In some ways, however, this is exactly what we would expect; there were no humans burning fossil fuels during the end of the last ice age, so CO2 served more as a feedback to orbital changes rather than the climate forcing that it is today. So, CO2 must be a feedback. Because the amount of sunshine — and the amount of ice — have direct and immediate effects on temperature, there should be places on Earth in which any change in CO2 lags rather than leads the orbital cause and the temperature change.
This should not bother anyone. The analogy I sometimes use is that, if I overspend my credit card and go into debt, interest will kick in and make my debt bigger. The interest lags debt — first I go into debt, then I pay interest, then I go further into debt. Pretty much everyone understands that this is a sensible, if unpleasant situation. When the orbits affect ice and temperature, this changes other things that, in turn, affect CO2, which, in turn, affects temperature some more — similarly sensible if the full story is understood.
That said, understanding of the interrelationship between CO2 and temperature at the end of ice ages has advanced in recent years with better reconstructions of both past temperature and CO2 levels in ice cores from Antarctica. While scientists used to think that CO2 lagged temperatures by to 1, years during deglaciation, a number of recent studies have suggested that the lag is considerably smaller or even too small to detect.
It is challenging to precisely match up CO2 records and temperature records from ice cores as there is a delay between new snowfall on an ice sheet that traps the air bubbles and then that snow slowly compressing into ice.
The figure below shows Antarctic temperatures red line and CO2 from recent proxy reconstructions blue line during the end of the last ice age — from 23,BC to 8,BC. While some periods might experience lags of a few hundred years, the relationship appears much more tightly coupled than was suggested by earlier reconstructions with larger uncertainties. In addition, looking at temperature data from Antarctica alone also obscures a more nuanced global picture.
A paper by Dr Jeremy Shakun of Boston College and colleagues examined a network of 80 climate proxy records around the world during the end of the last ice age. They found that while CO2 generally lagged temperatures in the southern hemisphere — consistent with Antarctic reconstructions — the same was not true for the rest of the world. Both the northern hemisphere and overall global temperatures actually lagged CO2; in other words, for the world as a whole, warming happened after atmospheric CO2 concentrations increased.
The reasons for this are complex and are driven in part by changes in ocean currents as ice ages end. The figure below shows the results from the Shakun et al paper for different regions of the world, along with the uncertainties in their estimate across different climate proxy locations and time periods during the end of the last ice age.
The onset of an ice age is related to the Milankovitch cycles - where regular changes in the Earth's tilt and orbit combine to affect which areas on Earth get more or less solar radiation. When all these factors align so the northern hemisphere gets less solar radiation in summer, an ice age can be started.
Based on previous cycles the Earth is probably due to go into an ice age about now. In fact, conditions were starting to line up for a new ice age at least 6, years ago. They've been getting colder for at least the last 6, years, so we were definitely on that trend," Dr Phipps said.
But that trend has now been comprehensively reversed because of greenhouse gas emissions, according to Dr Phipps. We may have delayed the onset of the next ice age for now, but if another one came it would have pretty big consequences for human civilisation.
Besides the fact it would be an awful lot colder, huge regions where hundreds of millions of people live would become completely uninhabitable. They'd be covered in thick ice sheets and subject to an inhospitable climate. There would be a lot less agricultural land available, so it would be very difficult to support the human population, Dr Phipps warned. And the physical shape of the continents would look completely different across the whole planet.
A huge drop in sea level of up to metres would close down marine channels - the Mediterranean Sea, Torres Strait, Bass Strait and Bering Strait - and create new areas of land that could be used for habitation or agriculture. Ocean ports would no longer be on the ocean, and anyone wanting water views would need to relocate large distances. During the last ice age, which ran from about , years ago to 10, years ago, the lower sea levels allowed humans to move out across the entire world.
While there was still some water between Asia and Australia it took just a few short canoe trips to bring the first humans to Australasia. There was no Torres Strait so humans could have just walked from New Guinea to the Australian mainland. And there was no Bass Strait so humans could have walked from the Australian mainland over to Tasmania," he said.
In the past 2. But what causes ice sheets and glaciers to expand periodically? Ice ages are driven by a complex, interconnected set of factors, involving Earth's position in the solar system and more local influences, like carbon dioxide levels.
Scientists are still trying to understand how this system works, especially because human-caused climate change may have permanently broken the cycle. It wasn't until a few centuries ago that scientists started recognizing hints of past deep freezes. In the midth century, Swiss-American naturalist Louis Agassiz documented the marks that glaciers had left on the Earth, such as out-of-place rocks and giant piles of debris, known as moraines, that he suspected ancient glaciers had carried and pushed over long distances.
By the end of the 19th century, scientists had named four ice ages that occurred during the Pleistocene Epoch , which lasted from about 2. Since the astronomical theory relied upon an increase in the sunlight falling on one hemisphere along with a decrease on the other hemisphere, many experts considered the world-wide pattern of ice ages a devastating refutation.
The computed variations in the angle and intensity of incoming sunlight were only tiny changes, "insufficient to explain the periods of glaciation. That same year, a leading American planetary scientist wrote a European colleague to ask how the astronomical theory stood over there, remarking that "People I have consulted in this country They are of the opinion that the theory cannot account for past changes.
The effects are too small and the chronology of the occurrence of glaciation is so uncertain that any correspondence That was still anybody's guess. The tool that would unlock the secret was constructed in the s, although it took scientists a decade to make full use of it.
This tool was radiocarbon dating. It could tell with surprising precision the age of features like a glacial moraine. You only needed to dig out fragments of trees or other organic material that had been buried thousands of years ago, and measure the fraction of the radioactive isotope carbon in them. Of course researchers had to devise and test a number of laboratory techniques before they could get trustworthy results. Once that was done, they could assign a timescale to the climate fluctuations that had previously been sketched out by various traditional means.
The best of these means, in the s, was pollen science. The study of ancient climates had turned out to be invaluable for identifying strata as an aid to oil exploration, and that had paid for specialists who brought the technique to a high degree of refinement. But other carbon dates seemed altogether out of step with the Milankovitch timetable.
The swift postwar development of nuclear science meanwhile fostered another highly promising technique. In , the nuclear chemist Harold Urey discovered a way to measure ancient temperatures. The key was in the oxygen built into fossil sea shells. Emiliani measured the oxygen isotopes in the microscopic shells of foraminifera, a kind of ocean plankton.
Tracking the shells layer by layer in long cores of clay extracted from the seabed, he found a record of temperature variations. Emiliani's paper, a landmark of paleoclimatology, provided the world's first high-quality record of ice age temperatures.
Historians usually treat techniques as a stodgy foundation, unseen beneath the more exciting story of scientific ideas. Yet techniques are often crucial, and controversial. The stories of two especially important cases are explored in short essays on Uses of Radiocarbon Dating and Temperatures from Fossil Shells.
Emiliani tentatively identified the rises and dips of temperatures with the geologists' traditional chronology of the past three ice ages. His efforts were motivated largely by a desire to learn something about the evolution of the human race, which had surely been powerfully influenced by the climate shocks of the ice ages. But his results turned out to tell less about the causes of human evolution than about the causes of climate change.
To get a timescale connecting the temperature changes with depth down the core, he made carbon measurements covering the top few tens of thousands of years farther back there was too little of the isotope to measure. That gave him an estimate for how fast sediments accumulated on the seabed at that point. Emiliani now found a rough correlation with the varying amount of sunlight that, according to Milankovitch's astronomical calculations, struck high northern latitudes in summer.
To get the match he had to figure in a lag of about five thousand years. That seemed reasonable, considering how long it would take a mass of ice to react. The chemist Hans Suess, another graduate of Urey's lab, took the lead in improving the carbon chronology. He reported, among other things, that the last ice age had come to a surprisingly abrupt end, starting sometime around 15, years ago. Looking farther back, Suess found hints of a roughly 40,year cycle, which sounded like the 41,year cycle that Milankovitch had computed for slight variations in the inclination of the Earth's axis.
To resolve the issue, Emiliani began urging colleagues to launch a major program and pull up truly long cores, a hundred-meter record covering many hundreds of thousands of years. But for a long time the drillers' crude techniques were incapable of extracting long, undisturbed cores from the slimy ooze.
As one of them remarked ruefully, "one does not make wood carvings with a butcher's knife. In the early s suggestive new evidence was dug up literally by the geochemist Wallace Broecker and collaborators. Ancient coral reefs were perched at various elevations above the present sea level on islands that geological forces were gradually uplifting. The fossil reefs gave witness to how sea level had risen and fallen as ice sheets built up on the continents and melted away.
The coral could be dated by hacking out samples and measuring their uranium and other radioactive isotopes. These isotopes decayed over millennia on a timescale that had been accurately measured in nuclear laboratories. Unlike carbon, the decay was slow enough so there was still enough left to measure after hundreds of thousands of years.
As a check, the sea level changes could be set alongside the oxygen-isotope temperature changes measured in deep-sea cores. Again the orbital cycles emerged, plainer than ever. At a conference on climate change held in Boulder, Colorado in , Broecker announced that "The Milankovitch hypothesis can no longer be considered just an interesting curiosity.
That could happen if the climate system were so delicately balanced that a small push could prompt it to switch between different states. Meanwhile oceanographers managed to extract a fine set of cores that reached back more than , years.
Analyzing the cores, Emiliani announced he could not make the data fit the traditional ice ages timetable at all. He rejected the entire scheme, painstakingly worked out around the end of the 19th century in Europe and accepted by generations of geologists, of a Pleistocene epoch comprising four major glacial advances alternating with long and equable interglacial periods.
Emiliani said the interglacials had been briefer, and had been complicated by irregular rises and falls of temperature, making dozens of ice ages. Most significantly, he believed the sequence correlated rather well with the complex Milankovitch curve of summer sunlight at high northern latitude. Calculating how the cycle should continue in the future, in Emiliani predicted that "a new glaciation will begin within a few thousand years.
Seldom was such work straightforward. Geologists defended their traditional chronology passionately and skillfully. For a few years they held their ground, for it turned out that Emiliani's data on oxygen isotopes taken up in plankton shells did not directly measure ocean temperatures after all. Emiliani fiercely defended his position, but other workers in the late s convinced the scientific community that he was mistaken. When water was withdrawn from the oceans to form continental ice sheets, the heavier and lighter isotopes evaporated and fell as rain or snow in different proportions.
The way plankton absorbed oxygen at a given temperature mattered less than what proportion of each isotope was available in the seawater as ice sheets came and went. Yet in a deeper sense Emiliani was vindicated. Whatever the forces that changed the isotope ratio, its rise and fall did represent the coming and going of ice ages.
These changes did turn out to correlate with ocean surface temperatures. New evidence for that came from scientists who took a census of the particular species of foraminifera, recognizing that the assemblage of different species varied with the temperature of the water where the animals had lived.
The data confirmed that there had been dozens of major glaciations during the past couple of million years, not the four or so enshrined in textbooks. Corroborating evidence came from a wholly different type of record. In a brick-clay quarry in Czechoslovakia, George Kukla noticed how wind-blown dust had built up into deep layers of soil what geologists call "loess". Although Kukla could not get dates that matched Emiliani's, the multiple repetitions of advance and retreat of ice sheets were immediately visible in the colored bands of different types of loess.
It was one of the few cases in this story where traditional field geology, tramping around with your eyes open, paid a big dividend. In , still more complete and convincing evidence came from an expedition that Broecker and a few others took to Barbados. Terraces of ancient coral covered much of the island, rising to hundreds of meters above the present sea level. The dates for when the coral reefs had been living ,, ,, and 82, years ago closely matched dates from Milankovitch cycles for times when the ice sheets should have been melted and the seas at their highest ,, ,, and 82, years ago.
The dating matched, that is, so long as one looked for the times when the maximum amount of sunlight struck a particular band of mid-northern latitudes during the summer. Since the Milankovitch cycles could be computed directly from celestial mechanics, one could project them forward in time, as Emiliani had done in In , presenting more Caribbean cores, he again advised that "the present episode of amiable climate is coming to an end.
However, he added, greenhouse effect warming caused by human emissions might overwhelm the orbital shifts, so we might instead face "a runaway deglaciation. Some other scientists agreed that the current interglacial warm period had peaked 6, or so years ago, and should be approaching its natural end.
A prominent example was Kukla, continuing his study of loess layers in Czechoslovakia. He could now date the layers thanks to a new technique provided by other scientists. Geological and oceanographic studies had shown that over the course of millions of years, from time to time the Earth's entire magnetic field flipped: the North magnetic pole became the South magnetic pole and vice-versa.
These reverses were recorded where layers of sediment or volcanic lava had entombed the direction of the magnetic field at the time. Geologists had worked out a chronology in lava flows, dated by the faint radioactivity of an isotope of potassium that decayed very slowly. When the loess layers were dated in this fashion, Milankovitch cycles turned up.
Extrapolating the cycles into the future, Kukla thought the next shift to an ice age "is due very soon. If the climate experts of the time seem to have been a bit preoccupied with ice ages, that fitted their training and interests.
For a hundred years their field had concerned itself above all with the ice ages. Their techniques, from pollen studies to sea floor drilling, were devoted to measuring the swings between interglacial and glacial periods. Home at their desks, they occupied themselves with figuring how ice age climates had differed from the present, and attacking the grand challenge of explaining what might cause the swings. Now that they were beginning to turn their attention from the past to the future, the most natural meaning to attach to "climate change" was the next swing into cold.
In , a group of leading ice-age experts met at Brown University to discuss how and when the present warm interglacial period might end.
A large majority agreed that "the natural end of our warm epoch is undoubtedly near.
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