How to interpret a percentage change

How to interpret a percentage change

Here is what the International Energy Authority said happened to carbon emissions last year across the globe.

Carbon emissions rose by 1.7 per cent in 2018 to a record 33.1 billion tonnes, with coal making up a third of the total increase

That is 560 million tCO2 more carbon emitted than the previous year.

That increase is equivalent to the total emissions of the international aviation industry or if you prefer, the annual emissions for Australia.

Recall that 1990 was a pivotal year for climate change issues. It was chosen, arbitrarily for those not in the know and all those with an ounce of common sense, as the benchmark year to compare targets for emission reduction.

In 1990 global greenhouse gas emissions were roughly 22.4 billion tCO2.

Rather than concern ourselves with the increase since then, yep it is half as much again, let’s focus on what a 1.7 per cent increase on 22.4 billion looks like.

It’s 381 million tCO2e

Buckets of water example

Now, let’s suppose that I have two buckets the size of laundry baskets. Each bucket is big enough to hold 50 litres of water.

The first bucket is the 1990 bucket. It contains 22.4 litres of water.

The second is the 2018 bucket and it contains 33.1 litres of water.

If each day I added 1.7% of the starting volume to each bucket (381 ml and 560 ml) in 72 days the 1990 bucket would be full.

The 2018 bucket is spilling water on the floor in 30 days.

Less than half the time!

Same percentage. Very different result.

The analogy is not quite reliable for the greenhouse gas issue. The atmosphere may be like a bucket in that it has a finite volume but it is a huge bucket unlikely to overflow with gas.

The issue for greenhouse gases is, of course, the way they alter the atmospheric composition and change the warming potential, retaining more of the sun’s energy as the proportion of greenhouse gases rises.

The per cent change result still holds. 1.7 per cent of the 2018 amount has a much bigger effect than 1.7 per cent of the 1990 amount.

Global population example

The global population growth rate in 2018 is around 1.1% or roughly 83 million people added to the mix each year.

This percentage gain is less than the 1.6% gain in 1990 that delivered roughly the same number of new people.

So again let’s do the bucket test. This time we’ll go with the 1.1 per cent gain and use slightly smaller, 10-litre buckets.

The 1990 bucket starts with 5.3 litres of water and the 2018 bucket has 7.2 litres.

If each day I added 1.1% of the starting volume to each bucket (58 ml and 79 ml) in 81 days the 1990 bucket would be full.

The 2018 bucket is spilling water on the floor in 35 days.

Lower percentage but a faster fill. Ouch.

Pay rise example

Is it fair to give a 1% pay rise to all employees in the company? Sounds fair.

Everyone gets the same proportional raise, all the boats get to float. Except that the CEOs 1% gets him a new fridge out of the first paycheck and the tea lady gets a coffee at Starbucks.

Be aware of percentages when the media spout them.

They are only useful if you know the amount the percentage refers to.



When you see the trajectory of Australia’s greenhouse gas emissions over the last decade or so, the pattern is a storyboard for the country’s political journey.

There was the ‘biggest moral challenge of our age’ in 2007 when a long period without emission reduction was obvious.

The carbon price that actually started to slow and then reverse emissions through the late 2000’s to the point that the country was tracking to meet its internationally agreed targets

Then the trashing of that ‘great big tax’ in 2014 to send the numbers upward again, and most recently, the apathy that has kept them climbing.

Here are the numbers as graphed.

The current numbers would have Australia with a cumulative failure to meet Paris commitments target by at least 40 million tCO2e a year or 7% of the annual emissions.

Not a good look. Arguably a renege.

It feels like right of centre governments can only understand graphs that project from bottom left to top right and so they create them, whatever the metric. And when it comes to emissions we have seen before how easy it is for them to have Lost the plot. So here we are with a government apparently unconcerned or oblivious to the combined facts that emissions are rising again, despite the growth in renewables, and the country is about to fail badly on an international commitment. Meantime the evidence continues to pile up that the planet is indeed warming and there are very tangible consequences for the people who live on it.

There are many Australians concerned about this tendency to abdicate on the issue of greenhouse gases. Even the medical profession who presumably have little interest in atmospheric physics are talking about the consequences of a warming world for health and safety.

So on the one hand there is evidence that denial has won and policies that do little and still question that there is even a climate problem, have won out, notwithstanding the rhetoric from podiums.

On the other, there is a growing sense of urgency that the problem is not only real but is with us in our daily lives, affecting our health and wellbeing.

This schizophrenic state is confusing to the majority of people, who, let’s face it, are not thinking much beyond their next Maccas or chai latte. And the handful of folk with part of an eye peering up from their screens toward the periphery of their personal bubble, don’t think with numbers.

So the coolaid speeches easily distract them.

Coolaid, the product of a tendency to spout excessive praise so as to massage the egos of anyone close enough to hear and in doing so ignore or deny any negativity

It is much easier to drink in the rhetoric than to question it. Especially as all the subtext is aimed at making you feel safe and eager to spend your money. Why else would you “vote for me”.

Instead the majority are able to ignore the reality of the numbers and the specifics that happen every day, even as we watch streaming shows like Homeland, The Handmaids Tale, and, ever so gently, Designated Survivor, that try to show us what is around the next corner.

It is actually rather sad. The human condition is so prone to being duped that almost anyone can do it. We can even believe the real housewives.

Unfortunately, sadness is an emotional blink from despair.