Q&A: how fast do we need to cut carbon emissions?

How hard will it be to transition away from fossil fuels?

Speed matters – the longer we leave it, the harder it gets. Scientists now know that achieving any climate outcome, such as the Paris agreement goal of limiting global heating to 1.5C above pre-industrial temperatures, is dependent on cumulative emissions over time.

In other words, there is a finite carbon budget, and we have already used most of it. The longer we continue with high emissions, the more drastic cuts will need to become in future years in order to stay within the overall budget. On the graph below, the total carbon budget is the area under the emissions line.

If we hit the end of the budget without having cut emissions at all, by about 2029 we would have to eliminate all carbon emissions within a single year. Obviously that won’t be possible, so what will happen instead is that we end up on a temperature trajectory associated with a higher carbon budget: 2C, 3C or even higher, with catastrophic consequences.

What about negative emissions? Can they help?

Many of the conventional 1.5C scenarios depend on negative emissions in the second half of the 21st century. In other words, we would overshoot our carbon budget and then have to remove the excess carbon from the atmosphere artificially. On the graph, this would be trending the emissions line underneath the X-axis into negative territory.

This is conceptually possible in that technologies already exist to do this, such as chemical scrubbers that dissolve CO₂ and pump it underground, or power stations burning biomass that sequester their CO₂.

However, using either of these techniques to get rid of CO₂ on the scale required – which would be potentially hundreds of billions of tonnes – is pretty implausible. It would be like reversing the global oil and gas industry for a few decades, and putting all the carbon we drilled out back in the ground.

But surely if we wait longer we’ll have better technologies to help with the transition?

This is a common argument, and it makes some sense. For example, if we had grid-scale electricity storage options or cheap advanced nuclear fission – both of which are in development but still some way off – we would potentially be able to substitute clean energy sources for fossil fuels in a way that does not reduce prosperity, and even contributes to economic growth as new markets open up.

The problem is the carbon budget: every year of high fossil fuel emissions – currently at 35bn tonnes of CO₂ equivalent a year – steadily eats away at the remaining carbon budget. At current emission rates our whole 1.5C carbon budget will be gone in about eight years, as the graph shows.

If we just build more wind and solar, won’t that solve the problem?

It would certainly help. We need to throw every clean energy technology we can at the problem given the urgency of the climate crisis. But the intermittency issue with renewables has not yet been solved, as the wind energy lull over the past few months in Europe – when coal-fired power plants had to be turned back on – demonstrated.

This has led to a renaissance of support for nuclear in the UK, France and many other countries. There are also some sectors of the economy that can’t be decarbonised even with clean electricity: think cement-making, chemicals, steel, fertilisers or long-distance transport. These will most likely need hydrogen – but in all these sectors clean hydrogen-based options are still in their infancy.

What happens if we miss the 1.5C target?

The carbon budget for 2C is larger, but will still be used up entirely within 25 years at current rates of emissions. Moreover, accepting that 1.5C is no longer possible means accepting the increased climate damages that ensue. These have been laid out for us by the Intergovernmental Panel on Climate Change (IPCC).

For example, at 2C as compared with 1.5C, an additional 1.7 billion people would be exposed to severe heatwaves, 420 million to extreme heatwaves and 65 million more to deadly heat. The world’s natural life would be devastated, and coral reefs virtually eliminated from the tropical oceans.

Rates of sea level rise would accelerate, displacing an additional 10 million people and putting 136 coastal megacities at increased risk of flooding.

How does Cop26 fit in to this picture?

One of the key demands of the countries most at risk – represented by the 48-member Climate Vulnerable Forum – is for an emergency pact to be agreed at Glasgow to keep the 1.5C goal alive. For some of these countries – such as the Maldives and Bangladesh – this is not so much a matter of policy as a matter of national survival.

• The graphic in this article was corrected on 5 November 2021: the original graphic incorrectly had annual emissions of carbon dioxide ranging up to 4bn metric tonnes, instead of 40bn metric tonnes.