(Although other studies suggest that ecosystems cannot adapt to a rate of change faster than 0.05–0.1 °C per decade, the globe is already committed to warming of around 0.2 °C per decade even with constant (year 2005) emissions. Given that, the authors decided, a more stringent target “would likely be unfeasible.”).

The cumulative target of 190 GtC for the period 2010–2030 is equal to an average of 9.5 GtC per year, compared to global emissions of 9.9 GtC in 2008. Taking the 190 GtC budget for 2010–2030 within the context of an overall one-trillion-tonne budget suggests that only about 300 GtC are available for post-2030 emissions, given that humans have already released around 500 GtC into the atmosphere in the past quarter century.

An alternative policy approach to limiting the rate of warming would be to focus on reducing short-lived atmospheric components. This may have a limited effect on long-term temperature change, however, since these components have atmospheric lifetimes in the order of days, in the case of black carbon, or a decade, in the case of methane. By focusing on CO₂, which has an impact over centuries or longer, our proposed mitigation strategy has the benefit of limiting the rate of warming in the short term, as well as addressing long-term temperature change.

—Kallbekken et al.

The implications are that global emissions must peak around 2015 and be cut by roughly half between the peak and the year 2030. Focusing climate policy on a long-term target, such as the EU 2-degree target, provides limited guidance for mitigation over the next few decades, and gives the impression that there is time to delay, says Steffen Kallbekken.

The researchers propose that, in addition to a long-term cumulative emissions budget, a maximum limit on the rate of warming should also be considered as an element in the design of climate policies. Required mitigation rates are 4-8% per year, which far exceeds anything achieved in history.

A short-term target provides clearer guidance on mitigation in the near term, limits potentially dangerous rates of warming, and allows easier inclusion of potent and toxic short-lived climate components. A short-term cumulative emissions target, for example 190 GtC for the period 2010-2030, is a useful approach to limit the rate of warming, while at the same time keeping the focus on what matters in the long term: reducing CO2 emissions.

—Steffen Kallbekken

Resources

• Steffen Kallbekken, Nathan Rive, Glen P. Peters & Jan S. Fuglestvedt (2009) Curbing emissions: cap and rate. Nature Reports Climate Change doi: 10.1038/climate.2009.121