In my opinion the greatest challenge facing the human race is overpopulation. We have become so successful as a species that our very success is threatening to destroy the planet. The current world population is something like seven billion, but it is projected to go up to eleven or twelve billion. Just the need for food is destroying the forests and destroying the soil. We are using up all the oil and will soon start using up all the coal. I have read somewhere that the population of the United States is projected to rise from around three-hundred million to around four-hundred million by 2040--and to around one billion by the end of this century. In the meantime there seems to be less and less need for human labor. What are all these people supposed to do when the robots take over? It is horrifying to see all those robotic arms assembling cars on television and to see the mechanization of agriculture and food processing.
A struggle for existence naturally follows from the high rate at which all organic beings tend to increase. There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair.
Charles Darwin, The Origin of Species
As with most questions based on philosophy, which is essentially someone's belief system, all answers are idiosyncratic, that is, they are based on an individual's perspective, so there will be as many answers as there are people to answer the question.
From my perspective, then, the greatest challenge we face as a race is existential--simply, are we going to survive or destroy ourselves? It seems to me that all other challenges, of which there are thousands, are secondary to the issue of our survival. I am assuming, of course, that our survival is still open to question. Many people would argue that, among all the challenges we face, our survival is really not at risk because we have sufficient self-control as a species not to wipe ourselves out. My answer to that argument is simply to look at every powerful group of people throughout history and ask the question, "Where are they now?" My philosophical perspective, grounded on my reading of history, tells me that our survival is not only at risk but also more than likely doomed. By doomed I do not necessarily mean that the entire human race will disappear from the earth but that society as it is presently constituted will be destroyed. We may survive at a Stone Age level, but we will not be texting each other with cell phones.
Because we have, for many decades, had the power to destroy ourselves, we are in the most precarious position in human history. Up to the mid-1940's, various powerful nations had the power to do tremendous damage to other nations, but no one had the power to destroy the entire human race. Since the mid-1940's, we have danced up to the point of annihilation--for example, the Cuban Missile Crisis in 1962--but have managed, barely, to avoid total destruction. Given the social tensions, political animosity, and religious intolerance that characterizes many societies that have access to nuclear weapons (and delivery systems), as well as almost unrestricted access to weapons of mass destruction in several countries, it seems logical to believe that, sooner or later, some group will attack another group with nuclear weapons, an event that will, in both calculable and incalculable ways, alter or destroy the human race.
According to my philosophy, then, our task as a race is to insure our survival first and then to address the other problems that may more slowly threaten our existence