US stocks may already be in bear market

The S&P 500 fell 4.6 percent last week. It was its biggest weekly decline since March and its worst start to a December since 2008.

Ed Clissold, Ned Davis Research's chief US market strategist, told CNBC last week that US stocks may be experiencing a bear market that is "going to take you down around 20 percent — maybe a little bit more".

Clissold thinks that earnings growth is becoming a "front-burner issue". He thinks it will slow down "more than expected" and that downward earnings revisions are "going to continue to plague the market for a few more months".

However, Clissold does not expect a recession and thinks that the bear market will last until early second quarter next year, "and then we can look for a bottoming process from there".

Doug Gordon, a senior portfolio manager at Russell Investments, does not expect a bear market though. "While you can certainly see a path that could get us to a bear market, I think it's more of a messy correction," he told CNBC last week.

Gordon sees the US-China trade war as a major economic risk and thinks the ceasefire achieved last week is an important marker.