The biggest threats to the Great Barrier Reef

Climate change is the single biggest threat to the Great Barrier Reef, as it is to many ecosystems around the world.  The cumulative impact of climate change, land run-off and other threats is testing the ability of the Reef to recover from major disturbances.

Climate change

The most dramatic impact of climate change is on coral and other species. Increasing water temperature is one of the main causes of coral bleaching, which is becoming more common. If these events are severe and frequent enough to hinder recovery, coral can die. Scientists know that sea surface temperatures of the Great Barrier Reef have increased by 0.8°C (on average) since the late 19th century and will continue to rise.

The indirect threats are just as great. Climate change is predicted to increase the intensity of extreme weather events such as cyclones and floods.

Linked with climate change is ocean ‘acidification’, which is caused by the oceans absorbing about a quarter of all carbon dioxide (CO2) released into the atmosphere. The higher the levels of atmospheric CO2, the greater the impact on water quality.

Land run-off

Poor water quality, including nutrients, sediments and pesticides flowing from the land to the Great Barrier Reef from activities like agriculture, is a major threat.


Nutrients as they occur naturally in Reef ecosystems are vital. They are the natural chemical elements and compounds that plants and animals need to grow. However, if excessive amounts of nutrients, notably nitrogen and phosphorus, are brought in through land run-off, this can upset the natural balance of the Reef systems.

Impacts can include increased coral-eating crown-of-thorns starfish outbreaks, increased macroalgae abundance and algal blooms which can take over and reduce coral diversity, and reduced light available for corals and seagrasses. Excess nutrients can also increase coral bleaching susceptibility and coral disease.

Studies show that most of the excess dissolved inorganic nitrogen and phosphorus comes from fertiliser use on land. These nutrients are of greatest concern because they are immediately and completely available for uptake by marine plants. Annual discharge of nutrients from catchment land use has more than doubled since European settlement.


Sediments, like nutrients, are a natural part of Reef ecosystems, but they are also one of the biggest pressures on the health of inshore reefs and seagrass. Again, problems arise when excessive amounts of the wrong type of sediments find their way into the system. In this case, ’wrong’ refers to the very fine sediments that remain suspended in the water and can be transported long distances.

This leads to increased turbidity and decreased water clarity (the water looks muddy), which in turn reduces the amount of light that reaches seagrasses and coral, stunting their growth. When this sediment settles, it can also have detrimental effects on the early life stages of corals – even smothering coral and seagrasses in more extreme conditions. Sediment can also carry nutrients into the Reef environment.

Studies have shown that the vast majority of these unwanted fine sediments are washed into the sea from grazing activities or streambank erosion, and the impact is greater during floods.


Pesticides are a threat because what they are designed to do on land – kill pests such as  weeds and insects – means they also impact plants and animals in rivers and creeks, as well as some coastal and inshore areas. The pesticides commonly used for weed control act by inhibiting photosynthesis, which is why they are so good at controlling weeds, and can affect non-target species such as seagrasses.

Pesticides, including herbicides, insecticides and fungicides, are generally not found in the natural environment and can take months or even years to break down. They are carried in river run-off and have been detected in Great Barrier Reef ecosystems at concentrations high enough to affect organisms.

The effects of ongoing low-level pesticide exposures in inshore environments are unknown but likely to impact coral fertility and reproduction. Less is known of the effect on freshwater, wetland and estuarine ecosystems, although the proximity of these ecosystems to pesticide sources suggests some impacts are likely.

Except for a few locations, monitoring of pesticides in marine waters shows they are below the level expected to cause significant risk to ecosystems.

The effects of ongoing low-level pesticide exposures are continuing to be researched.

Other threats

The other greatest threats to the Reef are coastal development, some remaining impacts of fishing and illegal fishing and poaching.

Outbreaks of coral-eating crown-of-thorns starfish are also a big concern.

Crown-of-thorns starfish are native to the Great Barrier Reef but when found in large numbers, and when coral is under stress, they can quite simply destroy corals by eating their living tissue or ‘skin’. Research shows that coral cover on surveyed reefs fell by 50% between 1985 and 2012 and that crown-of-thorns starfish were responsible for almost half of this decline.

There have been four documented outbreaks on the Reef since the 1960s, occurring on roughly a 17-year cycle. The latest started in 2010 and a control program is in place.

However, the threat of future damage is increasing because the Reef is now under greater stress than ever before, reducing its ability to recover. Scientists believe one of the causes of that stress – increased nutrient levels – may also increase crown-of-thorns outbreaks.