Extreme events and climate change
Consider the Paris Agreement where countries around the world have agreed to reduce greenhouse gas emissions with the goal of limiting global warming to well below 2°C, preferably to 1.5°C, compared to pre-industrial levels. Some people think “Who cares? It was 2°C warmer today than it was yesterday. 2°C is peanuts!”. Or consider sea-level rise. That could be around 3mm/year along the coast of Otago. That’s 3cm in a decade. Again peanuts, right? A wave could be 5 metres high so why are we worried about a 3cm rise over a decade?
In this piece I want to talk a little about why these apparently small numbers matter. Let’s start with temperature and consider the daily maximum temperature we experience each day in Alexandra, i.e., the highest temperature recorded throughout the day. There may be a few days where the daily maximum temperature hardly gets above 1 or 2°C (those nasty overcast days in winter) and then also a few days in each year when the hottest it gets during the day could be more than 30°C (those baking hot summer days when you just have to take the dogs to the lake). But it’s more likely that the most common daily maximum temperature is somewhere in the middle, i.e., around 15°C.
Let’s go and take a look at the data. In Figure 1 (below) I have taken hourly temperature data for Alexandra and calculated the daily maximum temperature for all days with 24 hours of data to arrive at 2,224 values. I have then counted the number of values in each 1°C bin (a histogram). You can see that the most typical daily maximum temperature value is somewhere around 14 or 15°C. On 44 days in that historical record the daily maximum surface temperature exceeded 30°C.
Now what happens if I go and add 2°C to every data point (simulating the threshold for the Paris Agreement). I have done that and plotted the new histogram (in red) behind the old histogram (in orange) in Figure 2 (below). Now when I go and count the number of days when the daily maximum temperature is above 30°C I find that there are 95 days! That 2°C of warming caused a 216% increase in the number of days where the daily maximum temperature exceeds 30°C. And it’s these hotter than 30°C days that have the greatest impact in terms of affecting crops, drying out the soil, causing heat stress to farm animals, etc. We’re not concerned about the days where the maximum temperature is 15°C and that goes to 17°C under global warming. It is these really hot days whose frequency can more than double that’s the problem.
And this is true for most climate variables. It is the changes in the extremes that really cause the damage, not the changes in the average. An apparently small increase in sea level can cause the number of times a protective seawall is breached to double. A 1°C warming of the atmosphere can cause a 7% increase in the capacity of the atmosphere to hold water, but the number of times severe rainfall exceeds some threshold to double. This is why we are seeing 1-in-100-year events becoming 1-in-50-year events; that’s what doubling the frequency of extreme weather events does.
When you see these apparently small changes in various climate variables, don’t confuse these with the much higher day-to-day variability that we see. Unlike day-to-day variability, these climate induced changes lift the entire data record, causing thresholds to be breached far more frequently. Beware the extremes, the jaws that bite the claws that catch.
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