We've got a climate goal of 1.5 degrees – so how do we get there?

The Paris climate agreement, which commits countries to “pursue efforts" to limit global warming to no more than 1.5℃ above pre-industrial temperatures, sends a much-needed political signal that the world is ready to take serious action on climate change.

But how do we actually go about limiting warming to 1.5℃? The Paris Agreement acknowledges that developed countries need to lead on reducing greenhouse gas emissions, but action in developing countries will also need to be swift, with poorer countries requiring support for a rapid transition to a clean energy future.

While the Paris Agreement has been interpreted as heralding the end of coal and ushering in a new age of renewable energy, it doesn’t explicitly say those things. But the more important question is to ask how the agreement sets the stage for a global energy transition of the scale and speed required to hit the 1.5℃ target.

Article 4.1 of the agreement (see page 22) outlines how global emissions should peak “as soon as possible” and should decline rapidly thereafter, to “achieve a balance between anthropogenic sources and removals by sinks of greenhouse gases in the second half of this century”.

This ambiguous wording is intended to reflect the recommendation from the latest Intergovernmental Panel on Climate Change (IPCC) Assessment Report that emissions will need to go to zero and then below (“net-℃zero”) in the second half of the century.

The Paris Agreement’s accompanying decision text suggests a specific pathway for getting below 2℃ of warming, which involves reining in global greenhouse emissions to 40 gigatonnes in 2030, rather than the currently projected 55 Gt. It also suggests commissioning a special report from the IPCC to look at the numbers that would get us to 1.5℃.

The problem with these numbers is that for 40 Gt by 2030 to be consistent with a 2℃ temperature limit, this would require large volumes of carbon to be removed from the atmosphere later in the century (known as negative emissions). Getting beyond this to ensure that we hit a 1.5℃ is likely to rely on even higher volumes of carbon removal later in the century.

What are negative emissions?

Achieving negative emissions involves a form of geoengineering known as Carbon Dioxide Removal (CDR). Options for negative emissions include large-scale forest plantations, bioenergy crops with carbon capture and storage, or directly capturing carbon from the atmosphere. As well as technology limitations, these options are severely limited by the scale of land required.

Of the scenarios in the IPCC database with a 50% or greater chance of limiting warming to below 2℃, around 85% assume large-scale uptake of negative emissions. For 1.5℃, all scenarios rely on even larger volumes of negative emissions.

Relying on taking carbon out of the atmosphere later in the century brings a risk that we might delay action in the next few critical decades while waiting for the technology to catch up). This could result in runaway warming if the negative emissions options prove to be unfeasible or too expensive, or socially unacceptable.

What next?

Pathways for delivering the 1.5℃ goal will require unprecedented action. If we carry on burning fossil fuels at current rates, our likely chance of achieving 1.5℃ would be blown in just 6 years (a likely chance of 2℃ gives us 20 years of emissions at current rates).

Carbon Countdown The Carbon Brief

This highlights that, rather than incremental action, immediate and aggressive emission reductions are needed in rich nations, in order to keep the need for negative emissions options to an absolute minimum, or (for a 2℃ pathway), to avoid relying on carbon drawdown at all.

The special report to be produced in 2018 by the IPCC will offer an opportunity for a more informed debate on the level of negative emissions that might be feasible, and the level of action that will be needed over the coming decade in order to limit our reliance on drawing carbon back out of the atmosphere in the second half of the century.

Keeping global warming below 2℃ or 1.5℃ of warming over pre-industrial levels is still within reach, but it will require an honest and informed picture of the scale of the challenge, and a clear-eyed appreciation of the risks if we delay.