GORHAM — The world stands at the brink of sweeping global energy transitions, and they should be transitions toward greater sustainability. This demands attention to sustainability’s two facets: meeting the needs of people today, while preserving opportunity for the future – all in a manner that is equitable for the world’s poor.

But what will the next energy transitions be like? What should they be like? Which options can best meet our needs, and in what time frames? And how do we cope with the need to expand energy use in poor regions?

Fossil fuels provide most of the world’s energy, but those supplies do face ultimate limits. We see enormous environmental impacts attendant with the enormity of energy they offer. Can we maximize the benefits derived from energy consumption, while minimizing the costs? Which paths offer the most promise?

We cannot know precisely when energy shortages will impose transitions. There is no clear consensus on which energy sources or production and conversion systems will dominate the new energy landscape. There are large questions about how to compare the merits and limitations of various systems. However, there are some things that we can know.

Energy is vital both to survival and to development. Developing countries will almost certainly demand more energy to support development. Growing population levels will also need more energy.

Despite common notions of “obsolete resources,” the prevailing fossil fuel systems will be called upon heavily to meet our needs for many decades to come. They provided tremendous benefit for the industrialized world as they moved us from raw biomass dependence to more modern energy systems.

That transition will continue in much of the developing world, because of the proven efficacy and cost-effectiveness of fossil fuels. Indeed, the fossil fuels, especially natural gas, are cleaner and less of a burden on the environment than firewood. The growth in fossil fuel use in the developing world will likely proceed even as the world embarks on new paths to new, nondepleting and ideally less polluting energy systems.

Generally, the “renewables” are viewed as the sustainable solution to the world energy problem, yet they each have their own constraints. Consider this. In 2010, the world consumed about 400 Quads of energy from fossil fuels. (A Quad is short for 1 quadrillion British thermal units, roughly 1 percent of U.S. annual energy demand.)

This was about 85 percent of total annual global energy consumption. The remaining 15 percent mostly came from nuclear electric power, biomass and hydropower, with a tiny amount coming from solar and wind energy. Renewables have to do a lot to catch up. But the constraints go beyond energy production numbers.

Biomass, for instance, has much in common with fossil fuels in terms of emissions and the potential for humans to deplete stocks. Even the engineered biofuels, especially corn ethanol, affect other resource systems like water and land.

Although solar and wind energy have enormous resource bases that cannot be depleted, they have only finite fluxes to tap, which is the reason that they supply such a tiny fraction of humanity’s energy.

We cannot use them on demand, like we do the stock-based resources that offer better stability. Imagine depending solely on the solar resource to run an economy, especially on a cloudy day. There is little certainty, little predictability.

Large-scale, dam-based hydropower provides more energy than the other sustainable alternatives, but its future development is probably limited by concerns over ecosystem disruption. Small-scale and run-of-the-river technologies may represent some more sustainable potential for the future of hydropower, along with forays into wave and tidal power. Like wave and tidal power, geothermal power is a resource waiting to be better utilized and deserves more attention than it receives.

So we find ourselves in the beginning of the 21st century with the world consuming exponentially growing amounts of energy. The commercial energy markets are dominated by petroleum, and many of us are wondering how long petroleum will be able to meet the growing appetites of a growing global population. When will there be an energy shortage? For half of humanity, the answer is simple:It’s already here.

For affluent countries, the question is how long our appetites may be sated – and what transition, both in energy systems and lifestyles, will be feasible. For the people in rapidly developing countries, the question is what energy systems can support their growth both now and into the future, while minimizing environmental costs.

Daniel M. Martinez, Ph.D., is an assistant research professor of environmental science at the University of Southern Maine in Gorham and co-author of “The Path to More Sustainable Energy Systems.”