This year, Michael Oppenheimer co-authored a study on the impacts on climate and ecosystems of overshooting CO2 concentration targets. Oppenheimer and colleague Brian O’Neill compared the consequences of three scenarios for stabilization targets of 500 – 700 ppm: a slow, almost linear increase toward stabilization over 200 years; a delayed but faster increase toward the same stabilization value; and a path that overshoots the target stabilization concentration by 100 ppm by 2100, then declines toward the stabilization value in the next 100 years.

All three approaches to the same CO2 target produce the same ultimate temperature change, but the authors found that overshooting the stabilization target causes significant additional warming in the short-term (Figure 9). Overshooting the desired CO2 concentration by 100 ppm caused temporary increases in global average temperature of up to 1.2ºC above those seen in the other scenarios in 2100, and up to 0.6ºC higher than the predicted stabilization temperature. The results suggest that even if ultimate CO2 targets are set low to avoid crossing climate thresholds, temporary overshoots could cause climate and ecosystem damage expected at significantly higher stabilization CO2 levels. They also showed that the differences in the rate of warming among these scenarios could lead to significant differences in environmental damage.

Figure 9. Global average temperature change, 2000–2300, for slow change (SC), rapid change (RC) and overshoot (OS) pathways to 500 (Top), 600 (Middle), and 700 (Bottom) ppm equivalent CO2, assuming a climate sensitivity of 2.5°C.