2011

The Representative Concentration Pathways: An Overview

D. P. van Vuuren, et al.

Observation and Context

Scientists and policymakers rely on socio-economic and emission scenarios to describe how the future may evolve and to study climate change impacts. In the past, older scenario sets like SRES were used, but the scientific community realized that a new generation of climate models required much more detailed information. Specifically, researchers observed that older models lacked high-resolution geographic details, ignored modern climate mitigation policies, and did not properly integrate information across different scientific disciplines, such as human adaptation and land-use change. This prompted the Intergovernmental Panel on Climate Change (IPCC) to ask the scientific community to develop a new set of pathways to facilitate future assessments.

Hypothesis

If experts from different disciplines — including integrated assessment modelers, climate modelers, and emission inventory experts — collaborate to harmonize existing scientific literature, they can create a standardized set of consistent, high-resolution pathways that accurately represent the full spectrum of future human emissions and land-use possibilities found in the open literature.

Experiment and Methodology

To test this approach, researchers reviewed 324 scenarios from published literature and selected four distinct pathways based on their year 2100 radiative forcing values. The teams updated these selected scenarios to reflect modern modeling advances and align them with common historical base-year data. The methodology involved data harmonization and downscaling, which organized land-use changes and air pollutant emissions into 12 human sectors on a highly detailed 0.5×0.5 degree geographic grid. Simple carbon-cycle climate models (like MAGICC6) and atmospheric chemistry models (like CAM3.5) were utilized to convert these human emission pathways into actual atmospheric concentrations. Finally, the teams used stylized math rules to extend these paths out to the year 2300, creating Extended Concentration Pathways (ECPs) to explore long-term climate system responses.

Results and Data

The process successfully produced a comprehensive dataset consisting of four distinct pathways named after their targeted radiative forcing levels by 2100: RCP2.6, RCP4.5, RCP6, and RCP8.5.

• RCP2.6 represents a strict mitigation future where emissions peak and decline, relying on bio-energy paired with carbon capture and storage (CCS) to achieve negative emissions by the end of the century.
• RCP4.5 and RCP6 represent intermediate stabilization scenarios where radiative forcing is stabilized shortly after 2100 without overshooting the long-term target.
• RCP8.5 describes a very high baseline emission scenario driven by high global population growth and a heavy reliance on fossil fuels without climate policy interventions.

Land-use projections showed massive variation across the set, ranging from extensive reforestation programs in RCP4.5 to significant agricultural expansion in RCP8.5. Air pollutants like SO₂ and NOx generally exhibited a declining global trend across all scenarios because of a shared assumption that rising global income levels bring stricter pollution control policies.

Conclusion and Climate Impact

The experiment proved that a collaborative, multi-disciplinary framework can build a highly detailed climate dataset. The resulting RCPs successfully provide a consistent analytical thread running through climate change research. Ultimately, these pathways allow global research groups to feed identical data into advanced climate models, allowing humans to project future temperatures, accurately map severe environmental impacts, and evaluate the economic costs and benefits of various global climate policies.