irreversibly impoverish our biosphere. Extinction rates are rising — we’re destroying the book of life before we’ve read it. Biodiversity is a crucial component of long-term human wellbeing. We're clearly harmed if fish stocks dwindle to extinction; there are plants in the rain forest whose gene pool might be useful to us. But for many environmentalists , preserving the richness of our biosphere has value in its own right, over and above what it means to us humans To quote the great ecologist EO Wilson ‘mass extinction is the sin that future generations will least forgive us for’. CLIMATE CHANGE So the world’s getting more crowded. And there’s a second firm prediction: it will gradually get warmer. In contract to population issues, climate change is certainly not under-discussed — though it is under-acted-upon The famous Keeling curve shows how the concentration of CO2 in the air is rising, mainly due to the burning of fossil fuels . The fifth IPCC report presented temperature- projections, for different assumptions about future rates of fossil fuel use. For each such assumption there’s a spread bevause it remains unclear how much the climatic effects of CO2 are amplified by associated changes in water vapour and clouds — the so-called sensitivity factor. IPCC PROJECTIONS But despite these uncertainties the science tells is that under ‘business as usual’ scenarios we can’t rule out, by 2100, really catastrophic warming, and tipping points triggering long-term trends like the melting of Greenland’s icecap. Sadly, many deny this. But even among those who accept that this threat is real, there is a range of views. These stem from differences in economics and ethics -- in particular, in how much obligation we should feel towards future generations. Bjorn Lomberg’s Copenhagen Consensus, for instance applies commercial-style discounting -- and in effect writes off what happens beyond 2050. So, unsurprisingly he downplays the priority of addressing climate change in c