The acronym RCP stands for ‘Representative Concentration Pathway’. RCPs are scenarios that describe how Earth’s climate could potentially change in the future up to the year 2100. Each scenario estimates alternative trajectories for future greenhouse gas emissions and the resulting atmospheric concentration of those gases. Once the concentrations from differing emissions scenarios are calculated, climate modelers can then estimate the effect this will have on near-surface air and water temperatures.

Radiative Forcing

The RCP scenarios’ names correspond with their radiative forcing target level for 2100. For example, in a scenario named “RCP 2.6”, the future radiative balance would be changed by 2.6 W/m2, causing Earth to warm to restore this balance. Radiative forcing is the effect that various components of the atmosphere (i.e greenhouse gases and air pollutants, solar irradiance) have on the balance of incoming and outgoing radiation. In other words, the radiative forcing estimates refer to changes in atmospheric heat caused by greenhouse gases and other forcing agents.

Future of Climate Change

There are numerous factors to account for when assessing future climate change. The amount of greenhouse gases in the atmosphere is perhaps the most significant among these factors because the atmosphere’s temperature responds to the total concentration of greenhouse gases. Therefore, RCP projections assume that temperature is linearly related to the cumulative total of human-caused greenhouse gas emissions.

By calculating the changes in the atmosphere’s components, predictions can be made about the temperature changes that would result. Other factors that must be considered for reliable future climate change assessments include developments in adaptation technologies, changes in land use, future energy production, population growth, and economic growth.

Some scenarios are optimistic, in that they predict future emissions to be much lower than they are today. The worst-case scenarios, on the other hand, are pathways that have the highest estimated future greenhouse gas concentrations; and therefore the highest temperature predictions.

RCP 2.6 is an example of a more optimistic scenario. RCP 2.6 assumes that humanity implements a variety of technologies and strategies for curbing emissions while also making ambitious emissions reductions across sectors by 2100. RCP 2.6 refers to the concentration of greenhouse gases that cause global warming at an average of 2.6 W/m2 (watts per square meter). RCP 2.6 results in the least amount of global warming of all the RCPs.

RCP 8.5, the so-called “business as usual scenario”, has the highest global mean temperature increases of all pathway scenarios. RCP8.5 is a high greenhouse gas emissions scenario that is likely to be the outcome if humanity makes few or no concerted efforts to decrease emissions, resulting in a warming average of 8.5 watts per square meter globally. Projected changes in climate under RCP 8.5 will be more severe than under RCP 2.6.

There are four main RCPs that extend to the end of the century (2100). RCP 4.5 and RCP 6 are intermediate scenarios between 2.6 (lowest) and 8.5 (highest). The RCP estimates are far from exact, as there are far too many societal and climatological uncertainties to factor into assessments. In reality, future greenhouse gas emissions can fall anywhere on the spectrum between RCP 2.6 and RCP 8.5, or perhaps outside the boundaries of these projections entirely. Also, no one can be sure exactly how sensitive Earth’s climate might be to increased concentrations of greenhouse gases and other forcing agents.

Inherent uncertainties aside, these limited sets of scenarios allow for a common language and way of thinking about future changes in climate for policymakers and climate modeling teams.

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