Near-term warming and our chances of staying within 1.5°C of global warming

Submitted by Debbie Rosen | published 29th Nov 2021 | last updated 21st Dec 2021
A man looking at the weather

A man looking at the weather.  Credit: Unsplash/Todd Trapini

Introduction

The annual ZERO IN reports by the CONSTRAIN project provide information on scientific topics that are fundamental to the Paris Agreement, as well as background and context on new developments at the science-policy interface.

This report includes new insights into the complex processes represented in climate models and what they mean for temperature change and other climate impacts over the coming decades. This third report also provides additional context and background on the latest IPCC report AR6 (2021) on the physical science basis of climate change, and addresses important questions around how likely we are to reach 1.5°C of global temperature increase.

The report shows how, even with strong emissions cuts, 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.

*Download the full report from the right hand column. The key messages from the report are provided below. See the full text for more detail.
 

Methodology

Climate model emulators are simple climate models or statistical methods, designed to reproduce the behaviour of complex Earth System Models (ESMs) without the same demands on computing time and power. To do so, they incorporate many aspects of the climate system, which can be adjusted within the emulator to explore how the climate system might respond to future emissions.

As emulators are simpler than ESMs, they can be run hundreds to millions of times with different values to explore a wider range of uncertainty, and produce future projections based on a larger set of emissions scenarios.

We used the climate emulators FaIR and MAGICC to look at how temperatures might change in future, both in the coming decades and by the end of the 21st century.  

What does the latest IPCC assessment say about near-term warming and our chances of staying within 1.5°C?

Limiting human-caused global warming to 1.5°C, in line with the Paris Agreement Long-Term Temperature Goal, requires deep and rapid reductions in greenhouse gas emissions. By the end of 2020, the world had experienced approximately 1.2°C of anthropogenic warming. Therefore the rate of global temperature increase over the next few decades is of key importance, not least because it greatly affects our ability to adapt to climate impacts.

The rate of near-term temperature change varies significantly across the five illustrative emissions scenarios assessed in the latest IPCC report, with warming clearly decelerating in scenarios with rapidly declining greenhouse gas emissions. While CO2 -driven warming is halved between the lowest and highest illustrative emissions scenarios, the overall warming contribution from CO2 continues to increase in all five over the next few decades, illustrating that cumulative CO2 emissions will cause further warming until net zero emissions are reached.


Warming contributions relative to pre-industrial levels (1850-1900) from groups of anthropogenic drivers, i.e. CO2 , non-CO2 greenhouse gases including CH4, aerosols and land-use reflectance, for 2020-2050.

Falling aerosol emissions, as a consequence of declining air pollution, also contribute to warming under four of the five emissions scenarios. Under the low and very low emissions scenarios, additional warming caused by declining aerosol concentrations will be largely compensated by cooling resulting from rapidly declining non-CO2 greenhouse gas emissions.

Under very low emissions, the best estimate is that we will reach 1.5°C warming in the mid-2030s. There is however still a chance that, if we implement strong emissions cuts, temperature rise in the coming decades might actually remain below 1.5°C.

How do uncertainties in the climate system response affect the likelihood of reaching the 1.5°C limit?

The climate system is highly complex and there are still uncertainties when it comes to how global temperatures will respond to rising greenhouse gas emissions. These include how temperatures will respond to a long-term doubling of atmospheric CO2 concentrations, and the effects on temperatures of both aerosols and carbon release from thawing permafrost.

Exploring these uncertainties using a climate emulator shows that they would clearly alter the peak temperatures we can expect to see this century. Such modelling experiments help us to understand how variations in complex climate processes lead to differences in projected warming under the same emissions pathway, highlighting the need to include and explain uncertainties when communicating temperature projections.

We also use the emulator to show how these uncertainties in the climate system can change the probability of staying within a given temperature limit, including the Paris Agreement Long-Term Temperature Goal. In our experiments, the probability of staying below 1.5°C varies from around 30% to just below 75% for the same emissions scenario.

Ultimately, given that they take into account the main uncertainties in the climate system, the temperature projections provided by climate models reflect a range of possible outcomes and should not be reduced to a single estimate which implies false certainty

Models give a range of temperatures projections, each with different chances of occurring

With deep and rapid emissions reductions, the best estimate by the Intergovernmental Panel on Climate Change 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 the simple climate model MAGICC, 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.   

Moreover, 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 (increasing and decreasing Equilibrium Climate Sensitivity (ECS) and aerosol forcing strength, turning the permafrost feedback off) under the same strong emissions reduction pathway.

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.  

 

Conclusions

The overall message is that, 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.