Proposed methodology for the Fourth Climate Change Risk Assessment – Independent Assessment (CCRA4-IA) 

In 2026 the Climate Change Committee (CCC) will deliver its assessment of the risks and opportunities from climate change and the potential for adaptation to address them.

Published:
15 May 2024

Type of publication:
UK climate risk

Country focus:
UK

Topics:
Adaptation

Contents

3. Proposed methodological approach for CCRA4-IA

In developing a methodology for the CCRA4-IA we have followed a set of key principles:

  • Robustness: the method should be consistent with the most up-to-date scientific understanding and based on up-to-date scenarios for plausible future global climate change and UK socioeconomic development. We will therefore anchor our assessment around the most up-to-date assessments of global emissions pathways and understanding of climate response uncertainties.
  • Policy needs: the method should align the development of evidence to policy and other user needs. We will use accepted datasets where possible to facilitate its relevance for policy. We are increasing our focus on the largely inevitable climate changes between the present day and the middle of the century to increase policy relevance, whilst acknowledging the need to prepare for a wide range of climates in the second half of the century.
  • Simplicity: the method should be as simple as possible, to aid communication and impact of the analysis, despite the need to make decisions against a backdrop of several dimensions of future uncertainty.

These have been informed by a set of inception reports aiming to help identify how climate risk information can be most useful for driving adaptation action in the CCRA4-IA. Key aspects of the method are detailed in the sub-sections below.

3.1 Outcome areas

For producing CCRA4-IA we will organise the production of our analysis of climate change risks and opportunities under a set of ‘outcome areas’, each corresponding to groupings of key assets or systems where risks from climate change threaten explicit or implicit societal outcomes or goals. These outcome areas serve as a working structure to ensure that an appropriate range and diversity of risks and adaptation are considered across the CCRA4-IA analysis programme. In the final outputs from CCRA4-IA we will seek to organise our presentation of the evidence to best serve the needs of the adaptation policy structures in use around the UK at the time of publication, several aspects of which are currently in development by the UK and devolved administration governments.

The outcome areas we are using for developing CCRA4-IA are:

  • Economy: business and industry sites, assets, employees, and supply chains; the UK’s financial system.
  • Health: health, and health and social care services.
  • Built environment: the built environment at all scales, the cultural heritage it contains, and the services and community functions that depend on it.
  • Infrastructure: energy, transport, communication and water/wastewater infrastructure.
  • Land, nature and food: the natural environment, rural land-use and landscapes, and the systems of food production.

Risks and opportunities from climate change (arising from both changes in the UK climate and internationally) to these outcome areas will be assessed in the CCRA4-IA Technical Report.

In the Well-Adapted UK Report we will ensure that we take forward analysis spread across these outcome areas to enable good coverage of the sectors of urgent adaptation assessed in our Adaptation Monitoring Framework. We are currently undertaking analysis of adaptation action on UK farmland to multiple hazards, adapting the UK’s urban environments for heatwaves, an analysis of the adaptation of water usage under periods of drought, adaptation to ensure effective health and social care delivery during heatwaves, and building heat resilience of the UK’s cold-supply chains. These correspond to many of the most urgent risks identified by the CCRA3-IA. We will seek to supplement these with further areas of analysis later in the CCRA4-IA production process.

3.2 Time periods for risk assessment

Risks and opportunity will be assessed across the CCRA4-IA for four different time periods, as the evidence allows.

  • Present: risk and opportunities from the range of possible weather and climate conditions possible today.
  • 2030s: a near-term reference period centred on 2035, to represent the climate for which the next round of national adaptation programmes will need to fully prepare for.
  • 2050s: a mid-century reference period, centred on 2055, consistent with the end of the period of ‘inevitable’ climate change, regardless of the trajectory of global greenhouse gas emissions over the next few decades.
  • 2080s: a late-century reference period, centred on 2085, used to consider the implications of further climate change beyond the middle of the century, particularly for long-lived assets.

These periods will allow CCRA4-IA to focus, more than in previous assessments, on near-term risks whilst also considering the wide range of climates and associated risks possible in the second half of the century.

3.3 Future climate change

There are many sources of uncertainties in future global and UK climate projections, all of which need to be considered when undertaking a risk assessment.

  • Uncertainty in future global greenhouse gas emissions: countries have established targets for emissions reduction by 2030 under the Paris Agreement but, even if delivered in full, these could be consistent with a wide range of global emissions trajectories over the rest of the century.
  • Uncertainty in the amount of global warming in response to greenhouse gas emissions: there remains a wide range of uncertainty in the amount of global warming expected due to an additional emission of carbon dioxide or other greenhouse gases into the atmosphere.
  • Uncertainty in changes in the UK weather and climate at a particular global warming level: expected changes in aspects of UK weather and climate can vary across projections even at the same level of global average warming. Low likelihood but high impact changes, such as a collapse in the Atlantic overturning circulation could also drastically change future UK weather patterns, as can natural climate variability for specific time periods.

CCRA4-IA will use an updated approach to considering the range of plausible climate futures consistent with the most recent assessments of global and UK climate outcomes published by the IPCC and other key global evidence providers. This will build on and extend the approach used in past CCRA-IAs.

Key aspects of this proposed approach include:

  • A focus on futures consistent with up-to-date projections for plausible future greenhouse gas emissions given current climate policy efforts in place around the world (Box 1).
  • Considering a range of possible global warming levels at each time period consistent credible global emissions pathways and IPCC assessments of uncertainty in the global climate response to greenhouse gas emissions (Box 2).
  • Where possible, undertaking a systematic sampling of the range of UK climate changes for a given global warming level.
Box 1

Plausible future pathways of global greenhouse gas emissions
Significant progress has been made on greenhouse gas emissions reduction ambition since the Paris Agreement in 2016, with many countries committing to Net Zero targets and providing more ambitious Nationally Determined Contributions (NDCs). This progress has reduced the modelled likelihood of extreme future climate outcomes and impacts to global society, for example those projected under scenarios of large increases in global emissions. However, there remains a significant shortfall between current global emissions reduction efforts and pathways consistent with the long-term temperature goal of the Paris Agreement of holding global warming well-below 2°C above preindustrial levels and pursing efforts to keep it below 1.5°C above preindustrial levels.

For CCRA4-IA we will endeavour to anchor our assessment to the range of near-term and long-term global emissions trajectories that remain plausible under the most up-to-date assessments of the global climate policy outlook (Figure 1).

Figure 1 Projections of future ranges of global greenhouse gas emissions consistent with current policy pathways. A line chart, with shaded uncertainty range, showing the projected global emissions of greenhouse gases from 2010 to 2100 that are consistent with current policy. The chart has five lines, representing each scenario variant under central assumptions. All five of these scenarios show relatively flat global greenhouse gas emissions between 2010 to 2100, ranging from 50 to 60 gigatonnes per year of carbon dioxide equivalent. The estimated uncertainty range increases significantly over time, spanning approximately zero to 100 gigatonnes per year of carbon dioxide equivalent, by the year 2100.

Box 2

Global Warming Levels consistent with current policy futures
Climate uncertainty means that a range of global warming levels are possible for credible future trajectories of global greenhouse gas emissions (Box 1). For CCRA4-IA we will anchor our assessment of climate uncertainty to that of the IPCC in its Sixth Assessment Report.

  • Between now and the middle of the century, continued climate change is largely inevitable. There is only a small variation across global emissions trajectories. Central estimates are for global warming averaged across the 2030s of around 1.5°C above preindustrial levels and around 2°C for the 2050s under current policy trajectories (Figure 2). Uncertainty in climate response means that it remains plausible that global warming might reach around 2°C above preindustrial levels as early as the 2030s and could be 2.5°C above preindustrial levels by the 2050s.
  • In the second half of the century, a wide range of global warming levels remain possible depending on both global emissions trajectories and climate response uncertainty. Central estimates under trajectories consistent with current policy indicate around 2.5-3°C of warming above preindustrial levels, but If current NDC and Net Zero targets are delivered central estimates of late-century warming would keep to levels of between 1.5–2°C above preindustrial – although sea-levels will keep rising for centuries to come. At the high-end of plausible climate responses, global warming of around 3.5°C above preindustrial in the 2080s remains possible with some current policy scenarios reaching and exceeding 4°C above preindustrial levels by 2100.

Figure 2 Projected 50th and 90th percentile global warming relative to 1850–1900 for the 2030s, 2050s, 2080s and 2100. A box and whisker diagram showing projected global temperature increases, relative to the Intergovernmental Panel on Climate Change baseline of 1850-1900, for the 50th percentile (central estimate of warming) and 90th percentile (the upper estimate of warming). There are four box and whiskers for each of the two percentiles, each representing a different decade (2030s, 2050s, 2080s and 2100). In the 2030s, the median temperature increase is approximately 1.5 °C for the central estimate and 2°C for the upper estimate, with a very narrow range of less than plus or minus 0.1°C. In both estimates, temperatures increase each decade up to 2100, with a sharper increase in the upper estimate of warming than in the central estimate. By 2100, the median temperature in the central estimate is 2.7°C with a range of 1.7°C to 3.5°C, and the median temperature for the upper estimate is 3.5°C with a range of 2.3°C to 4.5°C.

For analysis feeding into the Well-Adapted UK Report – which will focus on pathways of climate risk and adaptation action out to the middle of the century – we will consider climate uncertainty with two scenarios (Table 1) which sample UK as well as global climate uncertainty (Box 3).

  • Central scenario: a middle-of-the-road scenario where the UK climate and the associated changes in climate hazards are at the centre of the expected range projected by climate models for that global warming level.
  • High climate hazard sensitivity: a high global warming scenario where global temperatures increase more rapidly and changes to UK climate and hazards are towards the top of the projected climate model range at that global warming level.
Table 1
Global warming levels and sampling of UK climate hazards to be considered at each time period for the CCRA4-IA assessment
Central scenario High climate hazard sensitivity
Time period 2030s 2050s 2030s 2050s
Global warming level
(above preindustrial levels)		 1.5°C 2°C 2°C 2.5°C
UK climate hazards Median of UKCP18 at 1.5°C Median of UKCP18 at 2°C Upper-end UKCP18 at 2°C Upper-end UKCP18 at 2.5°C

To sample the potential for some of the most damaging changes in the UK’s climate hazards for bespoke research to feed into the Well-Adapted UK Report, we will sample climate hazards consistent with both the centre (Figure 3) and upper end (Figure 4) of the distribution for projected UK climates at given global warming levels.

For example, for projecting UK high temperature extremes for the 2030s, the high climate hazard sensitivity would be drawn from the upper end of the distribution of projected local temperature extreme changes for a global warming level of 2°C – the upper end of plausible global temperature changes over this time horizon (Figure 4).This approach to sampling more extreme variants in future UK climate and climate hazards will ensure that a wide range of possible climate futures are considered in analysis.

For hazards that do not scale well with global mean temperature – for example sea-level rise – a bespoke approach will be used.

Box 3

Sampling UK climate uncertainty for the Well-Adapted UK Report
To sample the potential for some of the most damaging changes in the UK’s climate hazards for bespoke research to feed into the Well-Adapted UK Report, we will sample climate hazards consistent with both the centre (Figure 3) and upper end (Figure 4) of the distribution for projected UK climates at given global warming levels.

 

For example, for projecting UK high temperature extremes for the 2030s, the high climate hazard sensitivity would be drawn from the upper end of the distribution of projected local temperature extreme changes for a global warming level of 2°C – the upper end of plausible global temperature changes over this time horizon (Figure 4).This approach to sampling more extreme variants in future UK climate and climate hazards will ensure that a wide range of possible climate futures are considered in analysis.

 

For hazards that do not scale well with global mean temperature – for example sea-level rise – a bespoke approach will be used.

Figure 3 Projected change in UK hottest day expressed relative to global warming levels for the central scenario. A line chart showing the change in the UK hottest day relative to global warming levels for the central scenario. There are many lines on the chart, with each line showing a different climate model. As global warming levels increase, the change in the temperature of the hottest day increases for each of the models, but at different rates. Vertical dashed lines at global warming levels of 1.5°C and 2°C show the projected warming for the central scenario in the 2030s and 2050s, respectively. The central climate model from the set of climate models is highlighted and shows that the projected change in temperature for the hottest day will increase by 1.45°C in the 2030s and 2.17°C in the 2050s, for the central scenario.

Figure 4 Projected change in UK hottest day expressed relative to global warming levels for high climate hazard sensitivity. A line chart showing the change in the UK hottest day relative to global warming levels, for the high climate hazard sensitivity scenario. There are many lines on the chart, with each line showing a different climate model. As global warming levels increase, the change in the temperature of the hottest day also increases for each of the models, but at different rates. Vertical dashed lines at global warming levels of 2°C and 2.5°C show the projected warming for the high climate hazard sensitivity scenario in the 2030s and 2050s, respectively. The upper climate model line from the set of climate models is highlighted and shows that the projected change in temperature for the hottest day will increase by 4.46°C in 2030s and 5.54°C in the 2050s, for the high climate hazard sensitivity scenario.

For the CCRA4-IA Technical Report, the evidence synthesis approach means a highly prescriptive approach to sampling climate futures is not possible. The CCRA4-IA Technical Report will instead aim to identify evidence consistent with the period of largely scenario-independent changes out to the middle-of-the-century and the wide ranges of climate futures that remain possible in the second half of the century (Box 2).

3.4 Future socio-economic development

Changes in the exposure and vulnerability to weather and climate-related impacts are also important determinants of future climate risk over the coming decades.

For the commissioned research feeding into the Well-Adapted UK Report we will consider changes in UK socio-economics (and their implications for aspects of exposure and vulnerability) alongside changes in climate hazards. We will explore relevant uncertainties in factors of exposure and vulnerability through two scenarios:

  • Central scenario: a middle of the road scenario in which key aspects of UK socio-economics evolve aligned with central projections from widely used official bodies (e.g. population projections from the Office for National Statistics (ONS), and GDP projections aligned with the projections of the Office for Budget Responsibility).
  • High exposure and vulnerability sensitivity: a sensitivity scenario in which relevant aspects of exposure and vulnerability are higher than in the central scenario. The most relevant aspects of exposure and vulnerability will be specific to the individual adaptation analyses but where possible we will standardise ‘high’ variants of factors of exposure and vulnerability relevant across multiple systems (e.g. ‘high’ population variants from the ONS).

The evidence synthesis undertaken in the CCRA4-IA Technical Report is constrained by the socio-economic assumptions already made in the existing evidence base therefore limiting the ability for a systematic assessment of the impact of alternative socio-economic futures in this product.

3.5 Baselines and evaluating costs and benefits

Within our commissioned analysis for the Well-Adapted UK Report, we will include an economic costing and appraisal framework, to enable a comparison of adaptation options and their costs, to inform what a well-adapted UK could look like.

These options will be compared to a ‘baseline’ future – where no additional adaptation actions are taken relative to present-day. The means that existing adaptation assets are assumed to be maintained up to the end of their planned lifetime, and then replaced like-for-like.

To consider possible future adaptation beyond this baseline – we will integrate costing of impacts and adaptation at several stages:

  • Costing ‘business-as-usual’ climate risks: these risks are based on changing hazard, exposure, and vulnerability in the absence of additional adaptation measures (beyond the baseline). Research will aim to comprehensively estimate how associated societal costs are expected to evolve.
  • Assessing the effectiveness of adaptation measures in reducing risks: research will aim to evidence the investment needs and the resulting benefits (avoided damage costs and co-benefits) for a set of the most relevant adaptation options that could be deployed for key systems in the UK over the coming decades. This will build on available evidence on how effective existing adaptation efforts have been in the UK and other relevant parts of the world.
  • Providing evidence on cost-effective levels of resilience: the evidence informing the Well-Adapted UK Report will focus on building national scale scenarios for what cost-effective adaptation looks like for key systems, recognising uncertainties. Building these scenarios of adaptation actions (spatially granular where relevant) will focus on evidencing a cost-effective approach to adaptation (i.e. at what point does additional spending on adaptation not provide sufficiently high additional benefits to justify additional spend within any given scenario) and resulting long-term resilience levels.

This can support the setting of appropriate resilience standards and identify an associated scale of near-term adaptation delivery in the next set of NAPs.

Although powerful, cost-benefit analysis in isolation has its limitations. To seek to address these limitations, we will ensure that broader consideration of distributional analysis, non-market costs and benefits, and uncertainty (including governments’ risk appetite) are considered in building pathways for sectoral adaptation where possible, steered by the input of an independent Economics Advisory Group

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