After three years of writing and two weeks of virtual negotiations to approve the final wording, the Sixth Assessment Report (AR6) of the Intergovernmental Panel on Climate Change (IPCC) confirms that changes are happening in Earth’s climate across every continent and every ocean.
My contribution was as one of 15 lead authors to a chapter about the oceans, the world’s icescapes and sea level change — and this is where we are now observing changes that have become irreversible over centuries, and even millennia.
Overall, the world is now 1.09℃ warmer than it was during the period between 1850 and 1900. The assessment shows the ocean surface has warmed slightly less, by about 0.9℃ as a global average, than the land surface since 1850, but about two-thirds of the ocean warming has taken place during the last 50 years.
We concluded that it is virtually certain the heat content of the ocean will continue to increase for the rest of the current century, and will likely continue until at least 2300, even under low-emissions scenarios.
We also concluded that carbon dioxide emissions are the main driver of acidification in the open ocean and that this has been increasing faster than any time in at least 26,000 years.
We can also say with high confidence that oxygen levels have dropped in many ocean regions since the mid-20th century and that marine heatwaves have doubled in frequency since 1980, also becoming longer and more intense.
Past greenhouse gas emissions, since 1750, mean we are now committed to future ocean warming throughout this century. The rate of change depends on our future emissions, but the process itself is now irreversible on centennial to millennial time scales.
Ice loss in Antarctica
All this heat is bad news for the area I work in: Antarctica. With a warming ocean, the Antarctic ice sheet is left vulnerable to melting because so much of it rests on bedrock below sea level.
As the ocean warms and the ice sheet melts, sea level goes up around the world. We have very high confidence that the ice lost from West Antarctica in recent decades has exceeded any gain in mass from snowfall. We are also confident this loss has largely been due to increased melting of ice below sea level, driven by warming ocean water.
This melting has allowed the acceleration and thinning of grounded ice further inland — and this is what contributes to sea level rise. On the other side of the world, the Greenland ice sheet has also been losing mass over recent decades, but in Greenland this is principally due to warmer air, rather than warming ocean water.
It is virtually certain that the melting of the two great ice sheets, in Greenland and Antarctica, as well as the many thousands of glaciers around the world, will continue to raise sea levels globally for the rest of the current century.
By 2100, we project global mean sea level to be between 0.4m (for the lowest emission scenario, in which CO₂ emissions would have to drop to net zero by 2050) and 0.8m (for the highest emissions scenario) above the 1995–2014 average. How high the seas rise this century clearly depends on how much and how quickly we manage to cut greenhouse gas emissions.
The time to act is now
There are processes at play which we still cannot fully capture in computer models, mostly because they take place over periods of time longer than we have direct (satellite-based) observations for. In Antarctica, some of these uncertain processes could greatly accelerate the loss of ice, and potentially add one metre to the projected sea level by 2100.
Whether or not this worst-case scenario plays out or not remains uncertain, but what is increasingly beyond doubt is that global mean sea level will continue to rise for centuries to come. The magnitude of this depends very much on the extent to which we are able, collectively, to reduce greenhouse gas emissions right now.
The scientific updates in our AR6 chapter are in line with those from previous assessments. That’s encouraging, because every assessment report brings in new authors with different expertise. The fact the scientific conclusions remain consistent reflects the overwhelming agreement within the global scientific community.
For our chapter, we have assessed 1500 research papers, but across the entire AR6, over 14,000 publications were considered, with an emphasis on recent research that hasn’t been assessed in previous IPCC reports.
The report has been scrutinised carefully at every stage of its evolution, attracting nearly 80,000 individual review comments from experts all over the world. Every single comment had to be addressed by the author team, with written responses provided and any changes to the text carefully noted and tracked.
What changes with each assessment is the clarity of the trends we are observing, and the increasing urgency with which we must act. While some aspects of AR6 are new, the underlying message remains the same. The longer we wait, the more devastating the consequences.
Click here to read more of The Conversation’s coverage of the IPCC report
Australia is experiencing widespread, rapid climate change not seen for thousands of years and may warm by 4℃ or more this century, according to a highly anticipated report by the Intergovernmental Panel on Climate Change (IPCC).
The assessment, released on Monday, also warns of unprecedented increases in climate extremes such as bushfires, floods and drought. But it says deep, rapid emissions cuts could spare Australia, and the world, from the most severe warming and associated harms.
The report is the sixth produced by the IPCC since it was founded in 1988 and provides more regional information than any previous version. This gives us a clearer picture of how climate change will play out in Australia specifically.
It confirms the effects of human-caused climate change have well and truly arrived in Australia. This includes in the region of the East Australia Current, where the ocean is warming at a rate more than four times the global average.
We are climate scientists with expertise across historical climate change, climate projections, climate impacts and the carbon budget. We have been part of the international effort to produce the IPCC report over the past three years.
The report finds even under a moderate emissions scenario, the global effects of climate change will worsen significantly over the coming years and decades. Every fraction of a degree of global warming increases the likelihood and severity of many extremes. That means every effort to reduce greenhouse gas emissions matters.
Australia is, without question, warming
Australia has warmed by about 1.4℃ since 1910. The IPCC assessment concludes the extent of warming in both Australia and globally are impossible to explain without accounting for the extra greenhouse gases in the atmosphere from human activities.
The report introduces the concept of Climate Impact-Drivers (CIDs): 30 climate averages, extremes and events that create climate impacts. These include heat, cold, drought and flood.
The report confirms global warming is driving a significant increase in the intensity and frequency of extremely hot temperatures in Australia, as well as a decrease in almost all cold extremes. The IPCC noted with high confidence that recent extreme heat events in Australia were made more likely or more severe due to human influence.
These events include:
the Australian summer of 2012–13, also known as the Angry Summer, when more than 70% of Australia experienced extreme temperatures
the heat leading into the Black Summer bushfires of 2019-20.
The IPCC report notes very high confidence in further warming and heat extremes through the 21st century – the extent of which depends on global efforts to reduce greenhouse gas emissions.
If global average warming is limited to 1.5℃ this century, Australia would warm to between 1.4℃ to 1.8℃. If global average warming reaches 4℃ this century, Australia would warm to between 3.9℃ and 4.8℃ .
The IPCC says as the planet warms, future heatwaves in Australia – and globally – will be hotter and last longer. Conversely, cold extremes will be both less intense and frequent.
Hotter temperatures, combined with reduced rainfall, will make parts of Australia more arid. A drying climate can lead to reduced river flows, drier soils, mass tree deaths, crop damage, bushfires and drought.
The southwest of Western Australia remains a globally notable hotspot for drying attributable to human influence. The IPCC says this drying is projected to continue as emissions rise and the climate warms. In southern and eastern Australia, drying in winter and spring is also likely to continue. This phenomenon is depicted in the graphic below.
Climate extremes on the rise
Heat and drying are not the only climate extremes set to hit Australia in the coming decades. The report also notes:
observed and projected increases in Australia’s dangerous fire weather
a projected increase in heavy and extreme rainfall in most places in Australia, particularly in the north
a projected increase in river flood risk almost everywhere in Australia.
Under a warmer climate, extreme rainfall in a single hour or day can become more intense or more frequent, even in areas where the average rainfall declines.
For the first time, the IPCC report provides regional projections of coastal hazards due to sea level rise, changing coastal storms and coastal erosion – changes highly relevant to beach-loving Australia.
This century, for example, sandy shorelines in places such as eastern Australia are projected to retreat by more than 100 metres, under moderate or high emissions pathways.
Hotter, more acidic oceans
The IPCC report says globally, climate change means oceans are becoming more acidic and losing oxygen. Ocean currents are becoming more variable and salinity patterns – the parts of the ocean that are saltiest and less salty – are changing.
It also means sea levels are rising and the oceans are becoming warmer. This is leading to an increase in marine heatwaves such as those which have contributed to mass coral bleaching on the Great Barrier Reef in recent decades.
Notably, the region of the East Australia Current which runs south along the continent’s east coast is warming at a rate more than four times the global average.
The phenomenon is playing out in all regions with so-called “western boundary currents” – fast, narrow ocean currents found in all major ocean gyres. This pronounced warming is affecting marine ecosystems and aquaculture and is projected to continue.
Like all regions of the world, Australia is already feeling the effects of a changing climate.
The IPCC confirms there is no going back from some changes in the climate system. However, the consequences can be slowed, and some effects stopped, through strong, rapid and sustained reductions in global greenhouse gas emissions.
And now is the time to start adapting to climate change at a large scale, through serious planning and on-ground action.
To find out more about how climate change will affect Australia, the latest IPCC report includes an Interactive Atlas. Use it to explore past trends and future projections for different emissions scenarios, and for the world at different levels of global warming.
Earth has warmed 1.09℃ since pre-industrial times and many changes such as sea-level rise and glacier melt are now virtually irreversible, according to the most sobering report yet by the Intergovernmental Panel on Climate Change (IPCC).
The report also found escape from human-caused climate change is no longer possible. Climate change is now affecting every continent, region and ocean on Earth, and every facet of the weather.
The long-awaited report is the sixth assessment of its kind since the panel was formed in 1988. It will give world leaders the most timely, accurate information about climate change ahead of a crucial international summit in Glasgow, Scotland in November.
The IPCC is the peak climate science body of the United Nations and the World Meteorological Organization. It is the global authority on the state of Earth’s climate and how human activities affect it. We are authors of the latest IPCC report and have drawn from the work of thousands of scientists from around the world to produce this new assessment.
Sadly, there is hardly any good news in the 3,900 pages of text released today. But there is still time to avert the worst damage, if humanity chooses to.
It’s unequivocal: humans are warming the planet
For the first time, the IPCC states unequivocally — leaving absolutely no room for doubt – humans are responsible for the observed warming of the atmosphere, lands and oceans.
The IPCC finds Earth’s global surface temperature warmed 1.09℃ between 1850-1900 and the last decade. This is 0.29℃ warmer than in the previous IPCC report in 2013. (It should be noted that 0.1℃ of the increase is due to data improvements.)
The IPCC recognises the role of natural changes to the Earth’s climate. However, it finds 1.07℃ of the 1.09℃ warming is due to greenhouse gases associated with human activities. In other words, pretty much all global warming is due to humans.
Global surface temperature has warmed faster since 1970 than in any other 50-year period over at least the last 2,000 years, with the warming also reaching ocean depths below 2,000 metres.
The IPCC says human activities have also affected global precipitation (rain and snow). Since 1950, total global precipitation has increased, but while some regions have become wetter, others have become drier.
The frequency and intensity of heavy precipitation events have increased over most land areas. This is because the warmer atmosphere is able to hold more moisture — about 7% more for each additional degree of temperature — which makes wet seasons and rainfall events wetter.
Higher concentrations of CO₂, growing faster
Present-day global concentrations of atmospheric carbon dioxide (CO₂) are higher and rising faster than at any time in at least the past two million years.
The speed at which atmospheric CO₂ has increased since the industrial revolution (1750) is at least ten times faster than at any other time during the last 800,000 years, and between four and five times faster than during the last 56 million years.
About 85% of CO₂ emissions are from burning fossil fuels. The remaining 15% are generated from land use change, such as deforestation and degradation.
Concentrations of other greenhouse gases are not doing any better. Both methane and nitrous oxide, the second and third biggest contributors to global warming after CO₂, have also increased more quickly.
Methane emissions from human activities largely come from livestock and the fossil fuel industry. Nitrous oxide emissions largely come from the use of nitrogen fertiliser on crops.
Extreme weather on the rise
Hot extremes, heatwaves and heavy rain have also become more frequent and intense across most land regions since 1950, the IPCC confirms.
The report highlights that some recently observed hot extremes, such as the Australian summer of 2012–2013, would have been extremely unlikely without human influence on the climate.
Human influence has also been detected for the first time in compounded extreme events. For example, incidences of heatwaves, droughts and fire weather happening at the same time are now more frequent. These compound events have been seen in Australia, Southern Europe, Northern Eurasia, parts of the Americas and African tropical forests.
Oceans: hotter, higher and more acidic
Oceans absorb 91% of the energy from the increased atmospheric greenhouse gases. This has led to ocean warming and more marine heatwaves, particularly over the past 15 years.
Marine heatwaves cause the mass death of marine life, such as from coral bleaching events. They also cause algal blooms and shifts in the composition of species. Even if the world restricts warming to 1.5-2℃, as is consistent with the Paris Agreement, marine heatwaves will become four times more frequent by the end of the century.
Melting ice sheets and glaciers, along with the expansion of the ocean as it warms, have led to a global mean sea level increase of 0.2 metres between 1901 and 2018. But, importantly, the speed sea level is rising is accelerating: 1.3 millimetres per year during 1901-1971, 1.9mm per year during 1971-2006, and 3.7mm per year during 2006-2018.
Ocean acidification, caused by the uptake of CO₂, has occurred over all oceans and is reaching depths beyond 2,000m in the Southern Ocean and North Atlantic.
Many changes are already irreversible
The IPCC says if Earth’s climate was stabilised soon, some climate change-induced damage could not be reversed within centuries, or even millennia. For example, global warming of 2℃ this century will lead to average global sea level rise of between two and six metres over 2,000 years, and much more for higher emission scenarios.
Globally, glaciers have been synchronously retreating since 1950 and are projected to continue to melt for decades after the global temperature is stabilised. Meanwhile the acidification of the deep ocean will remain for thousands of years after CO₂ emissions cease.
The report does not identify any possible abrupt changes that would lead to an acceleration of global warming during this century – but does not rule out such possibilities.
The prospect of permafrost (frozen soils) in Alaska, Canada, and Russia crossing a tipping point has been widely discussed. The concern is that as frozen ground thaws, large amounts of carbon accumulated over thousands of years from dead plants and animals could be released as they decompose.
The report does not identify any globally significant abrupt change in these regions over this century, based on currently available evidence. However, it projects permafrost areas will release about 66 billion tonnes of CO₂ for each additional degree of warming. These emissions are irreversible during this century under all warming scenarios.
How we can stabilise the climate
Earth’s surface temperature will continue to increase until at least 2050 under all emissions scenarios considered in the report. The assessment shows Earth could well exceed the 1.5℃ warming limit by early 2030s.
If we reduce emissions sufficiently, there is only a 50% chance global temperature rise will stay around 1.5℃ (including a temporary overshoot of up to 0.1℃). To get Earth back to below 1.5℃ warming, CO₂ would need to be removed from the atmosphere using negative emissions technologies or nature-based solutions.
Global warming stays below 2℃ during this century only under scenarios where CO₂ emissions reach net-zero around or after 2050.
The IPCC analysed future climate projections from dozens of climate models, produced by more than 50 modelling centres around the world. It showed global average surface temperature rises between 1-1.8℃ and 3.3-5.7℃ this century above pre-industrial levels for the lowest and highest emission scenarios, respectively. The exact increase the world experiences will depend on how much more greenhouse gases are emitted.
The report states, with high certainty, that to stabilise the climate, CO₂ emissions must reach net zero, and other greenhouse gas emissions must decline significantly.
We also know, for a given temperature target, there’s a finite amount of carbon we can emit before reaching net zero emissions. To have a 50:50 chance of halting warming at around 1.5℃, this quantity is about 500 billion tonnes of CO₂.
At current levels of CO₂ emissions this “carbon budget” would be used up within 12 years. Exhausting the budget will take longer if emissions begin to decline.
The IPCC’s latest findings are alarming. But no physical or environmental impediments exist to hold warming to well below 2℃ and limit it to around 1.5℃ – the globally agreed goals of the Paris Agreement. Humanity, however, must choose to act.
Click here to read more of The Conversation’s coverage of the IPCC report