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Explainer: Will global warming ‘stop’ as soon as net-zero emissions are reached?
Media reports frequently claim that the world is facing “committed warming” in the future as a result of past emissions, meaning higher temperatures are “locked in”, “in the pipeline” or “inevitable”, regardless of the choices society takes today.
The best available evidence shows that, on the contrary, warming is likely to more or less stop once carbon dioxide (CO2) emissions reach zero, meaning humans have the power to choose their climate future.
When scientists have pointed this out recently, it has been reported as a new scientific finding. However, the scientific community has recognised that zero CO2 emissions likely implied flat future temperatures since at least 2008. The Intergovernmental Panel on Climate Change (IPCC) 2018 special report on 1.5C also included a specific focus on zero-emissions scenarios with similar findings.
Much of the confusion around committed warming stems from mixing up two different concepts: a world where CO2 levels in the atmosphere remain at current levels; and a world where emissions reach net-zero and concentrations begin to fall.
Even in a world of zero CO2 emissions, however, there are large remaining uncertainties associated with what happens to non-CO2 greenhouse gases (GHGs), such as methane and nitrous oxide, emissions of sulphate aerosols that cool the planet and longer-term feedback processes and natural variability in the climate system.
Moreover, temperatures are expected to remain steady rather than dropping for a few centuries after emissions reach zero, meaning that the climate change that has already occurred will be difficult to reverse in the absence of large-scale net negative emissions.
Constant concentrations vs zero emissions
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UNEP: Net-zero pledges provide an ‘opening’ to close growing emissions ‘gap’
The recent net-zero pledges by major emitting countries and the potential for a “green recovery” from the Covid-19 pandemic “presents the opening” for the world to close the growing “gap” between existing commitments and what is needed to limit global warming to meet the Paris Agreement goals.
This is according to the latest UN Environment Programme (UNEP) emissions gap report, published today.
The annual report, now in its 11th year, finds that global emissions will fall in 2020 due to Covid-19 related disruptions. But it also shows starkly how quickly the 1.5C goal is slipping out of reach, as well as how limiting global warming to the “well below” 2C goal is becoming more difficult with every passing year in which emissions continue to grow.
However, UNEP highlights three areas – the recovery from Covid-19, a new willingness by countries to set ambitious mitigation targets, and the rapid advances in clean energy technologies – which together provide an opportunity to help close this “emissions gap”.
But, in the absence of more structural policy-driven changes, it suggests that emissions will rebound in coming years and the gap between commitments and necessary levels of mitigation will remain as large as it was last year. (Carbon Brief’s archives also include detailed coverage of the UNEP reports in 2014, 2015, 2016, 2017, 2018 and 2019).
This year’s report finds that, by 2030, global greenhouse gas emissions will need to fall by 23% from 2019 levels to put the world on track to “likely” (66% chance) avoid 2C warming above pre-industrial temperatures, by 33% to likely avoid 1.8C warming, and by 56% to likely avoid 1.5C warming.
The existing short-term commitments under the Paris Agreement, on the other hand, imply that emissions will simply plateau, remaining only slightly below 2019 levels by 2030.
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Analysis: How ‘carbon-cycle feedbacks’ could make global warming worse
Analysis: How ‘carbon-cycle feedbacks’ could make global warming worse
Scientists making climate-change projections have to deal with a number of uncertainties.
The amount of global warming will depend on the magnitude of future emissions, which, in turn, depends on how society grows and develops. The rate of warming will also depend on how sensitive the climate is to increased atmospheric greenhouse gases.
Yet climate change also depends on an under-appreciated factor known as “carbon-cycle feedbacks”. Accounting for uncertainties in carbon-cycle feedbacks means that the world could warm much more – or a bit less – than is commonly thought.
The carbon cycle is the collection of processes that sees carbon exchanged between the atmosphere, land, ocean and the organisms they contain. “Feedbacks” refer to how these processes could change as the Earth warms and atmospheric CO2 concentrations rise.
The commonly used warming projections – those highlighted in Intergovernmental Panel on Climate Change (IPCC) assessment reports – include a single best-estimate of carbon-cycle feedbacks. But they do not account for the large uncertainties in these estimates.
These uncertainties are “one of the dominant sources” of divergence between different model projections, according to Dr Ben Booth and colleagues at the Met Office Hadley Centre.
Climate campaigners, such as Greta Thunberg, have also expressed concern that climate projections typically do not fully incorporate the potential range of carbon-cycle feedbacks.
This article explores the implications of carbon-cycle feedback uncertainties by examining a number of modelling studies conducted by scientists over the past decade. These studies give a similar central estimate of carbon-cycle feedbacks to those used in IPCC projections.
But, at the high end, the results show these feedbacks could push atmospheric concentrations of greenhouse gases much higher – meaning more warming – from the same level of emissions.
Analysis for this article shows that feedbacks could result in up to 25% more warming than in the main IPCC projections.
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Factcheck: What Greenland ice cores say about past and present climate change
Factcheck: What Greenland ice cores say about past and present climate change
A misleading graph purporting to show that past changes in Greenland’s temperatures dwarf modern climate change has been circling the internet since at least 2010.
Based on an early Greenland ice core record produced back in 1997, versions of the graph have, variously, mislabeled the x-axis, excluded the modern observational temperature record and conflated a single location in Greenland with the whole world.
More recently, researchers have drilled numerous additional ice cores throughout Greenland and produced an updated estimate past Greenland temperatures.
This modern temperature reconstruction, combined with observational records over the past century, shows that current temperatures in Greenland are warmer than any period in the past 2,000 years. That said, they are likely still cooler than during the early part of the current geological epoch – the Holocene – which started around 11,000 years ago.
However, warming is expected to continue in the future as human actions continue to emit greenhouse gases, primarily from the combustion of fossil fuels.
Climate models project that if emissions continue, by 2050, Greenland temperatures will exceed anything seen since the last interglacial period, around 125,000 years ago.
Ice cores as climate ‘proxies’
Widespread thermometer measurements of temperatures only extend back to the mid-1700s. Scientists investigating how temperatures have changed prior to the invention of thermometers need to rely on a variety of climate “proxies”, which are correlated with temperature and can be used to infer, with some uncertainties, how it has changed in the past.
An ice core from Greenland is prepared for cutting at the National Ice Core Laboratory. Credit: Jim West / Alamy Stock Photo.
Climate proxies can be obtained from sources, such as tree rings, ice cores, fossil pollen, ocean sediments and corals. Ice cores are one of the best available climate proxies, providing a fairly high-resolution estimate of climate changes into the past.
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