4 min readNew DelhiFeb 27, 2024 12:21 PM IST
First published on: Feb 26, 2024 at 07:56 PM IST
There is no dearth of reports highlighting the consequences of climate change, including droughts, water scarcity, severe wildfires, rising sea levels, etc. Despite that, there are many myths and a lot of confusion around the subject. In this series of explainers, we answer some of the most fundamental questions about climate change, the science behind it, and its impact.
In the eighth instalment (you can scroll down to the end of this article for the first seven parts), we answer the question: ‘What are emissions scenarios and Representative Concentration Pathways (RCPs)?’
What are emission scenarios?
Essentially, emission scenarios are pathways that tell us about emissions of greenhouse gases and aerosol due to human activities over time. Scientists use these scenarios to feed into climate models, which then calculate things like future global temperatures or sea levels.
The latest approach to determining emission scenarios is known as Representative Concentration Pathways (RCPs). “RCPs specify concentrations of greenhouse gases that will result in total radiative forcing increasing by a target amount by 2100, relative to pre-industrial levels,” according to a report by the UK’s Meteorological Office.
Total radiative forcing is the difference between the amount of energy that enters the Earth’s atmosphere and the amount of energy that leaves it. Currently, as we know, more energy is entering the planet than leaving because of the increased levels of greenhouse gases and aerosols (for more details click here). That’s why we are witnessing global warming. Radiative forcing is measured in watts per square metre.
There are four pathways: RCP8.5, RCP6, RCP4.5, and RCP2.6 — the latter is also called RCP3PD, where ‘PD’ stands for Peak and Decline.
All forcing agents’ atmospheric CO2-equivalent concentrations (ppm) from the IPCC AR5 report. (Wikimedia Commons)
The numbers represent the expected change in radiative forcing from the year 1750 to 2100. So, RCP4.5 represents an increase of 4.5 watts per square metre of radiative forcing between 1750 and 2100. The Intergovernmental Panel on Climate Change (IPCC) considers 1750 as the base year as it predates the Industrial Revolution and the radiative forcing was quite stable. Moreover, a higher value of forcing means more concentration of greenhouse gases and other pollutants, which in turn, means higher global warming and a more pronounced impact of climate change.
To determine forcings and pathways, scientists relied on “the existing literature and synthesised values for a wide range of scientific and socioeconomic data, like population growth, GDP, air pollution, land use and energy sources,” according to a report by The Guardian. Notably, RCPs are neither forecasts nor policy recommendations. They are used to represent a broad range of climate outcomes.
What are the different RCP warming levels?
Let us start with RCP2.6, which assumes an additional radiative forcing of just 2.6 watts per square metre by the end of the 21st Century. This is the best-case scenario as the concentration of greenhouse gases is drastically reduced and the world takes stringent mitigation actions. RCP2.6 is also known as RCP3PD as emissions peak around 2050 and then decline. As a result, the global average temperature would increase by 1.6 degree Celsius compared to pre-industrial levels.
This figure shows both historical simulations and future projections in global average annual temperature from 1900 to 2100 under three emissions scenarios. Historical simulations are for the period from 1900 to 2005. Future projections are for the period from 2006 to 2100 based on three global emission scenarios: low (RCP2.6), moderate (RCP4.5), and high (RCP8.5). (Credit: Government of Canada)
RCP4.5 and RCP6 are intermediate scenarios, where . In RCP4.5, the global average temperature would increase by 2.4 degree Celsius. In RCP6, it shot up to 2.8 degree Celsius above the pre-industrial levels.
The worst case scenario is RCP8.5, where the concentration of greenhouse gases and other pollutants will be three times more than the present. In this case, the temperature would rise to 4.3 degree Celsius by 2100.
Here are the previous instalments of the series: part 1, part 2, part 3, part 4, part 5, part 6, and part 7.
A coal-fired power station near Bełchatów, Poland. (Wikimedia Commons) 
All forcing agents’ atmospheric CO2-equivalent concentrations (ppm) from the IPCC AR5 report. (Wikimedia Commons)
This figure shows both historical simulations and future projections in global average annual temperature from 1900 to 2100 under three emissions scenarios. Historical simulations are for the period from 1900 to 2005. Future projections are for the period from 2006 to 2100 based on three global emission scenarios: low (RCP2.6), moderate (RCP4.5), and high (RCP8.5). (Credit: Government of Canada)
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