With the data that have been gathered over the past 50-100 years on temperature, precipitation, ice melt, sea levels, and other factors related to climatic change, scientists can build models to represent the patterns that we have observed already and use these models to extrapolate predictions for what can be expected to occur in the near future. Frequently, scientists use multiple possible future scenarios, called Representative Concentration Pathways (RCPs) which are based on different choices that humans may make in the future and how these decisions affect future climatic change. RCPs differ primarily in the time period at which global GHG emissions peak and begin to decline and therefore how much warming we can expect. The commonly used RCPs are defined below:
- RCP 2.6: global annual GHG emissions peak 2010-2020
- RCP 4.5: global annual GHG emissions peak around 2040
- RCP 6.0: global annual GHG emissions peak around 2080
- RCP 8.5: global annual GHG emissions continue at their current level through the year 2100
Each RCP is named with a number that represents the difference in sunlight absorbed by the Earth’s surface versus sunlight reflected back to space (discussed in Fig 3.1.1). Larger numbers represent a larger increase in absorption of sunlight and, therefore, more warming. The ‘best case scenario’, RCP 2.6, requires a peak-and-decline of global GHG emissions to occur by 2020; consequently, the best case scenario is no longer possible. The most extreme scenario currently used in the projections, RCP 8.5, assumes no major declines in GHG emissions until after the year 2100. This scenario is commonly referred to as ‘business as usual.’ Given the lack of binding international agreements which would limit or reduce future global GHG emissions, we are currently following RCP 8.5. Scientists can use the different RCPs to consider the impact of different global political decision making. As seen in Figure 3.5.1, the differences can be profound. These decisions matter, and the longer we as a global community take to respond to the threats of climate change and alter our GHG emissions, the more severe the ramifications will be.