Research update

ZERO IN ON: Near-term warming and our chances of staying within 1.5°C

To avoid the most dangerous climate change impacts, we need to raise global ambition and take urgent action to cut greenhouse gas emissions. 
Depending on how the climate system responds to emissions, warming could still be substantially higher – or lower – than our current best estimates. But one thing is clear, the sooner we cut emissions, the better chance we have of meeting our climate goals. 

Our new ZERO IN report shows how the chances of global temperature rise staying within 1.5°C this century could range from around 75% to less than 30%, depending on how the climate system responds. We therefore need to look at the range of temperature projections provided by climate models, rather than just single best estimates, when assessing our chances of keeping warming below a certain temperature.

With world attention focused on COP26 in Glasgow, there are many questions around how temperatures will change in the near future, and whether we still have a chance of meeting the Paris Agreement’s 1.5°C ambition – and avoiding the most dangerous climate change as a result.  The third ZERO IN report by the CONSTRAIN project shines a light on these issues, unpicking some of the science behind the headlines and high-level statements.

How could temperatures change in the next two decades?

We already know that limiting global warming to 1.5°C will require deep and rapid cuts in greenhouse gas emissions.  So, our climate future depends on decisions taken now to curb emissions, plus actions taken over the next few decades to meet those commitments.  

These actions determine not just how far, but also how fast temperatures will rise. And the faster temperatures rise, the harder it will be for us to adapt to the impacts of climate change.

The science tells us that temperatures will continue to rise until greenhouse gas emissions reach net zero, but also that before halting temperature rise we can also slow down warming: hard and fast emissions cuts in the next 20 years could also cut warming from CO2 emissions by half, compared to what we would see in a fossil-fuelled world.   And cutting emissions of non-CO2 greenhouse gases such as methane (CH4) will also play a key role in slowing down warming in the next two decades.  

Average decadal warming rates over the next 20 years (2021-2040) by CO2, non-CO2 greenhouse gases including CH4, aerosols and land-use reflectance, for five different Shared Socio-economic Pathways (SSPs) ranging from one that reflects sustainability (SSP1-1.9) to one that reflects fossil-fueled development (SSP5-8.5).

Temperature projections fall within a range…. 

With deep and rapid emissions reductions, the best estimate is that we will reach 1.5°C warming in the mid-2030s.  But behind that number lies a range of possibilities, including that temperature rise actually stays below 1.5°C.  

Why a range?  Our ability to model the climate is improving all the time but, given all its complexities, pinpointing exactly how the climate will respond to future emissions is simply not possible.   

This is partly because there are still questions when it comes to understanding and modelling the climate system, such as precisely how temperatures will respond to a long-term doubling of atmospheric CO2 concentrations (Equilibrium Climate Sensitivity or ECS), and the roles that aerosols (which reflect sunlight back into space) and permafrost (which releases carbon as it thaws) will play.  

Using a simple climate model, we show how these factors could cause the peak temperature we can expect to see this century to vary, even if we follow the same emissions pathway.   

…and we have a range of chances of staying within a given temperature limit

These questions about the climate system also affect our chances of staying below any given temperature limit, including the Paris Agreement’s 1.5°C ambition.   

In our simple climate model experiments using strong emissions cuts, our chances of staying below 1.5°C this century could range from around 75% to less than 30%.

“Climate wheels” showing the probabilities of staying below 1.5°C, 1.75°C, 2°C, 2.5°C, and 3°C in the 21st century for the different simple climate model experiments (±10% Equilibrium Climate Sensitivity (ECS), ±10% aerosol forcing strength, permafrost off) under the same stringent emissions reduction pathway that reaches zero fossil and industrial CO2 emissions around 2050.

This doesn’t mean that it will be harder to stay within 1.5°C than we thought – instead, it shows that, alongside different emissions pathways, complex climate processes could lead us to different climate futures.  

The report’s message is that overall, instead of focusing on a single estimate of future temperature change, we need to prepare for a range of eventualities.  The more we are aware of these eventualities, the better we can plan for what lies ahead, particularly by helping the most vulnerable nations and communities build resilience to climate impacts. 

But we already know the most important message: to avoid the most dangerous climate change, we need to raise global ambition and take urgent action on cutting emissions.

The full report is:

ZERO IN ON: Near-term warming and our chances of staying within 1.5°C.  The CONSTRAIN Project Annual Report 2021, DOI:10.5281/zenodo.5552389.