New Scenarios Show How the World Could Limit Warming to 1.5C in 2100

A paper in Nature Climate Change presents the results from a new modeling exercise using six different “integrated assessment models” (IAMs) to limit global temperatures in 2100 to below 1.5C.

The results suggest that 1.5C is achievable if global emissions peak in the next few years and massive amounts of carbon are sucked out of the atmosphere in the second half of the century through a proposed technology known as bioenergy with carbon capture and storage (BECCS).

Defining the 1.5C target
One challenge with the goal of limiting warming to 1.5C above pre-industrial levels is that it was not clearly defined in the text of the Paris Agreement.  For example, scientists disagree on what, exactly, pre-industrial temperatures were and how best to define them, as well as what dataset to use.

There is also not a clear consensus if the target should be to aim to have even odds of the world reaching 1.5C warming by 2100, or seek to try and avoid having temperatures exceed 1.5C by aiming for an even lower warming amount.  Because uncertainties in climate sensitivity mean that we could have anything between 1.5C and 4.5C warming per doubling of CO2 emissions, scientists tend to plan to avoid the worst case where climate sensitivity ends up being on the higher end of the range.

In the case of the 2C target, the Paris Agreement’s “well below” language has been interpreted as ensuring that there is no more than a 33% chance of exceeding 2C – and, therefore, a 66% chance of staying below it.  But the 1.5C target could be interpreted as either aiming for a 50% chance of staying below 1.5C, or a 66% chance similar to the 2C target.  This may sound like a small distinction, but it has large impacts on the resulting carbon budget and ease of meeting the target.

In their new paper, a team of 23 energy researchers choose the stricter interpretation of the target, aiming for a 66% chance of avoiding more than 1.5C warming in the year 2100.  However, they allow for temperatures to exceed 1.5C over the course of the century as long as they fall back down to below 1.5C by the year 2100.  This is known as an “overshoot” scenario.

1.5C only possible in some future pathways
To assess viable pathways to limit warming to 1.5C, the researchers use the new Shared Socioeconomic Pathways (SSPs) developed in preparation for the next Intergovernmental Panel on Climate Change (IPCC) assessment report due early next decade.  These SSPs – which Carbon Brief will explore in more depth in the coming weeks – present five possible future worlds that differ in their population, economic growth, energy demand, equality and other factors.

Each world could have multiple different climate trajectories, though some will have a much easier time reducing emissions than others.  The new climate trajectory associated with avoiding more than 1.5C warming in 2100 is called Representative Concentration Pathway 1.9 (“RCP1.9”), which is a world where the radiative forcing from greenhouse gases is limited to no more than 1.9 watts per meter squared (W/m2) above pre-industrial levels.  This is lower than the range of RCPs previously used by climate modellers, which went from 2.6 up to 8.5W/m2.

The six IAMs all find viable 1.5C scenarios in SSP1, which is a pathway that focuses on “inclusive and sustainable development”.  Four of the six models find pathways in SSP2, which is a middle of the road scenario where trends largely follow historical patterns.  No models show viable 1.5C pathways in SSP3, which is a world of “regional rivalry” and “resurgent nationalism” with little international cooperation.

Finally, only one of the models has a 1.5C pathway in SSP4, which is a world of “high inequality”, while two models have viable pathways in SSP5, a world of “rapid economic growth” and “energy intensive lifestyles”.

Emissions must peak quickly
To limit warming to below 1.5C, all the models that the researchers examined require that global emissions peak by 2020 and decline precipitously thereafter.  After 2050, the world must reduce net CO2 emissions to zero and emissions must be increasingly negative throughout the second half of the 21st century.

Even with these rapid reductions, all the scenarios considered still overshoot 1.5C warming in the 2040s, before declining to around 1.3-1.4C above pre-industrial levels by 2100.  Models with more rapid reductions – generally associated with SSP1 – have less temperature overshoot than those with more gradual reductions.

Read more at New Scenarios Show How the World Could Limit Warming to 1.5C in 2100