Article
BAMOS October 2021
13
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 – 2oC , 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 2oC 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 1.5oC warming limit by early 2030s .
If we reduce emissions sufficiently , there is only a 50 % chance global temperature rise will stay around 1.5oC ( including a temporary overshoot of up to 0.1oC ). To get Earth back to below 1.5oC warming , CO₂ would need to be removed from the atmosphere using negative emissions technologies or naturebased solutions .
Global warming stays below 2oC 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 – 1.8oC and 3.3 – 5.7oC 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 1.5oC , 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 2oC and limit it to around 1.5oC — the globally agreed goals of the Paris Agreement . Humanity , however , must choose to act .