Icy time capsules

This is the 5th episode in a series recounting the history of measurements and data related to Global Climate Change. If you’re just joining, you can catch up on the previous episodes:

  • Episode 1: Beginnings (or two British scientists’ adventures with leaves and CO2 measurements)
  • Episode 2: First measurement of anthropogenic global warming
  • Episode 3: Our “large scale geophysical experiment” (1940-1960)
  • Episode 4: Dave Keeling persists in a great idea

Episode 5

In Episode 4, we saw Dave Keeling and coworkers discover the atmospheric CO2 concentration has been on a marked upward sweep, from about 290 ppm in 1900 to over 400 ppm now, and accelerating. Well, is that unusual? Is that a big swing? Or, does the CO2 concentration vary a lot due to natural causes?

Since Dave Keeling only began our continuous, high-accuracy CO2 measurements in 1958, it would seem we would need a time machine to figure that out. In some of the loneliest places on Earth, it turns out, nature has been quietly making time capsules for us.

In parts of Greenland and Antarctica, the snow never melts. In between the snowflakes, tiny volumes of air are trapped. As the years go by, each layer of snow is compacted under new layers. The snow is eventually compacted into ice, and the air is entrapped in minute, isolated bubbles. Geologists in heavy coats prospect for those historical bubbles, little bits of past atmospheres. Good spots to prospect are where it snows very often, such that the snow and ice are deep and the annual layers thick. One such place is Law Dome, Antarctica, a coastal location of Antarctica where the snowfall is as much as 225 lbs of snow per square foot per year.

(A) Field tents at Law Dome, Antarctica (Australian Antarctic Division). Ice core drilling was conducted in the tent in the foreground. (B) Slice from an ice core showing entrapped, ancient air bubbles (Norwegian Polar Institute). (C) Section of an ice core showing visible seasonal layers (Wikimedia Commons). (D) A researcher selects ice cores for greenhouse gas analysis at an Australian ice core storage facility (Australian Antarctic Division).

Ice cores are drilled out using cylindrical drills. Layers in the ice are dated, sometimes visually (see image C above), most times using more sophisticated methods. For example, a rare, heavy isotope of oxygen, O-18, is present in the frozen H2O of Antarctic precipitation at a higher concentration in summer than in winter. Thus, the years in an ice core can be counted as summer stripes and winter stripes, through isotopic analysis of the oxygen in ice layers using a mass spectrometer.

Scientists in the 1980’s expended considerable effort developing accurate methods of harvesting and measuring the composition of the old atmospheric air trapped in ice core bubbles. Since CO2 is water soluble, it’s important not to allow any of the ice to melt while you’re getting the air out. The figure below, from a 1988 paper, shows a schematic diagram of an apparatus used to measure the CO2 concentrations in gas samples retrieved from Law Dome ice cores. This has become known as the “cheese grater” technique, and is still used for CO2 analysis of ice cores.

Figure 1 of Etheridge, Pearman & de Silvia, 1988. Schematic diagram of “cheese grater” and associated gas condensing equipment for harvesting ice core air samples for analysis.

In a cold room (to prevent any melting), an ice core section is inserted in a cylinder with raised cutting blades on the inside, like an inside-out cheese grater. This is put inside a vacuum flask and shaken on a machine, crushing the ice inside. The released gases are sucked by a vacuum pump over, first, a water vapor trap, cooled to -100 degrees Celsius, to condense and remove water vapor. The dry sample is then made to flow over a “cold finger,” cooled by liquid helium to a frigid -269 degrees Celsius, cold enough to condense to liquid all the gases in the air sample. Once all the gas has been sucked out of the sample, the cold finger is isolated and warmed, and the accumulated gas sample is sucked into a gas chromatograph, a standard piece of analytical equipment for separating the gas constituents from each other and measuring their concentrations.

Between 1987 and 1993, Australian and French scientists working at Law Dome drilled 3 separate ice cores to depths of as much as three quarters of a mile. Samples of these ice cores have been analyzed by various groups. Below, in green, is a plot of data from a 2006 study of CO2 concentration from these ice cores going back over 2000 years.

Publicly available Scripps ice core-merged data, downloaded and plotted by me. Green: Ice core data from Law Dome, 0 C.E. to 1957 (see references here and here). Blue circles: Average yearly data from atmospheric sampling at Mauna Loa and South Pole, 1958-2016. Blue square: Mauna Loa measurement made on March 30, 2017. Human experience milestones added by me.

The data is publicly available; anyone can download it here. While this is a single data set, it is in agreement with data sets obtained from multiple ice cores, stored in multiple locations, by multiple scientific groups using a variety of methods (for a discussion of agreement between the various data sets, see here). The ice core data overlaps, with a high degree of agreement, with the Keeling Curve of direct atmospheric CO2 measurements made since 1958, shown in blue in the plot above. (Note that the blue data in the above plot are yearly averages, so the seasonal variations we saw in Episode 4 have been “smoothed out.”) No reasonable, well-informed person refutes this data, which has now been replicated by a multitude of independently sponsored research groups and reviewed extensively for years.

The historical CO2 data tells a story of remarkable stability for 90% of human experience since Biblical times. In fact, until around 1850, the atmospheric CO2 concentration averaged 279 ppm and never strayed outside a narrow range between 272 ppm and 284 ppm (see black lines on the plot below):

Plot of Scripps ice core-merged data showing the pre-industrial average (black dashed line) and range (black solid lines) of CO2 concentrations going back to 0 AD.

Around the time of the First and Second Industrial Revolutions (attended by the advent of coal-fired steam engines and the petroleum industry, respectively), atmospheric CO2 began its relentless upward sweep that continues today. By the time Dr. Brown and Mr. Escombe were doing CO2 measurements at the Royal Botanical Gardens around the year 1900, and certainly by the time Guy Callendar and Dave Keeling were publishing their CO2 measurements and analyses starting in the late 1930’s, the atmospheric CO2 concentration had already departed significantly from the pre-industrial range. The March 30, 2017 direct measurement at Mauna Loa was 47% higher than the average CO2 concentration that had persisted, until very recently, since classical antiquity.

The rate of increase of the atmospheric CO2 level is also strongly accelerating. The graph below shows the rate of change of CO2 concentration over the past two millenia. (If you remember your pre-calculus, I obtained the graph below by taking the derivative of the graph above.)

Rate of change of atmospheric CO2 concentration in parts per million per year (ppm/year).

Prior to the Industrial Revolutions, the atmospheric CO2 concentration changed very little from year to year, and the rate of change hovered around zero. Following the Industrial Revolutions, the rate of change was positive much more often than it was negative; the CO2 concentration was increasing. Immediately following World War II commenced an unprecedented period of positive and increasing rate of change of the CO2 concentration. Some climatologists have labelled the time period between the end of World War II and today as the “Great Acceleration.” During this period, the global population doubled in just 50 years, while the size of the global economy grew by a factor of 15 (Steffen, Crutzen & McNeill, 2007). At the same time, the global CO2 concentration has not only increased to levels unprecedented in previous human experience, but the rate of that increase has sped up from year to year. In 2016 (the hottest global year on record), the rate of increase reached 2.24 ppm/year.

The question for us is, how high do we wish to allow the atmospheric CO2 concentration to go? For me, I have to say the data shown above is alarming. The fact that, in spite of the data above, we are still having discussions about “putting coal miners back to work” is terrifying.

“It will bring back manufacturing jobs across the country, coal jobs across the country. Across the energy sector, we have so much opportunity, George. And the last administration had an idea of keeping it in the ground. We need to be more independent, less reliant upon foreign energy sources. And this is an opportunity.” (EPA Head, Scott Pruitt, explaining to ABC News Anchor, George Stephanopoulos, the merits of President Trump’s executive order of March 28, 2017, seeking to redefine the government’s role in protecting the environment)

In a future episode in this series, we will get into the details of how historical temperature records have been created and linked to the CO2 concentrations above. But there is already enough information on this website to show that our prodigious CO2 production, if unabated, will lead to prodigious warming. The physics of the greenhouse effect are well understood and have been refined by scientists since the effect was first proposed in 1824. It is a mathematical certainty that more CO2 in the atmosphere will cause warming. As we saw in Episode 3, physicist Gilbert Plass used this known math and some of the first computers to predict in 1956 that the combustion of fossil fuels would lead to a warming of about 1 degree Celsius by around the year 2000, and that has come to pass.

In a 2013 paper, respected climatologist, James Hansen, and co-workers calculated that the Earth’s fossil fuel reserves are sufficient to raise the average land surface temperature by 20 degrees Celsius (36 degrees Fahrenheit). Try adding that to the summer temperature where you live. Since humans require a wet bulb temperature less than 35 degrees Celsius (95 degrees Fahrenheit) to maintain body temperature, this temperature change would literally make most of the Earth uninhabitable for humans in the summer. As an engineer, it’s impossible for me to imagine a workable adaptation for this problem that could be accomplished on the short time scale over which this change is presently on track to occur. In fact, given the comfortable stability in CO2 concentration humans have “grown up” with, there is nothing to suggest our social systems are prepared to deal with many of the consequences of the rapid climate changes we would experience on the current trajectory. Our farm land will be moving toward the poles. (Will we then clear more carbon-absorbing forests as it moves?) Our most valuable coastal real estate will be submerged.

As for the consideration of jobs, I suspect it will always be plausible to make the argument that jobs in fossil fuel reliant sectors of our economy will be eliminated by shifting to more sustainable sources of energy. It seems to me that new jobs will be created making solar panels, solar concentrators, and wind turbines. With respect to energy independence, I would argue that the sun shines and wind blows in all regions of the Earth. In any case, given the conclusions of the last paragraph, it would seem the only reasonable conclusion is, yes, as much as it may pain us, we will need to leave much of our remaining fossil fuels in the ground.

To be continued…

Continue to 6th Episode

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