Commodity prices: Why the selloff may continue in 2015

Several of the world's key commodities -- including oil, gas, gold and corn -- have been suffering the worst months of trading since the commodities crash of 2008.

Back then, the main reason for downturn in prices was obvious: the credit crisis and subsequent panic about global economic growth.

Yet today, while global growth is more sluggish than hoped in certain parts of the world -- particularly in China -- the overall economic picture seems much brighter than in 2008.

In 2014, the focus seems to have switched to supply, as the Organization of Petroleum Exporting Countries (OPEC) pledges to keep supply constant despite plunging oil prices. As well as being interpreted as throwing down the supply gauntlet to the shale-rich U.S., the OPEC move has been criticised for apparently penalizing several of its members.

Read More Can oil fall all the way to $40?

Ty Wright | Bloomberg | Getty Images

Ultimately, it looks like investment decisions in the developed world may be causing the commodities glut.

"Increasingly the supply side counts more, as investment cycles are creating persistent gluts in some areas (e.g. oil, natural gas, iron ore, grains) and lagging investment is starting to result in tightening elsewhere (industrial metals in general, copper in particular)," commodities analysts at Citi wrote in a research note.

Read MoreWhy gold just can't win

Despite the focus on emerging markets, the Citi analysts argue that continuing weakness in 2015 will have a "Made in America" quality, and called an end to "the era of $100 a barrel oil."

With grains, better weather conditions than for years meant better-than-expected crops, which "should leave inventories chock-a-block for a good year or two," according to Citi.

The classic, straightforward analysis of commodity supply-demand dynamics would argue that, with cheaper commodities and cheaper prices, demand from consumers who feel like they're getting a bargain will subsequently grow, sending prices up again.

Unfortunately for miners and other commodities-linked companies, who have seen share price falls already this week, this may not be the case in 2015.

"This fall in prices seems demand rather than supply led and so any benefit (to consumers) will be negated by the declining world growth outlook," Rabobank strategists argue.

Follow us on Twitter: @CNBCWorld