After earnings season kicked off yesterday, the Dow Jones Industrial Average (Index: ^DJI) is falling lower during today's trading session. As of 12:30 p.m. EDT, the Dow sits at 13,381, down 0.68%. The story we have been hearing over and over again for the past week -- that global economic growth is slowing -- seemed to be echoed in last night's earnings releases. Although companies are still making money, the majority of market participants don't see the positive wording they would like.

The big losers today are Alcoa (NYSE: AA), Caterpillar (NYSE: CAT), Chevron (NYSE: CVX), and ExxonMobil (NYSE: XOM).

So why are they down?
Alcoa kicked off earning season after the bell last night. The initial reaction was good, but when investors let the report sink in, things turned south. Alcoa began to slide at the market open and has continued to decline throughout the day. Shares are down more than 4.7% as of 12:50 p.m. EDT. Alcoa is forecasting that demand for heavy-truck and trailer manufacturing is slowing. They also noted that can and packaging businesses are seeing less demand.

Alcoa's warning has hit heavy-equipment manufacturer Caterpillar particularly hard. Shares of Cat are down more than 1.75% on the news. Caterpillar has seen its shares move up and down wildly this year, with the 52-week range between $78.25 and $116.95. Today's share price of $83.15 is on the lower end, and the stock is down more than 6% year to date.

While Chevron has not released its earnings report, it did announce that it expects substantially lower third-quarter results. Lower oil prices and production rates falling during the beginning of the quarter were reasons for the lack of earnings. The share price has been reduced by more than 4% thus far.

Competitor ExxonMobil has also been affected by Chevron's statement and has lost 1.19% of its stock price. Although part of Chevron's lower production owed to a refinery fire, both companies were hit with lower oil prices and Hurricane Issac. Thus far there's no word that Exxon will cut its forecast, and the company is expected to announce earnings on Nov. 1, one day prior to Chevron's release date.

Matt Thalman does not own share of any company mentioned above. The Motley Fool owns shares of Exxon Mobil. Motley Fool newsletter services have recommended buying shares of Chevron. The Motley Fool has a disclosure policy. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. Try any of our Foolish newsletter services free for 30 days.