IPCC Sixth Assessment Report, Volume 1: The Physical Science Basis

Annette Cowie, NSW Department of Primary Industries

The Intergovernmental Panel on Climate Change (IPCC) released the first of three volumes that comprise its Sixth Assessment Report (AR6) on 9 August. The first volume Working Group I – The Physical Science Basis presents recent evidence for climate change, current understanding of the climate system, and future climate projections. The full report (over 1000 pages!) and summaries are available here.  

The report’s findings should come as no surprise: the evidence of climate change is stronger than ever. The report was described by UN Secretary-General António Guterres as “a code red for humanity”.  He went on to say “The evidence is irrefutable:  greenhouse‑gas emissions from fossil-fuel burning and deforestation are choking our planet and putting billions of people at immediate risk.  Global heating is affecting every region on Earth, with many of the changes becoming irreversible.”

Human influence has warmed the Earth at a rate that is unprecedented in at least the last 2000 years. Global average temperature over the past ten years was about 1.1°C warmer than in 1850-1900. Each of the past four decades has been the warmest on record since preindustrial times. There has been a larger average temperature increase over land (almost 1.6°C) than over the ocean (almost 0.9°C), and Australia has warmed by 1.4°C.

There is a greater than 50% chance that 1.5°C of warming will be reached in the early 2030s.This is earlier than projected in the 2018 IPCC Special Report on 1.5°C of warming, due to more accurate understanding of the temperature sensitivity to atmospheric greenhouse gas (GHG) concentrations, and updated data on historical warming. 

Unless there are immediate, rapid, and large-scale reductions in GHG emissions, limiting warming to 1.5°C will be beyond reach. But with rapid reductions of GHG emissions to reach net zero CO2 emissions around 2050, it is likely that global warming can stay below 2°C, probably below 1.6°C, and could decline to below 1.5°C by the end of the century. 

If global temperatures rise above 1.5°C then later decline, some changes can be reversed, but others, such as rise in sea-level, loss of ice-sheets, and release of permafrost carbon, would take centuries to millennia to reverse.
 
While CO2 is the dominant greenhouse gas, and reaching net zero CO2 will be necessary to limit global warming, strong reductions in other greenhouse gas emissions are needed, especially methane.

Failure to limit warming to 1.5°C will lead to climate impacts that are particularly severe for Australian agricultural systems. At 2°C of global warming, heat extremes would more often reach critical tolerance thresholds for agriculture and health. Even at 1.5°C warming, rainfall in southern Australia, which has already decreased significantly in the southwest, is expected to decrease, particularly in the winter cropping season, increasing aridity and incidence of droughts. In northern NSW and south east Queensland, a decrease in winter rainfall and more extreme rainfall events are expected.  In northern Australia, where increase in annual mean and heavy rainfall and decrease in droughts and tropical cyclones have already occurred, more heavy rainfall and flooding is expected. In future, cyclones are expected to decrease in frequency but increase in severity.

Massive amounts of CO2 will need to be removed from the atmosphere to reach net zero: 1–2 GtCO2 per year from 2050 onwards to as much as 20 GtCO2 per year, under different emission scenarios (Chapter 4). However, natural processes that sequester CO2 – the sinks associated with forests and the ocean– are weakening due to the effects of climate change.  Increasing drought will limit the growth of forests, and warmer temperatures will enhance breakdown of soil organic matter, releasing CO2.  The carbon dioxide removal (CDR) strategies anticipated are afforestation, bioenergy with carbon capture and storage (BECCS), soil carbon management, biochar, direct air capture, enhanced weathering and ocean alkalinisation. 

The report provides updated figures for the 100-year global warming potential (GWP), the metric used to convert emissions of the various greenhouse gases to CO2-equivalents. The new values are: 29.8 for fossil methane; 27.2 for non-fossil methane, and 273 for nitrous oxide (Table 7.15). Chapter 7 also presents several alternative metrics, including global temperature change potential (GTP) and GWP*, which give substantially different relative values for methane, due to its short atmospheric lifetime compared with CO2. 

New features in the AR6 include regional assessments of climate change, which are summarised in regional fact sheets, including a  factsheet for Australasia. Another novel feature is the Interactive Atlas which allows the user to explore effects of different levels of warming on temperature and rainfall.

This first volume of the AR6 does not discuss climate change mitigation measures in detail - that is the topic of Volume 3. But it does spell out the urgent need for deployment of all measures that reduce fossil fuel emissions. Bioenergy could play a key role in supporting the decarbonisation of Australia’s electricity supply. Other opportunities for abatement come from use of biomass residues for biogas, which can also avoid methane emissions, and production biofuels to displace fossil fuels in hard-to-abate applications such as aviation.  Indeed, the IPCC scenarios that limit warming to 1.5°C or 2°C assume a key role for bioenergy, as a renewable fuel source and in conjunction with CCS (BECCS), to deliver CDR, based on purpose-grown feedstock.  However, the potentially large scale of energy crops is noted as having significant implications for land and water requirements, as well as risks to food security and biodiversity. A challenge for bioenergy, discussed in detail in Volume 3, is the development of land use systems that integrate biomass production with agriculture and forestry.

The second and third volumes of the AR6, Working Group II – Impacts, adaptation and vulnerability and Working Group III – Mitigation of Climate Change, are due to be released in February and March 2022, respectively.