Lesson 6 Summary

In this lesson, we looked at the science underlying greenhouse gas emissions scenarios. We learned that:

• Up through the Fourth Assessment Report, the IPCC employed, for the purpose of projecting future greenhouse gas concentrations, a set of emissions scenarios, known as the SRES scenarios. These scenarios reflect a broad range of alternative assumptions about how future technology, economic growth, demographics, and energy policies will evolve over the next century, and, therefore, plausibly reflect the diversity of potential future global greenhouse emissions pathways;
• The SRES scenarios embody a range of projected increases in atmospheric ${\text{CO}}_{\text{2}}$ by 2100 from a lower end of approximately doubling the pre-industrial levels to reach 550 ppm (B1) to a near quadrupling of pre-industrial levels (A1FI). Current emissions place us on a pathway close to the upper-end A1FI scenario;
• In the Fifth Assessment Report, the IPCC switched to the use of Representative Concentration Pathways, or RCPs. These pathways (RCP 2.6, RCP 4.5, RCP 6.0 and RCP 8.5) were chosen to be representative scenarios named for their total radiative forcing in the year 2100 (in watts per meter squared), and reflect a range of policies, from strong mitigation (RCP 2.6) to approximately business-as-usual (RCP 8.5);
• The stabilization scenarios are designed to stabilize atmospheric ${\text{CO}}_{\text{2}}$ concentrations at a particular level. The lower the desired stabilization level, the lower and sooner the peak in emissions must be. To stabilize below twice the pre-industrial levels, emissions must be brought to a peak within the next few decades and rapidly brought down by the end of the century, falling below 1990 levels by mid-century. To stabilize below 450 ppm, ${\text{CO}}_{\text{2}}$ levels must be brought to a peak within the next decade, and brought down to 80% below 1990 levels by mid-century;
• An increasingly widely used approach to defining the required carbon emissions reductions is the Wedge approach. This approach involves freezing emissions at current rates by offsetting projected business-as-usual emissions over the next 50 years (roughly 7 gigatons), envisioned, e.g., as 7 strategies for 1 gigaton carbon emission reductions. After 50 years, emission rates are brought down, but how abruptly and rapidly depends on the stabilization targets desired. Additional wedges can be used to achieve lower stabilization targets by bringing down, rather than freezing, annual carbon emission rates over the next 50 years.

Reminder - Complete all of the lesson tasks!

You have finished Lesson 6. Double-check the list of requirements on the first page of this lesson to make sure you have completed all of the activities listed there before beginning the next lesson.