By the numbers: the measures of climate, energy and policy

In our new Synergy column, Cosmos writers explore how we’re dealing with the urgent issues of climate change.

According to the World Meteorological Organisation, 2023 was 1.45°C hotter than preindustrial levels. It’s a number that’s been plastered across headlines for a week – but what does it actually mean?

When studying science, I was trained to love numbers and to treat them as more objective and reliable than words. Numbers, I was told, don’t lie.

But stripped of context, numbers aren’t useful. How often do we hear claims like, ‘Our company has stopped 100 tonnes of CO2 from getting into the atmosphere’?

Is that a lot of carbon dioxide? How much CO2 was being emitted in the first place?

What about energy? How big does renewable infrastructure need to be to replace fossil fuels? And why do batteries always seem to have two numbers next to them to indicate size – like 100 MW/400 MWh?

What is a megawatt or a megawatt-hour, anyway?

To make it easier to understand all the climate news you’re reading, I’ve compiled a user’s guide to the numbers of climate and energy, what each one means and how each measure stacks up – what’s a lot, what’s not very much, where it might be used dishonestly.

So, put on your 1kW kettle and burn a couple of joules pouring yourself a cup of tea, here are the climate numbers you need to know.

Jump to:

Degrees above pre-industrial levels

Global temperature rise is measured in °C above “preindustrial” levels – the temperature the Earth was before the industrial revolution started around 1750. The Intergovernmental Panel on Climate Change (IPCC) uses the years 1850-1900 – a period before the vast majority of human-emitted greenhouse gases entered the atmosphere – as its baseline.

Now, changing the temperature a couple of degrees doesn’t sound like much – you’d be unlikely to notice the difference between a 23°C or 25°C room, right?

But we’re talking about an average across the whole planet, not a single room – a temperature rise everywhere is a lot of heat.

Graphic that reads: warming by degrees, the key climate targets. 4. 4°c worst case ipcc projections. 2. 5°c climate action tracker's prediction for 2100. 2°c the upper target of the paris climate agreement. 1. 5°c the lower target of the paris climate agreement. 1. 4°c how much hotter australia is than pre industrial levels on average. 1. 1°c the current global average of warming above pre industrial levels.

Putting this warming in context

  • The Paris Agreement aims to keep climate change to “well below” a 2°C global average temperature increase above preindustrial levels, and ideally below 1.5°C.
  • Earth is currently about 1.2°C above preindustrial levels on average. Australia has warmed even faster and is about 1.4°C above preindustrial levels.
  • The worst-case IPPC scenario, where carbon emissions rise steadily over the next century with little reduction effort, predicts warming of about 4.4°C by the end of the century.

This is not necessarily the most likely scenario. Climate Action Tracker predicts 2.5°C of warming by the end of the century, based on current country targets and actions.

And although 2023 was the hottest year on record at 1.45°C above preindustrial levels, this is just one year, and not the total average long-term temperature rise.

The rising tide: how much sea level rise can we really expect?

Global warming causes sea level rise in two main ways.

First, melting ice on landmasses, especially Antarctica and Greenland, adds water to the oceans.

Second, water expands as it warms – meaning the same amount of liquid takes up more space.

These rising sea levels will put more people at threat of flooding and inundation – Pacific nations are already being forced to relocate low-lying communities.

Putting sea level rise in context

  • From 1901 to 2018, global sea levels rose by 20cm.
  • Future sea level rise depends on emissions. The IPCC predicts 28-55cm to the end of the century in a ‘very low’ emissions scenario, and 98-188cm of sea level rise in a ‘very high’ emissions scenario. The real result will likely be somewhere between these extremes.

A 2020 study found that 267 million people currently live in areas that would be inundated by 100cm of sea level rise. This population is expected to grow to 410 million by the end of the century. In 2021, a report in Environmental Research Letters forecast what that rise would look like in three different cities.

What’s a watt? Making sense of Watts, kilowatts, kilowatt-hours, megawatts…

Australia produces – and uses – heaps of energy.

In 2022, 32% of that energy came from renewable sources that don’t release greenhouse gases. But that proportion hides a complicated mesh of everything from huge power stations to rooftop solar panels and home batteries. So how is it all measured?

Let’s start simply.

Scientists usually measure energy in joules. However, most of the time, you’ll see watts used by industry and government to describe how much electricity their products and services generate or consume.

A watt is a measure of power, which is energy over a certain amount of time – one joule of energy per second, to be precise.

A kilowatt (kW) is 1000 watts and a megawatt (MW) is 1000 kW: it goes up in increments of 1,000.

1 watt1,000 watts1 million watts1 billion watts
Low wattage
1 electric kettlePower output
of a sports car
power station

We see these ratings applied to products we buy, right through to major infrastructure projects.

Say a large-scale battery is rated at 100MW. That means it’s capable of pumping 100 megajoules into the system over a second.

If you have a 40-watt light bulb, that means it needs 40 joules each second to glow.

But what if we want to know how much energy is used, in total? Some organisations use joules, but it’s more common to see another unit: watt-hours (Wh).

The output of a power plant, or the energy your house consumes, is often measured using kilowatt hours (kWh) – the amount of energy a kilowatt of power generates over an hour.

Since a kilowatt is 1000 joules per second, and there are 3600 seconds in an hour, a kilowatt hour is 3.6 million joules.

You’d expend a joule of energy lifting an apple over your head. Imagine doing that 3.6 million times – that’s 1 kWh.

Putting these measures of energy in context

  • According to the Clean Energy Regulator, Australia’s two largest power stations Loy Yang and Bayswater – which both burn coal – each produced roughly 15 terawatt-hours (15 billion kWh) of energy in FY2021-22.
Loy yang
Loy Yang power station Credit: Marcus Wong CC BY-SA 3.0
  • The largest operating battery in Australia, the Victorian Big Battery, is a 300 MW (300,000 kW) power battery with 450 MWh (450,000 kWh) of storage. There are plans in the works for gigawatt (million kW)-scale batteries around the country.
  • Australia as a whole used 1,667 TWh (1.67 trillion kWh) of energy in 2022. The world used 179,000 TWh (179 trillion kWh).
  • All rooftop solar in Australia generated 22 TWh (22 billion kWh) of energy in 2021-2022.
  • Australian households used 254 terawatt-hours (254 billion kWh), in 2021-22.
  • According to the Australian Energy Regulator, the average customer in the eastern states’ energy grid used about 5,500 kWh in 2022-23.
  • Home batteries in Australia vary in size from roughly 5 kWh to 20 kWh. The Tesla Powerwall is 13.5 kWh, with 5-7 kW power.

Tonnes and tonnes and tonnes of carbon dioxide

Carbon dioxide (CO2) is not the only human-produced gas that’s warming our atmosphere, but it’s by far the most common. When people talk about emissions, they usually use CO2 as their yardstick.

We measure CO2 emissions by weight: metric tonnes, each of which is 1,000 kilograms.

1 tonne1,000 tonnes1 million tonnes1 billion tonnes

It’s worth noting that companies count emissions in scopes, which reflect different levels of responsibility for the emissions. If a business says they’ve cut, say, 80% of their carbon emissions, it’s always worth asking which scope they’re counting with – because they may not actually have cut 80% of the emissions they cause.

Putting carbon numbers in context

Humans have pumped a huge amount of carbon into the atmosphere since the wheels of industry started to turn:

  • Collectively, humans are currently emitting about 40 billion tonnes (40 Gt) of carbon dioxide each year, according to the IPCC.
  • Since 1850, humans have emitted 2.4 trillion tonnes (2,400 Gt) of carbon dioxide into the atmosphere.
  • As of 2019, a typical Australian car emits 181g of CO2 per kilometre travelled. An average Australian car travelled 11,100km in 2019-20, which is worth about 2t of CO2.
  • The average Australian’s activities emit 15t of CO2 each year.
  • In 2022, Australia emitted 392 million tonnes (392 megatonnes) of CO2. Since European settlement, Australia has emitted 19 billion tonnes (19 gigatonnes) of CO2.

Comparing apples with apples: CO2-equivalent

CO2 is not the only greenhouse gas and different greenhouse gases warm the atmosphere in different amounts.

Methane, for instance, warms the atmosphere significantly more than carbon dioxide but it also tends to stay in the atmosphere for a shorter amount of time. After roughly 12 years, it has typically reacted with other gases and turned into CO2, while CO2 itself stays stable for hundreds of years.

So, it’s not accurate to compare something that’s emitting 50 tonnes of methane to something that’s emitting 50 tonnes of CO2. While they’re both emitting the same amount of gas, they’re causing different levels of warming.

Enter CO2-equivalent. This is the method we use to compare the influence of different gases.

Since time matters too, we tend to measure CO2-equivalent over a century – enabling us to compare apples with apples.

Over those 100 years, methane is 28 times more potent than CO2. So a tonne of methane emissions is 28 tonnes of CO2-equivalent.

The US Environment Protection Agency has a calculator for other greenhouse gas equivalencies.

Putting CO2-equivalent in context

  • According to the IPCC’s most recent reports, the world at large emitted 59 billion tonnes of CO2-equivalent in 2019, 40bn tonnes of which was actually CO2.
  • According to its Paris Agreement accounts, Australia emitted 465 million tonnes (0.465gt) of CO2-equivalent in 2021.

Counting carbon in the sky: Parts per million (ppm)

Carbon dioxide that reaches the sky is described in terms of concentration rather than total weight.

Parts per million (ppm) is a way of describing concentration. In climate science, it’s most frequently used to measure carbon dioxide in the atmosphere.

A concentration of 400 parts per million CO2 means that for every million molecules in the air, 400 of them are CO2 molecules. It’s the same as saying the air is 0.04% CO2.

For other greenhouse gases with lower concentrations, parts per billion or parts per trillion (ppb or ppt) is used.

You might think that 400 parts in a million is a very low concentration – and it is. But low concentrations of dangerous substances can still have huge influences.

An Agatha Christie villain would only need to spike their victim’s tea with about six sand grains worth of strychnine to finish them off.  And you can smell chlorine gas in an area if it’s at a concentration of just 0.1ppm. It’s enough to be fatal if it reaches concentrations of 400ppm – good thing CO2 isn’t that toxic.

But 400ppm is still enough for carbon to trap heat and warm the atmosphere.

Putting ppm in context

  • In 2022, atmospheric CO2 concentrations were 417ppm on average. During the 2010s, it grew at a rate of 2.4ppm each year.
  • From 1850-1900, the concentrations were about 290ppm.
  • By 1980, they had made it to nearly 340ppm.
  • In May 2023, CO2 concentrations were 424ppm at Mauna Loa, Hawaii, breaking a new record. (CO2 concentrations fluctuate annually – so while they rise steadily year-on-year, they also tend to peak in May each year.)

So now you know

I want people to be able to understand what these climate numbers mean, to be able to contextualise them when they read a climate claim or an energy rating in the wild.

But I want to hear from you if you think there was something I missed, or something else you want explained. Climate science and the energy transition are numbers games – but they’re also things that people want, and need, to know more about.

So share this guide with friends, if you think it will help – and let us know if there are more values you want us to explain. Send your suggestions to [email protected].

(Oh, and for clarity, the numbers I used mostly come from the Working Group I section of the IPCC’s sixth assessment report – when I’ve used another source, I linked to it).

Correction, 30 January: An earlier version of this article featured incorrect kW measurements for some of the batteries mentioned in the ‘Putting these measures of energy in context’ section. We have updated this section with the correct values.

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