Emissions Scenarios

The last lesson showed that a projection is conditional - an 'if-then' whose 'if' is an emissions path. This lesson is about that 'if', because it is the single most consequential assumption in the whole chain. The reason is a fork built into the climate system: the warming a building will face depends overwhelmingly on the cumulative greenhouse gases humanity emits over the coming decades, and that quantity is not determined by nature. It is the sum of countless human decisions about energy, industry, transport, land and policy - decisions that have not been made and cannot be forecast like physics.

So the future genuinely forks. In a world that cuts emissions hard and fast, warming is held relatively low and the climate of 2100, while clearly changed, stays within a range buildings can be designed to survive. In a world that keeps burning fossil fuels heavily, warming by 2100 is far greater, pushing heat and humidity in places like India toward and past the limits of what passive design - or even human physiology - can handle. These are not small differences around a central estimate; by late century they are the difference between difficult and catastrophic. The gap is not scientific uncertainty about how the climate works; it is uncertainty about what people will choose to do.

This is why projections cannot be collapsed into one number, and why the honesty of the field starts here. A climate model is a machine for answering 'if we emit like *this*, then the climate does *that*' - it has nothing to say about which 'this' will happen, because that lives in politics and economics, not physics. The scenario is the assumption a human must supply. And because the scenarios diverge so far, the assumption is not a minor input to be defaulted past; it can dominate the entire result, easily outweighing the difference between two building designs. Note too that the fork is set by *cumulative* emissions - the running total we add over decades - not by the emissions of any single year, which is why choices made soon matter so much and why the branches keep spreading the longer a building lives. A designer who ignores the scenario question has not avoided it - they have simply let someone else, or a software default, make a value-laden choice on their behalf, invisibly.

To run models under agreed, comparable futures, scientists needed a common set of emissions stories. One widely used family is the Representative Concentration Pathways, or RCPs. An RCP is labelled by the extra heating it eventually adds to the planet - technically the radiative forcing, measured in watts per square metre, by 2100. So RCP2.6 describes a world of deep, rapid emissions cuts that adds about 2.6 watts per square metre and holds warming relatively low; RCP8.5 describes a very high-emissions world adding about 8.5 watts per square metre and warming far more; RCP4.5 sits in between. The number is not a temperature - it is a measure of how much extra energy the atmosphere traps, which then drives the warming.

The point of the RCP framework is comparability. Because every modelling group runs the same handful of pathways, their results can be compared like for like, and a designer or engineer can ask, 'under RCP4.5 versus RCP8.5, how differently would this building perform?' The pathways were deliberately chosen to span a wide range, from strong mitigation to little mitigation, so that the fan of outcomes brackets the plausible futures rather than guessing a single one. In effect the RCPs turn an unknowable future into a small, shared set of well-defined 'what ifs' that everyone can test against - which is exactly what makes them useful for design, where the goal is to bracket the risk rather than to name the outcome.

Two honest cautions travel with the RCPs. First, they are labelled by an end-of-century outcome, but the emissions and concentrations along the way matter too, and a building's mid-century decades are governed by the nearer part of the path. Second, the highest pathway, RCP8.5, is often loosely called 'business as usual', which is misleading: it represents a particularly high-emissions, coal-heavy future, and treating it as the single default - or, conversely, dismissing it as impossible - are both errors. RCP8.5 is best used as a high-end stress case, a way to ask 'is this building still survivable if warming runs hot?', not as a prediction. The RCPs are a shared language for the fork, not a forecast of which branch we will take.

The RCPs describe *how much* heat gets trapped but say little about *why* - what kind of world produced those emissions. A newer, complementary framework fills that gap: the Shared Socioeconomic Pathways, or SSPs. Each SSP is a narrative about how society might develop - population, economic growth, technology, inequality, energy choices, international cooperation - and therefore how easy or hard it would be to cut emissions. SSP1 sketches a sustainability-focused world that shifts to clean energy; SSP2 is a 'middle of the road' continuation of current trends; SSP5 is a world of rapid, fossil-fuelled economic growth; other SSPs describe fragmented or highly unequal worlds. The SSPs answer the question the RCPs leave open: what human story leads here?

In current climate assessments the two are combined, written as, for example, SSP1-2.6 or SSP5-8.5: the SSP names the socioeconomic story and the number gives the resulting radiative forcing, tying a plausible human world to a level of warming. This pairing is powerful because it makes the choice of scenario legible as a choice about the *world*, not just a dial of watts per square metre. It also makes clear which combinations are coherent - a high-mitigation narrative simply cannot produce the highest forcing, and vice versa.

For a designer, the value of the SSPs is that they reframe scenario choice honestly. Selecting a scenario is not picking a number off a chart; it is implicitly betting on which kind of future the building will live in - a cooperative, decarbonising world or a high-emissions one. That is exactly why the choice is value-laden rather than technical: two people can look at the same evidence and reasonably assume different worlds, because the disagreement is about the future of human society, not about physics. It is worth knowing the vocabulary well enough to read what a consultant's projection assumes - whether the future weather file you are handed rests on an optimistic SSP1 world or a pessimistic SSP5 one - because that single assumption can move the projected heat by more than any design decision you will make, and if it is buried in a footnote you may never see the bet that was placed on your behalf.

Everything above leads to an unavoidable, uncomfortable fact: to produce any future weather file, someone must choose a scenario, and there is no neutral, purely technical way to do it. The choice reflects values - how you weigh the cost of over-preparing against the risk of under-preparing, how much you trust the world to decarbonise, whose safety you prioritise. Picking a low scenario assumes an optimistic world and designs for less heat; picking a high scenario assumes a pessimistic world and designs for more. Neither is 'the truth'; both are bets, and the responsible course is to make the bet consciously rather than let a software default make it silently.

For buildings, the logic of the choice tilts toward caution, and it is worth being explicit about why. A building is long-lived, expensive to retrofit, and a place people must stay safe in during extremes - and the *cost of the two errors is asymmetric*. If you design for a hotter scenario than materializes, you have a building that is more resilient than strictly needed - some extra shading, ventilation, thermal mass. If you design for a cooler scenario than materializes, you have a building that overheats dangerously in heatwaves it was never prepared for, when retrofitting is hardest. Because passive survivability is a life-safety matter - acutely so in a hot, humid, populous country like India - designing against a higher-end scenario, or better, testing across a range that includes it, is the defensible stance. But this too is a value judgement about risk, made honestly, not a fact read off a model.

The disciplined practice, then, is not to hunt for the 'correct' scenario but to be transparent about the one assumed, to test the design across more than one - typically a moderate and a high pathway - and to design for robustness across that range rather than optimising to a single chosen future. And the binding results stay where they belong: the building-physics, energy, comfort and climate-risk engineering, and any compliance or life-safety determination, rest with qualified specialists, validated tools, verified data and the governing codes (NBC India, ECBC, IS). Your job is to make the scenario assumption visible and deliberate - because an invisible scenario is an invisible value judgement, and this field's honesty depends on refusing to hide it.