Our climate system is in trouble. It has warmed by over 0.7 degrees Celsius in the last 100 years. Most of the warming since at least the mid-twentieth century is very likely due to human activities. Warming's impacts on human and natural systems are now being observed nearly everywhere—perhaps most obviously in the recent loss of Arctic sea ice, which in 2007 and 2008 reached record low levels at the end of the northern summer. In spite of nearly 20 years of international attention, emissions of greenhouse gases (GHGs)—principally carbon dioxide (CO2) from the burning of fossil fuels—continue to grow rapidly. As a consequence, the concentration of carbon dioxide in the atmosphere has increased faster during the last 10 years than at any time since continuous measurements began in I960.1
Unabated, current increasing trends in emissions can be expected to raise Earth's temperature by a further 4-6 degrees Celsius
(7.2-10.8 degrees fahrenheit), if not more, by the end of this century. If even half that much warming occurs, it will bring huge damages and potentially catastrophic problems. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), which was released at the end of 2007, predicted serious risks and damages to species, ecosystems, human infrastructure, societies, and livelihoods in the future unless warming is reduced. The report's projected risks and damages are larger and more serious than previously estimated and threaten development in several regions of the world. The IPCC also found that reducing greenhouse gas emissions would lower the global temperature increase and consequently lessen the risks and damages. Yet it is also important to note at the outset that even reducing emissions 80 percent by 2050 will not eliminate all serious risks and damages.2
One of the great icons of the modern world,
W. L. Hare is a scientist in Earth System Analysis at the Potsdam Institute for Climate Impact Research in Germany and advises Greenpeace International on climate policy and science.
A Safe Landing for the Climate the jet aircraft, provides a telling metaphor for what the world faces in terms of climate change. Jet aircraft burn prodigious quantities of fossil fuels in order to move passengers and freight across vast distances in relative safety and luxury. Yet like the climate system, the rules of operating these machines are not widely understood by anyone except the few people whose job it is to know about such things. The climate system is like a jet aircraft that has become airborne safely but is now facing grave difficulty and must land as a matter of urgency before disaster becomes inevitable. If we do not reduce emissions fast enough and bring the warming of the climate system to a halt, we risk a major catastrophe.
This chapter is about how much and how fast the world needs to reduce greenhouse gas emissions in order to prevent or limit serious damage—in other words, to bring the climate system to a safe landing. But first it is important to review the current state of scientific knowledge on the risks, damages, and impacts estimated for different levels of warming in order to see what level might prevent dangerous changes and thus be "safe."
Preventing dangerous climate change is the universally agreed ultimate goal of climate policy established in the 1992 U.N. Framework Convention on Climate Change (UNFCCC). (See Box 2-1.) Once a dangerous level of change has been defined, scientists can calculate with reasonable confidence an emission pathway that can limit warming and other changes to this level, taking into account continuing uncertainties in their understanding of the climate system.3
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