SCIENCE FACT Where does all that knowledge for past climates come from? And what on Earth are isotopes??

Most of us know that the continuous measuring of climate variables (such as temperature, rainfall, ...) didn't start until roughly 150 years ago (see data sets on KNMI Climate Explorer). So then how are climate scientists able to know about climates in the past?... simply because the Earth has recored its climate history in various natural archives. Our job is it to look at these archives and understand the codes in which the different climate variables are encrypted... much like a detective.

The most common and famous natural archives are:

ice cores
sediment cores (either from the ocean or from lakes)
stalagtites + stalagmites
fossilized pollen
tree rings
corals

Why those?
All of the above named natural phenomena have one similarity: they all come with a record of time. Without a record of time, I will never be able to reconstruct the past. In all, time is represented as layers (except for pollen, which are normally found in a certain layer of a terrestrial core). If the layers are well kept, I simply have to go ahead and count backwards, thinking of each line as a year.
In the case of sediment cores, layers might not present an annual resolution meaning the lines here represent bigger time pieces. In that case I can use big global events that may be seen in the core and give those layers a date (i.e. Sarna-Wojcicki et al., 1985; Drexler et al, 1980; Machida, 1999). An example for such a big event is the explosion of the Yosemite Volcanoe. The event was so huge, that Yosemite ashes can be found in almost every record that goes this far back. In the more recent history, we can easily date the 1950s and 60s, due to the large amounts of atmic bombs that were tested. The material from those bombs is also visible in most modern archives (Picciotto & Wilgain, 1963; Eichler et al., 2000) .

Here are some examples of natural archives:

Ice core:

http://upload.wikimedia.org/wikipedia/commons/thumb/3/31/GISP2_1855m_ice_core_layers.png/384px-GISP2_1855m_ice_core_layers.png

Coral core:

http://www.fws.gov/coralreef/images/core_sample.jpg

Ocean deep sea sediment core:

http://ocean.si.edu/sites/default/files/photos/cores.jpg

Cave Stalagmite:

https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhdpIZNfwWE4gElOx3WXiK7_V9Bn4Ykenrwqu1JQMiRcKz1cX5G97S7ODZUuwg2D-gAeAAe0TsjRPZwh0JuBADhYZsNUL22ky2tZ1NTsEG88BzcuGr4lKFR5Si_jR8d1b8XFbFUcEeEAjUz/s320/StalagmiteCoreSlice_12_MNJensen72.preview.jpg

Soil core for Pollen:

http://www.sciencephoto.com/image/214061/350wm/G3550193-Soil_core_for_pollen_research-SPL.jpg

Tree rings:

http://cdn.desktopwallpapers4.me/wallpapers/photography/1680x1050/1/8788-tree-rings-1680x1050-photography-wallpaper.jpg

How can scientists extract data out of this? The isotope method:

The whole core is important for dating. The actual climate information however is extracted in the most genious and creative ways. The ones listed here are just a few ways of how one can do it.

One way is to look at the thickness of layers. Tree rings or ice cores for example (depending on where they are from) may differentiate between rain/snowyy vs dry years.

Another way is looking at isotopes. Whats that?

--> Every atom has electrons, protons and neutrons. The amount of electrons and protons (amount of electrons = amount of protons) defines what it is (either oxygen, or carbon, or iron...). The neutrons are defining how heavy it is. More neutrons = heavier. Less neutrons = less heavy.

So an oxygen atom with 16 neutrons is lighter than an oxygen atom with 18 neutrons.

Exactly this is what the scientists use.

Let me use the deep sea sediment core to show you how (bare in mind that this is just a simple way of explaining... incase you are a climate scientist):

Step 1: drill the deep sea sediment core

Step 2: count the layers and have an idea how old each layer is

Step 3: go to the layer you're interested in and look for little animals that lived in this time, then died, sank to the ocean floor and now are located in your core. Most scientists use little living beings called foraminifera. They build a calcium (chemically: CaCO3) shell and thus can provide you with carbon and oxygen atoms.

Step 4: get the atoms out of the little foraminifera

Step 5: count how many heavy oxygens and how many light oxygens you can find and calculate a ratio: all oxygens with 16 (16O)/ all oxygens with 18 (18O).

You are done! wow. what does that tell you about the climate?? a lot! Let me give you a little background information:

We know that 16O is lighter than 18O. So imagine you are at the equator. It is nice and warm and a lot of water from the ocean evaporates into the air.

What happenes: of course the lighter particles evaporate first meaning lots of 16O leaves the ocean and circles in the atmosphere.

Now imagine we travel to the poles. It is very cold and lots of moisture is in the air. It snows. All the 16O that we just collected from the oceans falls down as snow and gets stored on the big ice sheets. Do you see: the Atmosphere is a SORTING MACHINE for oxygen isotopes!!!!

Now if you search the isotopes in your sediment core: you see that there is layers with lots of 18O and layers where 18O = amount of 16O. From what we learned above: the layers with lots of 18O show ice ages (lots of snow keeps lots of 16O out of the ocean), while the layers where 16O and 18O are almost the same show warm periods (most 16O rains back into the ocean and only very little is turned into snow and stays on the poles).

That is quite amazing! Now you know why scientists love isotopes! :D

If you want to read it from the "discoverer of isotopes" himself: Shackelton (1987); Chappel & Shackelton (1987); Gat et al. (1981)

Good informatin about ice core sampling can be found here: Alley (2000)