By David Salt
A constant challenge for anyone writing about climate change is how to make the numbers involved mean anything to an average reader. Too often those numbers are either too big or too small to resonate with our daily lives, so we search for ways of making them meaningful. More often than not, we fail.
Consider this little gem from NASA on the loss of ice – since 2002, our planet has been losing 420 billion metric tons of ice every year from the Greenland and Antarctic ice sheets! (Figures 1 and 2)
Figure 1: Loss of ice mass over Antarctica
Holly molly! 420 billion metric tons!! That sounds like a lot but, really, who has any idea what that means. As soon as you say ‘billion’ you’ve effectively lost pretty much everyone because a billion – that’s 10 to the power of nine, or 1 with 9 zeroes after it – is too big to easily conceptualise.
So, how did NASA build a bridge to assist comprehension? First it pointed out that one billion metric tons is equal to 2.2 trillion pounds (the metric to Imperial conversion); which is even worse to your average (metrified) Aussie. (A metric ton, BTW, weighs 1,000 kilograms.)
And then NASA helpfully told us (in a tweet) that one billion tons is the mass of 10,000 fully-loaded US aircraft carriers. No joke, though it still left us to do the final calculation. That means each year we’re losing a mass of ice equivalent to 420 x 10,000 fully-loaded US aircraft carriers = 4,200,000 or 4.2 million fully-loaded US aircraft carriers. Over 20 years, that adds up to 84 million fully-loaded US aircraft carriers.
Figure 2: Loss of ice mass over Greenland
Feeling is believing
So, what our mind sees is a single aircraft carrier and we know it’s very big and heavy, even heavier when fully loaded. Then our mind says ‘wow, 10,000 of these things must be extremely heavy’ and ‘420 times this is very extremely heavy again’! However, if we’re being honest, we really have no idea at all what we’re talking about. All of us lost it as soon as we considered how big a single aircraft carrier weighs (regardless of its country of origin); none of us could then meaningfully upscale that immense weight by multiplying it by 4.2million; except to come away with an impression that the number is truly enormous, though what it means, who knows.
So, for this particular case study in science communication, I give the NASA a fail, and I really wondered why a US aircraft carrier would ever be used as a unit of weight comparison (unless you were secretly trying to intimidate other countries who lacked aircraft carriers).
However, it’s a trick that all science communicators and journalists will resort to from time to time; I know I do. If something is really big, we simply describe it as being 10X or 100X or a 1000X bigger than the biggest thing we think our readers can conceive of – a car, an elephant or a fully loaded US aircraft carrier.
And the same applies, but in reverse, for something really small. For example, how tiny is a nanometre? It’s very, very, very small. A billion of them lined up make up one metre. Still befuddled; well, a sheet of paper is about 100,000 nanometres thick. So, we know a sheet of paper is very thin, now imagine something 100,000 times thinner! In truth, that task is quite beyond your (and my) imagination. Neither a billion or 100,000 makes intuitive sense to us, but we ‘feel’ the description.
As I say, there’s not a science writer alive who hasn’t played these tricks but I sometimes wonder if, in so doing, we aren’t making it harder to get the message across. In attempting to convey an important idea that’s too big to grasp by using lurid and fanciful comparisons we risk trivialising the idea itself. If we do it all the time, we risk turning our efforts of engagement into a game or a circus, that can be more easily discounted.
NASA’s use of aircraft carrier units made me laugh out loud, yet the consequences of accelerating ice melt pose an existential threat to human civilisation. The most immediate consequence is rising sea levels. And maybe this is what we should be prioritising when communicating global change – that is, impact.
Water under the bridge
What happens when you add 4.2 million fully-loaded US aircraft carriers worth of water to the ocean? Believe it or not, it rises (see Figure 3). Once again, NASA comes to the rescue in telling me how much it will rise (noting that ice melt is only one component of sea level rise). In the past year the ocean has risen (on average) 4mm. In another tweet, NASA observes “that’s like adding water from a million Olympic-size swimming pools to the ocean every day for a year”! Wow, an Olympic swimming pool is big, so a million swimming pools is very, very big.
Figure 3: Sea level rise since 1900. Note the flattening out in the 1960s when massive dam development held back a lot of run off that would have entered the oceans. And also note the acceleration of the rising in recent decades. The best science available suggests this acceleration will continue.
Now I don’t want suggest for a split nanosecond that NASA is doing a poor job overall in science communication (even though I question their use of air carrier water units). They are world leaders in their communication efforts. But I do want to suggest that the relentless application of hyperbolic comparisons which is flourishing in all walks of science communication is not helping in getting society more engaged on climate change. (I’d also like to note that in Australia we’re always using the hyperbolic ‘SH’ water unit to colour our stories in which SH stand for a Sydney Harbour quantity of water; a unit close to completely meaningless but still commonly used.)
Since 1900 our oceans have risen over 200mm (20cm) and the modelling is all suggesting that this will accelerate in the coming decades. Four millimetres in one year doesn’t sound much and is difficult to notice against the ceaselessly changing sea level with tides and waves, something climate deniers always seize on. However, put 20cm of sea water on top of a king tide during a storm surge and you have disaster (maybe even a catastrophe) for low lying areas.
Consequences and impacts
And maybe that’s what irks me about loaded aircraft carrier and Sydney Harbour mass and volume comparisons. They’re fun and evocative but ultimately not really informative, and they possibly distract the reader from the consequences of what’s happening. Most of the human population lives by the sea. Around half a billion people live in low lying areas less than 2 metres above sea level. Sea level rise will be directly disrupting the lives of billions of people in the coming years, yet we rarely engage with these realities.
We know so much more about climate change and sea level rise in recent years yet regardless of what we know we don’t do anything about it. At the end of the day, no matter how many thousands of fully loaded aircraft carriers we have lined up defending our shores, we are powerless to turn back the tide.