Every feasibility study C4 prepares starts by fixing a reference climate: the observed baseline period, the emissions scenario and the planning horizon against which risk is assessed. UNEP’s Limiting Overshoot report challenges that starting point directly. On page 78, it states that pre-overshoot conditions — the climate conditions before global warming exceeds 1.5°C — are no longer valid reference conditions, requiring updates to design standards, safety margins and land-use planning. That sentence reaches into five decisions C4 makes on adaptation projects: whether an intervention is feasible, which climate-risk assumptions it rests on, how it is designed, which species it plants and how it adapts once implemented. The report sets out what has changed. This post asks what that change means for each of those decisions. It draws on C4’s work preparing feasibility studies and funding proposals for the Green Climate Fund (GCF), the Global Environment Facility (GEF) and the Adaptation Fund.

The report does not retire 1.5°C, and it is worth saying so, because a good deal of the coverage implied otherwise. UNEP’s Executive Director put it directly at launch: even if we do exceed 1.5°C, 1.5°C remains the goal. Andy Reisinger, a lead author on mitigation, made the same point, that overshoot is not a reason to give up on 1.5°C as a long-term goal. The report’s foreword also states that there are no good outcomes above 1.5°C. That is UNEP’s framing of the risk, and it sits alongside the position that the goal stands rather than against it. What changes is not the destination, but the conditions that projects will operate in on the way there.

Those conditions deserve precision, because the report’s warming projections of 2.6°C and 1.8°C are often quoted without the scenario behind each. For example, under current policies the report puts median projected warming at approximately 2.6°C by 2100, with a range of 1.9 to 3.6°C. Under one of UNEP’s most optimistic scenarios — which assumes implementation of all nationally determined contributions and long-term net-zero targets — central estimates peak at approximately 1.8°C, with a range of 1.7 to 2.2°C. Those are different scenarios and should not be used interchangeably. In all cases, warming exceeds 1.5°C within the next decade. The report finds that if warming peaks above approximately 1.8°C, a return to 1.5°C this century becomes increasingly challenging.

When C4 sets the climate-risk assumptions for a feasibility study, this is the first choice we make explicit. A design built to ~1.8°C assumes that every national pledge and net-zero target is met. It should also be tested against the current-policies range of 1.9 to 3.6°C, because that is the hazard profile the intervention will face if those pledges fall short.

For infrastructure, engineering standards already account for multiple hazards through safety factors, design return periods and service-life assumptions. An asset designed against a 1.5°C pathway will therefore not necessarily fail if warming exceeds 1.5°C. The technical question is which reference conditions those standards were calibrated against: an observed historical record or a projection of future climate. A drainage standard based on historical rainfall records, for example, assumes that the frequency and intensity of extreme rainfall in that record will hold over the asset’s service life. Under overshoot, the historical record becomes a weaker guide to the hazards an asset will face. UNEP’s report is explicit that decisions about where and how infrastructure is built could lock in long-term maladaptation or carry high opportunity costs for many years after temperature stabilisation. The report defines maladaptation as action that increases or shifts vulnerability, most often unintentionally. Whether a particular design results in maladaptation depends on its location, the hazard, the asset life and the risk its owner is prepared to carry. That is answered project by project, not settled by a single global number.

This is where much of C4’s design work sits. The GCF and the GEF expect adaptation finance to build on existing development investment rather than stand alone. Our designs therefore often climate-proof roads, water supply and irrigation infrastructure that governments and development banks have already planned against historical records. The first design question is which of those standards overshoot has made unreliable, and what it costs to revise each one.

Ecosystem-based Adaptation carries a version of the same question, and here the report is specific. It finds that even well-designed Ecosystem-based Adaptation is subject to timing constraints, because many interventions need long lead times to mature while exposure rises rapidly. The report also cites evidence that by 2050, 76% of tree species in urban areas will be at risk from projected changes in mean annual temperature and precipitation under 1.8°C of warming, which hampers the potential of using many of those species in urban nature-based solutions. That finding concerns urban forests rather than planted systems generally, and suitability at any site depends on elevation, soils, water availability, genetics and management. The lesson that transfers to other settings concerns timing. In species selection, a tree suited to a site’s climate at planting may be poorly suited to it by the time it matures. A stable-climate assumption hides that risk.

For the Ecosystem-based Adaptation interventions C4 designs, this changes two decisions. At feasibility stage, lead time becomes a selection criterion alongside cost and effectiveness. An intervention that takes years to deliver protection should be paired with actions that reduce exposure in the meantime. Candidate species lists should be screened against the climate projected at maturity as well as the climate at planting. Where local seed sources are projected to fall outside their suitable range, the list should include provenances from warmer sites 

The obvious objection is that overshoot is temporary. The report’s treatment of carbon dioxide removal complicates that, and it is worth being careful about what it does and does not say. It describes rapid upscaling of removal as critical, so nothing here argues against it. The constraint is scale and timing. Carbon dioxide removal can contribute credibly to a return below 1.5°C this century only if peak warming remains well below 2°C and remaining emissions of carbon dioxide, methane and other greenhouse gases are reduced. At the current scale, the report calculates that over 100 years of afforestation and forest management, with zero residual carbon dioxide emissions, would bring warming back by 0.1°C. That is arithmetic about global temperature at today’s deployment levels. It says nothing about the quality, permanence or value of an individual carbon project, and should not be read as if it did.

The financing position, meanwhile, has not moved. UNEP’s Adaptation Gap Report 2025 put developing-country adaptation needs above US$310 billion a year by 2035 against US$26 billion of international public adaptation finance in 2023. The benefit-cost argument usually offered in response is stronger than I have previously stated: WRI analysed 320 adaptation and resilience investments across 12 countries between 2014 and 2024 and estimated more than US$10.50 in benefits for every US$1 invested over 10 years. The qualification is what those benefits are, namely modelled and expected, not a realised return on every project. The report also highlights that weak mitigation will increase impacts and potentially divert funds to emergency responses instead of proactive climate action, creating a negative feedback loop.

For a feasibility study, this links the finance case to the climate case. The avoided losses that justify an intervention are calculated against reference conditions that overshoot has now moved. An economic analysis run against a single scenario therefore overstates its own certainty. Reporting benefit-cost results across more than one warming scenario gives a funder a more transparent basis for decision.

The report’s assessment of small island developing states highlights that habitability may be lost well before islands are submerged. A further 5–10 cm of sea-level rise, expected approximately between 2030 and 2050, is projected to double flooding frequency across much of the Indian Ocean and tropical Pacific. The magnitude of sea-level rise by 2050 is largely independent of whether warming exceeds 1.5°C, as much of this rise is already committed. For coastal interventions designed to a 2050 horizon, that removes one uncertainty: the additional 5–10 cm should be treated as a design input under any scenario, with overshoot shaping the hazard beyond it. The report also considers the implications for the continuity of statehood, citing the International Court of Justice’s 2025 advisory opinion, which affirms that the loss of territory does not necessarily entail the loss of statehood.

The report identifies agile planning as a practical approach to managing uncertainty. This involves maintaining long-term goals while incorporating monitoring, periodic reviews and defined decision points to adjust actions as conditions change. Flexible pathways, modular investment and decision triggers can reduce lock-in and maladaptation while enabling timely action. In project documentation, this approach requires explicit identification of the warming reference conditions assumed in the design, the scenarios from which they are derived, and the timing and criteria for reviewing those assumptions. 

In C4’s project designs, this is where adaptive management becomes concrete. Each decision trigger requires an observable indicator, a threshold and a pre-agreed response. Examples include a survival rate for planted species below which the species mix is revised, or a rainfall intensity above which drainage capacity is reassessed. Those triggers belong in the results framework and the budget, with a review at mid-term evaluation, rather than in a narrative risk section. Project teams cannot determine whether global warming returns to 1.5°C. They can state the reference conditions each design assumes, test it against more than one scenario and fix the points at which those assumptions are reviewed. For C4, that is what page 78 of the report asks of every project we design.

Sources in the text link to UNEP, Limiting Overshoot: Navigating exceedance of 1.5°C and pathways towards return (2 September 2026); the UNEP Executive Director’s launch statement; the Science Media Centre expert reaction; UNEP Adaptation Gap Report 2025; and WRI, Strengthening the Investment Case for Climate Adaptation (2025). Report page references are given in the accompanying verification note.