The Intergovernmental Panel on Climate Change (IPCC) has provided a range of possible scenarios for global warming by 2100, depending on greenhouse gas emission rates. The amount of median global warming predicted is seemingly small – only a few degrees separate the most optimistic from the most pessimistic scenario – so it is important to understand how even small changes in average temperatures have enormous consequences for the planet:

What warming means

1.5°C warming: The Paris Agreement has set a goal of limiting warming to less than 1.5°C above pre-industrial levels – achieving this goal will require rapid and significant reductions in greenhouse gas emissions. Even if we succeed in limiting warming to this level, there will still be significant impacts on the planet, including sea-level rise, more frequent heatwaves, and increased intensity of some extreme weather events. However, limiting warming to 1.5°C would still reduce the risk of many of the most severe impacts of climate change, such as widespread food and water shortages.

2°C warming: This is often considered the “dangerous” level of warming, as it would have significant impacts on human societies and natural systems. At this level of warming, there would be an increased risk of droughts, floods, and extreme weather events, and many species would be pushed to the brink of extinction. There would also be significant impacts on human health, with more heat-related illnesses and diseases spread by insects and other vectors.

3°C warming: If emissions continue at their current rate, we are likely to reach 3°C warming by 2100. At this level of warming, there would be significant impacts on the availability of water and food, with many regions facing water shortages and reduced crop yields. The frequency and intensity of extreme weather events would increase, leading to more frequent and severe droughts, floods, and storms. Sea levels would also continue to rise, threatening coastal communities. 

4°C warming or more: This level of warming would be catastrophic for the planet, with widespread impacts on ecosystems, food and water security, and human societies. At this level of warming, many regions would become uninhabitable due to extreme heat, and there would be widespread food and water shortages. There would also be a significant increase in the number of species that become extinct, as well as an increased risk of major global conflicts over resources.

Forecast Scenarios

Estimating future warming is a very challenging task.  The first part of the task is to come up with scenarios about how humanity will or will not work together in reducing emissions,  how economies will or will not grow, and how technology will develop.  The IPCC calls these shared socio-economic pathways and that is why all their forecast names start with the abbreviation “SSP”. The numbers after the dash refer to the amount of radiative forcing in the scenario by 2100 – a measure of the heat-trapping capacity of the atmosphere once it is thickened by increased concentrations of GHGs.

In their current assessment report, the IPCC highlights five forecasted potential pathways for the planet.

1. SSP1-1.9 – “Paris Agreement”: This scenario assumes a future where there is a rapid transition to a low-carbon economy, with strong international cooperation and ambitious climate policies leading to a decline in greenhouse gas emissions by mid-century. This scenario results in a global temperature rise of around 1.5°C above pre-industrial levels by the end of the century.
    
2. SSP1-2.6 – “Sustainability”: This scenario also assumes a future with strong climate policies and a transition to a low-carbon economy, but with a slower pace of change than SSP1-1.9. This scenario results in a global temperature rise of around 2.0°C above pre-industrial levels by the end of the century.
    
3. SSP2-4.5 – “Middle of the Road”: This scenario assumes a future where there is a moderate pace of economic and technological change, with some climate policies and moderate greenhouse gas emissions reductions. This scenario results in a global temperature rise of around 2.4°C above pre-industrial levels by the end of the century.
    
4. SSP3-7.0 – “Regional Rivalry”: This scenario assumes a future where geopolitical tensions and regional rivalries lead to slower economic growth and technological progress, with high greenhouse gas emissions and a global temperature rise of around 3.7°C above pre-industrial levels by the end of the century.
   
5. SSP5-8.5 – “Fossil Fuel Development”: This scenario assumes a future with rapid economic growth and technological progress, but also high levels of greenhouse gas emissions from fossil fuel use, leading to a global temperature rise of around 4.0°C above pre-industrial levels by the end of the century.

Best Estimate and Ranges

Forecasting how the climate will precisely change in response to a certain level of global emissions, over the next 80 or so years is a very complex task. That’s why the IPCC expresses their forecasts as a very likely range – which means that for a particular emissions forecast, the IPCC has a 90% to 100% confidence level that the global temperature increase will fall into that range.

But it is the best estimate temperature that is usually reported in headlines. Recognize that the best estimate is the median value of the forecast scenario – meaning that for a given emissions level, it is only 50% likely that warming will be limited to the forecasted best estimate temperature. It is just as likely that the temperature will fall into the higher end of the range.

Said another way, hitting the emission targets set out in the Paris Agreement means we have a 50% chance to limit warming to 1.5˚C. For the same level of emissions, the IPCC estimates warming could be as high as 1.8˚C by 2100. This range of uncertainty should drive us even more urgently to achieve the lowest level of emissions we can.

Timescales: How hot by when?

In all forecast scenarios, even the most optimistic, the world will continue to warm until at least 2050.  What happens beyond that depends largely on how we pull together to reduce future emissions – and whether technology can be developed and deployed that will pull excess CO2 from the atmosphere and sequester it in the ground.  Warming will exceed 1.5˚C and 2.0˚C by 2100 unless deep reductions in CO2 and other GHGs occur in the coming decades.  

Near term – 2021 – 2040: There is not a big difference between the various scenarios. Given my age, this is the world that I will experience – as will most policymakers and oil execs who are currently over 50 years old.

Mid Term – 2041-2060: Differences between the scenarios become important.  The two low-case scenarios remain in the 1.6˚C – 1.7˚C range  – the other 3 scenarios however continue growing to reach 2.0˚C to 2.4˚C degrees of warming.  Remember that even though the differences seem small, this level of warming is described by the IPCC as dangerous. Climate events such as droughts, floods, extreme storms, and fire weather will become more commonplace and increasingly threaten the well-being of humanity. This is the world that my kids will be inheriting.

Longer term – 2081 to 2100: By the end of the century global temperatures in the three status quo scenarios continue to grow, reaching 2.7˚C to 4.4˚C degrees of warming.  The upper end of the likely range in the most pessimistic scenario gets up to nearly +6˚C. This is the world we risk leaving to our grandchildren.

What the Forecast Scenarios Don’t Include

The IPCC forecasts are largely focused on elements of the climate model that are relatively well understood and predictable.  Scientists caution that there are potential events and phenomena that are either unpredictable or are not well enough understood, that if they were to occur, might completely upend all predictions – and not for the better. 

Some of the phenomena that are not well enough understood to be fully reflected in forecasting include:

• Fluctuations of greenhouse gas releases from wetlands
• How melting permafrost might produce large releases of methane
• Increased incidents of wildfires, releasing more CO2 and reducing the capacity of forests to act as carbon sinks
• The potential changing climate conditions to produce an overall dieback of forests, also resulting in reduced carbon uptake capacity and increased CO2 release.
• The impact of any large volcanic eruptions in the next 100 years
• The reduced global ice and snow surface, decreasing albedo, (reflection of sunlight back to space) further increasing the absorption of solar radiation and warming.

Finally, while the IPCC reports are frightening – both for their conclusions and for the clinical, purely factual manner in which they are described – one of the biggest concerns may be that they are overly conservative in their predictions.

Some observers have noted that criticism of prior assessments may have led the IPCC to even greater conservatism in their more recent assessments. 

It has also been reported that in the wrangling over how to characterize the description of various phenomena, some scientists from nations with a greater interest in fossil fuel production have influenced the character of some of the conclusions in the reporting.  For these reasons,  readers should recognize that these reports may err on the side of understatement.

Choosing Paris

The implications for the planet of the various degrees of warming forecasted by the IPCC by 2100 are frightening. Limiting warming to 1.5°C or 2°C will require significant and immediate action to reduce greenhouse gas emissions. Allowing warming to continue at its current rate or beyond will have catastrophic consequences for the planet.

The good news is that we do have the tools and technology today to realize the emission goals of the Paris Agreement. It is also good news that leaders around the world have begun to implement the policies required to transition the global economy away from fossil fuels – so much so, that most observers now believe that we are on track to at least avoid the very worst of the IPCC forecast scenarios.

But to limit warming to less than 1.5˚C, the breadth and pace of change need to increase rapidly. We all need to let our leaders know that we need them to do more and to do it faster.

Frankly, my dears, we all need to give a damn. ■