The 21st Century climate challenge

1The 21 st Century climate challengeOur carbon budget hasa single goal: keepingaverage global temperatureincreases (over preindustriallevels) below 2°Chouseholds or governments set budgets theytarget a range of objectives. Households have toavoid unsustainable spending patterns or facethe prospect of debt. Government budgets aregeared towards a range of public policy goalsin areas such as employment, inflation andeconomic growth. If public spending exceedsrevenues by large margins, the consequencesare reflected in large fiscal deficits, inflation andthe accumulation of debt. Ultimately, budgetsare about living within the bounds of financialsustainability.Carbon budgeting fora fragile planetCarbon budgets define the bounds of ecologicalsustainability. Our carbon budget has asingle goal: keeping average global temperatureincreases (over preindustrial levels) below 2°C.The rationale for this goal is, as we have seen,rooted in climate science and human developmentimperatives. Climate science identifies2°C as a potential ‘tipping point’ for long-runcatastrophic outcomes. More immediately, itrepresents a ‘tipping point’ for large scale humandevelopment reversals during the 21 st Century.Remaining within the 2°C threshold shouldbe seen as a reasonable and prudent long termobjective for avoiding dangerous climate change.Many governments have adopted that objective.Sustainable carbon budget management shouldbe seen as a means to that end.What is the upper limit on greenhouse gasemissions for a world committed to avoiding dangerousclimate change? We address that questionby using simulations carried out at the PotsdamInstitute for Climate Impact Research (PIK).Stabilization of greenhouse gas stocksrequires a balance between current emissionsand absorption. A specific stabilization targetcan be achieved through a number of possibleemission trajectories. In broad terms, emissionscan peak early and decline gradually, or theycan peak later and decline more rapidly. If theaim is to avoid dangerous climate change, thestarting point is to identify a stabilization targetconsistent with the world staying within the2°C dangerous climate change threshold.Keeping within 2°C—the ‘fifty–fifty’pointIn our simulation we set the bar at the lowestreasonable level. That is, we identify the levelof greenhouse gas stocks consistent with anapproximately even chance of avoiding dangerousclimate change. This level is around450 ppm CO 2e. It might be argued that thisis insufficiently ambitious: most people wouldnot stake their future well-being on the toss ofa coin. However, stabilizing at 450 ppm CO 2ewill entail a sustained global effort.Setting the bar above our target wouldlengthen the odds on avoiding dangerousclimate change. At greenhouse gas stock levels of550 ppm CO 2e the likelihood of overshootingthe dangerous climate change threshold of2°C increases to around 80 percent (figure1.9). Opting for a 550 ppm CO 2e target wouldbe taking a gamble at very long odds on thefuture of the planet and 21 st Century humandevelopment prospects. In fact, there would bea one-in-three chance of overshooting 3°C.Figure 1.9Probability of exceeding a 2ºC temperature increase (%)1009080706050403020100%Source: Meinshausen 2007.The risk of dangerous climatechange rises with greenhousegas stocks350 400 450 500 550 600 650 700 750CO 2e stabilization level (ppm)VerylikelyLikelyMediumlikelihoodUnlikelyVeryunlikelyNote: Data refer to the highest, lowest and midpoint estimateresulting from several different climate models. For details seeMeinshausen 2007.46 HUMAN DEVELOPMENT REPORT 2007/2008