Welcome to Carbon Brief’s DeBriefed.
An essential guide to the week’s key developments relating to climate change.

This week

UN adopts landmark opinion

ICJ OPINION: The UN has adopted a resolution backing a landmark world court opinion stating that countries have a legal obligation to address climate change, reported the Guardian. Some 141 countries voted in favour of the resolution, while only eight voted against: the US; Israel; Iran; Russia; Belarus; Saudi Arabia; Yemen; and Liberia. There were also 28 absentations, including India and Turkey, the host of COP31.

‘DETERMINED’: The text adopted by the UN general assembly “stresses” that “climate change is an unprecedented challenge of civilizational proportions” and says the assembly is “determined” to “translate the court’s findings into enhanced multilateral cooperation and accelerated climate action at all levels, consistent with international law”. The text “urges” states to implement measures including “transitioning away from fossil fuels in energy systems”. It also “requests” the next UN secretary general to report on progress in 2027 and adds a formal follow-up to the agenda of the UN general assembly in 2028.

AMENDMENTS REJECTED: A UN press summary detailed how countries rejected four proposed amendments to the text by a group of largely Arab nations. These amendments would have undercut the world court’s legal advice on countries’ climate obligations by saying its views should only be taken into account “as appropriate”. They also would have added a reference to 2C, instead of focusing on 1.5C alone, got rid of the formal follow-up process in 2028 and added a reference to the role of carbon capture and storage.

Scenario sceptic

‘GOOD RIDDANCE’: US president Donald Trump declared “good riddance” to a very high emissions modelling scenario in a Truth Social post on Saturday, misleadingly stating that “the United Nations TOP Climate Committee just admitted that its own projections (RCP8.5) were WRONG! WRONG! WRONG!” The post was quickly picked up by right-leaning media, including Fox News, the New York Post and the Australian.

NEW SCENARIOS: Trump’s claim follows the publication of a new set of emissions scenarios that will underpin research cited in the next set of reports from the Intergovernmental Panel on Climate Change (IPCC). In a guest post for Carbon Brief, scientists explained that the very high emissions scenario has “become implausible, based on trends in the costs of renewables, the emergence of climate policy and recent emission trends”.
TRUMP FACTCHECKED:Carbon Brief published a factcheck of Trump’s claims. It noted that the IPCC does not develop, control or own climate scenarios and has not published anything stating that any climate scenario is “wrong”. It added: “Projections suggest that the world is still on course for between 2.5C and 3C of warming…previously described as ‘catastrophic’ by the UN.”

Around the world

• ADAPTATION NEEDED: The UK’s Climate Change Committee outlined how investing in adaptation now could produce “long-term savings”, Carbon Brief reported. UK ministers are preparing to accept a CCC recommendation to “set a legally binding goal of cutting emissions 87% by 2040”, reported the Times.
• ELECTRIFY EVERYTHING: COP31 president-designate Murat Kurum told the Copenhagen climate ministerial that countries should be “decarbonising the way we generate electricity, but also expanding electrification into every sphere of life”, according to Climate Home News.
• STAFF CUT: Australia’s national science agency, CSIRO, is preparing to fire one-third of the team working on the national climate model that provides future projections, reported the Guardian.
• TARGET MISSED: An independent body has warned that Germany is expected to miss its 2030 climate goals and emit more CO2 than previously forecast, reported Reuters. According to Deutsche Welle, the country could breach its goal by up to 100m tonnes of CO2.
• PEAK POWER: India’s peak power demand “smashed all records” on Tuesday, after the country’s ongoing heatwave drove a “sharp rise” in electricity consumption, according to the Economic Times. The record fell again on Thursday, said Reuters.

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140

The number of countries in the world that have net-zero targets.

2

Major emitters that do not have a net-zero target – a group comprising Iran and the US, according to Carbon Brief analysis.

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Latest climate research

• Global warming above 4C is projected to cause large decreases in “climate connectivity” between habitats for land animals | Nature Climate Change
• Around 6% of respiratory deaths in Brazil from 2010-20 were attributable to “non-optimal temperatures”, accounting for more than 66,000 excess deaths | PLOS Climate
• Fungi that cause diseases in plants will approximately double in abundance around the Antarctic Peninsula by 2100 under a moderate emissions scenario | Global Change Biology

(For more, see Carbon Brief’s in-depth daily summaries of the top climate news stories on Monday, Tuesday, Wednesday, Thursday and Friday.)

Captured

The world added nearly 100 gigawatts (GW) of new coal-power capacity in 2025 – the equivalent of roughly 100 large coal plants – according to the latest annual report from Global Energy Monitor (GEM). This is a ten-year high, according to Carbon Brief’s coverage, which noted that the world’s coal plants nevertheless generated less electricity. The chart above shows that 95% of the new coal plants were built in India and China last year.

Spotlight

Climate migration

This week, Carbon Brief speaks to experts at a conference on migration and climate change in London about what their research could mean for how people move around the world in the future.

Prof Kerilyn Schewel, assistant professor of sociology at the University of North Carolina at Chapel Hill

We have moved beyond a ‘push factor’ narrative – that climate change is coming and uprooting communities – to a more nuanced perspective that recognises that people are already moving for all kinds of reasons… [For example] the more that young people are accessing formal education, the more they want to leave – particularly rural communities. We have to be very careful not to assume that when people want to leave, it is always driven by climate change. There are other developmental factors that are also shaping desires to move. This is a research frontier – seeing how environmental factors intersect with these other social or developmental outcomes.

Dr Aromar Revi, founding director of the Indian Institute for Human Settlements

The future of mobility is much more certain than [climate change is]. People have been mobile for a very long time. That’s been an important part of the transformation of societies and economies for centuries…mobility is part of the solution [to climate change]. It is not the full solution, but it’s part of the solution. People are voting with their feet and with their aspirations to make a change.

Prof Nitya Rao, a professor of gender and development at the University of East Anglia

There are many things that the system can do to welcome migrants and be more sensitive to different types of migrants and their needs… In the short term, [migrants] need piped water, a proper home, care for young children…In the longer term, we have to address structural inequality. There are still barriers to people accessing resources – especially productive assets such as land, capital and livestock…And these barriers are split by gender, class, ethnicity and so on. These need to be addressed, I think, to really make migration a case of [climate] adaptation and not just survival.

Prof Jon Barnett, professor in the school of geography, earth and atmospheric sciences at the University of Melbourne

In the Pacific islands, international migration isn’t driven by climate change. It’s enabled by the capacity of people to cross borders, so it’s all about migration agreements. As climate change amplifies pressures on people’s livelihoods, we may end up with a whole series of transnational populations that are kind of constantly in churn – where they’re not just living on the island, but also in Australia, New Zealand, the US.

Dr Maria Franco Gavonel, lecturer in global social policy and international development at the University of York

The migration response towards almost any climate event is short lived and short distance, so it will mostly affect internal movement rather than international…So all these narratives about climate refugees – like human rights related to international migration – are overstating the extent to which this is going to happen.

Dr Benoy Peter, the executive director of the Centre for Migration and Inclusive Development in India

Every one of us, including you and me, have benefited from migration. Migration is the fastest way for intergenerational upward social mobility for people from socially and economically disadvantaged populations. So I see migration as a [climate] solution.

Cecilia Keating also contributed to this spotlight. Read more of Carbon Brief’s coverage of the conference.

Watch, read, listen

TICE QUESTIONED: The Bloomberg Zero podcast interviewed Richard Tice, the deputy leader of the hard-right Reform UK party, who exposed his rejection of climate science and support for the oil and gas industry.

‘CLIMATE CROSSROADS’: The Guardian examined how Colombia’s upcoming election could leave the major oil-and-gas producer at a “climate crossroads”.

LAND GRAB: A Floodlight investigation for Inside Climate News examined “Trump officials, billionaires and the quiet reshaping of America’s public lands”.

Coming up

• 24 May: Cyprus elections
• 28-29 May: Blue economy and finance forum, Monaco
• 28 May: International Energy Agency (IEA) World Energy Investment 2026 report launch

Pick of the jobs

• Bulletin of the Atomic Scientists, editor in chief | Salary: $140,000-$160,000. Location: Washington DC, Chicago or New York City
• Climate Outreach, researcher | Salary: £44,000. Location: Remote (UK)
• University of Manchester, research associate, energy and climate governance | Salary: £37,694-£46,049. Location: Manchester, UK

DeBriefed is edited by Daisy Dunne. Please send any tips or feedback to debriefed@carbonbrief.org.

This is an online version of Carbon Brief’s weekly DeBriefed email newsletter. Subscribe for free here.

The post DeBriefed 22 May 2026: UN adopts landmark resolution | Trump takes on ‘RCP8.5’ | Climate migration appeared first on Carbon Brief.

DeBriefed 22 May 2026: UN adopts landmark resolution | Trump takes on ‘RCP8.5’ | Climate migration

Hundreds of scientists gathered in London this week to discuss the role of migration as a way for communities to adapt to climate change.

The impacts of a warming world, such as sea level rise and worsening extremes, are pushing many people around the world to leave their homes.

As a form of climate adaptation, a decision to migrate involves an array of different factors, such as politics, conflict and economic opportunity.

The conference unpacked these topics, as well as the impacts of climate change on livelihoods, relocation and gender norms across Africa and Asia.

The event had a strong focus on urban areas, with one co-convenor stating that “half of the world’s population now lives in the cities…A lot of the battles of climate adaptation will be won and lost in cities.”

Another co-convenor told Carbon Brief that the conference’s “focus really is on the climate change adaptation community, showing that migration is not a failure of adaptation – it is part of adaptation”.

Carbon Brief attended the conference to report on the sessions and speak to world-leading experts on climate-driven migration.

• Migration as adaptation
• Cities and livelihoods
• Immobility and relocation
• Legal pathways
• Changing narratives

Migration as adaptation

The two-day conference on “mobility in adaptation to climate change” was held at Wellcome’s headquarters in London. It gathered more than 100 leading experts in migration, adaptation and climate change from countries across Europe, Africa and Asia.

On day one of the conference, co-convenor Prof Neil Adger, a professor from the University of Exeter, told Carbon Brief:

“Our focus really is on the climate change adaptation community, showing that migration is not a failure of adaptation – it is part of adaptation.”

In his opening address, Adger highlighted that there were still many unknowns on climate migration – such as how and when it is an appropriate way to adapt to climate change, and who benefits and loses in these situations.

Dr Manuela Di Mauro – the head of climate-adaptation research at the UK Foreign Commonwealth and Development Office – took to the stage next. She told attendees that mobility has always been a part of human life, stating:

“We are all migrants. We are all part of the same history.”

She urged the scientific community to “learn the language and the political perspective” needed to support and engage with policymakers about climate-driven migration.

Conference co-convenor Dr Chandni Singh from the Indian Institute for Human Settlements (IIHS) then delivered the first in-depth talk of the conference, outlining the current state of knowledge on climate change and migration.

She explained that cross-border migration is “emotionally and economically arduous” adding “under a changing climate, people choose to move within national borders first”. (Estimates suggest that around three-quarters of total global migration is internal.)

Singh emphasised that “mobility choices are extremely complex and nuanced, based on one’s aspirations and capabilities, social norms and asset bases”. She continued:

“Some [people] are forced to move or are displaced, others are relocated preemptively to move people out of harm’s way and others choose to stay despite escalating risk – or because resilience-building measures allow people to stay.”

She stressed that people need resources to migrate, so the poorest people are often unable to move – leaving them in a state of “immobility”. However, she also noted that most people do not want to leave their homes, stressing the “visceral reality of place attachment”.

Singh explained that many families “live dual lives”, in which family members work in the city to save money for a life back in their village. This dynamic of living across two locations is often referred to as “translocality”.

For example, Singh shared the story of residents from the Indian village of Kolar, who travel more than 100km to and from Bangalore for work every day, or else live there in informal settlements.

These workers send the money they earn back home, where it is often used to dig bore wells to access water. However, Singh warned that climate change and poor water management mean these wells often fail year after year, trapping people in this cycle of travelling to Bangalore to earn more money.

Singh also stressed the prevalence of rural-to-urban migration. She cited UN estimates (that do not explicitly include climate-driven migration), which find that around 2.5 billion people are expected to migrate from rural to urban areas by 2050. It adds that 90% of the change occurring in Africa and Asia.

Singh added:

“Half of the world’s population now lives in the cities…A lot of the battles of climate adaptation will be won and lost in cities.”

She noted that although migration “helps to manage risks”, it also has “significant financial, personal and social costs”.

Singh went on to discuss the global goal on adaptation – a set of 59 indicators to measure global progress on adaptation. Singh said that “migration and mobility are completely invisible…and therefore completely overlooked” in the goals.

She concluded by discussing the importance of new narratives on climate change and migration, saying:

“It’s the narratives and stories we tell of this moment that can help us first acknowledge what is happening, help subvert misinformation and untruths, and really demand accountability.”

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Cities and livelihoods

Migration from villages to cities was a central theme of the conference. 

On day two of the conference, Dr Aromar Revi, founding director of the IIHS, told delegates that the “root cause of the climate emergency is maldevelopment” and emphasised the importance of pursuing adaptation, mitigation and development goals together.

He noted that the Intergovernmental Panel on Climate Change is currently working on a special report on climate change and cities and argued that “cities will play a decisive role in shaping global climate futures”.

He continued:

“Cities concentrate opportunities, but they also concentrate poverty, inequality and risk. And that’s something that we really don’t know how to understand, especially in a changing climate.”

Throughout the conference, many of the delegates presented nuanced stories of rural-to-urban migration from individual communities. These case studies highlighted the complex, interlinking factors that drive a person’s decision to move and the wide range of outcomes.

Dr Aysha Jennath from the IIHS presented the results from her research, which unpacks the experiences of migrants who have moved from rural to urban areas, for a range of reasons including the changing climate and for better livelihoods.

Jennath and her colleagues interviewed thousands of migrants living in informal settlements, or working in informal jobs, in large cities in Bangladesh, Bhutan, India and Nepal. The researchers’ questions aimed to understand the migrants’ “wellbeing, adaptive capacity and precarity”.

Overall, Jennath found that migrants in large cities are vulnerable to poor housing, unsafe working conditions and a lack of basic social services.

Dr Binaya Pasakhala and Dr Sabarnee Tuladhar from the International Centre for Integrated Mountain Development, presented initial results from the Climate Adaptation and Resilience (CLARE) project, in which researchers interviewed households across Bangladesh, Bhutan, India and Nepal about migration patterns.

They conducted hundreds of surveys to identify how households are adapting to the changing climate and grouped responses into a series of “pathways” describing the impacts of rural-to-urban migration on their livelihoods.

For example, Tuladhar noted that in Bhutan, there is a huge emphasis on education, which has “changed the aspirations of the community – especially the youth”. This drives “huge depopulation” from rural areas as young, educated people migrate to urban areas or internationally, she said.

This mass movement into the cities provides opportunities for young people. It also provides money for the families back home – a type of finance known as remittances.

However, it also “weakened resilience” in the villages through “gungtong” – a phrase which translates literally to “empty houses”.

However, they also described the case of Nepal’s Baragon mountain community, where remittances from people who moved to urban centres has allowed communities in the villages to shift livelihoods away from subsidence farming towards commercialised farming and tourism. In this case, “migration has actually strengthened the resilience of the community”, Tuladhar said.

Prof Nitya Rao is a researcher in gender and development at the University of East Anglia (UEA), also presented research funded by CLARE.

She told the conference that when men are forced to leave for work, due to a lack of other options, a lot of their earnings go towards “survival” and less is saved. On the other hand, “mixed migration” – such as the movement of a father and son – is often “aspirational”. It typically yields higher remittances and improves adaptive capacity back home, according to Rao.

Speaking to Carbon Brief, Rao argued that in order to “make migration a case of adaptation and not just survival in the short term”, destination cities need to do more to welcome migrants.

Dr Maria Franco Gavonel, a lecturer at the University of York and Prof Mumuni Abu, a senior lecturer from the University of Ghana, explored the concept of “social tipping points” in migration decision-making.

They suggested that as a drought intensifies, there may be a threshold at which households decide to leave. The authors compared drought indices to immigration patterns across communities in Ghana, Mali, Kenya and Ethiopia, but did not find evidence of a social tipping point.

This could be because households anticipate severe droughts and leave before they hit, the speakers suggested. They also noted that there are many government-led policy responses to drought that could affect a household’s decision to stay or leave.

For example, Kenya has a livestock-insurance policy to help families who lose animals during drought. Similarly the African Union uses satellite data to assess the severity of droughts and provide compensation to affected households.

In the final session of the conference, Dr Kasia Paprocki, an associate professor of environment at the London School of Economics and Political Science, provided a counterpoint to the idea that the vast majority of villagers want to abandon farming and move to the city.

She argued that people are often displaced from rural communities and unable to live farming lifestyles, even if they want to, adding:

“I have found that agrarian dispossession is being intensified through development interventions that are today being referred to as climate change adaptation.”

She argued for the need to “reorganise economies” to enable people to stay “if they would like to”, adding:

“Climate change adaptation and climate migration without meaningful agrarian reform will not produce climate justice.”

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Immobility and relocation

Movement from rural to urban areas was not the only migration pattern discussed in the conference. Experts also discussed movement patterns including planned relocation and immobility.

The graphic below – adapted from the 2021 Groundswell report and originally published in Carbon Brief’s 2024 explainer on climate-driven migration – shows different categories of mobility and immobility due to climate change.

Dr Roman Hoffmann from the International Institute for Applied Systems Analysis’s migration and sustainable development research group opened a session on “immobility” by presenting a way of defining and measuring the phenomenon.

He told Carbon Brief that immobility is “basically the absence of movement”, adding:

“The are different types of immobility. We have voluntary and involuntary immobility – and sometimes these different forms are not so clearly distinguishable, but there’s more sort of a continuum. Basically, the question is whether people are able to realise their aspirations to move or to stay.”

In his talk, Hoffman noted that media narratives around migration often focus on large movements of people, while the topic of immobility “falls between the cracks”.

Immobility is often seen as a problem experienced by the poorest and most vulnerable members of society – for example, because people cannot find or afford the resources they need, such as food or transportation, because they are not healthy enough to move or because they do not have the social network they require to make such a big change.

However, Dr Joyce Soo from the Lund University Centre for Sustainability Studies, explained that there are also instances when “wealth enables immobility”.

Soo explained that in coastal regions of Sweden that are exposed to extreme events, many residents there choose to stay, as there is “strong trust in government protection”, such as coastal defences. She explained that in this instance “immobility is linked to identity and status”.

A separate session at the conference focused on planned relocation – the organised movement of a group of people away from a site that is highly vulnerable to climate extremes.

Dr Ricardo Safra de Campos, a senior lecturer in human geography at the University of Exeter, told the delegates that planned relocation is “arguably the most controversial aspect of mobility as a response to climate change” and is usually implemented when “all other forms of in-situ adaptation have failed”.

Safra de Campos and Nihal Ranjit, a senior research associate at IIHS, worked with a team of researchers to interview people who underwent planned relocation programmes in India and Bangladesh.

They told delegates that planned relocation is often implemented when people feel unsafe – for example due to climate extremes – resulting in an “erosion of habitability”.

However, Ranjit explained “safety alone doesn’t make relocation successful”. He argued that the most important aspect of planned relocation is to ensure that migrants do not lose their livelihoods.

He presented the example of Ramayapatnam – a fishing village in India where houses were slowly being lost to coastal erosion. Ranjit explained that a planned relocation programme was set up to move people away from the coast, but that many people refused to move, as doing so would mean losing their only means of earning money.

He also noted the many Indian citizens hold a deep mistrust of the government and question the authorities’ intentions.

Relocation must be “rights-based, participatory, livelihood-centred and attentive to culture, community and long-term wellbeing”, Ranjit said.

Meanwhile, Dr Annah Pigott-McKellar, a human geographer at the Queensland University of Technology, compared two case studies of relocation in Australia.

When devastating flash floods hit Queensland in January 2011, a relocation programme led by the local government was set up to move people. The first houses were built within a year, and people were moved in “extremely fast”, Pigott-McKellar said. She explained that the goal was to keep the town together and “keep some level of social continuity”.

Conversely, when northern New South Wales faced severe flooding in 2022, the response was slow, according to Pigott-McKellar. She explained that different members of the community were offered varying levels of assistance by the state. For example, some households offered buybacks for their lost properties, while others were not.

The result was a “fragmented and dispersed mobility pathway” that saw the community split up and mistrust in the government grow.

Pigott-McKellar emphasised the importance of follow-through and continuity in relocation, stating:

“Relocation isn’t a moment in time. It is a process that unfolds over months or years”.

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Legal pathways

Most human migration happens within borders. However, conference delegates also discussed cases in which people move to other countries, with a focus on the possible legal pathways.

Prof Jon Barnett, professor in the school of geography, Earth and atmospheric sciences at the University of Melbourne, explained migration patterns in the south Pacific islands.

He told delegates that climate change is causing “significant social impacts” across the islands, adding:

“While we can’t say that climate change is a major factor in migration decisions…there is a “fingerprint of climate change in [all] migration decisions.”

Barnett outlined legal migration routes for Pacific islanders, such as Fiji’s climate relocation trust fund, which has already had more than 2,000 requests, or seasonal worker schemes to New Zealand, which have already issued 137,000 visas.

However, he noted that there is a “massive burden” for the women who stay on the Pacific islands when their husbands leave. He explained that not only do women substitute for the labour of the men, but climate change can also amplify their workload by making farming more difficult and illnesses more widespread.

He concluded:

“Migration cannot be the only adaptation strategy we offer to the Pacific Islands. It’s got to be one strategy in the portfolio.”

Speaking separately to Carbon Brief, he said:

“As climate change amplifies pressures on people’s livelihoods, we may end up with a whole series of transnational populations that are kind of constantly in churn – where they’re not just living on the island, but also in Australia, New Zealand, the US.

“That’s not necessarily a bad thing, I think, so long as people still have a right to return to their islands and can do so – and are making informed choices…to manage their climate risk.”

Demographer Prof Raya Muttarak, from the University of Bologna, told delegates that Italy is the only EU country with explicit legislation for climate-related protection.

This six-month residence permit was introduced in 2018, for people who are found to have faced a “contingent and exceptional calamity”. However, she noted that there are flaws in the evidence base for making these claims, which can make it difficult for people to obtain the permits.

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Changing narratives

Many speakers discussed the framing of climate change and migration in their talks. There was also a workshop on how to develop and promote “new narratives” around migration as an adaptation response to a changing climate on the first day of the conference.

Dr Reetika Subramanian, a senior research associate at UEA who helped to organise the conference, told Carbon Brief that many media narratives around migration are “alarmist” and “crisis-based”, with a focus on people from poorer countries illegally entering wealthier countries.

However, explained that the conference convenors wanted to begin work on developing a new framing for migration – both in response to climate change and more generally – focusing on its “adaptive aspects”.

Dr Benoy Peter, the executive director of the Centre for Migration and Inclusive Development, told Carbon Brief that “far right” media and politics often “leverage” migration to present a negative framing.

However, he said that he sees migration as a “solution”, describing it as the “fastest way for intergenerational upward social mobility for people from socially and economically disadvantaged populations”.

Prof Kerilyn Schewel, assistant professor of sociology at the University of North Carolina at Chapel Hill, told Carbon Brief that the migration community has “moved beyond a ‘push factor’ narrative – that climate change is coming and uprooting communities – to a more nuanced perspective that recognises that people are already moving for all kinds of reasons”.

She said the new “research frontier” is “seeing how environmental factors intersect with these other social or developmental outcomes”, such as education.

Liby Johnson, the executive director of development organisation Gram Vikas, told the conference his reason for hope:

“Communities are figuring this out. They are not rejecting mobility – they are asking for mobility that is safer, fairer and more dignified. Communities affected by climate uncertainty are not simply enduring crises – they are actively using mobility to diversify risk, protect dignity and build better futures.”

Revi, from the IIHS, told Carbon Brief:

“The future of mobility is much more certain than the climate futures are. People have been mobile for a very long time. That’s been an important part of the transformation of societies and economies for centuries…Mobility is part of the solution. It is not the full solution, but it’s part of the solution. People are voting with their feet and with their aspirations to make a change.”

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The post Experts: Why migration is ‘not a failure of adaptation’ in a warming world appeared first on Carbon Brief.

Experts: Why migration is ‘not a failure of adaptation’ in a warming world

Hundreds of scientists in dozens of institutions are embarking on the next phase of the world’s largest coordinated climate-modelling effort.

Climate-modelling groups use supercomputers to run climate models that simulate the physics, chemistry and biology of the Earth’s atmosphere, land and oceans.

These models play a crucial role in helping scientists understand how the climate is responding as greenhouse gases build up in the atmosphere.

For four decades, the Coupled Model Intercomparison Project (CMIP) has guided the work of the climate-modelling community by providing a framework that allows for millions of results to be collected together and compared.

The resulting projections are used extensively in climate science and policy and underpin the landmark reports of the Intergovernmental Panel on Climate Change (IPCC).

Now, the seventh phase of CMIP – CMIP7 – is underway, with more than 30 climate-modelling centres expected to contribute more than five million gigabytes of data – so much that downloading it using a fast internet connection would take two and a half years.

Here, we look at what is new for CMIP7, including its model experiments, updated emissions scenarios and “assessment fast track” process.

What is CMIP?

Around the world, climate models are developed by different institutions and groups, known as modelling centres.

Each model is built differently and, therefore, produces slightly different results.

To better understand these differences, CMIP coordinates a common set of climate-model experiments.

These are simulations that use the same inputs and conditions, allowing scientists to compare the results and see where models agree or differ.

The figure below shows the countries that have either produced or published CMIP simulations.

During this time, scientists use new and improved models to run experiments from previous CMIP phases for consistency, as well as new experiments to investigate fresh scientific questions.

These simulations produce a trove of data, in the form of variables – such as temperature, rainfall, winds, sea ice extent and ocean currents. This information helps scientists study past, present and future climate change.

As scientific understanding and technical capabilities improve, models are refined. As a result, each CMIP phase incorporates higher spatial resolutions, larger ensembles, improved representations of key processes and more efficient model designs.

CMIP7 objectives

Each CMIP phase has an “experimental design” that outlines which climate-model experiments should be run and their technical specifications, including the time period the models should simulate.

The CMIP7 experimental design has several components.

As in CMIP6, for a modelling centre to contribute, they are asked to produce a suite of experiments that maintain continuity across past and future CMIP phases.

This suite of experiments is known as the “diagnostic, evaluation and characterisation of klima” (DECK) and is used to understand how their model “behaves” under simple, standard conditions. These experiments are designed and requested directly by CMIP’s scientific governing panel.

Alongside the DECK, CMIP also incorporates experiments developed by model intercomparison projects (MIPs) run by different research communities. For example, experiments exploring what the climate could look like under different levels of emissions or those that explore how sea ice might have changed between the last two ice-ages.

Currently, CMIP is working with 40 MIPs. These groups investigate specific scientific questions at their own pace, rather than on timelines prescribed by CMIP.

Running a large number of simulations can take modelling centres a long time. To speed up the process, CMIP7 has launched the “assessment fast track”.

This is a small subset of CMIP7 experiments, drawn from past and present community MIPs, identified through community consultation as being critical for scientific and policy assessments.

Data from the assessment fast track will be used in the reports that will together form the seventh assessment (AR7) of the IPCC.

It will also be used as an input by other groups that create climate information, including organisations involved in regional downscaling and modelling climate impacts and ice-sheet changes.

The figure below shows the different components of CMIP7. It shows how a subset of CMIP7 experiments will be delivered on an accelerated timeline, while the majority of experiments will be led by MIPs.

CMIP7 experiments

There are three categories of experiments set to take place in CMIP7:

• Historical experiments, which are designed to improve scientific understanding of past climates. Model runs exploring the recent historical period also allow scientists to evaluate the performance of models by checking how well they replicate real-world observations.
• Prediction and projection experiments, which allow scientists to analyse what different climates could look like under varying levels of greenhouse gas emissions, as well as near-term (10-year) prediction experiments.
• Process understanding experiments, which are designed to better understand specific processes and isolate cause-and-effect relationships. For example, a set of experiments might change the emissions of one greenhouse gas at a time to see how much each pollutant contributes to warming or cooling the climate.

Modelling centres typically produce and publish their data for the historical and projection experiments first.

CMIP expects the first datasets to be available by this summer, with broader publication recommended by the end of the year, in time to be assessed by IPCC AR7 authors.

Drafting of the reports of AR7 is currently underway. However, countries are yet to agree on the timeline for when they will be published. This presents a challenge for the climate-modelling community, given the difficulties of working with a moving deadline.

(For more on the ongoing standoff between countries around the timing of publication of the reports, read Carbon Brief’s explainer.)

New emissions scenarios

Scientists use emissions scenarios to simulate the future climate according to how global energy systems and land use might change over the next century.

Crucially, these scenarios – also known as “pathways” – are not forecasts or predictions of the future.

The group tasked with designing the scenarios for CMIP phases, as well as producing the “input files” for climate models, is the “scenario model intercomparison project”, or ScenarioMIP.

In a new paper, the group has set out the new set of scenarios for CMIP7:

• High (H): Emissions grow to as high as deemed plausibly possible, consistent with a rollback of current climate policies. This scenario will result in strong warming.
• High-to-low (HL): Emissions rise as in the high scenario at first, but are cut sharply in the second half of the century to reach net-zero by 2100.
• Medium (M): Emissions consistent with current policies, frozen as of 2025, leading to a moderate level of warming.
• Medium-to-low (ML): Emissions are slowly reduced, eventually reaching net-zero emissions by the end of the century.
• Low (L): Emissions consistent with likely keeping warming below 2C and not returning to 1.5C before the end of the century.
• Very low (VL): Emissions are cut to keep temperatures “as low as plausible”, according to the paper. This scenario limits warming to close to 1.5C by the end of the century, with limited overshoot beforehand.
• Low-to-negative (LN): Emissions fall slightly slower than in the VL scenario, with temperatures just rising above 1.5C. Emissions then rapidly drop to negative to bring warming back down.

The figures below show the emissions (left) and the estimated global temperature changes (right) under the seven new scenarios for CMIP7, from the low-to-negative emissions scenario (turquoise) to a high-emissions scenario (brown).

As a set, the ScenarioMIP scenarios “cover plausible outcomes ranging from a high level of climate change (in the case of policy failure) to low levels of climate change resulting from stringent policies”, the paper says.

Compared to the scenarios in CMIP6, the range in future emissions they cover is now narrower, the authors say:

“On the high-end of the range, the CMIP6 high emission levels (quantified by SSP5-8.5) have become implausible, based on trends in the costs of renewables, the emergence of climate policy and recent emission trends…At the low end, many CMIP6 emission trajectories have become inconsistent with observed trends during the 2020-30 period.”

Put simply, progress on climate policies and cheaper renewable technologies means that scenarios of very high emissions have now been ruled out.

However, this progress has not been sufficient to keep society on track for the Paris Agreement’s 1.5C goal. The paper notes that, “at this point of time, some overshoot of the 1.5C seems unavoidable”.

The change to the high end of the scenarios has sparked misleading commentary in the media and on social media – even from US president Donald Trump. A Carbon Brief factcheck unpacks the debate.

Also notable in the new scenarios is the “low-to-negative” pathway, which has the explicit feature of emissions becoming “net-negative”. In other words, through carbon dioxide removal (CDR) techniques, society reaches the point at which more carbon is being taken out of the atmosphere than is being added through greenhouse gas emissions.

Reaching net-negative emissions is fundamental to “overshoot scenarios”, where global warming passes a target and then is brought back down by large-scale CDR.

Overshoot scenarios allow scientists and policymakers to investigate the impacts of a delay to emissions reductions and better understand how the world might respond to passing a warming target. This includes the question of whether some impacts of climate change, such as ice sheet melt, are reversible.

CMIP has encouraged modelling centres to run simulations using the “high” and “very low” scenarios first to ensure downstream users of the data – including groups working on regional climate projections (CORDEX), climate impacts modelling (ISIMIP) and ice-sheet modelling (ISMIP) – have enough time to produce their data for IPCC reports.

These two scenarios were selected as they sit at opposite ends of the spectrum of climate outcomes. The high scenario will demonstrate how models behave under high emissions, while the very low scenario will demonstrate how models behave when emissions are rapidly reduced.

CMIP has recommended that modelling centres then run the “medium” and “high-to-low” scenarios. The remaining scenarios should then follow and no official recommendation has been made yet on their production order.

Other new features

In addition to the assessment fast track and new scenarios, CMIP7 has a number of other new developments.

Updated data for simulations

Climate models use input datasets to define the set of external drivers – or “forcings” – that have caused the global warming observed so far. These drivers include greenhouse gases, changes to incoming solar radiation and volcanic eruptions.

CMIP recommends modelling groups use the same input datasets, as this makes it easier to compare model results.

In CMIP7, the historical forcing datasets available for modelling groups to use have been improved to better represent real-world changes and extended closer to the present day. The historical simulations will be able to simulate the past climate from 1850 through to the end of 2021, whereas CMIP6 only simulated the past climate through to 2014.

CMIP is also planning to extend these historical datasets through to 2025 and maybe further throughout the course of CMIP7.

Emissions-driven simulations

CMIP7 introduces a new focus on CO2 emissions-driven simulations, providing a more realistic representation of how the climate responds to changes in emissions.

In older generations of climate models, atmospheric levels of CO2 and other greenhouse gas concentrations have been needed as an input to the model. These levels would be produced by running scenarios of CO2 emissions through separate carbon cycle models. The resulting climate-model runs were known as “concentration-driven simulations”.

However, many of the latest generation of models are now able to run in “emissions-driven mode”. This means that they receive CO2 emissions as an input and the model itself simulates the carbon cycle and the resulting levels of CO2 in the atmosphere.

This development is important, as climate policies are typically defined in terms of emissions, rather than overall atmospheric concentrations.

This new development in modelling will enable a more realistic representation of the carbon cycle and a better understanding of how it might change under different levels of warming.

Enhanced model documentation and evaluation

All CMIP7 models will be required to supply standardised model documentation that ensures consistency across model descriptions and makes it easier for end users to understand the data.

Additionally, CMIP scientists have developed a new open-access tool that dramatically speeds up the evaluation of climate models.

This “rapid evaluation framework” allows researchers to compare model outputs with real-world observations, providing immediate insight into model performance.

The post Guest post: How CMIP7 will shape the next wave of climate science appeared first on Carbon Brief.

Guest post: How CMIP7 will shape the next wave of climate science