Most of us have heard the predictions: the meltdown of Arctic sea ice and mountain-top glaciers; extinction of species ranging from polar bears to coral reefs; catastrophic sea level rise that could eventually force the relocation of millions of coastal residents. Heat waves, malnutrition and famine, and wildfires would also be a greater risk to human communities if carbon dioxide levels in Earth’s atmosphere are allowed to rise too high. Specifically, these could be the characteristics of a world where carbon dioxide has risen to 1,000 parts per million by 2100, as described this week in a Nature opinion essay by Stephen Schneider of Stanford University. (Currently carbon dioxide is at about 384 parst per million.) Reaching this level of carbon dioxide by the end of the century was presented as a worst-case scenario if nothing is done to curtail emissions in a 2000 special report by the Intergovernmental Panel on Climate Change (IPCC). But ‘it’s not the worst-case scenario,’ Scheider now argues. ‘The worst-case scenario could be worse.’ Until the economic downturn late last year, actual emissions have been higher than those in the IPCC scenario. So without any mitigation, ‘that’s the track we’re on […]