Climate & Energy

So What Is “Net Zero”, Anyway?

Explainer5th June 2021

The UK’s major political parties perhaps don’t agree on much, but they do on one thing: the UK’s target of cutting our greenhouse gas emissions to “net zero” by 2050. The UK became, in 2019, the first major economy in the world to legislate for net zero, and since then many others – the US, EU, Japan, China, South Korea and many more – have set similar targets.

But while record numbers of people are concerned about climate change, public understanding of net zero is low: according to government data more than half of people don’t know what it means, and only 3 per cent have a good understanding.[_] 

So what does net zero mean?

At the moment, the UK emits around 500 million tonnes of greenhouse gases – mainly carbon dioxide and methane – into the atmosphere each year.

There is a near-universal scientific consensus that increasing the level of greenhouse gases in the atmosphere causes climate change. The Paris Agreement commits the world to limiting temperature rise to 2 degrees Celsius, and to aim to limit it to 1.5 degrees. To achieve that, we need to stop the concentration of greenhouse gases in the atmosphere rising.

To understand how we do that – and what net zero means – you can think of the atmosphere as a bathtub, and greenhouse gases as the water filling it[_].

At the moment, the bathtub is filling up quickly as we emit greenhouse gas emissions at a rapid rate. Turning the tap down – or slowing the flow of greenhouse gas emissions – is important; it will slow the rate at which the water rises. But to stop climate change, slowing it down isn’t enough. We need to stop the water in the bathtub rising. And because we’ve filled it up so quickly over the last 50 years, we need to do it fast.

Making that happen means doing one of two things.

First, we could turn the tap off completely. That would mean getting to “absolute zero” emissions – we emit no greenhouse gases. 

But doing that is virtually impossible. While many high carbon activities and technologies can be replaced by low carbon alternatives – for example, by replacing a petrol car with an electric vehicle – some others can’t. International flights, for example, can only get to zero emissions if we stop doing them. Agriculture will always cause some greenhouse gas emissions – to put it delicately, cows will always produce methane.

So we move to our second option: aiming not for “absolute zero”, but for “net zero”. That means that we cut emissions as much as we practically can, and every tonne of emissions we can’t cut is balanced out by removing emissions from the atmosphere, for example by using new technologies like Direct Air Capture.

So in our bathtub analogy, net zero means turning the tap right down so it’s only dripping a little water into the tub, and lifting the plug so the same amount of water is flowing out as is flowing in – meaning the overall level stops rising.

In the UK example, achieving that outcome means we rapidly reduce emissions, so they are close to zero by 2050 – for example by insulating our buildings and replacing gas boilers with low carbon alternatives; driving electric vehicles rather than petrol or diesel cars; using low carbon fuels in our industry; building low carbon electricity generation; planting more trees; and so on. And for the small remaining chunk of emissions, which we can’t cut, we invest in new technologies to remove an equivalent amount from the atmosphere.

And while our political leaders agree on the outcome we’re aiming for, helping the public understand what it means – and enabling proper engagement on the action required to meet it – is a vital next step on the path to net zero.


  1. 1.

    In numbers: The public's understanding of net-zero and green policies (

  2. 2.

    The “bathtub” analogy has been used elsewhere e.g. the Grantham Institute –

    What is net zero?


    Grantham Research Institute on climate change and the environment (

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