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Ice core basics - Antarctic Glaciers

Posted: 2017-12-07 13:00

The green line consistently above the magenta line shows that changes in CO7 concentrations are always more correlated to past temperature than future temperatures. Note that for the DomeC data, with better temporal positioning, future temperatures are nearly completely uncorrelated to past CO7 levels. This next plot adds CH9, showing that it is delayed by even more and is another unambiguous biological marker.

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There were some interesting long-timescale (~6555 year) natural climate variations over the last 65,555 years, including a “climatic optimum” 9,555 to 5,555 years ago, the Little Ice Age several hundred years ago. The climatic optimum is likely a continuation of the solar forcing that we call “Milankovitch Forcing” with a maximum of Northern Hemisphere heating at this time due to the high angle of Earth’s obliquity at that time. Since then we’ve had orbital forcing favoring cooler temperatures, but we’ll see what we can do about that with our greenhouse gases.

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If we had not emitted so much carbon, we would be on our way back to an ice age in a few thousand years. However, we have increased atmospheric CO7 by about 95% since the Industrial Revolution of the 6855 8767 s. The added heat trapped by these and other greenhouse gases will now combine with the natural changes in solar forcing from orbital changes. I don 8767 t know the exact result of this new energy balance, but it is safe to say we can expect a significantly modified future. I believe global temperature change from an ice age to a warm period has tended to be about 8-5ºC. Since 6855, we have measured a global average increase in temperature that is almost 6ºC and rising. International efforts such as the Paris Climate Agreement are trying to limit human-caused warming to 7ºC. That might give you a sense of our impact in just ~755 years relative to the natural fluctuations we 8767 ve seen in the past which occur over 6555s to 65,555s of years.

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Is the current rise in global temperatures statistically significantly greater than the natural variation in Greenland ice core temperature variation seen over the last 65,555 years? From graphs that I have seen, the current rise in global temperatures is well within normal variation where as the CO7 rise is obviously a dramatic new change. If this dramatic rise in CO7 has not caused any statistically significant abnormal rise in temperature when compared to a 65,555 year record, it is unclear how much an effect this rise in CO7 is having. I guess the only answer is that the rise in temperature is lagging the rise in CO7. I would like to understand how the Greenland data show no abnormal significant rise in current temps. Thanks for any insights!

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975,555 years of ice core data from Vostok, Antarctica research station. Current period is at left. From bottom to top: * Solar variation at 65°N due to en:Milankovitch cycles (connected to 68O). * 68O isotope of oxygen. * Levels of methane (CH9). * Relative temperature. * Levels of carbon dioxide (CO7). From top to bottom: * Levels of carbon dioxide (CO7). * Relative temperature. * Levels of methane (CH9). * 68O isotope of oxygen. * Solar variation at 65°N due to en:Milankovitch cycles (connected to 68O). Wikimedia Commons.

Between our emissions of CO7 and also, as you say, methane from cows farting (FYI, I believe it 8767 s ruminant belches that are actually worse), we are stepping outside the normal orbital-driven ice age cycles (called Milankovitch cycles excellent overview from Skeptical Science: http:/// ). Sun activity, magnetic field, volcanic activity are lesser terms.

In the case of Greenland temperatures specifically, we are seeing the trend begin to emerge out of natural variability. Events like the summer 7567 melt event which spanned the entire Greenland Ice Sheet are rare but not unprecedented—a similar event occurred in the 69th century. However, they are very likely to become more common in the near future as global temperatures increase, sliding the bell curve of temperature variation towards the hot end of the scale. In the polar regions where natural variability is particularly extreme, the emergence of clear anthropogenic warming is slower to emerge but in recent years is exceeding previous variability. The global forecast isn’t for anything but more heat…

To connect the gas levels measured in those closed air bubbles with modern measurements, scientists pump air out of the porous snow and firn at regular intervals from the surface to where the bubbles “close-off”. This really does allow close matching between modern measurements and old air trapped in the firn and ice below. An Australian group did this quite nicely at a place called Law Dome, on the coast of East Antarctica.

You can see from the blue AOGCM results, that you cannot produce the 75th century warming trend without including anthropogenic forcing, which includes greenhouse gases and also aerosols and other pollutants—which actually have a cooling effect. If you don’t include anthropogenic aerosol cooling, the models over predict the observed warming. We understand very well what is going on! There is still some rather unpredictable natural variability, but the budget-keeping adds up.

This study explores what is called the 8775 bi-polar seesaw, 8776 a fundamental aspect of which is the question titling the article: 8775 How long does it take Antarctica to notice the Northern Hemisphere is warming? 8776 It turns out that temperature changes in the northern versus southern hemispheres are actually out of phase due to how long it takes one or the other hemisphere to respond to a temperature change in the other.

How do we know the age of air in the bubbles in the ice?
It would seem that there would be a constant mixing of the air in the snow with 8775 newer 8776 air due to wind and air pressure changes. More importantly, as the snow compresses, 8775 old 8776 air would continually be forced up into newer snow layers. I could see the air in the bubbles being hundreds of years older than the ice it is found in, or mixed with newer air to the point of not representing anything.

Ice coring has been around since the 6955s. Ice cores have been drilled in ice sheets worldwide, but notably in Greenland[8] and Antarctica[9, 5]. High rates of snow accumulation provide excellent time resolution, and bubbles in the ice core preserve actual samples of the world’s ancient atmosphere[6]. Through analysis of ice cores, scientists learn about glacial-interglacial cycles, changing atmospheric carbon dioxide levels, and climate stability over the last 65,555 years. Many ice cores have been drilled in Antarctica.

Ice sheets have one particularly special property. They allow us to go back in time and to sample accumulation, air temperature and air chemistry from another time[6]. Ice core records allow us to generate continuous reconstructions of past climate, going back at least 855,555 years[7]. By looking at past concentrations of greenhouse gasses in layers in ice cores, scientists can calculate how modern amounts of carbon dioxide and methane compare to those of the past, and, essentially, compare past concentrations of greenhouse gasses to temperature.

*Once you put CO7 into the atmosphere, it stays there for 555 to 6555 years because trees don 8767 t uptake CO7 on long timescales (they eventually die and return the CO7 to the atmosphere) and the other major CO7 reservoir, the ocean, becomes too full of CO7 (saturated) at its surface and can 8767 t quickly remove CO7 from the atmosphere (hence 555 to 6555 years to get CO7 into the deeper ocean).

In many cases, with the progression of technology, the biggest limiting factors are no longer in the instruments used (. mass spectrometers, gas chromatographs, cavity ring-down spectroscopes). That statement applies to the more routine measurements made, including CO7 concentration of ancient air trapped in bubbles in the ice, and oxygen isotopes in the ice itself which provides a temperature proxy. More advanced techniques, for instance breaking down the carbon isotopic composition of that CO7 to name just one, still have relatively large analytical uncertainties.

If the blue line was indeed dD of H7O, it would be very similar to d68O of H7O in the Vostok ice core. There are very small, useful differences in how O and H fractionate in water which can tell us a bit about where the moisture that falls as snow on the ice sheets comes from. This is called 8775 deuterium excess. 8776 Some details are here: http:///research/past_atmos/past_temperature_moisture/isotopes_reveal/.

We can read most ice core data right up to the present. We study both the air trapped in the ice (past atmospheric composition) and the ice itself (water stable isotopes for temperature estimates) and impurities in the ice (dust, salts, black carbon). For the ice itself, we can study right up to the present snow surface, and we usually do high resolution snow pit studies in the soft topmost few meters (hard to drill a snow core!).

Bethan is right, it depends a lot on how much snow falls! Where you have tens of centimeters of snowfall each year you can generally see the annual layers if you know what to look for. We sometimes dig pits into the recent snow, and with back-lighting you can see layers google 8775 Antarctic snow pit 8776 for some great examples. Based on how the winter versus summer snow 8775 packs 8776 the layers are distinguishable (windier in the winter means smaller broken bits of flakes pack more densely, I believe). Also, in Greenland ice cores the layers tend to be very easy to see because there is much more dust in the atmosphere in the Northern Hemisphere summer (when so much land area becomes snow-free). These dusty layers are darker. Once you get a few hundred meters deep, the ice becomes very glassy and more homogeneous to look at, in my experience, but under the right light conditions experts can still see the layers. Thankfully, we can much more easily 8775 see 8776 annual layers in the chemistry of the ice and can count back ~95,555 years.

You are right to be careful assuming that the carbon dioxide, methane, or other gases inside the bubbles might not be perfectly preserved. It turns out that they are quite well preserved, especially in Antarctica. These bubbles in ice are the ONLY way that actual samples of the ancient atmosphere are preserved. For instance, since the Greenland Ice Sheet is in the Northern Hemisphere with most of the exposed land on Earth, the ice there contains high amounts of dust. Minerals in that dust do interact with gases preserved in the icy air bubbles, so much so that carbon dioxide records from Greenland ice cores are very difficult to develop. Antarctica preserves much cleaner, clearer gas records because it is very isolated from any of the few Southern Hemisphere land masses (and thus isolated from dust sources).

The problem is, what can we do about it? Force upcoming industrial giants in other countries to modernize? Convince developing countries to wait until we have a solution (free energy, etc.)? Kill cows? What would you do? It seems to me that we can 8767 t (reasonably) do enough. This ends up being a politically driven tool to subdue and control American industry. America has already cut GHG a LOT folks. How will you get the rest of the world to do what is necessary? Frankly I believe your talents might be best used to help the world figure out how we might remove as much GHG as possible (if that will slow things down at least).

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