a) Energy -- the global economy runs on energy (indeed, everything runs on energy). We have reached the limits of the cheap oil energy that has shaped the global economy for the past 150 years. The basic issue is the cost of a unit of energy. Perhaps more accurately, the cost of a replacement unit of energy. While there is plenty of reduced carbon in the lithosphere (coal, natural gas, oil, etc.), these are increasingly expensive to extract. There is also ample direct solar energy if we can harvest it economically. A number of sub-issues belong here: cost and volume of energy storage; inertia of existing capital investment; technical familiarity with existing energy sources versus rapid technical advances with new energies; needing to change social perceptions based on 150 years of social mind-sets that have lived in a "cheap oil" world.
b) Climate and sustainability -- the evidence is clear that global climate is changing. This is an issue that cannot be contained by nation-state boundaries. The relationship between humans and the natural environment is thus called into question. Our societies have evolved over a few thousand years of relative environmental stability, and a major environmental change is likely to require major human societal changes. Time scale is an issue here, as the millennial changes are very rapid from an earth-systems perspective, but human individuals operate from an approximately single decade time framework. The existing human-environment relationship, which presumes growth in many areas, is self-evidently not sustainable over long periods.
c) Economy and finance -- numerous writers have suggested that our species should be homo economicus (or some variant) rather than homo sapiens. For most of the global population, some trade in an economic system is allowing a life that is significantly removed from existence in a hunter-gatherer pack. At the top of the economic pyramid stand the OECD nations and their populations. But unlike a pyramid, the global economy is becoming increasingly unstable. Communications expose inequities in wealth distribution. The amount of capital available for reinvestment seems insufficient for current societal needs, let alone future needs; at the same time the economic horizon becomes increasingly short. And political structures, which are fundamentally just the organizational base of economic and financial structures, have significantly lagged so that the entire foundations of the system are faltering.
These three areas are orthogonal but intimately interrelated. Hence an attempt to analyse, let alone "solve", an issue by looking at just one axis can at best be but partial. And, as noted in the introduction, there are almost certainly other areas of major concern. These are probably represented along additional orthogonal axes. The future is always messy and will have unexpected turns; I think the next several decades will be even more so.
