Climate Change — A Glimpse Into the Science of Cataclysm (Part II)
An Explanation of What the Scientists Told Us Decades Ago but Which We Chose To Ignore Anyway (Yeah, Very Bad Idea)
This series on climate change has FIVE parts.
In Part I of this series we were introduced to the essence of the unfolding climate change crisis and saw how its greenhouse gas scientific underpinnings could be traced back more than 150 years.
In Part II (this page) we look for clues as to what we might expect to happen in the coming decades based on what has happened in times past when the concentration of greenhouse gases in the atmosphere was similar to what we see today.
PLANETARY WARMING — WHAT DOES THE HISTORICAL RECORD TELL US?
If we accept the planet really is warming up and we have a reasonable handle on the science behind it, is this something we should truly be concerned about? After all, we know the planet has warmed and cooled repeatedly in the past. Isn’t this basically a natural phenomenon?
Skeptics of climate change take great comfort from being able to point out that the climate has changed in the past, with temperatures sometimes rising significantly. This is true, but never in the presence of humankind.
It is not uncommon for a skeptic to put up a temperature plot for the last few thousand years and claim that the temperature has “swung wildly” in that time frame. But how does this claim hold up when we look more closely at it?
Below is a plot of two independent reconstructions of the global temperature record for the last two thousand years (up to 2013). We are going to start with this (geologically) short time window, then expand it backwards.
The green dots, from the Past Global Changes program (PAGES), represent continental (land) temperatures estimated from examining “511 climate archives from around the world, from sediments, ice cores, tree rings, corals, stalagmites, pollen or historical documents and measurements” [Ref. PAGES].
The blue line, the Marcott data, represents estimates of temperatures derived from deep sea sediments.
To within the estimated accuracy of the latter measurements (light blue shaded band) the two sets of data are in close agreement. They show, excluding the rise in the last century, that average global temperature has fluctuated about 0.2 degrees Celsius.
If we extend this time frame back another 10,000 years (see following graph) the temperature variation is greater still [Ref. MARCOTT2]. It appears to have swung as much as 0.7 degrees Celsius. Surely then, the rise of about 1.2 degrees Celsius since pre-industrial times is within the realm of possibility due to nothing more than a statistical fluctuation. One that might well correct itself given a sufficiently long time frame. So contends the climate change skeptic.
There are at least two very good reasons for thinking this is NOT the case.
The first has to do with the rate and projected magnitude of the swing we are now witnessing.
For the past 10,000 or so years the temperature has remained remarkably stable, never leaving that narrow range of about 0.7 degrees Celsius.
That period allowed for the development of human agriculture, metal-working, and then the rise of the first large-scale civilizations. We got the Sumerians and the first written language, the Egyptians and their pyramids, the Chinese dynasties, the Minoans, dozens of cultures that began exploding across the world and significant enough in size and duration to leave their mark. All because the temperature remained in a relatively stable range conducive to consistent annual crop yields while the sea level neither rose nor fell, allowing for coastal cities to take root and flourish (more on the dependence of this fact on average global temperature later).
But according to climate scientists we may see a temperature rise in the next few decades of as much as 3 to 6 degrees Celsius. We will be going where neither agriculture nor human beings have ever gone before. And compared to the 5,000 year interval between the stabilization of the temperature and the appearance of the first agriculturally-driven mass societies, we will be doing it in the relative blink of an eye. We have no record of the climate changing this rapidly for at least the last 10,000 years, and possibly the last million or so years (the duration over which ice ages have come and gone).
The second reason for thinking the recent temperature changes represent something far from a natural anomaly comes from considering the reconstructed temperature record out to 20,000 years, back to the end of the last ice age.
Now it becomes apparent that the temperature fluctuations of the last 10,000 years are minor in comparison to the millenia scale changes. Also that the “fluctuations” of the last 2,000 years are less fluctuations than bendings to a trend. The shape of the (20,000 year duration) temperature curve is characteristic of the final stage of all ice ages, each of which spans a period of roughly 100,000 years. The final millenia of ice ages all show a general rise in temperature of about 4 or 5 degrees Celsius, followed by a slow cooling phase that lasts until the temperature has dropped back down by the same amount, initiating the next cycle of rapid thaw and slow refreeze.
It becomes apparent on re-examining these curves that currently we should be on the slope of a gently cooling climate, slowly headed towards the next ice age in the space of the next few tens of thousands of years. Instead, we have completely erased the natural cycle of climate variability (due to variations in the orbit of the Earth around the Sun and the tilt of its axis of rotation) and now we are driving the temperature of the planet in the opposite direction at great speed.
There is nothing natural about the sharp upturn seen in the temperature graph shown above.
If we continue to follow this trend of zooming out to greater time scales we can use ice core data to look back as far as 800,000 years to see the saw-tooth pattern of rising and plunging global temperatures which characterize each of the ice age periods of roughly 100,000 years:
It is evident from the graph that temperature and carbon dioxide levels have closely tracked each other for at least the last eight ice ages. This record, taken from ice cores drilled into the Antarctic ice sheet leave no doubt of some kind of definite causal relationship. When C02 levels go up, it appears temperature goes up too…
SKEPTIC: Uh-huh! Now I’ve got you. This graph absolutely proves that carbon dioxide levels simply cannot be the cause of global warming. Because if you look close enough you actually find that the rise in temperature at the end of each ice age slightly precedes the rise in CO2 levels. The warming is driving the increase in greenhouse gas concentrations. Not the other way around!SCIENTIST: Well, you have it half right. The temperature in the Antarctic region DID go up ahead of the increase in carbon dioxide levels. As you rightly suspect, it was the CAUSE of those emissions. But this observation does not prove that globally (the world over) the temperature drove the rise in C02 concentration. In fact, it was the other way around. Ice ages end when rising CO2 levels cause the planet to warm and melt ice across the world (see below for details).
Our doubting friend is correct about the finer details of the correlation between temperature and C02 levels.
When scientists looked closely at these ice core data records it did indeed appear that each rise in temperature in the Antarctic was FOLLOWED very shortly by a rise in carbon dioxide around the world.
Did that mean the theories of Arrhenius and Plass were incorrect? Does carbon dioxide not actually drive the average global temperature?
A team of climate science modelers suspected the established science would turn out to be correct when all the evidence was collected. So they decided to run an exceedingly sophisticated computer model of the period covering the end of the last ice age [Refs. SIM1,SIM2]. They included the interactions of all the physical entities across the globe which drive climate — the atmosphere, the oceans, the land masses and the ice sheets — and used as input the initial conditions determined by looking at the reconstructed temperature and C02 records determined from ice cores and oceans and lake sediments representative of the period.
They ran their simulation, modeling 15,000 years of ice age evolution, for a full 3 years on one of the most powerful supercomputers available in 2009. And after 14 million processor hours they had their answer.
In graphical form it looks something like the following (the interpretation of which I will explain in a moment). What it tells us is that the century-old theory of a greenhouse gas-driven climate, backed by more than six decades of computer model simulations [Ref. MODELLING], is still on rock solid footing.
As expected, the model showed it was carbon dioxide that causes global warming. The intuition and mathematical prowess of Arrhenius, Plass and other climate scientists over the years has been borne out.
The graphic is a little difficult to interpret, but it is well worth making the effort to understand it — especially if you have ever doubted that climate scientists really have any handle on what is happening with climate.
First, let us just admit that modeling something as complex as the energy flow of the entire planet is not the least bit trivial. We should not expect painfully obvious smoking gun signatures to appear as the answers to questions posed about such an intricate system. And yet, the curves on this graphic are as close to “smoking gun” as you are ever likely to see. So let me describe what we are looking at.
The green curve is the average global temperature record reconstructed from paleoclimate data. It corresponds to the same temperature data (also in green) appearing in the 20,000 year record two graphs back. The goal of the modelers was to try to reproduce this temperature record using nothing more than reconstructed carbon dioxide levels (from paleoclimate data) shown as the blue dots.
Also present on the graph, for reference, is the reconstructed temperature record (from paleoclimate data) for Antarctica (in red).
Finally there is the dashed line, the global temperature as determined by the computer modeling. This is the “result” and I will explain the significance of it in a moment.
Skeptics have looked at the red curve (temperature) and the blue dots (CO2) and declared that it shows the temperature rising before the corresponding rise in CO2. They are correct. At key points in the history of the last ice age you can see the Antarctic temperature clearly increasing before C02 — most evidently at about 17,500 years ago, and 15,000, and 12,000. This is because temperature IS driving the release of CO2.
At least, it is doing so around Antarctica.
The modelers point out that at the very end of an ice age the summers in the Northern Hemisphere become unusually hot. This is due to cyclical changes in the Earth’s orbit around the Sun lasting about 100,000 years. The glaciers in the north begin melting, sending cold fresh water into the Atlantic ocean which inhibits the northward flow of warm water from the southern oceans. This backs up heat in the Antarctic region and causes the Antarctic oceans to become hotter and less able to contain dissolved carbon dioxide which bubbles to the surface of the water and enters the atmosphere.
The newly-released CO2 circles the globe and the entire Earth slowly begins to warm in response to the increased levels of greenhouse gas.
The surprise perhaps is that each time CO2 stabilized in concentration (around 14,000 and 11,000 years ago) it took about 1000 years for the global temperature to also stabilize at a new level. But it turns out there is a very good reason for that. The short answer is the oceans take a very long time to warm up in response to an applied source of heat (more details on this point shortly). The presence of all the ice at the end of an ice age provides yet another clue as to why the temperature rise might have lagged as long as it did.
So the reconstructed records tell us the average global temperature lagged the rise in C02 by centuries. This is very strong historical evidence that greenhouse gases are the CAUSE of global warming.
Moreover, if we look at the results of the computer simulation (the dashed black line) we see that the behavior and magnitude of the average global temperature changes predicted by the model match fairly well the historical record (the green curve).
Where the historical temperature record stabilizes, so too does the simulation. Where it rises significantly, so too does the simulation, at about the same time. By no means is it a perfect match. But considering the enormous complexity of the model the agreement between the prediction and the historical record is very compelling.
Carbon dioxide acts as a greenhouse gas and drives the average global temperature.
The next time you come across a climate skeptic who tells you climate scientists have little understanding of the physics of climate change, keep in mind this 14 million hour computer simulation and the result which came out of it. Besides all the blood, sweat and tears university researchers put into developing a robust and comprehensive model which could withstand international scrutiny, the simulation also cost a pretty penny to run. It never would have received funding if the National Science Foundation (NSF), the Department of Energy (DOE), and the National Aeronautics and Space Administration (NASA) did not believe their money was being well spent [Ref. SIM1].
In short, the majority of climate scientists are not bumpkins. The work they perform is very precise, is carried out with the utmost care and regard for the seriousness of the governmental policy decisions that may one day be based upon its predictions, and they have absolutely no desire to be associated with misleading claims about the results of their research. When potentially their lives, and even more so the lives of their children depend on these pronunciations, they try damned hard to come up with the right answer.
DRIVERS OF PLANETARY TEMPERATURE — FAST AND SLOW TIME SCALES
We study the historical temperature records for clues as to how global warming may play out in the years ahead. I would be attempting to sweep under the rug the enormous complexity of climate change science if I said it was easy to infer future warming trends by looking at the past data.
On the other hand, if we are willing to accept that someone has already done the hard work of figuring out how to use the past data to predict roughly how the future will shape up there IS a relatively straight forward way to “see” the bigger picture. I will use this section to expand upon that idea.
In our simplified model of the world we can picture TWO main driving forces that control climate change.
One causes relatively RAPID responses in the environment and results in temperature changes which occur within a period of several years to a few decades. It is the spatial differential between temperatures of large bodies of air and water that drive all the extreme weather events that get our attention in the short term, but it is the overall change in temperature from decade to decade, century to century, that matters most to us. So we concentrate on temperature when talking about climate change, knowing that ultimately this is what drives the extreme weather events that could upend the world as we know it.
The other driving force is a SLOW response which can take several centuries, or even millenia, to push the temperature in one direction or another.
Greenhouse gases fall into the category of RAPID response drivers. Others include aerosols due to volcanic eruptions, water vapor, clouds and sea ice (which waxes and wanes seasonally).
As an example of a rapid climate response scenario you can imagine a sudden massive volcanic eruption which spews millions of tons of sulphur dioxide (SO2) into the atmosphere. SO2 combines with water and air to form sulphuric acid which reflects a portion of sunlight back into space before it can interact with the surface of the Earth and warm it. This causes a global cooling effect as the SO2 spreads widely in the atmosphere — a cooling that can last for several years.
Oceans, and ice sheets which sit on land masses (like the ice over Greenland and Antarctica), are the prototypic SLOW response drivers. Oceans can take centuries to mix heat into their depths. Ice sheets can take up to thousands of years to melt, and tens of thousands of years to form.
You can begin to appreciate the kind of time scales involved due to the thermal inertia of the oceans by looking at a graph showing the (simulated) average global temperature response due to an instantaneous doubling of the amount of greenhouse gases in the atmosphere (a rapid driver). In this case the climate model assumes that other (non-ocean) slow response environmental factors remain essentially unchanged over time (for example, the ice sheets do not melt).
The graph shows, compared to the eventual equilibrium temperature, the fractional amount of temperature change for the time that has elapsed since the C02 doubling event at the beginning of the simulation.
This exercise, of doubling the C02 concentration and seeing how much the temperature of the system (Earth) changes, is known as the climate sensitivity problem. The question being, how sensitive is climate to the presence of CO2 in the atmosphere?
The ultimate amount of warming depends on the dynamics of the model and the strength of the various parameters used to quantify them. In the graph above the ultimate temperature change is not shown because here only the nature of the response of the system over time is of interest — the time scales.
In this case, and it is typical of realistic climate models with a combination of RAPID and SLOW response drivers, you can see from the graph how it takes more than a thousand years for the equilibrium (stabilized) average global temperature to be achieved. At the other end of the scale, within just a few years, about 40 percent of the warming is achieved, about 50 percent within the first 40 years, and about 60 percent within 100 years.
It is the slow mixing of heat into the layers of the ocean on the time scale of a century or more — and in more realistic models involving millenia-scale melting of the ice sheets and other slow drivers — that allows one to think about the overall warming process as dividing into two parts: the RAPID response that gives half the warming within about 40 years, and the SLOW response that completes the process on a time scale of a millenia or more.
The distinction between RAPID and SLOW is not likely to seem all that important or consequential for a planet in energy equilibrium (as has been the case for most of Earth’s history). This is because, in a state maintained in near equilibrium, changes in energy flux take place at a rate slower than the system is able to adjust itself so as to restore the energy balance and remain in equilibrium.
It is only when a sudden and large change in energy flux takes place (for example, a drop in solar radiation due to a large sulphur dioxide plume from a volcano, or an increase in greenhouse gas warming caused by a massive release of methane from the bottom of the ocean) that RAPID and SLOW responses need to be distinguished to understand how the system will fully respond.
This last point is important because essentially what we have done in the last century and a half is introduce a HUGE influx into the atmosphere of a RAPID driver of global warming (carbon dioxide emissions). This has short-term AND long-term warming implications, both of which need to be addressed.
But before you can figure out what those implications might be you first need to calibrate in some sense the climate response of RAPID and SLOW drivers.
This is what Dr. James Hansen, Adjunct Professor at Columbia University’s Earth Institute, and his colleagues set out to do using paleoclimate records to determine the effects of fast and slow drivers of climate during the last 400 thousand years — a time period long enough for the Earth to have cycled through four ice ages.
The key to understanding the analysis is that throughout most of that period the Earth was never far from energy equilibrium. In this case the underlying theory describing a system in thermodynamic equilibrium tells us the change in the equilibrium temperature due to applied sources of heat is directly proportional to the sum of those heating effects (forcings).
In plain math:
ΔT = λ ΔF
The term ΔF, the applied heating effect, is known as a “radiative forcing” in climate science because it always derives from energy coming or going as electromagnetic radiation (incoming sunlight, or outgoing infrared).
ΔT is the change in the average global temperature caused by the radiative forcing after equilibrium has been restored, and λ is the proportionality constant measured in degrees Celsius per watt per square meter. Equilibrium is restored when the radiative forcing returns to zero.
Hansen’s team looked at the temperature record for the Antarctic going back 400 thousand years, and attempted to relate the temperature to the dominant radiative forcings throughout that period.
One of these was the RAPID forcing responsible for causing each of the ice ages to come to a relatively abrupt end. It has as its origin a periodicity in the Earth’s orbit around the Sun (and the tilt of Earth’s axis of rotation) which causes an increased amount of sunlight to warm the Northern Hemisphere during the spring for a period of several thousand years. This happens roughly once every 100,000 years, the last time occurring between about 18,000 and 11,000 years ago.
It was at these times, every spring, while the Earth was significantly out of energy equilibrium that the planet was repeatedly nudged to make a transition from deep freeze to thawed state. These short RAPID injections of heat into the top of the planet (which produced copious ice-sheet meltwater in the Northern Hemisphere) eventually caused the oceans in the Southern Hemisphere to warm and release CO2. This secondary much SLOWER process, occurring on a time scale of thousands of years, kicked the warming of the atmosphere into high gear.
In turn, the most vulnerable ice sheets (the Laurentide over North America and the Eurasian covering Scandinavia and the British Isles) melted and the planet finally reached a new equilibrium state with the average global temperature stabilized about 5 degrees Celsius warmer than the coldest days of the ice age.
All throughout this period the processes responsible for SLOW climate change left traces of their influence in the paleoclimate record, as did the RAPID influence of spring heating which kick started the process. But it was the SLOW causes of temperature change that, by far, drove the change of climate.
It was the slow release from the ocean of the greenhouse gases carbon dioxide and methane that raised the temperature enough to cause the ice sheets to begin to melt.
By examining concentrations of these trace greenhouse gases in bubbles collected from ice cores drilled from the Antarctic one can get a record of atmospheric carbon dioxide and methane concentrations going back at least twice the period of time studied by Hansen’s team.
The other radiative forcing (or the absence of it) which caused the temperature to rise was due to the reflectivity of all the sea ice and land surface ice sheets present throughout these periods. More ice means more reflected sunlight, and less radiative forcing. By looking at sea level records determined by sediment cores taken from the Red Sea it was possible to infer how much ice existed based on how far the surface of the oceans had dropped over the same period (evaporated ocean water turns into snow over the land, reducing the amount of water remaining in the oceans).
Because large amounts of ice (like ice sheets over the Canadian and North American land masses) take a very long time to form (and melt), ice sheets can be thought of as SLOW forcings of the climate.
Both RAPID and SLOW radiative forcings are measured in watts per square meter. They express how much energy per unit area over the surface of the Earth is going into heating the planet.
Depending on where you set your reference value, these radiative forcings can be positive (causes heating) or negative (causes cooling). Compared to today (our reference point) radiative forcings throughout an ice age are negative (less greenhouse gases to warm the planet, less sunlight getting past all the reflective ice).
By calculating the sum of the RAPID (seasonal/orbital) and SLOW (gradually changing greenhouse gas concentration and reflective ice) radiative forcings Hansen knew that it should be possible to calculate the expected equilibrium temperature of the planet for the entire history of the ice ages and then compare that with the observed (indirectly measured) temperature to see if they agreed. If they did, it would be strong evidence that our understanding of climate dynamics is on solid footing.
The RAPID forcing turns out to be small in comparison to the SLOW forcings, so it is ignored in Hansen’s calculation of the equilibrium temperature.
This was arrived at by figuring out the sum of the radiative forcings for the greenhouse gases and the ice sheets, and then dividing by the proportionality constant λ.
You can see the result of Hansen’s calculations in the following graphs (explained on the other side):
How do we decipher what’s going on in these graphs?
It is perhaps simpler than it appears.
In the top graph we see concentrations of the greenhouse gases — carbon dioxide and methane (red and green curves). From these Hansen calculates the radiative forcing, or heating per unit area (green curve in second graph). It is negative for all periods deep into an ice age because those gases were present in lower concentrations than they are today. For example, CO2 concentration has been as low as 200 ppm during the coldest part of an ice age (today it is above 400 ppm).
We also see the reconstructed sea level in the top graph (blue curve). The lower the sea level, the more frozen water is bound up in the ice sheets covering the North American and European continents, and the larger is the amount of sunlight that never makes it to the surface of the Earth to warm it. This albedo-derived forcing (blue curve in second graph) is in almost one-to-one correspondence with the drop in sea level.
By summing the two radiative forcings in the second graph and asking what is the forcing-to-temperature conversion factor λ needed to best scale the calculated temperature (grey curve in lower graph) to the independently reconstructed Antarctica temperature record (red curve) Hansen comes up with the following value:
λ = 3/4 degrees Celsius x squared meter / Watt
This expression for λ says that for every extra one watt per square meter of radiative forcing applied to the planet the equilibrium average global temperature will rise by 3/4 of a degree Celsius. Or to say it another way, a radiative forcing of 4 watts per square meter will result in a temperature increase of 3 degrees Celsius.
You can see that the final calculated temperature in the third graph shown above is in very good agreement with the indirectly determined temperature.
This latter (reconstructed) temperature record is based on ice-core measurements of oxygen-18, an isotope of oxygen whose concentration in the water molecules of ice is highly correlated with average local temperature of the surface of the ocean (and thus the air above it). In this case local refers to the region of the Antarctic.
NOTE: Why is oxygen-18 abundance in preserved ice used to determine the local temperature record for periods of the long past? It is because to be captured as snow over land and then compressed into ice, oxygen-18 must first escape the surface of the ocean as a component of a water molecule. Because it is slightly heavier than oxygen-16, it needs more thermal energy to get aloft in the atmosphere, so it is less likely to be captured as a component of snow in colder weather. It is also more likely to be lost as precipitation in cold air before finding its way over land. So the oxygen-18 to oxygen-16 ratio found in ice cores can be used to determine the temperature in the region quite well [Ref. OXYGEN18].Should we expect the reconstructed (actual) temperature record and the calculated equilibrium temperature to agree as well as they do?
Only if the theory behind climate change, one based on the notion that greenhouse gases are a major driver of climate change, is correct.
Even then the close agreement, given the relatively few parameters that went into determining the temperature on the basis of concentrations of trace greenhouse gases and reflective ice sheets (sea level drop) is fairly remarkable.
Hansen has said of this agreement between theory and measurement:
The remarkable coincidence of calculated and observed temperatures cannot be accidental. The close agreement has dramatic implications for interpretation of past climate change and for expectation of future climate change due to human-made climate forcings.
[Ref. HANSEN2007]
What he means by this is the following.
You can take the derived value of λ and now ask what it implies for the climate sensitivity problem. If you double the concentration of C02 in the atmosphere, so as to trap more outgoing long wave radiation from the surface of the Earth, by how much does the temperature of the atmosphere rise?
Given our copious burning of fossil fuels since the dawn of the industrial age the answer to this question is going to reveal how long we have left before we can expect serious climatic consequences.
Someone has already calculated the radiative forcing for a doubling of CO2 in the atmosphere. The commonly accepted value is around 3.7 watts per square meter.
Multiplying this figure by Hansen’s nominal value of 3/4 degrees Celsius times one square meter per watt gives an average global temperature increase of around 2.8 degrees Celsius.
So if we were to double the pre-industrial concentration of CO2 from 280 ppm to 560 ppm we would tentatively expect the average global temperature to rise by 2.8 degrees Celsius. About half of this would happen in the first few decades and the rest would follow within a time span approaching 1000 years.
But we know from the Antarctica temperature records that in addition to greenhouse gas forcings ice sheet forcings also played a part in changes of the equilibrium temperature. And of a comparable size.
Because plenty of ice remains on the surfaces of Greenland and Antarctica, their continued melting and disintegration should lead to an additional 2.8 or so degrees of eventual warming, for a total of 5.6 degrees Celsius (about 4.5 degrees Celsius hotter than today).
Remember, this is assuming we simply double the concentration of C02 from pre-industrial and take it to 560 ppm (today in 2021 we are at around 420 ppm).
This expected change in average global temperature can probably be assumed to include the effect of the radiative forcing due to the melting of sea ice in the Arctic (see the next section) but not extra forcings due to the release of additional greenhouse gases from increased incidence of wildfires, thawing of permafrost in the far north, or methane releases in warming oceans.
Assuming we hold constant the current rate at which we are emitting carbon dioxide into the atmosphere we will be at 560 ppm by the year 2080.
Just to confirm this CO2 doubling implies a world eventually ice-free, with both the Greenland and Antarctic ice sheets fully melted, the concentration of CO2 in the atmosphere when the Antarctic ice sheet formed 14 million years ago was just 450 ppm.
We are currently on target to pass that atmospheric CO2 concentration and commit to an eventual fully ice-free world (thousands of years from now) by the year 2035.
Next Up, in Part III:
● LIFE ON AN ICE-FREE PLANET EARTHClick Here to Proceed to Part III
REFERENCES[HAWKINS] Warming stripes (by Ed Hawkins)[PAGES] Most Comprehensive Paleoclimate Reconstruction Confirms Hockey Stick[MARCOTT1] MARCOTT graphs[MARCOTT2] 20,000 Years of Global Temperatures: Asst. Prof Shaun Marcott (Youtube June 2015)[SIM1] simulated ice-age ending temperature rise[SIM2] Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation[MODELLING] Timeline: The history of climate modelling[HANSEN2008] Target Atmospheric CO2: Where Should Humanity Aim?[EURASIAN] The build-up, configuration, and dynamical sensitivity of the Eurasian ice-sheet complex to Late Weichselian climatic and oceanic forcing[OXYGEN18] How are past temperatures determined from an ice core?[HANSEN2007] Climate change and trace gases
