Understanding climate change

There are multiple lines of evidence that show the climate system is changing. These include:

• record high surface air temperatures
• increased average number of hot days per year
• decreased average number of cold days per year
• increasing intensity and frequency of extreme events (e.g. fires, floods)
• changing rainfall patterns
• increasing sea surface temperatures
• rising sea levels
• increasing ocean heat content
• increasing ocean acidification
• changing Southern Ocean currents
• melting ice caps and glaciers
• decreasing Arctic sea ice

Air temperatures have increased globally by around 1.2 degrees Celsius since the late 1800s. This is when modern meteorological record keeping became widespread and reliable enough to produce global datasets. Most of the warming from 1880 to present has occurred since the 1970s. The observed increase in temperatures has occurred across the globe, with rising temperatures recorded on all continents and in the ocean. 2016 and 2020 are tied for the world’s warmest year on record, and years 2015 and 2017 were the equal second warmest years on record globally.

Australia’s weather and climate are changing in response to a warming global climate. Australia has warmed on average by 1.44 ± 0.24 °C since 1910, with most warming occurring since 1950 and every decade since then being warmer than the ones before.

Australia’s warmest year on record was 2019, with an average temperature 1.52°C above the 1961-1990 average. The seven years from 2013 to 2019 all rank in the warmest years on record. This long-term warming trend means that most years are now warmer than almost any observed during the 20th century. When relatively cooler years do occur, it is because natural drivers that typically cool Australia’s climate, such as La Niña, act to partially offset the background warming trend (Figure 1).

Figure 1. Annual mean air temperature anomaly for Australia over the period 1910-2020. Annual average air temperatures have warmed by about 1.44 degrees Celsius since 1910 (when our surface air temperature record began), and each decade has been warmer than the previous decade since the 1950s.

Source: Bureau of Meteorology, licensed under the Creative Commons Attribution Australia Licence.

One of the strongest indicators of climate change is the amount of heat stored in the world’s oceans. The heat content of oceans has increased during recent decades and accounts for more than 90 per cent of the total heat trapped by added greenhouse gases and accumulated by the land, air and ocean since the 1970s. Ocean warming is continuing, especially in the top several hundred metres of the ocean. In the Australian region, the average sea surface temperature for each decade since 1900 has been warmer than the previous decade (Figure 2).

Figure 2. Sea surface temperature changes. In the Australian region, the average sea surface temperature for each decade since 1900 has been warmer than the previous decade.

Source: Bureau of Meteorology, licensed under the Creative Commons Attribution Australia Licence.

Average sea surface temperature in the Australian region has warmed by more than 1°C since 1900, with eight of the ten warmest years on record occurring since 2010. The warmest year on record was 2016, associated with one of the strongest negative Indian Ocean Dipole events on record.

The greatest ocean warming in the Australian region since 1970 has occurred around southeastern Australia and Tasmania. The East Australian Current now extends farther south than in earlier decades, creating an area in the Tasman Sea where the warming rate is now twice the global average. There has also been warming across large areas of the Indian Ocean region to the southwest of Australia. Warming of the ocean has contributed to longer and more frequent marine heatwaves.

The increasing frequency of marine heatwaves around Australia in recent years has caused significant impacts on marine ecosystem health, marine habitats and species. These impacts include depleting kelp forests and sea grasses, a poleward shift in some marine species, and increased occurrence of disease. Recent marine heatwaves are the primary cause of mass coral bleaching and widespread damage to coral reefs around Australia, including the Great Barrier and Ningaloo reefs. Other pressures such as tropical cyclones, nutrient runoff and disease also affect the health of some areas of the Great Barrier Reef.

There is clear evidence that sea levels have risen as a result of climate change, based on observations from tide gauges, measurements of our past climate and satellite measurements.

Global mean sea level rise is accelerating. Tide gauge and satellite altimetry observations show that the rate of global mean sea level rise increased from 1.5 ± 0.2 cm per decade (1901–2000) to 3.5 ± 0.4 cm per decade (1993–2019). The dominant cause of global mean sea level rise since 1970 is anthropogenic climate change. Australia, like other nations, is already experiencing sea level rise. Sea level varies from year to year and from place to place, partly due to the natural variability of the climate system from the effect of climate drivers such as El Niño and La Niña. Based on satellite altimetry observations since 1993, the rates of sea level rise to the north and southeast of Australia have been significantly higher than the global average, whereas rates of sea level rise along the other coasts of the continent have been closer to the global average. (Figure 3).

Figure 3. The rate of sea level rise around Australia measured using satellite altimetry, from 1993 to 2019. Rates of sea-level rise vary from year to year and spatially. This is partly due to the natural variability of the climate system from influences such as El Niño and La Niña. On average, sea level has risen around Australia in line with the global rise in sea level. The largest rate of rise in sea level around Australia is to the north and west of the continent and at the New South Wales coast.

Source: CSIRO, State of the Climate 2016.

Ocean currents are also changing, particularly in the Southern Ocean. Studies have shown that the deepest ocean water in the Antarctic, which is normally the coldest water, has warmed and become less saline since the 1980s. The world’s deep ocean currents play a critical role in transporting heat around the planet, thus regulating the climate. Warming of these currents alters their ability to perform this function.

Another serious impact of the increasing concentration of atmospheric carbon dioxide is ocean acidification. Around a quarter of the carbon dioxide produced by humans is absorbed by the oceans. As the carbon dioxide dissolves in sea water it forms carbonic acid, making the ocean more acidic. There are early indications that some marine organisms are already being affected by ocean acidification.

Many marine species that have low mobility or rely on specific habitats or narrow temperature ranges for survival, such as marine plants, corals, and other invertebrates, may not be able to move to new or more suitable habitats. If they cannot move, then as the oceans become warmer and more acidic it becomes harder for these organisms to evolve quickly enough for populations to remain viable. For those marine organisms that form shells or bodies of calcium carbonate minerals, (for example, molluscs and corals), it also becomes more difficult for them to capture and absorb the minerals they need from the sea water to maintain and grow their bodies.

Initially these changes are likely to have the greatest impacts on marine environments such as coral reefs as well as the Arctic and Antarctic ecosystems. Eventually almost all aquatic ecosystems will be affected by increasing levels of ocean warming and acidification if greenhouse gas emissions are not curbed and reversed.

Extreme weather and climate events have serious impacts on our economy, society and environment. Extreme weather events include heatwaves, bushfires, tropical cyclones, cold snaps, extreme rainfall including flash flooding, and droughts.

There is increasing evidence that the frequency and intensity of many types of extreme weather events are changing. Extreme hot days in Australia are getting hotter, with the frequency of very hot (greater than 40°C) daytime temperatures increasing since the 1990s (Figure 4).

Figure 4. Number of days each year where the Australian area-averaged daily mean temperature for each month is extreme. Extreme daily mean temperatures are the warmest 1 per cent of days for each month, calculated for the period from 1910 to 2019.These extreme events typically occur over a large area, with generally more than 40 per cent of Australia experiencing temperatures in the warmest 10 per cent for that month. The frequency of very hot (greater than 40°C) daytime temperatures has been increasing since the 1990s.

Source: Bureau of Meteorology, licensed under the Creative Commons Attribution Australia Licence.

In addition, there has been an increase in extreme fire weather and fire season length across large parts of Australia since the 1950s, with a rapid increase in the late 1990s to early 2000s at many locations in south-eastern Australia.

Individual extreme events occur as a result of a number of contributing climatic factors. A growing body of research provides estimates of the relative contribution of natural variability and anthropogenic (human-caused) climate change to individual extreme events. For example, a study by the ARC Centre of Excellence for Climate System Science analysed the record heat and record low rainfall in winter 2017. It found that conditions were 60 times more likely to have occurred because of climate change than they would have otherwise.

Understanding the influences on such events helps us to better understand how and why extreme events are changing and allows us to plan for the future impacts of these events in Australia.

Rainfall patterns are changing around the world. Research shows the global water cycle is intensifying with a warming climate, which means wet areas are likely to get wetter and dry regions are likely to be drier in response to climate change.

Australian rainfall is highly variable. Average annual rainfall has slightly increased since 1900, with a large increase in northwest Australia since 1970. Trends in rainfall over recent decades include:

• increased spring and summer monsoonal rainfall across the north
• higher than normal rainfall across the centre
• decreased late autumn and winter rainfall across the south.

A climate change signal has been identified in the observed long-term reduction of cool season rainfall in southern Australia, with significant declines since the 1970s in southwest Western Australia (17 per cent), and since the mid-1990s in south-eastern Australia. South-eastern Australia experienced the most persistent rainfall deficit between 1997 and 2009 since records began at the start of the 20th Century.

More information on general climate impacts across Australia is provided in the State of the Climate 2020 report or on the website Climate Change in Australia.