A quick glance at recent headlines would lead a reasonable person to assume that this year’s big losers are Greek and Chinese stocks. Yet, despite all the furor in the news, the Athens Stock Exchange is down less than 5 percent year-to-date, while the Shanghai Composite remains up more than 10 percent, according to Bloomberg data.

The real damage has been in the commodity complex. Through late July, year-to-date crude oil prices were down around 10 percent, platinum prices were off nearly 20 percent and coffee prices were down almost 30 percent, Bloomberg data shows. Based on the Bloomberg Commodity Index of 22 commodities, the overall complex is now trading at a 13-year low. Several factors account for the sell-off.

Slowing global growth

So far, 2015 is shaping up to be another disappointing year. The International Monetary Fund (IMF) recently lowered its full year estimate for global growth to a bit below last year. In the United States, 2015 gross domestic product (GDP) growth estimates have fallen by nearly a full percentage point since February, according to Bloomberg data. Slower growth negatively impacts cyclical commodities, like energy and industrial metals. As economic activity decelerates, so too does demand for these commodities.

Expectations for a Federal Reserve (Fed) hike

While growth is disappointing, it’s arguably still strong enough to justify an initial Fed hike later this fall. Expectations for tighter monetary policy are impacting commodities in a few related ways. Central bank divergence, i.e. the Fed hiking while most other central banks are easing, is likely to push the dollar, already up 8 percent year-to-date, higher. In addition, certain commodities, notably precious metals, are being negatively impacted by rising real rates.