I do worry about whether we will get through this century without severe setbacks.  I think there are two different classes of risk we need to worry about.  The first is the category which we are imposing collectively because there are more of us on this planet, a growing population, and each of us is more demanding of energy and resources.  There are now nearly seven-and-a-half billion people on the planet; there will be nine billion by mid-century.  We’re not sure what will happen after that; it could go up even more.  But also, we’ll all be more demanding in terms of energy and resources. And of course, we hope that the developing world will close the gap with more fortunate countries like ours, and that will, of course, increase the pressure on energy and resources.  So there’s a risk that this is going to lead to despoliation of the environment —  climate change; it could have severe long-term effects — and to disruption of ecologies, leading to mass extinctions.  And of course, those will be irreversible degradations of our environment.  So we will not be leaving an environment for future generations as good as the one we inherited.  So that’s a serious concern, our collective effects.