The IPCC’s 6th Assessment Report WG1 – findings and background

Introduction

This month the UN’s IPCC published the first part of its 6th Assessment Report (AR6). Entitled The Physical Science Basis (for Climate Change) it was written by the IPCC’s Working Group l (WG1), comprising 700 authors from 66 countries, and involving the assessment of 14,000 peer-reviewed studies up to 31 January 2021.

Quoting the IPCC, the report; “assesses the current evidence on the physical science of climate change, evaluating knowledge gained from observations, reanalyses, palaeoclimate archives and climate model simulations, as well as physical, chemical and biological climate processes”. 

There is actually no new science in AR6, it confirms what we already know and are seeing, but with more advanced methodologies and comprehensive data than was available for AR5 (2013). Consequently, there’s more granularity on the consequences and geographical impacts and its  opinions are more robust, with greater degrees of certainty, and without the cautious language of the past.

The report itself runs to 4,000 pages but the IPCC has issued a 40 page “Summary for Policymakers” (SPM) each line of which has been agreed by representatives of the 195 member governments.

Three more reports associated with AR6 are due next year:

• Working Group ll is set to publish Impacts, adaptation and vulnerability in February 2022. 
• Working Group lll is set to publish Mitigation of climate change in March 2022. 
• The IPCC will release a Synthesis Report, in September 2022, bringing together the findings of all three working groups.

Key findings

1. It is now unequivocal that humans have warmed the planet, causing widespread and rapid changes to Earth’s oceans, ice and land surface, with the present state of many parts of the climate system being unprecedented over many centuries to many thousands of years.
2. Many of these changes, particularly to the oceans, ice sheets and global sea levels, are irreversible and abrupt changes and “tipping points”, such as rapid Antarctic ice sheet melt and forest dieback, cannot be ruled out.
3. The links between human-caused warming and the increasing frequency and severity of  extreme weather events, is now established as fact.
4. With increased global warming previously rare “compound” extreme events, eg a heat wave followed by fire or flood, will become more frequent and intense with longer durations.
5. Nowhere on the Planet is safe from the impacts of global warming.
6. In almost all emissions scenarios, global warming is expected to hit 1.5C (The Paris target for 2100) in the 2030s and, without reaching net-zero CO2 emissions, along with strong reductions in other greenhouse gases, the climate system will continue to warm.
7. Near-term emissions cuts can reduce the rate of unprecedented warming, and net-zero will  work for stabilising or even reducing surface temperatures.

Models & Scenarios

The report uses the output from the latest generation of about 100 global climate models run by modelling groups around the world within five distinct scenarios (pathways) that describe how global society, demographics and economics might change in the future.

These recently developed scenarios, ​​known as Shared Socio-economic Pathways (SSPs), have been used, with the models, to develop projections of energy use, air pollution control, land use and greenhouse gas (GHG) emissions over this century but in the absence of new climate policies, beyond those already in place today.. 

For the purposes of the report the results were further categorised into bands of expected global warming effects (radiative forcing) at the end of the century, measured in Watts per square metre. This measurement represents the net amount of the sun’s energy being absorbed by the planet, ie the energy arriving from the sun, less the amount reflected back into space, say by atmospheric aerosols. 

The report’s scenarios (chosen from 4 of the 5 SSPs) and their effects can be briefly described as follows, with each one showing the original SSP (SSP1, SSP2 etc) and its radiative forcing at the end of the century (1.9, 2.6 etc):

• SSP1-1.9: ​After an initial overshoot, warming is held to approximately 1.5C above 1850-1900 in 2100 (2C is extremely unlikely to be exceeded) and implies net-zero CO2 emissions around the middle of the century.
• SSP1-2.6: 2C warming is unlikely to be exceeded, with implied net-zero emissions in the second half of the century.
• SSP2-4.5: Is approximately in line with the upper end of combined pledges from countries under the Paris Agreement. The scenario “deviates mildly from a ‘no-additional climate-policy’ reference scenario, resulting in a best-estimate warming around 2.7C by the end of the 21st century”.
• SSP3-7.0: A medium-to-high reference scenario resulting from no additional climate policy, with “particularly high non-CO2 emissions, including high aerosols emissions”.
• SSP5-8.5: A high reference scenario with no additional climate policy. Emissions this high are only achieved within the fossil-fuelled SSP5.

Whilst the authors are sceptical about the likelihood of SSP5-8.5, they say the projections “can still be valuable” and that the concentrations of greenhouse gases it contains “cannot be ruled out”.

The report says warming is very likely to be within ranges for each scenario so, for example, by 2081-2100, SSP1-1.9 has a range of 1.0-1.8C, SSP1-2.6: 1.3-2.4C and SSP5-8.5: 3.3-5.7C.

Here is a graph of all five pathways, using the report’s best estimates of target temperatures:

To put these temperature ranges into perspective the SPM points out (with medium confidence) that; “The last time global surface temperature was sustained at or above 2.5C higher than 1850–1900 was over 3m years ago.”

Given the weather extremes already being experienced at just over 1C and the uncertainty over the various tipping points, eg irreversible melting of land ice, ACT and most academics now consider SSP-1.9 the only, but still worst case, scenario worth considering.

Comparisons with the Paris Agreement & 1.5C

Following the Paris Agreement in 2015, and the pledge to “pursue efforts” to keep global warming below 1.5C (over pre-industrial levels), the IPCC published a special report on 1.5C (SR15) in 2018 that looked at questions including how close the world was to breaching the limit, what it would take to avoid doing so and what might happen if those efforts failed.

AR6 WG1 examines the first question and its findings are similar to SR15 however the SPM states that the predictions on when 1.5C is first breached are far more detailed in the AR6 than the SR15 and so are not directly comparable. 

Nevertheless, AR6 states that :

“In all scenarios assessed here, except SSP5-8.5, the central estimate of crossing the 1.5C threshold lies in the early 2030s.”

Carbon Budgets and Net-Zero

A carbon budget is an estimate of the maximum amount of CO2 emissions that can still be released into the atmosphere before a particular temperature increase is reached.  In accordance with the Paris Agreement the two key increases are 1.5C or 2C, since pre-industrial times (around 1870). Measurements are in Gigatonnes (one billion tonnes) of CO2 (GtCO2).

Temperature increases are roughly aligned with accumulating emissions and so scientists calculate the total emissions, that will result in say a 1.5C increase, and then deduct what has already been emitted since 1870, the difference is the remaining budget that we can emit before reaching the particular temperature threshold.

An important consideration is that, as we have already accumulated so much CO2, the amount left to emit is quite small and so is sensitive to differing assumptions in the calculations.  It’s a bit like filling a bath to just below the brim and trying to estimate how long, and with what force, we can leave the taps running before it overflows whilst incorporating an estimate of the loss of water from a badly fitted plug.

The modelling results and statements carry a degree of uncertainty which are expressed in terms such as “extremely likely” and associated with a % likelihood. In AR6 they calculate that a 460GtCO2 budget would give the world a 50% chance of limiting the increase to 1.5C and 360GtCO2 a 67% chance (The budgets for a 2C increase, with the same probabilities are 1,310GtCO2 and 1,110GtCO2 respectively).

The world is currently emitting about 40GtCO2 a year which is why there is an urgent need for “substantial and sustained reductions”, with the next decade being crucial. 

Scenarios are increasingly dependent upon the availability of facilities for CO2 removal (CDR), ie natural or industrial methods for removing CO2 from the atmosphere, and the report covers the effects of these in detail, including some negative side effects on land use, food production, water quality and biodiversity. 

The reliance on “net” emissions has grown in recent years with SR15 calling for Net-Zero by around 2050, in other words making sure that, by that date, what the world is emitting is cancelled out by what the world is removing. AR6 WG1 has not changed this.

Whilst AR6 WG1 concentrates on CO2, as the primary source of human caused warming, it also considers the effects of “non-CO2” GreenHouse Gasses (GHG) both in terms of those that warm and those that cool. Reducing some warming non-CO2 GHG will be possible but they will not be eliminated and so the report states that additional CO2 removals will be necessary to compensate for these.

Given the inherent uncertainties ACT believes that we owe it to future generations not only to strive to hit Net-Zero well in advance of 2050, by concentrating on emission reductions rather than significant CO2 removals, but also to use the carbon budget most closely aligned with achieving at least a 67% chance of limiting temperature increase to 1.5C.

Other aspects

The report goes into detail over many other effects on the climate and environment, covering rainfall, ice cover, oceans (including sea level rise), tipping points and air pollution and, for the first time, has  “a far greater emphasis on regional climate change” including an online interactive atlas. 

These aspects are reviewed in detail in the following links:

IPCC AR6 WG1 Summary for Policymakers the SPM

Carbon Brief in depth Q&A on AR6 WG1 (the source of much of this article)

YouTube “Just have a think” summary

YouTube discussion between Dr. Alison Green, Sir Robert Watson and Dr. James Dyke

Article by Professor Simon Lewis on Fossil Fuels and AR6National Geographic article