Australia: The Land Where Time Began

A biography of the Australian continent 

Global Warming - the Difficulty of Recovering from Dangerous Levels

Compelling evidence from climate models suggests that key global temperature thresholds, such as the EU limit of 2oC warming since pre-industrial times, will likely be crossed in the next few decades if the present rates of release of greenhouse gases continue. The authors1 say that in spite of this, relatively little attention continues to be paid to whether it would then be feasible for the global temperature to return to safer levels in a time period that is usefully short if a temperature level that is dangerous is exceeded. By the use of a state-of-the-art general circulation model the author1 focuses on the timescales that would be needed to reduce the levels of atmospheric greenhouse gases, and the associated temperatures, below the thresholds that are potentially dangerous. Uncertainty bounds were provided by a simple climate model that extended this analysis. The author1 found that only a low rate of temperature reduction is likely to occur even for reductions of emissions that are very large. He suggests it is necessary to consider timescales of recovery, that are implicit in the Earth system, that are very long when formulating emission pathways for the future that have potential to overshoot particular concentrations of atmospheric greenhouse gases, and what is more important, related temperature levels that could be considered dangerous.

The author's1 conclusions

In has been demonstrated by previous studies using climate-carbon cycles, that are simple models of intermediate complexity, that timescales for global temperatures to decline, that are potentially long, even after large reductions of carbon dioxide emissions have been achieved. According to the author1 this result is particularly important to policy makers, especially if dangerous levels of climate change are realised. It is necessary to know that to return to safer levels of global warming may not be easy. The first key outcome in this letter is that the robustness of this result by attaining it by using the most complex class of Earth system model, a GCM, that is complete with spatially resolved carbon cycle. It has been shown by analysis with a particular carbon cycle that after rapid emissions decreases, the result may be even slower carbon dioxide and temperature decreases compared to previously published results. The author1 recommends that in future their analysis should be extended using additional GCMs.

Large ensembles of simulations were next used with a coupled simple climate-carbon cycle model, each ensemble member being given a different paramiterisation together with an associated probability that was based on the existing knowledge base. This allowed estimates of probability to be made for the length of time for which the global surface temperature might exceed critical warming thresholds. They found that for a multi-gas scenario with emissions peaking in 2015 before adjusting for a long-term reduction rate of 3%/yr the probability of exceeding a 2oC target above pre-industrial levels about 55%. They fond that there was a probability of 30 % that the temperatures would remain above this warming level for at least 100 years, and a 10 % probability of remaining above 2oC for up to 300 years. According to this particular scenario a greenhouse gas emissions reduction that approaches 50 % of the 1990 values by 2050, that is similar to the G8 statement in 2008 to be able to consider the goal of at least 50 % of global emissions by 2050. It has been shown that it would also be required to further reduce emissions beyond 2050 to limit the temperature increases in the long term and this ongoing emissions reduction is included here (House et al., 2008).

It is noted by the author1 that the MAGICC simulations show there are values of key climate parameters that allow a faster recovery, though the GCM simulation suggests global temperature would decline very little during the century after a warming of 2oC. Timescales for recovery that are possibly long for society to deal with are represented by these more optimistic possibilities. The author1 suggests that motivation for narrowing the uncertainty bounds on climate sensitivity, the diffusivity of the ocean and and the climate-carbon cycle feedback, is provided by the remaining uncertainty. It is also implied by these results that future research needs to be focused on quantifying the resiliency of the Earth system components, such as the Greenland ice sheet, major ecosystems or the thermohaline circulation, as well as on thresholds of dangerous climate change, to exceeding critical thresholds temporarily for a range of different periods. It may be appropriate according to cost-benefit analysis of temporary  adaptation to levels above an eventual desired target temperature to consider the extra cost incurred by such adaption, with analyses of the type presented here providing estimates of the time above such targets. Earlier work has shown that such temporary resilience is important to assess (Schneider & Mastrandrea, 2005; O'Neill & Oppenheimer, 2004). Their concepts to the most complex climate models have been extended here and by using in parallel large ensembles of simulations with a simpler climate model the authors1 team were able place the overshoot issue into a probabilistic framework that is direct for planning in the future within the debate on climate change.

Sources & Further reading

  1. Lowe, J. A. "How Difficult Is It to Recover from Dangerous Levels of Global Warming?". Environ. Res. Lett. 4 (2009 2009).


Author: M. H. Monroe
Last Updated 10/07/2013
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