The Greenhouse Effect
How do(es) carbon dioxide (and other greenhouse gases) cause warmer temperatures? Sunlight comes through the atmosphere and when that happens, some of that sunlight is directly reflected into space (through ice in the poles) and some hits the ground. When that happens some of the energy is absorbed in the ground, and then it reflects back into space with a different wavelength (in the infrared radiation part of the electromagnetic spectrum). For that reflected energy in infrared part of the spectrum, some goes straight into space and some gets absorbed by certain gases called greenhouse gases. When that energy gets absorbed, it gets trapped into our atmosphere and it warms our atmosphere. This phenomenon is not a bad thing inherently; in fact, it is necessary for our survival. Without it, the Earth’s temperature would plummet. However, too much of it can also be a bad thing, because it causes the Earth to warm too much.
But, why are certain gases greenhouse gases, but not others? Certain molecules are nonpolar (meaning that the electron density is equally distributed throughout the molecule) but through vibrations in the molecule, it can become polar momentarily. However, some molecules cannot do this. For example, CO2 and H2O can vibrate in many different ways, but gases like N2, which only have two atoms, cannot vibrate in the same ways:
Gases with only two atoms cannot do these vibrations.
The energy from that infrared radiation that is being reflected from the ground gets absorbed by these gases only because it causes them to vibrate, causing the molecule to become polar from nonpolar or nonpolar from polar (this essentially just means that the electrons, as the molecule vibrates, become unequally distributed throughout the molecule). This causes the molecule to gain kinetic energy and vibrate, absorbed the heat. More carbon dioxide in the atmosphere would cause more infrared radiation to be absorbed by these molecules, causing more heat to be retained in the atmosphere.
Watch out for a post about the evidence for climate change coming soon!
How do we know climate change is occurring: Ice Core Stratigraphy
There are many ways to create a paleotemperature record and form a chart of temperature and atmosphere concentrations dating back thousands of years. One of the most common ways, and most commonly known, is ice core stratigraphy, or the use of ice core data to determine temperature and atmosphere conditions.
So how does this work? When ice forms, it traps bubbles of gas inside, and as this ice accumulates on top of more ice it compresses, retaining this trapped gas. Therefore, scientists can go to Greenland or Antarctica and drill a core of ice dating back hundreds of thousands of years, formed from compressed snow and ice. Using very careful methods, they can extract the gas from the core and analyze it for concentrations of carbon dioxide, methane, and other greenhouse gases.
Then scientists have to “date” the ice cores, or determine when the gases they are analyzing came from, to accurately get an understanding of how the atmospheric conditions have changed over time. This is why it is known as ice core stratigraphy; they can use the strata, or layers, in the ice as an indicator of time. There are layers of annual freezing and melting that are identifiable in the ice core; by looking at these layers we can get an estimate of when the gases were trapped in the ice.
However, this is not always accurate; it’s hard to count year-by-year and sometimes the individual layers are not visible enough to create an accurate estimate. So, scientists look at impurities in the ice. Scientists can cut out a portion of the ice, melt it down, and then analyze the water for impurities. These impurities vary annually. For example, in the spring in Greenland storms bring in large amounts of dust, which can be identified by high calcium concentration and insoluble dust (the dust that does not dissolve in water) can be seen by shining light at the water and looking at how the light bounces off the insoluble dust particles. Other elements and compounds, such as sodium, ammonium, and nitrate, also vary by season. Looking at the relative concentrations of these impurities gives scientists a more accurate estimate of when the gases were trapped in the ice.
So now, we have atmospheric conditions and when this atmosphere existed. Now, to create a chart of how these atmospheric conditions relate to temperature, we need a paleotemperature record, or an idea of how temperatures fluctuated over time. To do this, scientists look at isotopic concentrations. We have discussed the concept of an isotope in previous posts; isotopes of elements differ only in number of neutrons (which leads to a difference in mass). Isotopes of elements exist in different amounts. The most common isotope of oxygen is oxygen-16, which means an atom of this oxygen isotope weighs 16 atomic mass units. There does exist an isotope of oxygen known as oxygen-18. Water molecules can have either oxygen-16 or oxygen-18, and those molecules with oxygen-18 are heavier than those with oxygen-16. Thus, when those molecules evaporate, they are the first to rain back down, because they are heavier.
So, oxygen-18 and oxygen-16-carrying water molecules evaporate (mostly from the equator because that is where the waters are warmest) and as they move away from the equator in clouds, they cool. As they cool, they begin to precipitate out some of that water from the clouds, and the first types of water to precipitate out, as we discussed above, would be the water molecules containing oxygen-18. So, we would see the highest concentration of oxygen-18 near the equator, and as those clouds move farther to the poles, they are much more concentrated in oxygen-16-carrying water molecules. Thus, all the ice cores that we are digging up in Greenland and Antarctica are very heavily concentrated in oxygen-16.
However, this cycle does change with temperature. When the temperature is much warmer, we should see more oxygen-18 making it to the poles because the cooling phase, when most of they oxygen-18 precipitates out, happens later and later in the cycle because the atmosphere is so warm. Thus, scientists can use the isotope concentrations at the poles to create a temperature record.
Some scientists can also use fossils of marine creatures from a long time ago to date these temperatures. At the equator, when it is warmer, we will see a lower concentration of oxygen-18 because it is carried all the way to the poles (or close to the poles). Marine creatures can take oxygen from the water and incorporate that into their skeletons. Therefore, when they are fossilized, those isotopes are trapped in their skeletons. Scientists can then extract those isotopes and use radiocarbon dating (see the previous post) to date when that temperature was.
This is just one of the ways we can get an understanding of how the atmosphere and temperature changes over thousands of years.
How we know climate change is occurring: Vostok Ice Core Data
Many researchers in 1999 published a paper examining ice core data from the Vostok lake, which allows us to see 400,000 years into the past (see the previous post about how ice core stratigraphy works).
Here is one graph from their paper (http://www.nature.com/nature/journal/v399/n6735/full/399429a0.html) representing their results:
The graph is a little tough to read, but if you can see on the sides there are 5 lines, representing CO2, temperature, CH4 (methane), oxygen-18 isotope (for explanation see the previous post about ice core stratigraphy) and insolation, respectively.
One of the important parts of this graph is how the greenhouse gases and temperature are related; the graphs seem to be very, very similar, further suggesting a relationship between them.
A very noticeable part of this graph, though, is its cyclical nature. Temperature and greenhouse gas concentrations fluctuate on a cycle, which could fuel the argument from climate change skeptics that climate change is just a natural cycle.
But what cause these cycles? Well, for the most part they are caused by different tilting and spinning of the earth. First, the actual tilt of the Earth’s axis changes in a cyclic nature and this influences the temperature of the Earth. Second, the radius of the orbit around the Sun also changes, affecting temperature. Third, precession, which is the “wobbling” of the Earth around its axis, also affects temperature.
Antarctic Glaciers (via Wikimedia Commons) have an extension of this graph showing new data, and it is clear that the current levels of carbon dioxide far exceed that of the natural cycles in the past.
This cycle is known as the Milankovitch cycle, and it lasts about 100,000 years. However, we should be currently in the cooling part of the cycle, but the levels of carbon dioxide put in by humans have caused us to deviate from this cycle.
This graph is from the NOAA and shows where we should be in the cycle and how different the last cycle was from our current position. (For more graphs and visuals, see http://ossfoundation.us/projects/environment/global-warming/natural-cycle).
So far, current data suggests that climate change is anthropogenic and is a problem that we are currently facing.
Svensson, et al, “A 60,000 year Greenland stratigraphic ice core chronology,” http://www.clim-past.net/4/47/2008/cp-4-47-2008.pdf