This statement provides a brief summary of climate change and the role of human action in causing this change, with a particular focus on Australia.[1] It has been compiled by atmospheric and oceanographic scientists, reviewed by members of the Australian Meteorological and Oceanographic Society (AMOS), and approved by the AMOS Council as an official AMOS Position Statement on 2 November, 2021. This AMOS statement will expire 5 years from its approval, or earlier as determined by AMOS Council.

The paper was developed following the release of the IPCC 6th Assessment Report in 2021. It was circulated for review and consideration by the Royal Society of Victoria’s membership in August 2022 as notice of the Society’s intention to endorse this AMOS Position Statement as the Society’s Official Position. In line with the Society’s Rules and By-Laws, this was adopted as the Society’s Official Position by the Society’s governing Council in September 2022. This paper supersedes the Society’s earlier positions on this matter.

Key points:

• Global climate is changing rapidly due to human activities, with global mean temperatures having increased by about 1.1°C since the second half of the 19th century. For Australia specifically, temperatures have warmed by more than 1.4°C since national records began in 1910.
• Warming is already leading to dramatic changes in the global and Australian climate, with impacts on ecosystems and most aspects of human health, economic activity and wellbeing. These impacts will intensify with future warming.
• Australia is highly vulnerable to the impacts of climate change, with many regions expected to experience intensified droughts or floods, increased heatwaves and extended bushfire seasons as well as increased coastal erosion and inundation due to sea level rise. Ocean warming and acidification will threaten coral reefs and other marine ecosystems.
• Climate projections indicate that significant, urgent and sustained reductions in greenhouse gas emissions and fossil fuel production are required to limit global warming to the Paris Climate Agreement targets of well below 2°C above pre-industrial temperatures with efforts to limit the temperature increase to 1.5°C.
• Delays in reducing emissions will increase the practical and economic costs of avoiding dangerous climate change and place a greater burden on future generations to adapt to higher levels of warming.

Climate science provides rigorous evidence

Climate science is based on the scientific method, using rigorous and thorough comparison of observations and theory underpinned by the independent peer review process. There is an ongoing effort of refinement as new data are collected and technologies developed. Australian climate scientists, including AMOS members, conduct research and assessments as part of global efforts to improve and refine understanding of how the climate system operates, including interactions between the atmosphere, ocean, land and cryosphere (frozen water on Earth’s surface). The availability of high-quality climate data, the methodologies employed in data analysis and climate assessments, and climate models have improved over the past 50 years, such that we now have a high degree of confidence in the findings from climate science. While some uncertainties persist in the climate system response to human influence, there is high confidence in most of the information summarised in this statement. Quantifying remaining uncertainties will help further inform decision making.

Global climate has changed substantially since 1850

Global warming due to human-induced greenhouse gas increases is real and observable and the rate of warming has been the largest in recent decades. Each of the last four decades has been successively warmer than any previous decade since 1850, with the global mean surface temperature of the Earth for 2011-2020 1.1°C above the 1850–1900 average (which is generally used to represent the preindustrial period). The years 2020 and 2016 were the equal warmest years on record, while the six years from 2015 to 2020 were the warmest six years on record.

A summary of major observed climate changes include:

• • Since the mid-20th century, increasing temperatures have been observed throughout the Earth system, including over land and in the oceans, in rural areas and cities, at the surface and in the lower atmosphere.
  • Increases in the frequency of warm temperature extremes and heatwave events and decreases in the frequency of cool temperature extremes have accompanied the rise in mean temperatures over most areas.
  • There has also been a decrease in the number of frosts, a rapid contraction of almost all alpine glaciers, significant mass loss of the Greenland and West Antarctic ice sheets and a reduction in Arctic sea ice and global snow cover.
  • The absorption of carbon dioxide by the oceans has reduced near-surface pH by approximately 0.1 units compared to pre-industrial levels – a process known as ocean acidification.
  • In the centuries prior to 1850, the rate of sea level change was only a few tenths of a mm per year. Since then, sea level rise has accelerated to reach a rate of 3.7 ± 0.5 mm per year over the period 2006-2018, with a net rise in global average sea level of 20 cm from 1901 to 2018.[2]

Australia’s climate is changing

Temperatures

Since the introduction of robust instrumental surface temperature measurements in the early 20^th century, the mean surface temperature of Australia has increased by about 1.4°C, larger than the global average increase. The warming has been concentrated in the post-1950 period with over 1°C of warming since 1960. Warming is observed in all months with both day and night‑time temperatures increasing. Based on records to 2020, Australia’s warmest year on record was 2019, and the eight years from 2013 to 2020 all rank in the ten warmest years since at least 1910.

The warming over Australia has been accompanied by marked changes in the frequency of extreme temperatures at a variety of timescales, with warm extremes generally becoming more frequent and cold extremes less frequent. There have been an increased numbers of individual hot days and extreme warm months and decreased numbers of cool nights and extreme cool months. There has also been an increase in the frequency, intensity and duration of heatwaves in many parts of the country.

Ocean

Sea surface temperatures in the Australian region have increased by more than 1°C since 1900. Since 1993, sea level has risen by 2–4 mm per year over much of the southern coastline of Australia, in line with the global average. Northern coastlines of Australia experienced sea-level rise more than twice the global average since 1993, though much of this enhanced rise may be related to natural variability. Sea level extremes that result in episodic coastal flooding have increased in frequency on the east and west coasts of Australia.

Rainfall

Rainfall in Australia is highly variable from year to year, and from region to region. Nevertheless, long term trends are evident in Australia’s regional rainfall.

In the southwest and southeast there has been a trend towards drier conditions since the 1970s, especially for the cool season (April to October). In 17 of the last 20 years, rainfall in southern Australia in these months has been below average. The trend is particularly strong for the period from May to July over southwest Western Australia, with rainfall since 1970 around 20 per cent less than the 1900-1969 average. For the southeast of the continent, April to October rainfall for the period 2000 to 2019 has decreased by around 12 per cent when compared to 1900–1999. The reduction in cool season rainfall in the southwest and southeast of Australia has led to an increase in droughts in these regions.

In contrast with southern Australia, average northern Australian rainfall has exhibited an increasing trend since the 1970s across all seasons, especially in the northwest during the northern wet season (October to April). However, year-to-year rainfall variability remains high, with, for example, below average rainfall totals in northern Australia during both the 2018–19 and 2019–20 wet seasons.

Observations indicate that short-duration (hourly) rainfall events have become around 10 per cent more intense in some regions in recent decades, with larger increases seen in the north of the country.

Tropical cyclones

Tropical cyclone activity in Australia’s cyclone region varies substantially from year to year. This is partially due to the influence of oceanic conditions and large-scale climate drivers; the number of cyclones in our region generally declines during El Niño events and increases during La Niña events. There has been a downward trend in the number of tropical cyclones observed in the Australian region since 1982. The historical trend in tropical cyclone intensity is harder to quantify due to large natural variability and limited observations.

Human influence has increased greenhouse gases leading to global warming

The physical role of greenhouse gases in the atmosphere has been understood for more than a century. Shortwave radiation from the Sun passes through the atmosphere and is absorbed by the Earth’s surface which thereby warms. The surface emits infrared radiation. In the absence of greenhouse gases, this radiation is emitted direct to space, and approximately balances the solar radiation reaching the Earth from the Sun. But the greenhouse gases (water vapour, carbon dioxide, methane and other gases present in small amounts in the atmosphere) absorb some of the infrared radiation emitted from the surface. These greenhouse gases also emit infrared radiation in all directions, including back to the Earth’s surface thereby increasing the warming of the surface. The warm surface causes the overlying atmosphere to warm, through convection, conduction, and radiation. The surface temperature, and the temperature of the lower atmosphere, increases as the atmospheric concentration of the greenhouse gases increases, because of these processes.

Human activities have increased the concentration of greenhouse gases in the atmosphere since 1750, and it is now certain that this human influence has warmed the atmosphere, ocean and land.

The warming associated with increases in greenhouse gases originating from human activity is called the enhanced greenhouse effect. The average atmospheric concentration of carbon dioxide exceeded 410 ppm in 2019, higher than at any time in at least the last 2 million years. Atmospheric concentrations of methane and nitrous oxide (other major greenhouse gases) are higher than at any time in at least 800,000 years.

The increase in atmospheric carbon dioxide is a direct result of burning fossil fuels, large-scale deforestation and other human activities. Concentrations of a range of other greenhouse gases, such as methane, nitrous oxide and CFCs, have also contributed to the observed warming.[3] Some other by-products of human activity, most notably industrial aerosols4, have had a net cooling effect on the atmosphere, offsetting some of the warming from the enhanced greenhouse effect.[4]

Increased greenhouse gas concentrations due to human activities have led to warming of each of the inhabited continents, including Australia. As well as a direct link between increases in greenhouse gases and mean temperatures, evidence increasingly suggests that human activities have also substantially increased the risk of very hot years and seasons at the continental scale, for example the Australian record hot year of 2019. Human influence on ocean warming and sea level rise is also clear.

Future changes to global climate depend on amount of greenhouse gas emissions

While some future increase in global temperature is unavoidable due to past and current greenhouse gas emissions, the magnitude and rate of this warming is highly dependent on our greenhouse gas emissions over coming decades. On shorter time scales, global average warming by 2021-40 is very likely to be in the range 1.2°C to 1.9°C relative to 1850-1900. Uncertainty on this time scale is dominated by natural climate variability and model uncertainty.

Under the Paris Climate Agreement, international efforts aim to keep warming to below 1.5°C, or at least well below 2°C relative to pre-industrial levels. As global warming has already exceeded 1°C, the remaining carbon budget (total amount of carbon dioxide that can be emitted) available to avoid exceeding these thresholds is limited. At current rates of greenhouse gas emissions and temperature rise, there is a high risk of exceeding 1.5°C during the 2030s and 2°C by 2040-2060.

Future greenhouse gas emission trajectories can be summarised by a range of low, medium and high emission “scenarios”, described in detail in the IPCC Assessment Reports. Low emission scenarios represent the best opportunity to meet the Paris Agreement targets. To achieve the reductions in greenhouse gas emissions consistent with a low emission future pathway, a rapid transition to ending the production and burning of all fossil fuels is required globally and in Australia. A wide range of technological solutions already exist to facilitate this rapid transition from fossil fuels to renewable energy.

Some low and very low emission pathways that provide a high probability of meeting the Paris Agreement temperature targets assume negative emissions from the middle of this century, with more CO2 being drawn out of the atmosphere than is emitted. Negative emissions technologies, including methods to increase carbon storage in natural reservoirs (land or ocean) as well as to capture and store carbon from fossil fuel burning, are not proven to be practically or economically viable at the scale required. Beyond carbon removal, another form of proposed “geoengineering” (climate intervention) is to directly reduce surface temperatures by blocking incoming solar radiation, e.g., through adding reflective particles such as aerosols to the upper atmosphere. Caution is required as the risks of such approaches may outweigh the benefits. Further research is urgently needed to evaluate the full range of carbon removal and geoengineering technologies to fully examine their possible climatic, ecological and geopolitical impacts.

Projected warming and sea-level rise in the second half of the 21st century depend on the emission scenario, with additional uncertainty due to the range of climate model projections for a given scenario. By 2081-2100 under very low or low emission scenarios, best estimates are warming ranging from 1.0–2.4°C (relative to 1850-1900) and sea-level rise ranging from 0.43–0.78 m (relative to 1900). Under high and very high emission scenarios, the projected likely ranges of global warming and sea-level rise by end of century are 2.8–5.7°C (relative to 1850-1900) and 0.79–1.20 m (relative to 1900). It is also important to note that warming and sea level rise will continue beyond 2100 for hundreds of years as the climate system reaches a new equilibrium. Uncertainty about ice sheet processes means that sea level rise of 2 m by 2100, and many metres more over coming centuries, cannot be excluded. When human greenhouse gas emissions reach zero (or net-zero, taking into account carbon removal technologies), the excess carbon dioxide in the atmosphere will gradually be taken up by the land and ocean over hundreds to thousands of years.

Projections indicate larger rates of warming over land than over ocean, and greater warming at high latitudes than in the mid-latitudes and tropics. For rainfall change, there is more uncertainty. Most models agree on an average increased rainfall in the tropics and mid-latitudes (generally wet regions in the current climate), with decreased rainfall in the subtropics (generally arid or semi-arid regions in the current climate). However, rainfall changes at a regional and local scale are driven by changes in atmospheric circulation as well as a warmer atmosphere, resulting in complex patterns of change with higher levels of uncertainty.

There are a number of sources of uncertainty in global climate projections. The largest source of uncertainty beyond the next few decades is the trajectory of greenhouse gas emissions, which is due to the range of future economic, demographic and technological pathways that society may choose. There is also uncertainty due to natural variability of the climate system, which can cause global temperature variability on decadal time scales. This natural variability is overwhelmed by the larger human-induced warming trend when longer time scales are considered. Finally, there is uncertainty in the way climate models represent some components of the climate system (e.g., due to the limited spatial resolution of the model grid, constrained by computer resources). This model uncertainty results in a range of different representations of regional temperature and rainfall and a range of sensitivity to increased greenhouse gases. Note that other lines of evidence apart from climate models are used in producing projections which adds confidence. These sources of uncertainty are included in projections of future climate change and discussed in detail in the IPCC Assessment Reports.

Australia is highly vulnerable to the impacts of climate change

The average Australian surface temperature is likely to increase by between 0.6 and 1.3°C by 2030 above the climate of the recent past (1986-2005) under all emission scenarios. Warming over Australia beyond the next few decades depends strongly on the emission scenario followed. Continued high emissions are likely to produce an increase in Australian average temperatures of 2.8°C –5.1°C by 2090. A rapid reduction in emissions (low emission scenario) is likely to result in temperature increases that are limited to between 0.6°C and 1.7°C by 2090.

Climate models suggest that the warming in inland Australia will be larger than coastal areas, with the least warming (on an annual mean basis) expected in southern Australia. The number of days classified as extremely hot, including multi-day heatwaves, is projected to increase, and the temperatures on the hottest days will typically be hotter than at present. In contrast, there will generally be a reduction in frost events. Many areas where frost typically occurs only a few times a year are likely to be nearly frost-free on average by 2030. The projected changes in extremes will be especially important as many of the most significant impacts of climate change are manifested through the occurrence of extreme events.

Rainfall in Australia will continue to vary from year to year and decade to decade due to natural variability, including large-scale circulation features such as El Niño-Southern Oscillation. In addition, there are significant rainfall trends projected for some regions. These include a decrease in cool season rainfall across much of the south and east, with more time spent in drought. Drying combined with warmer temperatures in the south and east will lead to a longer fire season with more dangerous fire weather days. There are likely to be more intense short-duration heavy rainfall events throughout the country. There are also likely to be fewer tropical cyclones, but a greater proportion are projected to be of high intensity, with ongoing year to year variability.

In the oceans, projections indicate more frequent, extensive, intense and longer-lasting marine heatwaves leading to increased risk of more frequent and severe bleaching events for coral reefs, including the Great Barrier Reef and Ningaloo Reef. There will be continued warming and acidification of their surrounding oceans. Higher sea levels will result in more frequent extreme sea level events, leading to more frequent occurrences of coastal flooding. For most of the Australian coast, extreme sea levels that had a probability of occurring once in a hundred years are projected to become an annual event by the end of this century under low emissions, and by mid-century under high emissions.

Analysis of the impacts of extreme weather and climate events has shown how risks associated with those events increase disproportionately as the temperature increases. These disproportionate risks can arise from extreme heat, floods, droughts, fire weather, strong winds and coastal oceanic events, all of which have the potential to adversely affect communities and ecosystems. Altered risk of extreme events also changes the probability of compound events (e.g., concurrent severe drought, heatwaves and fire weather, as occurred in 2019). While the magnitude of climate change expected in the next decade is similar under all plausible global emission scenarios, by the mid-21st century, higher emissions of greenhouse gases will lead to greater warming and associated impacts, while reducing emissions will lead to less warming and fewer impacts.

Urgent action is needed to avoid dangerous climate change

In summary, climate science provides overwhelming evidence that significant, urgent and sustained reduction in greenhouse gas emissions, reaching greenhouse gas neutrality by 2050, is required to limit global warming to the Paris Agreement targets of well below 2°C, and preferably below 1.5°C, above pre-industrial temperatures. A target of 50% reduction relative to 2005 levels by 2030 for Australia would be consistent with the required rate of emissions reductions to meet the Paris Agreement targets.

Warming will lead to more extreme climate impacts on Australia, including increases in extreme heat, fire weather, floods, droughts and coastal erosion and inundation. We will have to adapt to that part of climate change we can no longer avoid. The 2021 IPCC 6th Assessment Report concluded recently “Every tonne of CO₂ emissions adds to global warming.” Any delay in reducing emissions will increase the practical and economic costs of avoiding dangerous climate change and place a greater burden on future generations to adapt to higher levels of warming.

Sources of Information and Further Reading:

IPCC 6th Assessment Report: https://www.ipcc.ch/assessment-report/ar6/

IPCC Special Report on Global Warming of 1.5°C: https://www.ipcc.ch/sr15/

IPCC Special Report on the Ocean and Cryosphere in a Changing Climate: https://www.ipcc.ch/srocc/

State of the Climate 2020: http://www.bom.gov.au/state-of-the-climate/index.shtml

WMO State of the Global Climate 2020 Report: https://public.wmo.int/en/ourmandate/climate/wmo-statement-state-of-global-climate

Climate Change in Australia: http://www.climatechangeinaustralia.gov.au

The Risks to Australia of a 3°C Warmer World – Australian Academy of Science Report (March2021): https://www.science.org.au/files/userfiles/support/reports-and-plans/2021/risks-australia-three-deg-warmer-world-report.pdf

Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (2022): https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_SummaryForPolicymakers.pdf

Footnotes

[1] Climate change here refers to ‘modern’ climate change, i.e., since the Industrial Revolution.

[2] The sea level rise is caused by melting of alpine glaciers, loss of mass of the Greenland and West Antarctic ice sheets, and expansion of the warming ocean.

[3] CFCs (chlorofluorocarbons), the gases responsible for the ozone hole, are also greenhouse gases.

[4] Aerosols are tiny solid or liquid particles suspended in the atmosphere.