Gradual falls and sharp rises in temperature for millions of years have profoundly affected living conditions on the planet and, consequently, our own evolution.
Mark Maslin is a professor of Earth system science in the department of geography at University College London.
Physics Today 73, 5, 48 (2020); https://doi.org/10.1063/PT.3.4474
Milutin Milanković, a brilliant Serbian mathematician and climatologist, postulated in 1941 that variations in Earth’s orbit could push the planet’s climate in or out of an ice age.1 Vital to that idea is the amount of insolation—incoming solar radiation—at 65° N, a bit south of the Arctic Circle. At that latitude, insolation can vary seasonally by 25%. Milanković argued that reductions in summer insolation allow some winter ice to survive. Each year for thousands of years, ice accumulates around 65° N and eventually forms sheets large enough to trigger an ice age.
Three scientists joined forces 30 years later to verify Milanković’s theory using deep-sea sediment cores collected by the international Ocean Drilling Program. James Hays examined marine microfossils in the cores to estimate past sea-surface temperatures. Nicholas Shackleton measured the oxygen isotope composition in the sediment’s layers, which showed changes in past global ice volume. And the last member of the team, John Imbrie, brought an expertise in time-series analysis to the project. In 1976 they published a seminal paper showing that their climate record contained the same temporal cycles as three parameters, summarized in figure 1, that describe Earth’s orbit: eccentricity, obliquity, and precession.2
Eccentricity describes the shape of Earth’s orbit around the Sun. As Earth experiences a gravitational force from Jupiter, its orbit adjusts during a 96 000-year period from nearly a perfect circle to an ellipse, which causes minor variations in total insolation. Obliquity—the tilt of Earth’s axis of rotation with respect to the plane of its orbit—fluctuates during a period of 41 000 years between 21.8˚ and 24.4˚ and is currently at 23.4˚. A larger obliquity generates a greater difference in the insolation Earth receives during summer and winter.
The third orbital parameter, precession, occurs every 21 700 years and influences Earth’s closest approach to the Sun. During each hemisphere’s summer, precession has the greatest effect in the tropics. Tidal forces of the Sun and Moon, amplified by Earth’s oblate spheroid shape, cause one component of precession. Those forces exert gyroscopic motion on the planet that changes the orientation of its rotational axis. The second component of precession moves Earth’s entire orbit around the Sun in space and resembles the petals of a flower, as shown in figure 1c.
The great ice ages
Over the past 2.5 million years, Earth has undergone some 50 major ice ages and each has substantially changed the planet’s climate.3 During the last one 21 000 years ago, a nearly continuous ice sheet spanned North America. At its thickest, across what is now Hudson Bay, it was more than two miles deep and reached as far south as New York City and Cincinnati, Ohio. The British–Irish ice sheet spread as far south as Norfolk, and the Scandinavian ice sheet extended from Norway to the Ural Mountains in Russia. In the Southern Hemisphere, large ice sheets covered Patagonia, South Africa, southern Australia, and New Zealand. So much water was locked in all those ice sheets that global sea level dropped 120 m, yet if all the Antarctic and Greenland ice melted today, sea level would rise only by 70 m.
How did small wobbles in Earth’s orbit cause those ice ages? Summer temperatures must first decrease a little bit. The consequent accumulation of snow and ice increases Earth’s albedo—the reflection of sunlight to space. Reflecting more sunlight suppresses local temperatures and promotes more snow and ice accumulation, which increases the albedo further. The process, called an ice–albedo feedback, is responsible for building increasingly bigger ice sheets.
Another positive feedback cycle triggers when ice sheets, such as the Laurentide sheet that once covered much of North America, become big enough to deflect atmospheric planetary waves. The change redirects storm paths across the North Atlantic Ocean and prevents the Gulf Stream and its northeastward arm, the North Atlantic Drift, from penetrating as far north as they do today. The surface ocean effects, combined with melt-water increase in the Nordic Seas and the Atlantic, cause a decrease in the sinking of cold, salty water (see Physics Today, April 2019, page 19). As less water in the North Atlantic is driven to the deep ocean, the Gulf Stream pulls less warm water northward, and increased cooling in the Northern Hemisphere expands the ice sheets.
Greenhouse gases (GHGs) in the atmosphere reinforce ice-sheet feedbacks. Analyses of air bubbles trapped in polar ice indicate that during glacial periods carbon dioxide concentrations dropped by a third and methane by half. Changes in GHGs always precede variations in global temperatures and are therefore a clear driving force of climate change, not a response to it.4
Runaway positive feedbacks froze most of Earth’s water billions of years ago during snowball Earth events, but moisture limitation has prevented a more recent episode. Forming an ice sheet requires a cold, wet climate. But as an ice sheet forces warm surface water farther south, the supply of moisture decreases. By changing the atmosphere and ocean circulation, ice sheets starve themselves of moisture, and that negative feedback loop limits the effects of positive ones.
For the past million years, ice sheets have taken at least 80 000 years to reach their maximum extent, which occurred most recently about 21 000 years ago. However, ice melts much quicker than that: 80% of expanded ice sheets can be lost in 4000 years. Summer sunshine at 65° N triggers deglaciation and starts the melting of Northern Hemisphere ice sheets. Rising concentrations of carbon dioxide and methane in the atmosphere promote climate change and further melt large continental ice sheets. Such processes work against the ice–albedo effect, which acts to keep the ice sheets intact by producing a cooler microclimate.
Ultimately, rising sea levels diminish large ice sheets because the coldest that seawater can be is −1.8 ˚C, whereas the temperature of the ice sheet’s base is −30 ˚C. As seawater melts the ice sheets by undercutting them, ice calves into the ocean. The calving raises sea level further and causes more undercutting (see Physics Today, October 2019, page 14). The sea-level feedback mechanism can be extremely rapid. Once the ice sheets are retreating, the other feedback mechanisms—albedo, atmospheric and ocean circulation, and GHGs—are reversed. That’s why glaciologists and climatologists worry about future climate change: It will activate those feedbacks and cause irreversible instability to the Greenland and West Antarctic ice sheets (see Physics Today, July 2014, page 10).
The eccentricity myth
The last million years of glacial–interglacial cycles, each lasting about 100 000 years, have a saw-toothed pattern with a long period of cooling followed by a short, warm one of rapid melting. More than a million years ago, the cycles were smoother, and each lasted only 41 000 years, as shown in figure 2. That period corresponds to the length of the orbital change associated with obliquity, which controls the heat transfer between low and high latitudes and thus regulates ice growth.
Figure 2. Many glacial–interglacial cycles (red solid line) during the last 5 million years can be seen from measurements of the oxygen isotope composition of lake records. Large ice sheets started to grow in North America 2.5 million years ago during the intensification of Northern Hemisphere glaciation (iNHG). The development of the atmospheric Walker Circulation (DWC) started 1.7 million years ago in the Pacific Ocean and is sustained by a large east-to-west sea-surface-temperature gradient. About 1 million years ago, during the Mid-Pleistocene Revolution (MPR), the polar ice caps expanded toward the equator, and the glacial–interglacial cycle period increased from an average of 41 000 years to 100 000 years. (Adapted from ref. 3.)
For many years, scientists struggled to explain the 100 000-year glacial–interglacial cycles because the 96 000-year eccentricity mechanism has a similar length. But eccentricity is by far the weakest of the orbital variations, and many thought it predominantly modulated precession, so scientists suggested several nonlinear feedbacks to explain the discrepancy. But they found an answer when they realized that the 100 000-year cycle is a statistical artifact.
The average length of the last eight cycles is indeed 100 000 years, but each one varies from 80 000 to 120 000 years. Every fourth or fifth precessional cycle is weak enough that ice sheets can grow bigger and thus more vulnerable to sea-level rise during deglaciation. The next precessional cycle is always much stronger than the previous one and initiates rapid, extreme deglaciation through the sea-level feedback.5 Although the timing of deglaciation seems to better match precession, some researchers have argued that the long glacial–interglacial cycles may correspond to every second or third obliquity cycle.6
HT/Leif Svalgaard
The CO2 political statements in this article turn it from an informative read to an awkward one (“Changes in GHGs always precede variations in global temperatures and are therefore a clear driving force of climate change, not a response to it.”). It always happens when they Frankenstein in the obligatory political correctness.
Many in the west worry about melting glaciers, but few consider how old they are, but if you do you will see that many modern glaciers first formed over the last 4000 years ago as the Holocene began to cool. The oldest ice in the Hans Tausen Glacier in Northern Greenland first formed ca. 4,000 years ago; Mt Churchill in Alaska ice some 2,500 years ago; Quelccaya in Peru began its current existence 1,500 years ago; Kilimanjaro dates are less certain and are somewhere between Quelccaya and the Fremont Glacier in Wyoming, which is only 300 years old. New glaciers are forming closer and closer to the equator as the planet begins to cool inexorably into the next ice age. This is from John Kehr’s Inconvenient Skepic and to date, I have not seen it seriously questioned, let alone discussed. And this cooling has been accompanied by rising CO2 …. Curiouser and curiouser said Alice …
When glaciers grow in Tennessee.
H/t to the late, great Freeman Dyson
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“Milutin Milanković, a brilliant Serbian mathematician and climatologist, postulated in 1941 that variations in Earth’s orbit could push the planet’s climate in or out of an ice age.1 Vital to that idea is the amount of insolation—incoming solar radiation—at 65° N, a bit south of the Arctic Circle. At that latitude, insolation can vary seasonally by 25%. Milanković argued that reductions in summer insolation allow some winter ice to survive. Each year for thousands of years, ice accumulates around 65° N and eventually forms sheets large enough to trigger an ice age.”
That “…insolation can vary seasonally by 25%.” might sound impressive, but it’s not. That amount sunlight reaching the land surface at 65 degree latitude is very low.
If you go from 65 degree latitude to 60 degree latitude there will be significant increase in the amount sunlight striking the surface. But there not much sunlight striking the surface at 60 degree latitude or even at 50 degree latitude.
To collect solar energy at 65 degree latitude, you tilt the angle of solar panels to collect more sunlight, as compared to having level with the surface. I would guess the tilt would be somewhere near 45 degree so that the sunlight hit panel surface near perpendicular. So at 65 degree latitude if have a surface at angle where sunlight hit somewhere near perpendicular, the surface will get more energy per square meter, as compare to the surface of the land- much more 25% difference. If did same thing at 60 degree latitude, the tilt could be about 40 degree rather than 45 degree {or at 40 degree latitude the tilt would 20 degree rather 40}.
Now don’t have to go up to Arctic to see this. It happens every day where ever you are, you get the sun low above the horizon in the morning and when near sunset.
When Sun is directly above the equator it is the equinox, sun at noon at will at 90 degree and if at 65 degree latitude and at noon it is 90 – 65 or 25 degree above the horizon at noon.
Back to equator at equinox, you will have 12 hour day and night {roughly speaking this also true all over the world- a equal amount of day and night} anyhow at equator, the sun rise 15 degree per hour. And if at noon,
in 6 hours it will be sunset: 6 times 15 = 90, sun will from 90 degree to 0 degree in 6 hours. Every else in world will be sunset in 6 hours. But at 65 degree latitude, the degrees it lowers per hour is roughly, 25 / 6 = about 4.1
degrees per hour, the sun lowers {or it’s flatter arc].
With that is mind, one has rough guide call peak solar hour. If at equator is easier, you have 6 hours in middle of day in the sunlight is the strongest- 3 hour before and after noon. Or since noon is 90 degree, when sun is 45 degree or more above the horizon. At noon the sunlight could be about 1050 watts per square meter, and 3 before or later it about 800 watts per square meter and doesn’t matter much whether you talking about level ground or pointing something at the Sun. But it does matter more when sun is before and after the peak hours. At 4 hours before or after noon, the sun is at 30 degree above the horizon, and sunlight must pass thru about 2 times more atmosphere as compared to when sun is at noon, and there ever increasing amount of atmosphere the sun has to pass thru. And terms of level surface, the sunlight spread out over large area, or someone standing will cast ever longer shadows when sun get lower 30 degrees above horizon.
Now at equinox, at 60 degree latitude, and at noon, the sun at 30 degree above the horizon {and when goes towards summer it peaks + 23.5 degrees, or 53.5 degrees above horizon at noon and you long daylight hours
and significant amount of time {hours} when the sunlight is at or above 45 degree above the horizon- or sun does NOT lower by 15 degrees per hour- it’s a flatten arc [as it was at the equinox}, And if sunlight is 45 or higher above horizon, the sunlight is not spread out over surface much {or your shadows are not to too long}
or guess one getting about 700 watts per square meter of insolation upon the surface- or ground may warm up to 30 to 40 C. But if go 5 degree up to 65 degrees latitude, it’s 53.5 – 5 = 48.5 degree above the horizon.
So you two factor and these factors quite significant when sun is lower than 30 degrees above horizon and in transitional zone from 30 to 45 degree. Or the 5 degrees different in the tilt is significant difference in terms of
insolation.
But I don’t think it’s a significant effect on whether one has glaciers or not. The average air temperature is important- and how dry it is and both these factors can be far more important.
Now, article does say this:
–Runaway positive feedbacks froze most of Earth’s water billions of years ago during snowball Earth events, but moisture limitation has prevented a more recent episode. Forming an ice sheet requires a cold, wet climate. —
I don’t agree about snowball earth myth, but this part:
“Forming an ice sheet requires a cold, wet climate. ”
I might say even “cool”, wet climate could give ice sheet. But we go with “cold” especially since think most
of Earth presently is pretty cold {it’s just in the context that others are imagining Earth is too warm- or we aren’t in Ice Age and people imagine that 15 C is warm}.
So how do get more “cold, wet climate”.
I think if entire ocean was warmer, one would get more cold, wet climate. And you get warmer, wet climate. Or Canada average year temp is minus 4 C, and if Canada got wetter, it’s average could rise to 0 C, and in northern parts of Canada, one could get more glaciers.
Celestial Mechanics, better said
Et ex machina caeli tempestatibus.
Deus ex machina?
Sure the solar system is a mechanical system, and that appeals to our modern Cartesian machine culture.
Only problem is, there is a much bigger, grander system, of which this solar system is but a tiny cog – the galaxy. And Svensmark, Shaviv found that is quite something else – a vast cosmic radiation system.
Milanković in 1941 simply predates our nuclear age.
Just exactly what is an “ice age”? Presently there are almost 200,000 glaciers on planet Earth and the poles retain a great deal of ice year around. In northern hemisphere winter 30% of the landmass is covered with ice and snow. So, what’s the deal, does NYC have to be under ice to qualify as an ice age?
Ice Age or icehouse climate is cold oceans and polar ice caps, we have in Ice Age for millions of year and the last million has been the coldest.
Some people say glacial periods are ice ages.
Some people claim there has been colder periods on Earth which are called Snowball or Slushball Earth.
But most of last 500 million year of Earth history has had much warmer Oceans then we have currently.
The warmest states are called Hothouse climate and they have very warm oceans.
I am unaware of any Hothouse climate which didn’t involve a very unusually large amount of volcanic activity- and would think the type of volcanic activity which is forming and has form the Hawaii island chain as not a very large amount of volcanic activity.
But would count the island formation of Iceland as involving unusually large amount of volcanic activity:
“The pocket of magma that sits beneath Iceland is thought to be what created the island, as hot lava rose to the surface of the ocean, where it cooled and gradually accumulated into an island beginning about 70 million years ago, according to San Francisco’s Exploratorium museum.”
And Hothouse climates, geologically speaking are short periods of time.
There were ice ages during the Late Ordovician-Early Silurian when CO2 was ca.7,000ppm
A very long ice age during the Late Carboniferous – Early Permian and a small one in the Late Jurassic when CO2 was 1,500 ppm
The coldest temperature ever recorded was −89.2 °C at the Soviet Vostok Station in Antarctica on 21 July 1983 by ground measurements.
How cold do imagine temperatures have ever got to on land on Earth.
I would guess it occurred during our last glaciation period when polar sea ice was at it’s greatest extent at southern polar region.
Since the Pleistocene Ice Ages started ~ 2.5 million years ago and for all of the preceding ~65 million year long Tertiary period Earth’s climate did not experience any Ice Ages, it seems as if something happened at the end of the Tertiary period that changed 1 or all 3 of the variables of Earth’s orbit Obliquity Eccentricity and Precession. Any thoughts on this?
Isthmus of Panama. As for the premature death of Milankovic, a glance at the ice core data vindicates him beyond reasonable doubt:
–AGF
“Changes in GHGs always precede variations in global temperatures and are therefore a clear driving force of climate change, not a response to it.”
Uh…No.
Clearly GHGs are responding to changes in ocean temperature – just look at the lags. If you can cool off even ocean water that it can absorb a substantial amount of CO2, then you have the correlation observed in the record. Reduced CO2 *might* be a feedback to further cooling, but there is no mechanism where it can start the entire chain off. Think about it, where does the atmospheric CO2 disappear to if it isn’t into a cooler ocean? And if you can’t have cooling with lot’s of atmospheric CO2, how the heck does the entire process start? So, first there is cooling and then CO2 is absorbed – very straightforward.
Changes in GHGs always precede variations in global temperatures and are therefore a clear driving force of climate change, not a response to it.
A shameful lie.
Javier, who has commented at WWUT in the past, wrote an extensive series of posts at Judith Curry’s site under the title “Nature Unbound”. The first of the series which has at least 10 parts begins as follows:
Nature Unbound I: The Glacial Cycle
Posted on October 24, 2016 by curryja | 269 Comments
https://judithcurry.com/2016/10/24/nature-unbound-i-the-glacial-cycle/
by Javier
Insights into the debate on whether the Holocene will be long or short.
Summary: Milankovitch Theory on the effects of Earth’s orbital variations on insolation remains the most popular explanation for the glacial cycle since the early 1970’s. According to its defenders, the main determinant of a glacial period termination is high 65° N summer insolation, and a 100 kyr cycle in eccentricity induces a non-linear response that determines the pacing of interglacials. Based on this theory some authors propose that the current interglacial is going to be a very long one due to a favorable evolution of 65° N summer insolation. Available evidence, however, supports that the pacing of interglacials is determined by obliquity, that the 100 kyr spacing of interglacials is not real, and that the orbital configuration and thermal evolution of the Holocene does not significantly depart from the average interglacial of the past 800,000 years, so there is no orbital support for a long Holocene.
If any of you are in communication with Javier, please ask him to read the post above and comment on it.
It has always seemed to me that astronomical theories of the cycles of stadials and interstadials have been a problem for GHG driven Climate Change theorists.
The occurrence of a stadial would be a far bigger problem for humanity and the biosphere than any plausible warming scenario.
I could easily adjust to the world being 2 or 3 °C warmer than it is now. I would just have to invest in a backyard swimming pool.
But having my house covered by a mile thick sheet of ice, as it was during the last stadial. That would be a problem.
If humanity by burning fossil fuels is preventing the return of the next stadial, we are doing a very good thing. And, we ought to pick up the pace.
Changes in GHGs always precede variations in global temperatures and are therefore a clear driving force of climate change, not a response to it.
Maslin backs up this statement with a single reference only, by Jeremy Shakun of Oregon State University:
https://www.nature.com/articles/nature10915
Shakun’s notorious “study” is an egregious hatchet job on palaeo climate data. This study looked at only the single most recent deglaciation – the Holocene inception. Two elements combine to allow Shakun et al to flourish like a rabbit out of a hat, the bizarre conclusion that CO2 increase has preceded glacial termination.
1. Shakun trawled through no fewer than 80 climate “proxies” of the last deglaciatuon to the Holocene. These included a host of highly dubious biological proxies – pollen, insects, plant based etc. The timelines of these proxies in the full version of Shakun’s research show that many of them are so weak that they scarcely resolve a difference between the last glacial maximum and the Holocene optimum. Critically, some biological proxies show delayed change after the glacial melt and rising sea level. Many are very imprecise. The effect of including so many such proxies is to delay the apparent beginning of the Holocene to allow the conclusion that CO2 increased before temperature increased.
2. The second part of Shakun’s trick was to take advantage of the unusual nature of the last deglaciation. Namely, the Younger Dryas (YD). There was an interrupted start to the Holocene (casting doubt on when it actually began). Northern Hemisphere and Greenland warming began sharply about 14,500 years ago with the episode called the Bolling-Allerod. After this there was a sharp reversal to cooling – the YD – a northern hemisphere only phenomenon in which the North Atlantic cooled back to near glacial temperatures for 1000 years or so, due to a cut-off of the AMOC. This eventually came to an end about 12,000 years ago when the Holocene finally started in earnest in the NH.
However, hidden behind the magician’s cloak is the face that the real initiation and causative driver of the Holocene glacial termination was not in the northern hemisphere at all, but was Antarctica. All major climate shifts start in the ocean and this was no exception. Southern ocean warming around Antarctica had begun as early as 20,000 years ago – while the northern hemisphere was still in the grip of the last glacial maximum. This is shown by this paper by Weaver et al:
http://home.sandiego.edu/~sgray/MARS350/deglaciation.pdf
This is the real start of the Holocene. The blogger Javier who has posted extensively on this subject previously here and on climate Etc., showed convincingly that interglacial initiation is always preceded about 6000 years earlier by a peak in obliquity (accompanied by an eccentricity maximum, following the MPT). This gradually starts deep ocean warming. The world’s largest ocean is the southern Ocean (there is less land in the SH). A full 35% of the ocean’s volume is the Antarctic bottom water (AABW).
Eventually slow Antarctic Ocean warming caused the Bolling Allerod (BA) warming in the NH. But after 1000 years this reversed to YD cooling. The important thing is that the YD cooling was in the NH only – the Antarctic continued to warm during the YD. The cause of the YD reversal is disputed – there is an impact hypothesis, although it is more likely that it was an oceanographic process linked to a catastrophic Antarctic ice sheet collapse, which caused the brief “Antarctic cold reversal” about the time of the BA and perturbed deep Atlantic circulation.
An important feature of ocean driven climate especially at around Holocene inception, is the “bipolar seesaw” the phenomenon of alternating warming and cooling in inverse phase between the SH and NH. This is caused partly by the delay in starting of slow ocean processes, and also a big difference between the NH and SH in their stability – the NH is more unstable due to the AMOC (Atlantic meridional overturning circulation) which has the habit of unexpectedly turning off and on again. This instability is due to the salinity-Arctic downwelling feedback. By contrast, ocean changes are more smooth and slow in the SH.
Putting all this together, you have a scenario where Antarctica warmed steadily from about 20,000 years ago. Albeit with a brief interruption at the ice sheet collapse and the cold reversal. The warming of the huge volume of the southern ocean released CO2 into the atmosphere. This is why CO2 continued to rise throughout the YD, while NH glacial termination was experiencing a “false start”. (The YD could be regarded as simply the last of about 20 Dansgaard-Oeschger DA warming excursions that happened throughout the last 80,000 year glacial interval – all arising from AMOC instability.)
So combine this ongoing Antarctic ocean warming during the anomalous YD, together with a spurious delay to the northern hemisphere Holocene inception by mixing in so many dubious biological proxies, and Shakun and colleague are able to get to the “Prestige” moment of their conjuring trick: the false conclusion that CO2 preceded Holocene inception. This was achieved by deliberately blurring over the complexity and SH-NH interplay of the Holocene inception.
The result is this fraudulent marker in the literature – Shakun et al 2012, allowing Maslin and others to make the one reference only claim that CO2 precedes warming, created by smoke and mirrors at the last deglaciation and then generalised to all deglaciations.
Aside from that flawed study, the consensus of geology including ice cores at both poles from all deglaciations is that warming happens first, then CO2 increase from the outgassing oceans.
Another reference to early Antarctic Holocene inception:
https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97GL02658
A review of the Maslin paper
https://tambonthongchai.com/2020/05/03/the-co2-theory-of-everything/
I have a hunch that Milankoviç cycles need to be overlaid with sun spot activity, in order to improve the fit where it seems to go slightly astray.
Unfortunately I do not have the wit to go about this.
It would be nice to blow this GHG nonsense out in one fell swoop. I’m getting very bored of it now – it is highly damaging as much as tiresome.
Almost anything out of most universities these days are lies, but at least the mechanics of Maslin’s study seem pretty good — obliquity does indeed rule. But this has already been well-analyzed on Climate Etc.
https://science.sciencemag.org/content/299/5613/1728
CLICK ABOVE – NOT QUITE CORRECT – WAY OFF ACTUALLY – a much longer way off – temperatures very likely increase 1st – then CO2 gas concentrations increase in the ice cores – yes back to the soft drink bottle – in and out of the fridge – basic chemistry of gases in cold oceans as opposed to warming oceans
The sequence of events during Termination III suggests that the CO2 increase lagged Antarctic deglacial warming by 800 ± 200 years and preceded the Northern Hemisphere deglaciation.
https://science.sciencemag.org/content/299/5613/1728
Global warming preceded by increasing
carbon dioxide concentrations during the
last deglaciation – from Harvard via Nature – https://projects.iq.harvard.edu/files/climate/files/shakunetal2012.pdf
note these are old references – the very information no one broadcasts any more – or if they do – they risk losing their jobs or not getting any more funding – which can equate to the same thing
WHOOPS yes – they still do – 2016 – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4822573/