"Headed toward the danger zone Human activity is warming the planet. For the past millennium the Earth?s average temperature varied within a range of less than 0.7°C ... however, man-made greenhouse gas emissions have resulted in a dramatic increase in the planet?s temperature over the past century (shown in yellow). The projected future increase over the next 100 years (shown in red) due to growing emissions could possibly warm the planet by 5°C relative to the preindustrial period. Such warming has never been experienced by mankind and the resulting physical impacts would severely limit develop- ment. Only through immediate and ambitious actions to curb greenhouse gas emissions may dangerous warming be avoided. The evolution of the planet?s tempera- ture for the past 1,000 years is based on a range of proxy estimates (such as tree ring analysis or ice core sampling) that define the envelope of long-term temperature variation. With modern weather observations starting in the nineteenth century, global temperature could be estimated more precisely; thermometer data for the past 150 years or so docu- ment a global temperature increase of nearly 1°C since the preindustrial period. Global climate models that estimate the effect of different future emission scenarios on Earth?s climate predict a range of possible global temperatures for this century. These estimates show that even the most aggressive mitigation efforts may lead to warming of 2°C or more (a level already considered dangerous), and most models project that less mitigation would lead to warming of 3°C or even up to 5°C and beyond (though with less certainty around these higher amounts of warming)..."