Sen. Amy Klobuchar, D-Minn., told delegates at the Democratic convention that Hillary Clinton showed compassion to the powerless across the world, especially victims of human trafficking. Klobuchar highlighted the enormous scope of the problem.

"Human trafficking is the third-biggest criminal enterprise in the world," she said.

We found that comparison intriguing and looked into it.

We reached out to Klobuchar’s office and did not hear back. From our research, the claim is plausible, with a significant caveat: It’s always enormously hard to put a dollar figure to illegal activity.

However, most estimates suggest that drug trafficking is larger than all other crime. Whether human trafficking falls second or third is a matter of debate, but the consensus view is that it ranks in the top three.

Definitions matter when talking about trafficking.

The Government Accountability Office wrote that "human trafficking involves the exploitation of a person typically through force, fraud or coercion for the purpose of forced labor, involuntary servitude or commercial sex."

This can take a number of forms. Desperate people might go into debt to smugglers who place them in jobs. Their only option is to work off that debt on terms dictated by their employer. They might be sold by their parents and have no money to get back home. Or they might be tricked into prostitution and find themselves living in the shadows of an illicit enterprise.

The International Labor Organization, an arm of the United Nations, estimated the profits associated with human trafficking reached about $150 billion in 2012. Of that, about $100 billion was tied to sexual exploitation.

How does that compare to money from illegal drugs?

The U.N. Office on Drugs and Crime tackled that question in 2011. Their rough estimate was that drugs accounted for about a fifth of all criminal proceeds worldwide, or about $420 billion.

The research group Global Financial Integrity assessed all forms of transnational crime in 2011. Its report said that "there is little doubt that, of all the illicit markets, the drug market is king." It leaned toward a total value of $320 billion, but gave a nod to another report that put the range between $45 billion and $280 billion.

The issue is simply a lack of consistent or comprehensive data.

That report also looked at profits from counterfeiting, whether that is fake Rolex watches or fake $100 bills. It cited a crude estimate from the Organization for Economic Cooperation and Development of $250 billion.

So where does that put human trafficking in the ranking?

Without putting too much credence in the precision of any figure, human trafficking seems to come in third behind the drug trade and counterfeiting.

A big red flag

The International Labor Organization’s report is full of cautionary notes.

"In the case of forced sexual exploitation, information ... is unavailable because of the nature of the work," it said.

A key formula in the report’s model is based on data from just seven countries — India, Brazil, Peru, Bolivia, Cote d’Ivoire, Kenya and Uganda.

It identified three kinds of human trafficking, including forced labor, forced sexual exploitation, and state imposed forced labor. But to apply that breakdown across Asia, Latin America, the industrialized nations, Africa, the Middle East and Central Europe, it wrote "in absence of more detailed information, this report assumes that the distribution of victims across the three forms ... is uniform across the regions."

At the end of the day, assumptions fill in missing data right and left.

So no one should take these figures to the bank, but Dominique Roe-Sepowitz, a human trafficking researcher at Arizona State University, said that doesn’t mean they have no value.

"These are as good of an estimate as we know," Roe-Sepowitz said. "There are so many curves and complexities in estimating the cost of human trafficking globally I don't think we can create a better estimate."

The same uncertainties apply to the estimated value of the drug and counterfeiting trade.

Our ruling

Klobuchar said that human trafficking is the third-biggest criminal enterprise in the world.

A U.N. agency estimated the total value of human trafficking at $150 billion. The comparable estimates for the drug trade range from about $280 billion to $420 billion. There is one dicey estimate for counterfeiting of $250 billion.

By those measures, human trafficking does rank third. However, all of these numbers hinge on sweeping assumptions and limited data.

With that caveat in mind, we rank this claim Mostly True.