Warren Buffett is bullish on America. He made that perfectly clear when he wrote "Buy American. I Am" during the depths of the financial crisis. And he reaffirmed that belief in America's promising future earlier this year in a profile by Time titled "The Optimist."

His argument is pretty compelling, too, for long-term investors. He notes that the Dow Jones Industrial Average (DJINDICES:^DJI) rose from 66 to 11,497 in the 20th century, despite four wars, a Great Depression, and several recessions. That observation is further strengthened by the fact that the Dow is up 65% since he wrote his "Buy American" piece five years ago.

Back in January, Buffett said he's "100% sure that people in this country will be doing more business 10 years from now than they are today." He argues that basic demographics favor the U.S. when compared to other rich countries.

Despite that solid case for America's future, I think there are some troubling long-term trends that could impede economic growth in the coming years. Given that Buffett himself invited someone to lay out the bear case for Berkshire Hathaway (NYSE:BRK.B) at this year's annual meeting, I thought it might be helpful for investors to consider the bear case for America. Here are three trends to be concerned about.

1. Rising Economic Inequality
After considering all of the recent Census data, The Economist reported that, "The most unequal country in the rich world is thus becoming even more so." Currently, the share of overall income held by the richest 1% in the United States is close to the highest level in a century. Meanwhile, the middle class saw its share of the nation's income decline considerably since 1970. And lower-income households are becoming an increasing share of the population.

At a recent talk at Georgetown University, Buffett noted that inequality is getting "wider" and that 24 million or so Americans live on $22,000 per year. He added, "I don't want to try to live on $22,000 with a couple of kids."

Leaving aside the notion of fairness, it's important to consider what inequality might mean for the economy. Throughout much of American history, our relatively high wages resulted in a healthy demand for goods and services, which in turn helped grow the overall economy. If the middle class and poor continue to see their incomes decline, will decreased consumption result in lower economic growth in the future?

2. Climate Change
Last month, the United Nations released its latest assessment of climate science. You can read a summary of the report here.

One of the co-chairs of the committee introduced the report by saying, "Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions." [Bolding in the original.]

Earlier this year, The Washington Post identified another interesting fact about climate change. Since 1991, "roughly 97 percent of all published scientific papers that take a position on the question agree that humans are warming the planet." And yet many Americans seem to believe that the scientific community is divided on the cause of global warming.

It seems to be a topic that is beyond our society's ability to address. First of all, it's a complex topic that is playing out over an extremely long time frame. Secondly, it's hard to put a price tag on the worst-case scenario facing our economy if we do nothing. Finally, our short-term-oriented political system isn't very good at addressing long-term problems.

Looked at this way, it's not surprising that the issue of climate change wasn't seriously addressed during the presidential campaign last year. But here's the thing. If the scientists are right and we need to do something meaningful soon, it's very unlikely that our government would be able to agree on what must be done. At the very least, that's a major risk factor facing our economy in the future.

3. We're falling behind the rest of the world in key skills.
Ever since 1776, old guys like me have been ranting about how the younger generation in America doesn't know anything. This time it's true, however.

A new study by the Organization for Economic Cooperation and Development, as reported by The Atlantic, found that the "United States performed worse than nearly every other country in the group of developed nations" when it came to math, reading, and technology-driven problem solving.

When broken out by age group, American young adults fared poorly compared with the other countries. But before I get too judgmental about the younger generation, I should note that American 45 to 54 year olds performed just average. That's probably not something to celebrate. Responding to the report, our education secretary said that "our education system hasn't done enough to help Americans compete" in the global economy.

This is bad news, and our economy will likely suffer the side effects of this for quite a long time. Unsurprisingly, America had lots of high-performers and lots of low-performers, which further illustrates the increasing economic inequality in this country. You can read the entire study here.

The decline and fall of America?
It's not easy to feel optimistic about America's economic future, after considering the long-term trends discussed above. And yet, those trends are only one part of the story.

America remains a leader in technology and innovation, and it continues to attract some of the brightest minds from around the world. Our companies also continue to be the most profitable, with 132 of Fortune's most profitable corporations in the world coming from the United States. ExxonMobil (NYSE:XOM), with $45 billion in profit, and Apple (NASDAQ:AAPL), with $42 billion in profit, came in Nos. 1 and 2 respectively on the global list for 2012.

We've had much darker times in our history than today, and investors were rewarded for buying equities during those periods. For example, I was born a month after the Cuban Missile Crisis – a considerably more perilous time than nowadays. I wish I'd been buying stocks back then as a 1 month old.

So maybe Buffett is right after all: "Bad news is an investor's best friend. It lets you buy a slice of America's future at a marked-down price."

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.