I believe the challenge of the rising and out of control U.S. national debt is the most significant issue facing the United States today. The country is in a straitjacket because of this debt. Initiatives that keep the country's infrastructure, health care system, education system, and more progressive and healthy, cannot be pursued to their fullest because the money just isn't there. Therefore, America is falling behind in their standard of living, in innovation, and the debt keeps climbing.

Think of a typical family that is in deep debt and continues to spend more than they are taking in. They continue to rack up debt but eventually they have to pay up or go completely bankrupt. In their debt they cannot buy a new car, have major repairs done on their home, take a vacation and travel, or provide all the necessities their family requires. They must make hard choices and cut spending in certain areas; it is no different for the government. Debt hampers national progress just as it does the progress of a family. This is the greatest challenge facing America today. 

I think our greatest challenge is going to be the very boring but crucial issue of fiscal responsibility. We are going to somehow have to remain intact as a nation without spending more than we earn. This is going to be difficult because nobody is willing to give up their slice of the federal pie. Sooner or later it's going to happen, either by choice or disaster.

The economy is certainly a huge issue in the US and around the globe, but since that has been covered above, I'd add that climate change looks to be another global challenge. Regardless of your views on the causes of climate change and global warming, we are experiencing extreme weather this summer that is in perfect accord with the predictions and models of climate change. 

In our response to climate change, we will be making many decisions that will drive or be driven by economic considerations/concerns. Recently a mandate was announced for minimum fuel efficiency in vehicles produced in the US. This mandate will reduce the demand for fossil fuels but it will also raise the price of cars. 

Though this mandate is not solely aimed at reducing greenhouse emissions (it is aimed, in part, at reducing dependence on "foreign oil"), there is a direct connection between raising fuel efficiency and reducing emissions. 

It would seem that the biggest challenge that America faces is in admitting to the country's problems and addressing them. There is failure in adhering to the US Constitution, failure to address the facts of tremendous unemployment, massive National Debt, political corruption, and real threats to the democratic process as executive privilege is often invoked or procedures defied by those in very high positions. 

I'd say that our biggest challenge is our need to come up with some sort of economic system that is going to allow us to keep having a strong middle class.  We are experiencing something of a "hollowing out" that leaves us with richer and poorer people but with not as much of a middle class (particularly for those lacking a lot of education).  This is a real danger to our entire society as it degrades our trust in our entire system, both economic and political.