The 2021 report by Working Group 1 of the Intergovernmental Panel on Climate Change (IPCC), the 6th Assessment report (AR6), is a comprehensive assessment of the physical science associated with climate change.

Australian researchers in the ARC Centre of Excellence for Climate Extremes have made major contributions to this report, through the authorship of the report, review of the report and the many scientific papers cited in the report.

AR6 provides an assessment of thousands of scientific papers, each independently reviewed, and builds on earlier IPCC assessments in 1990, 1995, 2001, 2007, 2013, and three special reports released in 2018–2019.

Scientific advances have enabled many statements on the causes and effects of climate change to be made with increased confidence in AR6 compared with previous reports.

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The current state of our climate

  • Global surface temperatures are now about 1.1oC higher than in 1850-1900 with human activities explaining the observed warming. Climate change is already affecting people across the world, particularly through worsening weather and climate extremes.
  • Extreme heat and heatwaves have become more frequent and intense over both land1, 2 and the oceans3, 4. For Australia, warming over land has exceeded 1.4oC and consequently heat extremes have increased5.
  • Globally averaged rainfall has increased since 1950, and the frequency and intensity of heavy rainfall has increased over most land areas where we have good observations6, including Northern Australia7.
  • Human-induced climate change has increased droughts in some regions, including southern Australia.
  • The risk of extreme events acting simultaneously (compound events, eg. drought and extreme heat, conditions associated with extreme fire weather8) has increased in many regions, including Australia.
  • Based on multiple lines of evidence, new science has improved our confidence that equilibrium climate sensitivity is between 2.5oC-4.0oC, with a best estimate of 3oC9, 10.
  • The recent speed of global surface temperature increases and sea level rise are unprecedented over many thousands of years. Temperatures in the last decade are now warmer than at any time during the last 100,000 years.

CLIMATE MODELS
Global climate models are research tools based at their core on the laws of physics. To explore future climate, global climate models are run under a range of greenhouse gas emission scenarios.  The global climate models are rigorously evaluated and have generally improved since the last IPCC assessment, including in their simulation of many elements of the Australian climate11, 12, 13. Some of the models assessed in AR6 simulate warming due to increases in greenhouse gases that are lower than expected based on multiple lines of evidence, while some simulate more warming than evidence suggests14, 15. This has been widely reported in the scientific literature, and the causes of some models simulating too much warming has been traced to a cloud feedback that is too strong. Issues of this kind are taken into account when assessing future climate change. Moreover, the science community’s concerns over global warming are not based on climate models alone; they are also based on observations, paleoclimate reconstructions and supported by scientific understanding. The basic recognition that the Earth’s climate warms as greenhouse gases increase can be traced back to Fourier in the 1820s, Foote in the 1850s, Tyndall in the 1860s, Arrhenius in 1895 and Callendar in the 1930s. Robust projections for a given level of global warming allow assessments to be made independent of when that warming level is reached.

Our future climate and consequences for Australia

  • All regions, including Australia, will experience further increases in climate change impacts, and the frequency and intensity of these impacts increases with each increment of additional warming.
  • Global surface temperatures will continue to rise until at least 2050 under all emission scenarios. The amount of warming globally depends on the amount of future global greenhouse gas emissions and the ability to deploy large-scale greenhouse gas removal from the atmosphere. The same is true for warming of Australia (Figure 1).
Figure 1: Australian mean annual temperature changes 1850-2100 from climate model simulations (models that are thought to simulate warming above or below that expected based on multiple lines of evidence have been removed – see “Climate models” box). The observations are sourced from the Australian Bureau of Meteorology (ACORN-SAT v2).
  • Even under a very low emissions scenario (assuming global reductions in emissions from the 2020s reaching net zero emissions by 2050, and negative emissions thereafter) there is now less than a 50% chance of limiting warming to 1.5oC. If a very low emission future were achieved, temperatures could drop back to below 1.5oC by the end of the century (i.e. the warming is reversible).
  • Under medium and high emission scenarios, the 2.0oC ceiling for warming under the Paris Agreement will be exceeded by around 2050.
  • The effectiveness of natural carbon sinks into the ocean and land is expected to decrease in the second half of the century. Any reduced effectiveness of natural sinks would necessitate deeper cuts in human emissions to stabilise global temperatures.
  • The positive effects of a low emission future as distinct from a high emission future (e.g. reduced warming and smaller changes in many extremes) would become clearly evident within around 20 years, and are large by the end of this century.
  • Further increases in climate extremes, relative to those already observed16, are inevitable. The intensity and frequency of most extremes get worse at higher levels of warming17, 18. The increased intensity of heatwaves is larger than the associated increase in global average temperature, and increasing numbers of extreme events that are unprecedented in the observational record can be expected even if warming is limited to 1.5°C.
  • For Australia these changes imply increases in many climate extremes19 including increased risk of ecological and agricultural drought20, more intense short-duration rainfall21, less frequent but more intense tropical cyclones, intensifying land22 and marine heatwaves23, 24, and an increase in the intensity, frequency and duration of fire weather25.
  • For Australia, under a high emissions scenario, an increased vulnerability associated with sea level rise is projected, with the potential loss of 50-200 metres of shoreline along sandy coasts.
  • Figure 2 shows climate model estimates of changes in climate indices related to extreme heat and drought by the end of the century under a high emissions future.
Figure 2: Projections of changes in Australian climate extremes by the end of the century for a high emissions scenario, generated using a large number of climate models26.
  • A key risk to Australia is that rainfall variability associated with the El Niño-Southern Oscillation is very likely to increase in the second half of the century27, 28, leading to more very wet and very dry years. This risk is mitigated under very low emission scenarios.
  • A further risk to Australia is a continued southward (poleward) shift of the storm tracks29 which would lead to continued wintertime drying in southern Australia30. This risk is minimised in summer due to the offsetting effects of ozone recovery and reduced under very low emission scenarios.
  • Sea level rise will continue, but the speed of change is dependent on the emission scenario. While likely limited to 1 metre this century, an increase of 5 metres by 2150 cannot be excluded if very high greenhouse gas emissions continue.
  • Sea level rise is now irreversible even if warming is limited to 1.5°C, and will continue for centuries to millennia due to committed changes in the deep ocean and ice sheets.
  • Risks to Australia’s climate and coasts associated with ice sheet collapse and abrupt changes in ocean circulation, which would have catastrophic consequences, cannot be discounted and their chances of occurring increase under higher emission scenarios.
  • Ocean acidification and ocean deoxygenation will continue to worsen and are probably irreversible on timescales of centuries to millennium.

Actions required to achieve Paris targets

There is a near-linear relationship between the cumulative emissions of CO2 by humans and global warming. This allows a carbon budget to be defined for a given warming target, with the proviso that this budget depends on assumptions including climate system feedbacks.

  • To limit warming to 1.5oC with a high probability (80%), there is about 300 billion tonnes of CO2 left in the global carbon budget. This would be fully spent in 7–8 years at current rates of emissions.
  • Limiting warming to 2oC with a high probability (80%), there is about 900 billion tonnes of CO2 left in the global carbon budget (22 years at current rates of emission).
  • Large-scale deployment of carbon removal methods, if feasible, could extend these timelines, but have the potential to have wide-ranging effects on water availability and food production and will be partly compensated by changes in natural sinks.
  • Low and very low emission pathways that give a good chance of meeting the Paris Agreement temperature targets also assume negative emissions from the middle of this century, with more CO2 being drawn out of the atmosphere than is emitted.

Overall, the AR6 report is an important step in further solidifying our understanding of climate risk. The conclusions of the report echo strongly previous reports that near immediate and very deep cuts to greenhouse gas emissions are now overdue and any further delay commits us to an escalating risk of climate extremes. At a global scale, current commitments to reductions in emissions remain too small, and the reduction in emissions too slow, to avoid in excess of 2oC warming and multiple consequences to the climate of Australia31.

Contact

ARC Centre of Excellence for Climate Extremes: clex@unsw.edu.au

Written by Andrew Pitman, Nerilie Abram, Lisa Alexander, Julie Arblaster, Andrew King, Ian Macadam, Shayne McGregor, Sarah Perkins-Kirkpatrick, Steven Sherwood, Alvin Stone.

References

The ARC Centre of Excellence for Climate Extremes has contributed to a large number of scientific papers that support the conclusions of the report, a selection of which are listed below. Further details of some relevant issues can also be found in the Centre briefing notes also listed below.

1. Perkins-Kirkpatrick & Lewis, 2020, Increasing trends in regional heatwaves, Nature Communications, https://doi.org/10.1038/s41467-020-16970-7

2. Dunn et al., 2020, Development of an Updated Global Land In Situ-Based Data Set of Temperature and Precipitation Extremes: HadEX3, J. Geophysical Research, https://doi.org/10.1029/2019JD032263

3. See briefing note written by the ARC Centre of Excellence for Climate Extremes: Heatwaves in the ocean threaten marine ecosystems across the world

4. Oliver et al., 2018, Longer and more frequent marine heatwaves over the past century, Nature Communications, https://doi.org/10.1038/s41467-018-03732-9

5. Perkins-Kirkpatrick & Lewis, 2020, Increasing trends in regional heatwaves, Nature Communications, https://doi.org/10.1038/s41467-020-16970-7

6. Contractor et al., 2021, Changes in Observed Daily Precipitation over Global Land Areas since 1950, J. Climate, https://doi.org/10.1175/JCLI-D-19-0965.1

7.Dey et al., 2019, Investigating observed northwest Australian rainfall trends in Coupled Model Intercomparison Project phase 5 detection and attribution experiments, International J. Climatology,  https://doi.org/10.1002/joc.5788

8. Abram et al., 2021, Connections of climate change and variability to large and extreme forest fires in southeast Australia, Communications Earth & Environment, https://doi.org/10.1038/s43247-020-00065-8.

9. See briefing note written by the ARC Centre of Excellence for Climate Extremes: How sensitive is the Earth’s temperature to the amount of carbon dioxide in the atmosphere?

10. Sherwood et al., 2020, An Assessment of Earth’s Climate Sensitivity Using Multiple Lines of Evidence, Reviews of Geophysics, https://doi.org/10.1029/2019RG000678

11. Grose et al., 2020, Insights From CMIP6 for Australia’s Future Climate, Earth’s Future, https://doi.org/10.1029/2019EF001469

12. Deng et al., 2021, Evaluation of Extreme Temperatures Over Australia in the Historical Simulations of CMIP5 and CMIP6 Models, Earth’s Future, https://doi.org/10.1029/2020EF001902

13. Ridder et al., 2020, Do CMIP6 Climate Models Simulate Global or Regional Compound Events Skillfully? Geophysical Research Letters, https://doi.org/10.1029/2020GL091152

14. See briefing note written by the ARC Centre of Excellence for Climate Extremes: The latest global climate models present challenges for generating climate projections

15. Article in The Conversation written by the ARC Centre of Excellence for Climate Extremes: Yes, a few climate models give unexpected predictions – but the technology remains a powerful tool

16. Alexander & Arblaster, 2017, Historical and projected trends in temperature and precipitation extremes in Australia in observations and CMIP5, Weather and Climate Extremes, https://doi.org/10.1016/j.wace.2017.02.001

17. King et al., 2020, Global and regional impacts differ between transient and equilibrium warmer worlds, Nature Climate Change, https://doi.org/10.1038/s41558-019-0658-7

18. Perkins-Kirkpatrick & Gibson, 2017, Changes in regional heatwave characteristics as a function of increasing global temperature. Scientific Reports,  https://doi.org/10.1038/s41598-017-12520-2

19. Herold et al., 2021, Projected changes in the frequency of climate extremes over southeast Australia, Environmental Research Communications, https://doi.org/10.1088/2515-7620/abe6b1

20. Ukkola et al., 2020, Robust Future Changes in Meteorological Drought in CMIP6 Projections Despite Uncertainty in Precipitation, Geophysical Research Letters, https://doi.org/10.1029/2020GL087820

21. Fowler et al., 2021, Towards advancing scientific knowledge of climate change impacts on short-duration rainfall extremes, Phil. Trans. R. Soc. A., https://doi.org/10.1098/rsta.2019.0542

22. Perkins-Kirkpatrick & Lewis, 2020, Increasing trends in regional heatwaves, Nature Communications, https://doi.org/10.1038/s41467-020-16970-7

23. Misra et al., 2021, Projected Changes to Australian Marine Heatwaves, Geophysical Research Letters, https://doi.org/10.1029/2020GL091323

24. Holbrook et al., 2020, Keeping pace with marine heatwaves, Nature Reviews Earth & Environment,  https://doi.org/10.1038/s43017-020-0068-4

25. Abram et al., 2021, Connections of climate change and variability to large and extreme forest fires in southeast Australia, Communications Earth & Environment, https://doi.org/10.1038/s43247-020-00065-8.

26. Grose et al., 2020, Insights From CMIP6 for Australia’s Future Climate, Earth’s Future, https://doi.org/10.1029/2019EF001469

27. Perry et al., 2017, Future Changes to El Niño–Southern Oscillation Temperature and Precipitation Teleconnections,  Geophysical Research Letters, https://doi.org/10.1002/2017GL074509

28. Taschetto et al., 2020, ENSO Atmospheric Teleconnections, https://doi.org/10.1002/9781119548164.ch14

29. Goyal et al., 2021, Historical and Projected Changes in the Southern Hemisphere Surface Westerlies, Geophysical Research Letters, https://doi.org/10.1029/2020GL090849

30. Dey et al., 2019, A review of past and projected changes in Australia’s rainfall, WIREs Climate Change, https://doi.org/10.1002/wcc.577

31. See briefing note written by the ARC Centre of Excellence for Climate Extremes: Can we limit global warming to 1.5ºC?