Recent Climate Change

 

Experiment 7 — Causes of Recent Climate Change

Things that can cause the climate to change are sometimes called climate forcings.  It is generally agreed upon that on relatively short timescales like the last 1000 years, there are 4 main forcings:

  1. •    solar variability

  2. •    volcanic eruptions (whose erupted particles and gases block sunlight)

  3. •    aerosols (tiny particles suspended in the air) from pollution

  4. •    greenhouse gases.


In this experiment, we will add the history of these forcings over the last 1000 years (figuring out these forcing histories is not a trivial thing, what is shown here is the result of lots of research) and see how our climate system responds, comparing the model temperature with the best estimates for what the temperature actually was over that time period.  This estimate of past temperature of the planet is partly based in the instrumental record (from thermometers at weather stations) and partly from reconstructions based on corals, tree rings, ice cores, stalactites, and other “paleothermometers”.


We can turn the individual forcings on and off by flicking some switches, and thus get a clear sense of what each of them does and which of them is the most important at various points in time.


First, open the model with the forcings built in, and study the Model Diagram to get a sense of how the forcings are applied to the model.  If you run the model with all of the switches in the off position, you will see our familiar steady state model temperature of 15°C over the whole length of time.  The model time goes from 0 to 998 years, but in reality, this is meant to be the year 1000 to 1998 (it ends in 1998, because the forcings are from a paper published in 2000).


7a. First, try each of the forcings separately, and describe what each of them does to the model temperature?







7b.  Now, turn all of the forcings on together and run the model. What you see is considered to be a very good match, which suggests that these 4 forcings as represented here do a good job of explaining recent climate. Which of them is the most important in controlling the model temperature?


The forcings used here are from a paper by Crowley, 2000, Science, v.289 , p. 270-277