Since its March 2009 lows, the Dow Jones industrial average- as represented by the SPDR Dow Jones Industrial Average (NYSE:DIA) – has risen an impressive 145% over the past four years. At the same time, the S&P 500 continues to hit news. All in all, the current bull market is one of the strongest in recent history. 

And that has many market pundits worried.

Cracks are beginning to how themselves as recent earnings reports from a variety of firms have either missed expectations or relied on extreme cost cutting- rather than sales growth- to get the job down. At the same time, slowing economic growth in key emerging markets like China and Brazil are having their ways with global GDP. Let’s not forget that the Eurozone still remains in a quagmire of debt and austerity.

For investors, the best offense could be a good defense for the rest of summer.

Trouble Brewing
While the bull market has been a great ride, things are starting to look a bit dour for the global economy as a host of issues could bring the beast back down to earth over the next few months.

First, emerging market leader China seems to finally be slowing down. According to a new survey, China's manufacturing sector shrank in July and the nation’s job market weakened further. Investment bank, HSBC’s (NYSE:HBC) Purchasing Managers' Index (PMI) showed that output, employment and new orders all declined at a faster pace in July. The index of business conditions fell to 47.7. That’s down from June's 48.2 reading and represents the third straight month below the critical 50 mark. That indicates a contraction. Additionally, this is the weakest level since August of 2012. That contracting PMI reading has repercussions for the entire global economy. 

Already, those repercussions are starting to be felt.

Here in the U.S., earnings reports have generally faired to the negative side so far. Key bellwethers for their industries- like Microsoft (NASDAQ: MSFT) and McDonalds (NYSE: MCD) -have reported lower than expected numbers. With the S&P 500’s current P/E at 18.8 - above historical norms- these lower earnings reports don’t bode well for stock prices going forward. 

Finally, while conditions in Europe seem to be getting better, the export reliant economy could catch a cold if the conditions in the U.S. and China falter further.

Get Defensive
With some storm clouds beginning to form over the global economy, investors may want to consider thinking about moving some money into more “defensive” areas. While the bull could continue, there is a real chance that things could at least drift sideways for quite a bit of time. By focusing on these defensive measures, investors could avoid some of the worst effects of a possible summer downturn. Here are some top picks:

With “defense” right in its name, the Guggenheim Defensive Equity (NYSE: DEF) could be a great starting point. The fund tracks the Sabrient Defensive Equity Index- which selects stocks based on low relative valuations, conservative accounting and a history of out-performance during market downswings. The ETF currently has 101 different holdings including supermarket chain Kroger (NYSE:KR) and pipeline firm Enterprise Products Partners (NYSE: EPD). More importantly, DEF held up pretty well during the crisis in 2008 and fell less than the overall market.

Another defensive option could be to focus on lower volatility stocks. Funds like the iShares MSCI USA Min Volatility (NASDAQ: USMV) and the PowerShares S&P International Developed Low Volatility (NASDAQ: IDLV) bet on stocks that should move "less" - up or down- than their respective indexes. This should help mitigate any wide market moves and create a smoother ride for investors.

SEE: Guard Your Portfolio with Defensive Stocks

Perhaps the defense in these times is simply cash. Taking some gains off the table and parking it while waiting for more bargains is a good strategy in any market. And considering that interest rates could rise any day now, this cash could be worth even more tomorrow. The PIMCO Enhanced Short Maturity ETF (NASDAQ: MINT) represents a perfect money-market option for investors, while the Vanguard Short-Term Bond ETF (NYSE: BSV) gives a bit more in the yield department.

The Bottom Line 
Some signs are starting show that the bull market may be a bit long in the tooth. For investors, moving some assets into more defensive funds could make a ton of sense right now. The previous picks should help still provide returns if the market keeps gaining as well as protect to the downside.

Disclosure: At the time of writing, the author did not own shares of companies mentioned in this article.