Reef-wrecking numbers: What will a 3°C rise in global temperature do ?

Cosmos Magazine


Cosmos is a quarterly science magazine. We aim to inspire curiosity in ‘The Science of Everything’ and make the world of science accessible to everyone.

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Both the continent Australia and its inhabitants are well accustomed to extreme weather and climate events. 

In the past few years we’ve seen record-breaking heat, severe drought, bushfires and, just recently, exceptional flooding. In each instance, we’ve struggled to cope: it’s clear that we’re not fully adapted to the weather and climate extremes we experience in today’s climate.

So how are we going to manage with further global warming and more people exposed to these sorts of events? And what would it look like if global warming isn’t limited by prompt action, but allowed to grow at current rates?

Where are we at today?

The aim of the 2015 Paris Agreement is to limit global warming well below 2°C above pre-industrial levels, and preferably below 1.5°C. The world has warmed by around 1.1°C since the late 19th century. 

Increasing trend of global temperature chart
Global temperatures have increased markedly over the last 170 years. Credit: Berkeley Earth.

The 1850–1900 period is widely used as a proxy for a pre-industrial climate, although of course some industrialisation had previously occurred. As a result, some studies suggest that we may be slightly underestimating global warming relative to a pre-industrial state.

As the global community aims to keep global warming to low levels this doesn’t leave us much wiggle room. The world is warming at a rate of around 0.2°C per decade, giving little time to turn things around.

Up until 2019 annual emissions of carbon were still rising. To be confident of limiting global warming to below 2°C above pre-industrial levels, we need to see net-zero emissions within decades. The COVID-19 pandemic and associated reductions in economic activity caused a large, but perhaps temporary, drop in global carbon emissions. We can’t assume we’ll get another “leg-up” like it, and even if we did we must recognise that it came with severe consequences in many other ways.

If we revert to our pre-COVID emissions trajectory, even with existing pledges for carbon reductions from many countries, we’ll be on track to see a whopping 3°C global warming relative to pre-industrial levels – well beyond the Paris Agreement aims of holding global warming below 2°C.

Chart showing global carbon emissions continued to rise pre-covid, but the pandemic saw a dramatic drop
Global carbon emissions continued to rise pre-COVID, but the pandemic saw a dramatic drop. Credit: Le Quéré et al. (2020).

Have temperature-rise estimates changed over time?

Estimates of the temperature rise between now and 2100 haven’t changed that much. The scenarios used to project future climates have changed in their design to some extent, but they still yield outcomes that see high-emissions estimates associated with about 4°C global warming, and low-emissions estimates in line with the Paris Agreement 2°C limit.

The main uncertainty about climate projections for the end of the 21st century is to do with emissions. Scientists can construct pathways with big differences in global warming based on either increasing or reducing carbon emissions. There’s some uncertainty still due to other factors (eg. models warming more or less due to same CO2 increase).

If we revert to our pre-COVID emissions trajectory, even with existing pledges for carbon reductions from many countries, we’ll be on track to see nearer 3°C global warming relative to pre-industrial levels – well beyond the Paris Agreement aims. That’s why we chose a 3°C rise by 2100 as the marker for the new Australian Academy of Science report.

What would 3°C global warming mean for Australia?

There have been several studies examining implications for Australian climate of 1.5°C and 2°C global warmingbut fewer looking at specific higher levels.

Even at 1.5°C global warming we know that there would be major impacts of climate change for Australia. It is estimated that with a 1.5°C rise 70–90% of the world’s coral reefs would not survive,primarily driven by warming of the ocean and acidification. Australia’s Great Barrier Reef is already being severely damaged by the effects of climate change, with marine heatwaves in recent years causing mass bleaching and coral death, leaving a much less ecologically diverse reef system.

At 2°C global warming, 99% of coral reefs are projected to disappear, while at 3°C global warming it’s likely the entire ecosystem will be gone.

Sea level rise, which is a slow process expected to continue for centuries regardless of our carbon emissions pathway, will continue and will exceed one metre if we go beyond 1.5°C global warming. This would cause large-scale coastal inundation; storm surges would be more damaging without widespread and costly sea defences.

Among the other key impacts of 3°C global warming for Australia, heatwaves would continue to become more frequent and intense; weather associated with severe bushfire conditions is projected to become more common; short-duration extreme rainfall events – where rain falls in the space of an hour or less and causes flash floods – are expected to become more frequent.

Try to imagine this: if 3°C global warming comes to pass in the Australia our grandchildren and great-grandchildren inhabit, an unusually coldsummer will be the same as the hottestsummers in today’s climate.

The Australian Academy of Science report is meant to act as a warning on what will happen to our climate if we continue down the path of limited ambition for greening our economy and reducing our carbon emissions.

If we seriously ramp our actions, we can avoid the worst effects of climate change and bequeath a safer country and a safer world to future generations.

Dr Andrew King is a lecturer in climate science and an ARC DECRA Fellow at the School of Earth Sciences and ARC Centre of Excellence for Climate Extremes, University of Melbourne. He was one of 15 scientists on the expert panel that created the Australian Academy of Science’s report The Risks to Australia of a 3°C warmer world.

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