals are the normal “heartbeat” of the planet and not the primary cause of the hot Eocene climate – what was?

Volcanic CO2 pushes

Superimposed on the background ‘hum’ of regular orbital cycles are several distinct spikes of strong climate change, too big or with too long a recovery time to be forced purely by orbital wobbles.

The earliest of these spikes is known as “ELPE” (Early Late Paleocene Event – 58.9 million years ago), which marks the end of the long term cooling trend in the Paleocene. It corresponds with a 4°C jump in ocean bottom water temperatures, swings in carbon isotope values, and ocean chemistry changes suggesting acidification. Intriguingly the ELPE coincides with a particularly violent phase of North Atlantic volcanic activity, resulting in the deposition of ash deposits like the Kettla Tuff in the Faroe-Shetland area, and contemporary volcanic ash deposits in western, and eastern Scotland, and igneous activity in Greenland and offshore Ireland.

Another sharp swing in ocean chemistry, carbon isotopes and abrupt ocean warming occurred at 57.7 Million years ago, known as the “Peak Paleocene Carbon Isotope Maximum” or “Peak PCIM.” Littler et al attribute the long term warming trend that began then and continued into the Eocene, to sustained carbon emissions resulting from North Atlantic Igneous Province activity.

The PETM's big push to warming

The PETM (Paleocene-Eocene Thermal Maximum) is chief among the exceptional, non-cyclical, dramatic warming events, as these papers agree. Even though it began coincident with the 100,000 year orbital pacing, it was not in phase with the 405,000 year cycle that dominates the other Hyperthermals, suggesting an “extra push” was provided, probably by the spectacular eruptions from Canada to Norway that accompanied the opening of the North Atlantic at that time.

Bank withdrawal

Rising temperatures beginning in the late Paleocene must have triggered the slow release of carbon reservoirs, (the peat, permafrost, methane hydrates that were ‘banked’ in the cooling part of the Paleocene) into the atmosphere, until they were exhausted by around the Early Eocene Climate Optimum. All that extra carbon was mobilized into the dynamic biosphere-ocean-atmosphere system, where it amplified and prolonged the warming.

These 3 discrete events, chiefly the PETM, seem to have provided the global warming “push,” that combined with feedbacks from carbon reservoirs on land and the oceans, which switched the cooling Paleocene over to the long, hot Eocene.

Long, hot tail – then and now

If those 3 episodes each represent major singular additions of CO2 to the atmosphere that caused the hot Eocene climate, then the duration of the Eocene hothouse is extraordinarily long. Mathematical modelling for a single pulse of carbon emissions like the PETM shows that, without additional carbon release, we expect a return to background temperatures after a few tens of thousands of years. The observed duration of the PETM is much longer than predicted by those models, requiring prolonged additional carbon input. It would appear that the finger is increasingly pointing at the oceans and their long term influence on the carbon cycle as the main sustaining factor.

uncomfortable parallels to today

As was the case during the Paleocene, our modern Earth has reserves of permafrost and methane hydrates that were ‘banked’ during the cool Pleistocene ice age all the way up to the pre-industrial era. And even though some interglacials may have been warmer than the preindustrial era, deep-ocean temperatures appear to have been only slightly elevated, keeping most deep sea methane locked away - until now. A Paleocene-to-Eocene-like methane release is expected from modern climate change because the deep oceans are now warming by a magnitude unprecedented in the past several million years!

Last year Zeebe and Zachos compared the long tail of the PETM with our present human-caused global warming. They concluded that our own long hot tail will last tens of thousands to hundreds of thousands of years. But they argued that future environmental effects will:

“more likely resemble the end-Permian and end-Cretaceous catastrophes, rather than the PETM,”

due to the far more abrupt nature of modern carbon emissions and warming.

Posted by howardlee on Thursday, 9 October, 2014

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