1.1 The global whodunnit

Attributing the causes of the climate crisis is like solving a ‘whodunnit’, that is, a crime mystery. Thinking about it in this way is a useful way to unpack the different stages of the issue.

You will study this here in the context of past GMST changes. The six steps in our whodunnit to establish the causes of climate change are:

Step 1: Record the scene (measure climate change over time)

Here, the evidence is the global surface temperature record, but you could measure other aspects of climate change.

Step 2: Identify the suspects (what are the possible factors that can change climate?)

We consider all external factors that could warm surface temperatures.

Step 3: Identify the red herrings (are there any factors that may confuse your investigation?)

Mysteries can be made more difficult to solve by the presence and actions of ‘innocent parties’ which serve to confuse the investigation.

For the attribution of climate change, these ‘red herrings’ could be factors that cool surface temperatures, or random fluctuations that are not caused by any external factor but that might cause warming or cooling.

You will look here at three red herrings, both natural and human-caused.

Step 4: Establish what everyone was doing at the time (how much were each of the factors changing?)

The whodunnit analogy is perhaps a little more stretched here, but the next step is to estimate the strength of each factor (both ‘suspects’ and ‘red herrings’) through time, either by direct measurements or by other methods. If a suspect was not present, then they could not be responsible.

Step 5: Take everyone’s fingerprints (describe the pattern of change each factor causes)

Look for fingerprints. Luckily for us, each of the suspects and red herrings has a different fingerprint on global surface temperatures: a characteristic spatial pattern of temperature changes.

Step 6: Infer who has the most fingerprints on the scene (how strongly does each factor affect the temperature record?)

You will see how climate models – mathematical representations of the climate system are used to quantify the contribution of each of the fingerprint spatial patterns (Step 5) to the temperature record (Step 1).

In Session 2, you saw summaries of the evidence for Step 1 of the whodunnit, the measurement of climate change over time. You will now consider the suspects (Step 2) and the red herrings (Step 3). In climate science, these suspects and red herrings are termed ‘radiative forcings’, or just ‘forcings’, and act to change the Earth’s energy budget.