RCP Scenarios Quiz: IPCC Pathways and Climate Projections

Representative Concentration Pathways are standardized scenarios developed by the IPCC that describe four different possible trajectories of atmospheric greenhouse gas concentrations through the year 2100. Rather than prescribing specific policies, they are defined by their radiative forcing outcome at 2100 in watts per square meter. They allow climate modelers worldwide to run consistent projections using the same concentration inputs, enabling meaningful comparison of model outputs and assessment of a range of possible futures.

The number in each RCP name refers to the additional radiative forcing in watts per square meter at the year 2100 compared to pre-industrial conditions. RCP 2.6 projects a forcing of 2.6 watts per square meter, requiring deep emissions reductions and leading to relatively limited warming. RCP 8.5 projects 8.5 watts per square meter, associated with continued high fossil fuel use and the most severe projected warming. The different values allow scientists to bracket the range of plausible future climate outcomes from ambitious mitigation to high emissions.

RCP 2.6 is the most ambitious and challenging pathway, requiring global greenhouse gas emissions to peak soon, decline rapidly, and potentially reach net negative levels in the second half of the century. This pathway is broadly consistent with the temperature targets of the Paris Agreement and requires transformative changes across energy, transport, land use, and industry. It implies major deployment of renewable energy, energy efficiency improvements, and potentially negative emissions technologies to remove carbon dioxide already accumulated in the atmosphere.

RCP 8.5 is the highest-emissions scenario, describing a pathway in which global greenhouse gas emissions continue to rise throughout the 21st century driven by continued fossil fuel use, population growth, and limited climate policy action. It projects atmospheric carbon dioxide reaching approximately 935 parts per million by 2100 and is associated with global average temperature increases of approximately 3.2 to 5.4 degrees Celsius above pre-industrial levels, with severe consequences for ecosystems, sea levels, and human societies worldwide.

RCP 8.5 is the highest emissions scenario in the IPCC Representative Concentration Pathway framework, representing a future in which global greenhouse gas emissions continue to rise strongly through the 21st century with very limited mitigation action. Under this scenario, atmospheric carbon dioxide concentrations could reach approximately 940 parts per million by 2100, roughly three times pre-industrial levels. RCP 8.5 serves as the high-end benchmark for assessing worst-case climate projections and the potential consequences of failing to reduce global emissions.

The Shared Socioeconomic Pathways were introduced alongside the updated scenario framework for the IPCC Sixth Assessment Report to add socioeconomic context to concentration pathways. SSPs describe different worlds with different challenges for climate mitigation and adaptation, ranging from sustainable development to fossil fuel dependence. Each SSP can be combined with different forcing levels, creating scenario pairings such as SSP1-2.6 or SSP5-8.5 that link both the socioeconomic story and the resulting greenhouse gas concentration trajectory together.

Future greenhouse gas concentrations depend critically on human choices about energy systems, land use, and policy, none of which are determined by physical laws. Using multiple RCP scenarios allows planners and policymakers to understand the range of possible futures and design strategies that are robust across different outcomes. Planning for only the best-case scenario would leave societies unprepared for higher-warming outcomes, while planning only for the worst case may lead to unnecessarily pessimistic decisions. The scenario range provides essential information for risk-based climate planning.

Under the high-emissions RCP 8.5 pathway, climate models consistently project that the Arctic Ocean will experience essentially ice-free summer conditions by mid-century or earlier. The rapid warming under RCP 8.5, with Arctic temperatures rising far faster than the global average due to Arctic amplification, makes near-complete summer ice loss a near-certainty. Even under more moderate pathways such as RCP 4.5, significant reductions in summer sea ice extent are projected, with ice-free summers possible later in the century depending on the pace of warming.

Climate projections based on RCP scenarios consistently show that higher emissions pathways are associated with more severe changes to extreme weather. Under RCP 8.5, heatwaves that currently occur once every 50 years are projected to occur every 5 to 10 years. Heavy precipitation events are projected to intensify in many regions. The proportion of tropical cyclones reaching the highest intensity categories is expected to increase. Under RCP 2.6, these changes are substantially reduced but not eliminated, demonstrating the significant difference that emissions choices make for extreme weather risk globally.

The carbon budget identifies the total cumulative amount of carbon dioxide that can be emitted globally while maintaining a specific probability of keeping warming below a given temperature threshold. For example, to have a 50 percent chance of limiting warming to 1.5 degrees Celsius, the remaining budget from 2020 onward is approximately 500 billion tons of carbon dioxide. RCP 2.6 is broadly consistent with staying within this budget, while RCP 8.5 would exhaust it within about a decade. Carbon budgets translate temperature targets into concrete emissions constraints.

The IPCC Fifth Assessment Report projects warming ranges that vary widely across scenarios. Under the stringent mitigation RCP 2.6, global average temperatures are projected to increase by approximately 0.3 to 1.7 degrees Celsius above pre-industrial levels by 2100. Under the high-emissions RCP 8.5, the projected range is approximately 2.6 to 4.8 degrees Celsius. The large difference between outcomes illustrates the significant climate benefits of aggressive emissions reduction compared to continued high fossil fuel use throughout the century.

RCP-based climate projections are widely used in real-world planning and policy at national and international levels. Coastal planners use sea level rise projections under different RCPs to design flood defenses. Water managers use precipitation projections to plan reservoir capacity. National governments use temperature and extreme weather projections to assess agricultural risks. International bodies such as the IPCC and UNFCCC use RCP scenarios to evaluate progress toward emissions targets and to communicate the differences in risk between high- and low-emissions futures to policymakers worldwide.

The Paris Agreement's most ambitious target of limiting warming to 1.5 degrees Celsius above pre-industrial levels is broadly consistent with the RCP 2.6 emissions pathway, which requires steep and rapid reductions in global greenhouse gas emissions followed by net negative emissions in the second half of the century. The 2 degree Celsius target is consistent with the lower end of RCP 4.5. These correspondences allow scientists to use climate model projections under RCP 2.6 to estimate the feasibility and consequences of meeting the Paris Agreement goals.

Global sea level rise projections scale significantly with the RCP scenario. Under RCP 2.6, sea level rise by 2100 is projected at approximately 0.29 to 0.59 meters. Under RCP 8.5, projections rise to approximately 0.61 to 1.1 meters or potentially more if ice sheet instabilities are triggered by accelerated warming. The difference between scenarios has enormous implications for low-lying coastal communities and small island states, highlighting the direct relationship between emissions choices and the magnitude of future sea level rise.

Mitigation strategies aim to reduce greenhouse gas emissions and limit the magnitude of future climate change, addressing the root cause of the problem. Adaptation strategies prepare human societies and natural systems for the climate changes that are already locked in given existing atmospheric greenhouse gas concentrations. RCP scenarios inform mitigation policy by quantifying the climate benefits of different levels of emission reduction, and they inform adaptation planning by projecting the magnitude and timing of impacts that communities must prepare for under different future emissions trajectories.