Impacts of Climate Change
In 2022, the Australian Institute of Marine Science recorded the Great Barrier Reef's worst-ever mass bleaching, affecting 91% of surveyed reefs as sea temperatures reached 1.5°C above average.
● Know
- Oceans have absorbed over 90% of excess heat; current warming rate ~3.6 mm/year sea-level rise
- Two sources of sea-level rise: thermal expansion (~50%) and ice melt (~50%)
- Ocean acidification: CO₂ + H₂O → H₂CO₃; pH fallen from 8.2 to 8.1 since pre-industrial (~30% more acidic)
● Understand
- How coral bleaching occurs and why mass events are becoming more frequent
- Why decreasing ocean pH harms calcifying organisms (corals, oysters, pteropods)
- How climate change affects Australian ecosystems: GBR, Black Summer fires, Murray-Darling
● Can do
- Match climate change impacts to their mechanisms
- Explain ocean acidification using a chemical equation
- Describe how multiple Australian ecosystems are being affected by climate change
In February 2022, aerial surveys over the Great Barrier Reef recorded water temperatures reaching 32°C in shallow northern sections, 3°C above typical summer maximums. Within weeks, dive teams confirmed widespread bleaching across over 650 reefs. The same thermal energy driving that bleaching is part of a massive planetary buffer: the world's oceans have absorbed approximately 90% of the excess energy trapped by greenhouse gases since 1971. This buffering effect has slowed surface warming, but it means ocean heat content has been increasing consistently in every decade of measurement. The top 700 m of ocean has warmed by an average of 0.1°C since the 1970s, a small number that represents an enormous quantity of energy given the mass of the oceans.
Coral bleaching occurs when sea surface temperatures rise by as little as 1–2°C above the seasonal maximum for several weeks. Corals live in a tight symbiotic relationship with photosynthetic algae called zooxanthellae, which live inside coral tissue and provide the coral with up to 90% of its energy through photosynthesis. Under heat stress, corals expel these algae, turning ghostly white (bleached). If temperatures remain elevated for more than a few weeks, the coral starves and dies.
The Great Barrier Reef (GBR) has experienced mass bleaching events in 1998, 2002, 2016, 2017, 2020 and 2022 with the 2016 and 2022 events being the worst on record. The interval between bleaching events is shortening, giving corals less time to recover. A 2022 study found that two-thirds of the GBR's reefs showed coral bleaching during the 2022 event.
The Great Barrier Reef is the world's largest coral reef system, 2,300 km long, visible from space, home to over 1,500 species of fish, 4,000 types of mollusc, 239 species of birds and 30 species of whales and dolphins. It generates approximately $6.4 billion annually for Australia's economy through tourism and fishing. Its health is directly tied to sea surface temperatures in the Coral Sea, which have been rising at a rate of approximately 0.12°C per decade.
Australian Institute of Marine Science (AIMS) conducts long-term monitoring of the GBR, publishing the annual "Reef Snapshot" report. AIMS surveys show that coral cover on the GBR has fluctuated dramatically, with major losses following bleaching events and cyclones, and partial recovery during cooler periods. Their data is central to Australian government assessments of GBR health and to international discussions about coral reef futures under different emissions scenarios.
"Corals have survived warm periods before, so they'll adapt." While corals have survived warmer periods in deep geological time, the current rate of change is far faster than evolutionary adaptation can match. Adaptation through natural selection requires many generations. Even if some corals are more heat-tolerant, the frequency of bleaching events means reefs no longer have time to recover between events, resulting in net reef degradation regardless of adaptation potential.
Sea level is rising for two distinct physical reasons, each contributing approximately equally to the current rate:
1. Thermal expansion (~50%): As ocean water warms, it expands in volume. This is a direct physical consequence of the temperature-density relationship of water. Even a small temperature increase across the vast volume of the ocean adds significantly to sea level.
2. Ice melt (~50%): Land-based ice (glaciers and ice sheets in Greenland and Antarctica) is melting, adding liquid water to the ocean. Sea ice (floating on the ocean already) does not contribute to sea-level rise when it melts, only land-based ice does.
The current rate of sea-level rise is approximately 3.6 mm per year (satellite altimetry data, 1993–present). This rate has itself been accelerating: the 20th-century average was ~1.5 mm/year; the current rate is more than double that. IPCC projections for 2100 range from 0.3 m (low emissions) to 1.0 m (high emissions), with higher values possible if ice sheet instabilities are triggered.
If the West Antarctic Ice Sheet were to collapse entirely, it would raise sea levels by approximately 3.3 metres. If the Greenland Ice Sheet melted completely, it would add about 7 metres. Neither is expected to happen within this century under current projections, but instability mechanisms (marine ice sheet instability, ice cliff instability) could trigger faster-than-expected loss. This is why scientists are monitoring outlet glaciers like Thwaites Glacier in Antarctica very closely, some researchers call it the "Doomsday Glacier."
Australian impacts of sea-level rise: Australia has more than 85% of its population within 50 km of the coast. Low-lying areas including parts of coastal Sydney, Melbourne's Port Phillip Bay, Brisbane's river floodplain, and major Pacific island neighbours (Tuvalu, Kiribati, Marshall Islands) face significant risks. The Australian government has provided support for Tuvalu residents to migrate to Australia under a climate mobility agreement, one of the first of its kind globally.
"Sea ice melting (e.g. Arctic sea ice) raises sea levels." This is false. Sea ice is already floating on the ocean, it displaces its own weight in water. When it melts, there is no net addition of water volume (same principle as an ice cube melting in a full glass of water). Only land-based ice (glaciers, ice sheets) adds new water to the ocean when it melts.
About 25–30% of all CO₂ emitted by humans is absorbed by the oceans. While this helps slow atmospheric CO₂ accumulation, it comes at a cost: the dissolved CO₂ reacts with seawater to form carbonic acid:
CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
The released hydrogen ions (H⁺) lower the pH of seawater. Before the Industrial Revolution, average ocean surface pH was approximately 8.2. Today it is approximately 8.1 a seemingly small change, but because pH is a logarithmic scale, this represents a 30% increase in hydrogen ion concentration (i.e., 30% more acidic).
This is a serious problem for calcifying organisms marine animals that build calcium carbonate (CaCO₃) shells and skeletons, including corals, oysters, mussels, sea urchins, and tiny sea snails called pteropods. In more acidic conditions, CaCO₃ dissolves, making shell-building energetically more expensive and eventually impossible. Lab experiments have shown pteropod shells beginning to dissolve within 45 days in water at projected 2100 pH levels.
pH is a logarithmic scale: each unit change represents a 10-fold change in hydrogen ion concentration. Ocean pH has fallen from 8.2 to 8.1, that is 0.1 units, which corresponds to a 26% increase in [H⁺]. If emissions continue on current trajectories, ocean pH could reach 7.9–7.8 by 2100, a 100–150% increase in acidity relative to pre-industrial levels. The ocean has not been this acidic for at least 20 million years.
Australian oyster industry: Southern Australia supports a major oyster aquaculture industry (Sydney rock oysters, Pacific oysters). Research by the CSIRO and universities has found that oyster larvae in increasingly acidified water struggle to form their initial shells, reducing survival rates. Australian shellfish farmers have noticed changes in their stock health consistent with acidification. This is not a future problem, it is affecting Australian aquaculture now, with economic consequences for coastal communities.
"The ocean is still basic (above pH 7), so it's not really acidifying, that's misleading." The term "ocean acidification" refers to the direction of change (becoming more acidic = lower pH), not to whether the ocean is crossing the neutral point of pH 7. Changing from pH 8.2 to 8.1 is genuine acidification in the chemical sense, and the organisms that evolved over millions of years for pH 8.2 are affected by that change, just as a human body is affected if blood pH shifts by even 0.1 units.
Climate change affects biodiversity through multiple pathways:
Range shifts: As temperatures warm, species move toward cooler higher latitudes or altitudes. In Australia, species like the golden bowerbird and mountain pygmy possum are being pushed to higher elevations, eventually running out of mountain to climb. Marine species are shifting southward along Australia's east coast.
Phenological mismatch: When the timing of biological events (flowering, insect hatching, animal breeding) shifts at different rates, ecological relationships break down. For example, if flowers bloom earlier but their pollinators emerge at the same time as before, plant reproduction fails. In Australia, changes in timing of native bee activity and flowering plants have been documented.
Australian context, the 2019-20 Black Summer: The 2019-20 bushfire season burned approximately 18.6 million hectares, an area larger than England and Scotland combined, killing or displacing an estimated 3 billion animals. Climate attribution studies showed the extreme heat and drought conditions that enabled the fires were made at least 30% more likely by climate change. Smoke reached New Zealand, South America and the southern hemisphere stratosphere.
Murray-Darling Basin: Australia's most important agricultural waterway is experiencing declining rainfall reliability, more frequent drought, and increased evaporation under warming conditions. Water security for agriculture and communities in inland Australia is increasingly under pressure.
A WWF Australia commissioned study estimated that approximately 3 billion animals were killed or displaced by the 2019-20 fires. This included ~143 million mammals, ~2.46 billion reptiles, ~180 million birds and ~51 million frogs. For context, koala populations in some regions were reduced by more than 60%. The fires directly affected over 800 vertebrate species, including 300 classified as threatened. This is the largest wildlife disaster attributed to climate change ever documented for a single event.
Indigenous ecological knowledge and climate change: Aboriginal and Torres Strait Islander peoples across Australia have observed dramatic changes to country: altered fire seasons, changed rainfall patterns, plant species flowering at unusual times, and animals behaving differently. On Arnhem Land, Yolngu elders describe changes to the seasonal calendar, traditional seasons based on wind, flowering, and animal behaviour are shifting. This long-term observational record complements scientific data and provides invaluable localised evidence of climate change impacts that monitoring instruments often miss.
"Australia has always had bushfires, so the 2019-20 fires were just normal." While fire is part of Australia's ecology, the Black Summer was abnormal in scale, intensity and timing. The fire season began in winter (months earlier than normal), and the areas burned were unprecedented in modern records. Attribution science shows these conditions were made far more likely by human-caused climate change, even though individual fires have multiple contributing factors including land management decisions.
Wrong: "Arctic sea ice melting causes sea-level rise." Sea ice is already floating in the ocean, it displaces its own weight in water. Melting floating ice does not raise sea levels. Only land-based glaciers and ice sheets add new water.
Right: Sea-level rise comes from land-based ice melt (glaciers, Greenland and Antarctic ice sheets) plus thermal expansion of warming ocean water. The melting of Arctic sea ice, while an important indicator of climate change, does not itself cause sea-level rise.
Wrong: "The ocean is still alkaline so ocean 'acidification' is just alarmist language." The term describes the direction of change (becoming more acidic). A shift from 8.2 to 8.1 represents a 30% increase in hydrogen ion concentration, a chemically significant change for organisms that evolved over millions of years at a stable pH.
Right: Ocean acidification is the correct scientific term for the directional decrease in ocean pH caused by CO₂ absorption. The ocean remains alkaline, but the change in acidity is real and measurable. Marine organisms are sensitive to even small pH changes because biological systems (enzymes, shell formation) depend on specific chemical conditions.
Wrong: "Coral bleaching always kills the coral." Bleaching is a stress response, not immediate death. If water temperatures return to normal quickly enough (within weeks), corals can recover and re-acquire zooxanthellae. The problem is that bleaching events are becoming more frequent and severe, leaving insufficient recovery time.
Right: Bleached corals are severely stressed but not immediately dead. If temperatures return to normal within 4–6 weeks, recovery is possible. The increasing frequency of bleaching events, from once per decade in the 1980s to near-annual now on parts of the GBR, means reefs cannot recover between events, causing cumulative damage.
Australia's Climate Vulnerability Hotspots
Great Barrier Reef: 2,300 km of coral reefs facing warming, acidification and cyclones simultaneously. The reef is listed as a World Heritage Area, and in 2021 UNESCO recommended it be listed as "in danger", a recommendation Australia successfully opposed through diplomatic engagement but which reflects genuine international concern.
Murray-Darling Basin: Covers 14% of Australia's land area and produces 40% of agricultural output. Climate projections indicate a ~20% reduction in average runoff into the basin by 2050, threatening water security for 3 million people and Australia's food export industry.
Changing fire seasons: CSIRO projects that extreme fire weather days will increase by 5–65% by 2050 across southern Australia, with the fire season extending earlier into spring and later into autumn. This compounds pressures on biodiversity, air quality, infrastructure and mental health in rural communities.
Cause-and-Effect Chains
Understanding Acidification Numbers
If global greenhouse gas emissions were suddenly stopped today (zero emissions from tomorrow), would coral bleaching on the Great Barrier Reef stop immediately? Predict what you think would happen and why.
How close was your prediction?
The hook for this lesson revealed something striking: Earth's oceans have absorbed over 90% of the excess heat trapped by greenhouse gases. Without that ocean buffer, surface temperatures would already be far higher, but all that absorbed heat comes at a cost to the ocean itself.
Now that you've explored the cascading effects of a warming, acidifying, and expanding ocean, look back at your initial thinking. Was the scale of the ocean's role as a heat buffer what you expected? And does knowing what the ocean is "doing for us" change how you think about coral bleaching and sea-level rise?
Earlier you described what you thought coral bleaching was and listed sources of sea-level rise.
Now that you've worked through the lesson, has your understanding of either changed? What was the most surprising impact you learned about?
Q1. Explain the process of ocean acidification. Write the chemical equation for the reaction and explain why it is harmful to calcifying organisms such as corals and oysters.
Q2. Explain why melting Arctic sea ice does NOT raise sea levels, but the melting of the Greenland Ice Sheet does. Use the concept of water displacement in your answer.
Q3. Describe TWO specific impacts of climate change on Australian ecosystems and explain the mechanism by which each impact occurs. For at least one impact, refer to specific data or examples from this lesson.
Revisit Your Thinking
Go back to your Think First answer about sea-level rise. Were you right about the sources?
- Did you include both thermal expansion and ice melt?
- Did you correctly identify that floating sea ice does not contribute?
Model answers (click to reveal)
Answers
▾MCQ 1
B Oceans have absorbed approximately 90% of the excess heat trapped by greenhouse gases, dramatically slowing the rate of surface warming but driving ocean temperature increases, thermal expansion and coral bleaching.
MCQ 2
C The pH scale is logarithmic: each 0.1 unit decrease represents approximately a 26% increase in hydrogen ion concentration. The change from 8.2 to 8.1 therefore represents ~30% more acidic, a significant chemical change for marine organisms.
MCQ 3
D The shortening interval (from once per decade in the 1980s to near-annual now) means corals cannot fully recover between bleaching events. Each event causes cumulative damage, preventing reefs from sustaining healthy coral cover over time.
MCQ 4
A Thermal expansion of warming seawater (~50%) and melting of land-based glaciers and ice sheets (~50%) each contribute approximately equally to current sea-level rise of ~3.6 mm/year.
MCQ 5
C Pteropods are calcifying organisms that build thin calcium carbonate shells. In more acidic seawater, CaCO₃ dissolves, making shell maintenance increasingly difficult and eventually impossible.
Short Answer 1
Model answer: Ocean acidification occurs when atmospheric CO₂ is absorbed by seawater. The reaction is: CO₂ + H₂O → H₂CO₃ (carbonic acid), which then dissociates to release hydrogen ions (H⁺) and bicarbonate: H₂CO₃ → H⁺ + HCO₃⁻. The released H⁺ ions lower the pH of the ocean. Since pre-industrial times, ocean pH has fallen from 8.2 to 8.1, which represents approximately a 30% increase in hydrogen ion concentration due to the logarithmic nature of the pH scale. For calcifying organisms like corals and oysters that build calcium carbonate (CaCO₃) shells and skeletons, lower pH is harmful because CaCO₃ dissolves in acidic conditions. This makes shell formation energetically more expensive and eventually impossible, threatening the survival of these organisms and the broader ecosystems that depend on them.
Short Answer 2
Model answer: Arctic sea ice is already floating in the ocean. By Archimedes' principle, a floating object displaces its own weight in water. When floating ice melts, the liquid water it produces has the same volume as the water it was displacing, so there is no net change in sea level. This is the same principle as an ice cube melting in a full glass of water: the level does not rise. The Greenland Ice Sheet, by contrast, rests on land. When it melts, the liquid water flows into the ocean, adding new volume that was not previously in the ocean system. This is equivalent to pouring extra water into a full glass, it overflows. The same principle applies to all land-based ice: mountain glaciers and the Antarctic Ice Sheet.
Short Answer 3
Model answer (example): Impact 1, Great Barrier Reef bleaching. Mechanism: When sea surface temperatures rise 1–2°C above the seasonal maximum, corals expel their symbiotic zooxanthellae algae. Without these algae, the coral loses up to 90% of its energy supply and turns white. If temperatures remain elevated for more than a few weeks, corals die. The GBR has experienced six mass bleaching events since 1998 (including 2016, 2017, 2020, 2022), with intervals shortening due to persistent ocean warming. Impact 2-2019-20 Black Summer bushfires. Mechanism: Climate change increases the frequency and intensity of heatwaves and droughts, creating conditions highly conducive to fire ignition and spread. The 2019-20 season burned approximately 18.6 million hectares and killed or displaced an estimated 3 billion animals. Attribution studies showed that extreme heat and drought conditions were made at least 30% more likely by human-caused climate change, though individual fires have multiple contributing factors.