Monday, April 3, 2017

The most convincing evidence for CO2 increase

Today global warming and climate change are widely discussed. Many people refer to "scientific evidence" that proves that climate change is caused by humans, by burning fossil fuel in particular. However, I rarely hear on public media discussion of the actual data and interpretations that serve as the scientific evidence for human-induced climate change. May be there would be more productive conversation between different members of our society on climate change if the public media discussions included some background on the scientific evidence. 

Human-induced climate change has expressed itself in elevated concentrations of CO2 in the atmosphere. Elevated CO2 is a consequence of releasing carbon in the atmosphere through burning fossil fuel like coal and petrol. Among multiple evidences for such change, glacial ice cores represent the most convincing indication of human-induced elevated CO2 levels. As glacial ice forms every year it trap atmospheric air in the form of bubbles. Extracted from drill holes, glacial ice cores can be accurately dated to the recent time line (back to several thousand years). Extracted air bubbles are analyzed for concentration of gases and isotopic composition of the gases. This graph below copied from "Stable Isotope Geochemistry" by Hoefs represents the most convincing evidence for human-induced climate change.


 

Clear increase in CO2 concentration (plot a) starting at about 1850 marks the bloom of the industrial era. Plot B shows the carbon isotopic composition (δ13C, ‰; delta carbon thirteen) of CO2 from the atmosphere. Starting from industrial era, it becomes more negative, meaning CO2 molecules in the atmosphere are increasingly depleted in the heavier isotope of carbon - 13C - with respect to the lighter carbon, 12C. Organic matter, like coal, is extremely "light" carbon, means it is depleted in heavy carbon 13C. Burning such "light" source of carbon makes CO2 in the atmosphere "light" as well. Just for clarification, I include here a diagram from the same book on isotopic composition of all carbon sources known to Earth. It show that organic matter (such as fossil fuel) is the source of isotopically "light" carbon.

 


EDIT:
It was pointed out that the isotopic composition of CO2 from volcanoes should be shown too since volcanic emanations contribute to the level of atmospheric CO2. Valid point. The δ13C from volcanoes is almost identical to the δ13C of the air. See attached diagram. 
The image is taken from a Pennsylvania State University website
 

Monday, March 6, 2017

How it's done: fluid inslusion measurements (VIDEO)

Imagine a crystal of quartz growing from a hot hydrothermal fluid. Since no mineral grows without imperfections, quartz will trap some of the surrounding hydrothermal fluid (see picture below). As quartz cools down, the trapped fluid inside cools down too and it shrinks. That volume change is expressed in appearance of vapor bubble. Trapped fluid now consists of two phases - vapor and liquid. Fluid inclusions like that have a bubble and liquid now and are commonly found in hydrothermal quartz. Samples of quartz in which such fluid inclusions can be found are used as thermometers. One can heated up a fluid inclusion with bubble and liquid to make liquid and vapor become one. Such transition is called homogenization. The temperature of this transition represents the temperature when the quartz was formed. This method is well established and applied by a wide range of geoscientists, mostly economic geologists and geochemists.


 Healing of a imperfection (crack) in quartz. Surrounding fluid gets trapped. Adopted from Roedder, 1984.

An example of a fluid inclusion with bubble

Moreover, information about salinity of the hydrothermal fluid can be extracted from fluid inclusions. The freezing point of pure water is ~0°C. The freezing point of saline water is lower than that. For example seawater freezes at -2°C. Fluids with higher salinity freeze at lower freezing points. Here's the set up at Mark Reed's lab, University of Oregon for conducting heating and freezing measurements. 
 Fluid inclusion thermometry set up at University of Oregon, Mark Reed's lab

For homogenization measurements a heating element is used in combination with air flow. The element warms up the sample and thermocouple is used for measuring the temperature. A researcher regulates the temperature and observes a fluid inclusion with a bubble and liquid under the microscope. When the bubble and liquid become one (homogenize), researcher presses the footswitch and the temperature reading on the screen freezes. Careful measurements and keeping good track of inclusions (logging, taking pictures, drawings) produces the best results. Here how the stage for these measurements look like.
 Amount 300 microns thick double-polished sample of quartz is held in the camera, between 6 layers of glass and pinned down by thermocouple tip. The heat blows from the left (where the paper label was burnt) 

For conducting freezing experiments, compressed nitrogen gas is used to transfer liquid nitrogen through a set of tubing into the same stage. The sample experiences liquid nitrogen temperature of -195°C. After a fluid inclusion of interest becomes frozen (which is seen under microscope), a researcher needs to warm up the stage until ice crystals start to melt away and record when the last ice crystal disappears. That temperature is the freezing point.

Here's videos showing both heating and freezing experiments. Different inclusions are used. The stage of the microscope moves slightly so the field of view merges a little bit. I tried to maintain good focus through out the videos.The field of view in these videos is about
 
 Heating a fluid inclusion that homogenizes via vapor+liquid->liquid at 241°C.

Freezing an inclusion that have freezing point of -8°C which corresponds to salinity of about 10 wt. % NaCl.