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Three degrees does not sound like much but it a rise in temperature compatible with the global heating that occurred between the last ice age, some 15,000 years ago, and the warmth of the eighteenth century. When Earth was cold, giant glaciers sometimes extended from the polar-regions as far south as St Louis in the US and the Alps in Europe. Later this century when it is three degree hotter glaciers everywhere will be melting in a climate of often heat and drought, punctuated with storms and floods. The for humanity could be truly horrific; if we fail to act swiftly, the full impact of global heating could cull us along with vast populations of the plant and animals with whom we share Earth. In a worst case scenario, there might - in the 22nd century - be only a remnant of humanity eking out a existence in the polar-regions and the few remaining oases left on a hot and arid Earth.