ROSENBERG: The world has a 'reliable buyer of last resort' - it's smaller than the US economy, but bigger than China

2015 hasn't been a great year for China.

The dramatic slowdown in China, technically the world's second largest economy behind the US, has policymakers around the world nervous as they assess how the massive emerging market affects their local businesses.

The US economy also isn't without its problems as legislatures push the government closer to the brink of a shutdown.

But some of Wall Street's biggest bulls write that there's one sub-economy that deserves more attention.

The US consumer.

In a recent note to clients, David Rosenberg, chief economist and strategist at Gluskin Sheff, pointed out that the US consumers alone make up an "economy" that's larger than China's entire economy. And, furthermore, it has more positives than China has negatives.

As he wrote in the note:

Yes, we hear constantly about how China's share of global GDP has risen inexorably over the decades, but that obscures a huge point.

China's contribution to global producers has been in the basic material sphere as the country absorbed so much of the world's resources in its quest to build mega cities and build a world-class industrial infrastructure, but that is about it.

China, for years, racked up massive trade surpluses as these mega cities became home to low-cost export regions.

The reliable buyer of last resort, outside resources, was never China. It was and continues to be the United States.

The American consumer, if it were a country of its own, would be the largest economic base in the world. That's right - even bigger than China's entire economy.

Piggybacking on Rosenberg's analysis, Fundstrat's Tom Lee then re-created a chart that Rosenberg shared showing this. As you can see below, the US consumer economy makes up 15% of the global GDP, while China's entire economy makes up 13% of it. (For what it's worth, the Chinese consumer economy makes up 5%.)

Lee then goes on to suggest that the fact that the US consumer economy is both large and robust suggests that there could be "life after China."

"[T]hink about the tailwinds for US consumers - jobs market is strong. Wages could rise. Housing is only early stages of recovery. Gasoline is lower. All pointing to upside surprises," writes Lee. "In fact, one could argue there is more upside surprises to the US consumer than downside to China at the moment."

However, it's worth noting that - when looking at this from the longer-point of view - the robust US consumer economy could also be a red flag.

Last week at a press briefing, Mike Wilson, chief investment officer of Morgan Stanley wealth management, said consumer behavior was starting to show signs of excess as the economic recovery was reaching its later stages:

Consumers are feeling pretty good, and they are starting to spend money again, and they're starting to do dumb things. They're starting to borrow money, they're starting to maybe buy that house they shouldn't or that car they shouldn't. [...] And now, the clock is ticking. We're into the final part of this recovery. It could last three years, it could last five years, it could last two years, I don't know. But that excess sort of behavior is starting to happen.

Either way, China's not the only economy worth paying attention to.