Response: Predicting the future – climate models
Image: Caltech
Models used by the IPCC
Since 1992, several models to predict future scenarios have been used by the IPCC, each improving on previous versions.
- 1992: six Future Scenarios were used in the IPCC second assessment report
- 2001 and 2007: six Special Report on Emission Scenarios (SRES) were used in the IPCC Third (TAR) and Fourth (AR4) assessment reports (Fig. 4).
- 2013/2014: four Representative Concentration Pathways (RCP) scenarios used in the IPCC Special Report on Emission Scenarios (AR5) assessment report (Fig. 3)
- 2022: Nine forcing scenarios being developed for the upcoming IPCC Sixth Assessment Report (AR6) are based on Special Report on Emission Scenarios (SSPs).
Interpreting IPCC graphs
Emission scenarios used in earlier IPCC reports.
These considered what the climate would be like based on different scenarios: how much greenhouse gasses we might emit and what priorities we gave to the environment vs the economy, called ‘Representative Concentration Pathways’ or ‘RCPs’ (see Fig 9. for a more detailed explanation).
These pathways were presented on graphs as an alphanumeric code: A2, AB1, etc (Fig. 2). Figure 9 explains what these mean, and why different emissions pathway ‘storylines’ were considered.
In the Fifth Assessment Report 2013/2014, Representative Concentration Pathways (RCPs):
The represent the concentration of greenhouse gases in the atmosphere based on how these gases retain heat.
- Heat is measured in watts per metre squared, written as W⋅m−2
- In most graphs, the numbers 2.6, 4.5, 6.0, and 8.5 are W⋅m−2 however W⋅m−2 is implied, and the four units are written as four scenarios: RCP2.6 etc. (for example, Fig. 3).
- In some graphs, this is written without a decimal place: RCP26, RCP45 etc.
Some graphs show the Representative Concentration Pathways (RCPs) over time, based on the emissions of fossil fuels. The coloured lines in Figure 4, for example, show dozens of model runs for the different RCPs and the potential temperature range for each scenario. Temperatures are shown as a range rather than an exact figure, because complex feedback effects and the temperatures at which irreversible tipping points are breached is uncertain.
Note: these are average global temperatures; the tropics are not warming as rapidly while the Arctic is warming much faster.
Shortcomings and criticisms
The complexity of climate change, the careful pace at which research is reviewed by other scientists before being published, the assumptions that governments would act jointly to reduce carbon emissions, and the limitations of modelling complex global systems about which we still have only limited knowledge, means that models are limited in terms of how well they can predict the future (Video 1).
This was certainly the case for the 2007 Fourth Assessment Report (AR4). The modelling for sea-level rise, for example, was based in part on NIWA generated reports published six years earlier in 2001, themselves based in part on 2001 IPCC projections based on twentieth-century understanding (or lack) of how quickly ice sheets could melt.
The worst-case scenario, A1F1, was considered least likely, in part because it was assumed that governments would act to reduce carbon emissions.
At the 2009 Climate Change Conference (Copenhagen Summit or COP15), unequivocal evidence was presented showing that the worst-case scenario for rising sea levels were being met or exceeded (Fig. 5), while Arctic Sea ice loss (see here for why this is so important) was literally falling off the charts (Fig. 6).
The Summit concluded that unless governments acted to reduce carbon emissions the planet is committed to temperature increases of 3–7°C by 2100, with 7°C most likely if the projected temperatures match the upper limit of A1F1 sea-level predictions.
In defence of the models that resulted in these graphs, the AR4 IPCC report clearly stated that these sea level predictions largely excluded contributions from melting ice caps because ‘the physics of ice sheet dynamics was not sufficiently well understood’.
In short, they included a disclaimer, one that was almost universally ignored in the media and by politicians and policy makers.
The basis for higher projections of global mean sea level rise in the 21st century has been considered and it has been concluded that there is currently insufficient evidence to evaluate the probability of specific levels above the assessed likely range. – IPCC 2013
Older models
The models used by the IPCC (2013/2014), are the basis for the 2015 Paris Climate Accord and also based on assumptions that have not come to pass:
- Technology to capture and store carbon underground will be invented and widely used (spoiler: it’s not).
- Governments and society would recognise the peril and act to drastically reduce greenhouse gas emissions (spoiler: we haven’t).
And crucially:
- Do not take into considerations dangerous tipping points (already underway) such as melting permafrost and methane clathrates leading to irreversible climate change, and the rapid disintegration of Greenland and Antarctic ice sheets leading to faster rising sea levels. The reason is because the models can’t assess the probabilities.
New generation of models
New models contributed to the 6th Coupled Model Intercomparison Project (CMIP6), which informed the Sixth Assessment Report of the IPCC. However, these new models still cannot include data, for example, on the speed at which ice shelves are disintegrating and ice sheets are melting, and therefore these cannot be included in projected modelling of sea level rise.
