6. Factors influencing the Region’s values

Changes to the global climate interact with existing climate drivers (Box 6.1) to determine climate patterns in the Region.¹⁵³⁸ Changes brought about by global warming can either amplify or reduce the effects of these regional drivers, leading to altered weather patterns and changes in the timing, duration and intensity of extreme events.^1539,1540 There is also evidence some drivers are themselves being altered as the climate changes, affecting the way regional climates are linked to distant climate phenomena and causing permanent shifts to drier or wetter conditions in some areas.^540,569,1540 For example, recent increases in both the strength and frequency of El Niño–Southern Oscillation events have been observed over various timescales, though it is not yet clear whether these are outside the range of historical variability.^{1540,1541,1542} It is also projected that future El Niño and La Niña events may become more intense as the climate continues to warm.¹⁵⁴³ Other potential effects, such as shifts in the timing and duration of these events, remain subject to considerable uncertainty.¹⁵⁴⁴ 

Projecting climate change involves addressing uncertainties arising from limitations in understanding both the physical climate system and societal trends.¹⁵³⁸ The Sixth Assessment Report of the IPCC1538 incorporates both factors using an expanded set of Shared Socio-economic Pathways (SSPs) to model future greenhouse gas concentrations in the atmosphere.^{1545,1546,1547}  These illustrative pathways span the range of possible future emissions scenarios, from very low to very high emissions. For example, SSP1–2.6 portrays a sustainable, low-emissions future resulting in limited warming of 1.8 (very likely range 1.3 to 2.4) degrees Celsius this century; SSP5–8.5 depicts a fossil fuel-driven development path of intensifying emissions resulting in severe global warming in the order of 4 degrees Celsius; while SSP2–4.5 represents a middle-ground scenario in which social, economic and technological trends do not shift markedly from historical patterns.¹⁵⁴⁶ 

Developments such as emerging climate policy and the rapid growth of renewable energy deployment make very high emissions scenarios like SSP5–8.5 less plausible now than in 2019.¹⁵⁴⁸ Under all considered emissions pathways, the best estimate is for global average temperature to reach 1.5 degrees Celsius above pre-industrial levels between the late 2020s and mid-2030s.¹⁵³⁸ The risk of exceeding 2 degrees Celsius depends strongly on emissions pathways, but under medium to high emissions this is likely by the middle of the twenty-first century.¹⁵⁴⁴

Warming of the global climate system is driving climate change. This warming is unequivocal, as confirmed by all major temperature monitoring datasets.^{608,1549,1550, 1551,1552} It is primarily driven by the release of carbon dioxide and other greenhouse gases due to human activities.¹⁵³⁸ In 2023, the carbon dioxide concentration reached a global average of 419.3 parts per million in the atmosphere,¹⁵⁵³ the highest levels on Earth in at least 2 million years ⁵⁴⁰ (Figure 6.4). Globally, 2023 was approximately 1.45 degrees Celsius warmer than pre-industrial levels (with a margin of uncertainty of 1.33 to 1.57) and the past 9 years have been the hottest in the 174-year instrumental record ^582,1554 (Figure 6.5).

Warming is generally higher over land than over the ocean, but the pace of warming in the oceans is accelerating.^1555,540 At the ocean surface, temperature has increased by around 0.9 degrees Celsius since the late nineteenth century, with around two-thirds of this warming occurring since 1980.⁶⁰⁸ Global surface temperature has increased faster since 1970 than in any other 50-year period over the past 2000 years.¹⁵³⁸ The magnitude of these changes exceeds the extent of natural variation over glacial–interglacial time scales.^{1556,1557,1558} A recent temperature-proxy study on long-lived sponges in the Caribbean indicates that human-caused warming may have begun earlier and may be greater than previously recognised.¹⁵⁵⁹ 

Around 90 per cent of the extra heat generated by global warming is absorbed by the oceans,¹⁵⁶⁰ causing the heat content of the global ocean to increase since at least 1970.^540,582,608 The average warming rate for the surface layers of the ocean (to 2000 metres depth) is 71 per cent higher from 2006 to 2022 compared to a longer baseline (1971 to 2022).⁵⁸² Ocean heat content is certain to continue to increase over the twenty-first century. Associated warming of the climate system will likely continue until at least 2300, even for low-emissions scenarios, because of the slow circulation of the deep ocean.⁶⁰⁸ Although continued warming is inevitable, the extent to which the climate will change in coming decades depends on current and future greenhouse gas emissions.^1039,1539

The most direct impact of climate change in Australia has been an increase in air and sea surface temperatures.^540,1539 Temperatures over Australia have increased by an average of 1.47 degrees Celsius since 1910, at a rate considerably higher than the global average.⁵⁴⁰ Future warming is likely to be similar to the global average.¹⁵³⁹ The number of days with dangerous weather conditions for bushfires has increased in nearly all locations across Queensland ¹⁵⁷⁰ and this trend is projected to continue.^540,566 Across the Region, sea surface temperatures are projected to increase by between 0.3 and 1.2 degrees Celsius under a low-emissions pathway by 2030 (relative to a baseline of average temperatures between 1986 and 2005), with little further increase during the twenty-first century.^1571,565 However, in a very high emissions pathway, warming would range from 0.6 to 1.3 degrees Celsius by 2030 and 2.3 to 4.7 degrees Celsius by 2090.^565,566,1571

Sea temperature increases at the ocean surface in the Australian region have been around 1.05 degrees Celsius since 1900,¹⁵⁷² with the Reef warming by 0.94 degrees Celsius over the same period.⁶⁶⁰ Since the 1980s, sea surface temperatures have warmed at a rate of between 0.1 and 0.3 degrees Celsius every decade (Figure 3.8). Annual sea surface temperature anomalies for the Region since 1910 are shown in Figure 3.9. Globally, the sea surface temperature is projected to increase on average by about 1.3 degrees Celsius above pre-industrial levels under a low-emissions pathway and by about 3.2 degrees Celsius under a very high emissions pathway.^1573,1574 Projected changes to sea surface temperatures across the Region range from 2.1 to 3.5 degrees Celsius by 2090 under very high emissions.^565,566,1571 Changes to temperature extremes often lead to greater impacts than changes to the average climate, and these extreme events are becoming both stronger and more frequent because of climate change.^582,1538 This trend is projected to continue.¹⁵⁷⁵ A return to near present-day conditions by 2100 is only plausible under a scenario that embraces large net negative emissions to limit warming to 1.5 degrees Celsius.¹⁵⁹

Globally, marine heatwaves have doubled in frequency since the 1980s and become longer-lasting, as well as more intense and extensive.^{608,1572,1576,1577} Despite La Niña conditions (Box 6.1), 58 per cent of the global ocean surface experienced at least one marine heatwave during 2022.⁵⁸² This trend will continue, with marine heatwaves at the global scale expected to increase four- to eight-fold by the end of this century, depending on emissions pathways.⁶⁰⁸ In the Reef and Coral Sea, marine heatwaves are increasing in frequency, intensity and extent.²⁴⁶

Globally, average sea levels are rising as a direct consequence of anthropogenic global warming.⁶¹³ Two main processes are driving sea-level rise: thermal expansion (an increase in the volume of sea water as it warms) and increased melting of land-based ice.^540,614

Global average sea level rose faster during the twentieth century than in any prior century during the past 3000 years. Since the late 1960s, sea level rise has accelerated from 3.7 (2006 to 2018)^608,615,1578 to 4.6 (2013 to 2022) ⁵⁸² millimetres per year. Global average sea level reached a new record high for the satellite altimeter record (1993 to 2022) in 2022.⁵⁸²

Increasing loss of land ice from glaciers and ice sheets, including a four-fold increase in the contribution of meltwater from Greenland and Antarctica in 2010 to 2019 compared to 1992 to 1999, has made this the largest contributor to global sea-level rise since 2006.⁶⁰⁸ Recent research suggests that a continued trend of ice sheet loss from West Antarctica is now inevitable well beyond 2100, irrespective of future emissions pathways.¹⁵⁷⁹ Global average sea level is projected to rise by 0.15 to 0.29 metres by 2050 and by 0.28 to 1.1 metres by 2100. Sea-level rise could be several tenths of a metre higher (and approaching 2 metres under very high emissions) if Greenland and Antarctic ice sheets disintegrate faster than anticipated.^608,1580 Projected sea-level rise across the Region is similar to the global average.^565,1571

Sea-level rise caused by climate change is superimposed on the normal, frequent fluctuations in sea level associated with tides, storm surges, and ocean waves (swell and wind waves) in the order of metres. Sea-level fluctuations are also affected by cyclical variations of 20 to 30 centimetres over years or decades caused by climate drivers, such as El Niño–Southern Oscillation.⁶⁰⁸ As these sea level fluctuations occur on a higher baseline, they will lead to increases in the frequency of sea level extremes,⁶⁰⁸ including storm surges associated with the passage of cyclones. If increases in the Region (Figure 6.6) are similar to the global average, extreme sea level events that previously occurred around once per century will occur about 20 to 30 times as often by 2050 and 160 to 530 times as often by 2100 compared to the recent past.^608,629 Sea-level rise is also expected to interact with the complex bathymetry of the Reef to locally increase or decrease tidal ranges such that some coastal areas will be more affected than others.⁶³⁰

The absorption of carbon dioxide alters ocean chemistry in a process commonly referred to as ocean acidification.¹⁵⁸² The ocean takes up about a quarter of annual global carbon dioxide emissions ¹⁵⁴¹ and has absorbed roughly one-third of anthropogenic carbon dioxide emissions since the beginning of the Industrial Revolution.^1572,1583 The absorption of carbon dioxide leads to a shift in carbonate chemistry involving a decrease in the concentration of carbonate ions and an increase in both bicarbonate and hydrogen ions.¹⁵⁸² This process also lowers the saturation state of aragonite, the form of calcium carbonate used by many marine organisms including reef-building corals, to produce skeletons and shells.¹⁵⁷² One consequence of this process has been a global decrease in ocean pH by approximately 0.1 units (a 26 per cent rise in acidity) since the pre-industrial period (Section 3.3.2)^{1541,1584,1585} to its lowest level in at least 25,000 years.¹⁵⁴¹ The increase in acidity is occurring at a rate estimated to be occurring at least 10 times faster than at any time within the past 65 million years.^{1541,1582,1586} The rapid rate of contemporary changes in ocean chemistry is important in determining the magnitude and impacts of change because the processes by which oceans buffer these changes occur over many thousands of years.¹⁵⁸²

Ocean acidification is also progressing rapidly in the Region, with long-term trends predominantly driven by increasing atmospheric carbon dioxide.^{1539,1587,739} These changes are superimposed on increasing daily and seasonal fluctuations linked to patterns in sea temperatures and stratification, changes in net production and in seasonal transport of offshore waters onto the Reef.⁵³⁵ Concentrations of dissolved carbon dioxide increased by an estimated 6 per cent from 2009 to 2019, and by approximately 28 per cent since 1958.⁷³⁹ The extensively carbonate seafloor has been unable to buffer the seawater against atmospheric changes.⁷³⁹ There are indications that the rate and magnitude of increases of carbon dioxide concentrations are more significant on inshore reefs than those further offshore.^1588,1589 For the Reef’s coastal waters, average annual seawater pH is projected to decrease by 2090, compared to that for the period between 1986 and 2005, by an additional 0.15 to 0.33 units under moderate to very high emissions (a 40 to 100 per cent rise in carbon dioxide concentrations, respectively).^1571,565 Between 1960 and 2009, the aragonite saturation state likely decreased by 0.33 units.⁷³⁹ Further median declines (compared to 1986 to 2005) of 0.34 to 0.41 units by 2030 and 0.34 to 1.55 units by 2090 are projected to values below critical levels for coral reefs to be able to maintain net calcification, depending on emissions trajectories (Section 6.3.2).^535,1571,565

Global atmospheric and ocean circulation patterns and rising temperatures influence ocean currents (Section 3.2.1).⁶⁰⁸ Australian boundary currents and their associated eddies have immediate impacts on the circulation of water over the relatively narrow continental shelf, with flow-on effects for the entire marine ecosystem.¹⁵⁷² The major boundary current affecting the Region, the East Australian Current, has been intensifying in recent decades,^1572,608 and this trend is expected to continue.⁶⁰⁸ This intensification has been associated with an increase in both current strength and the frequency of its southward extensions,¹⁵⁹⁰ contributing to marine heatwaves in south-eastern Australia.¹⁵⁷² More detailed projections of future ocean circulation and altered ocean currents remain a key knowledge gap. The present generation of climate models have a limited ability to forecast the future state of boundary currents, such as the East Australian Current, and do not adequately represent associated eddies or deep ocean and continental shelf interactions.^535,1572 Downscaled modelling of climate change projections at resolutions that may better resolve these effects in the Region is underway.⁵³⁵

The number of cyclones has decreased globally during the twentieth century.¹⁵⁹¹ They have also decreased across northern Australia since reliable satellite observations began in 1982.⁵⁴⁰ Additional non-satellite observations suggest there has also been a longer-term reduction since 1900.⁵⁴⁰ The number of cyclones is projected to decrease in the future, but it is expected that a greater proportion will be higher intensity, with continuing large variations in cyclone numbers from year to year (Section 3.2.2).^{540,1592,1593} Whether or not cyclone-generated waves will become more damaging to coral reefs depends on cyclone size and forward speed as well as intensity. Modelled data are not yet robust enough to make reasonable predictions of these characteristics, especially at fine scales.⁵⁴⁹ It is projected that average peak wind speeds will increase globally with warming.¹⁵⁷⁵ No climate change effect on spatial extent (size) of cyclones has yet been detected, in part because cyclone size is highly variable, but potential effects are a subject of active research interest.¹⁵⁷⁵ Since 1979, the proportion of intense cyclones has increased by around 25 per cent, which is around 6 per cent per decade and consistent with physical process understanding and climate model predictions.^1594,1595 Although observed trends in cyclone intensity remain inconclusive,¹⁵⁹⁶ in part due to the limitations of satellite-based estimates, indirect measurements suggest that cyclones at all intensity categories have strengthened over the past 30 years.⁵⁴²

Model projections indicate that, relative to pre-industrial conditions, cyclones in north-east Australia will have both higher wind speeds (by 5 to 10 per cent on average) and heavier rainfall (by up to 27 per cent for average hourly rainfall rates) under moderate- to high-emissions future climate change.^565,1571 Predicted increases in peak rainfall rates have already been substantiated by attribution studies, which indicate that higher temperatures have augmented cyclone-associated rainfall.¹⁵⁹⁶ As a result of this effect, rainfall totals in particular locations are likely to increase in the future, bringing a higher risk of associated flooding.¹⁵⁶⁶ An observed slowdown of forward speed over Australia (by 22 per cent since 1946) suggests flooding risk may also be increased by cyclones remaining over a given location for longer,¹⁵⁹⁷ but future trends in the speed of travel and spatial extent of cyclones remain uncertain.⁵⁴⁹ In contrast, one of the most assured future impacts from cyclones will be an increase in coastal flooding when storm surges are superimposed on higher sea levels (Section 3.2.5).^540,553

Changes in patterns of cyclone activity from year to year may also change over the course of this century.^1544,1598 The number of cyclones making landfall in the Region is highly variable over different timescales.⁵³⁷ The frequency of landfall is higher during La Niña years than El Niño years.^538,539,1599 This pattern may change; there are indications that cyclones will become more frequent during El Niño events (and less frequent during La Niña events) by the end of the twenty-first century.^1544,1598

Observed and projected changes to broadscale rainfall patterns occurring globally ^582,1541 and in some regions of Australia ^540,1539 are not as evident or well resolved in the Region.^565,1571 Recent research has shown that seasonal rainfall has changed in northern Queensland as the climate has warmed, with wet season rainfall increasing and becoming more variable since the pre-industrial period.⁵⁶⁹ Continued warming of the climate will have important effects on rainfall, particularly through the amplification of extremes of drought and flood. Rainfall in the Region is strongly seasonal, with around 60 to 80 per cent occurring in the wet season from December to April.¹⁵⁶⁴ This rainfall is often associated with the monsoon trough and the passage of low-pressure systems and cyclones, which can bring flooding across the Region (Box 6.1).^1564,1600 It is modulated in part by climate drivers, such as the Madden–Julian Oscillation.¹⁵⁶⁵ Variation from year to year is strongly linked to other major climate drivers, especially the El Niño–Southern Oscillation.^{1565,1601,1602}

The varied temporal and spatial patterns of rainfall across the Region are superimposed on decadal rainfall trends occurring due to climate change.^1539,1541 One of the most important of these is that heavy rainfall events are becoming more intense.⁵⁴⁰ Daily rainfall totals associated with thunderstorms have increased since 1979, particularly in northern Australia and this trend is expected to continue.⁵⁴⁰ On a global scale, approximately 20 per cent of global heavy rainfall events over land are attributable to the observed temperature increase since pre-industrial times.¹⁶⁰³ For heavy rain days, total rainfall is expected to increase by around 7 per cent for each degree of warming.^540,1566 More-intense rainfall increases the likelihood of surface-water and flash floods ¹⁵⁶⁶ (Section 3.2.3) and large flood events are occurring more frequently than in the past (Section 6.5.1).⁵⁷⁵

Climate across the Region is changing. Changes have already been widely documented in terms of rising air and sea temperatures as well as changing ocean chemistry. Trends for rising sea levels and increasing intensity of storms are increasingly evident. Anticipated changes, such as increases in the intensity of El Niño and La Niña events, vary in certainty but are potentially consequential for the Region’s values.