Just for fun (and because I'm a paranoid alarmist asshole), I'll try and make the argument that negative thinking could kill us all.
The Maharishi Effect (also called "super radiance") is the supposed benefit that a society receives when a small number of practitioners of Transcendental Meditation techniques are active. In 1960, Maharishi Mahesh Yogi predicted that if one percent of the population practiced Transcendental Meditation, the quality of life for the entire population would be affected by reducing violence, crime, disease, deadly storms, and other destructive natural forces. The threshold required to see such a Maharishi Effect is training 1% of the population in Transcendental Meditation. In 1976, a study conducted by researchers with Maharishi University of Management claimed to show the Maharishi Effect because they said that crime was reduced an average of 16% in communities where 1% of the population was practicing the TM technique.
According to literature from the Canadian branch of this guy's school, "by creating this effect, Canada will radiate a peaceful influence to all nations, naturally disallowing the birth of an enemy...the result is an invincible armour for the nation, which automatically prevents incoherent influences from disturbing the country's internal peace and harmony." (It obviously, obviously has.)
A study conducted in the Middle East in 1983 by David Orme-Johnson, et al., was published in Journal of Conflict Resolution and presented statistical evidence for the Maharishi Effect. This was a prospective experiment (one in which the outcome is predicted in advance). All the variables were publicly available data, and a list of the variables used in the study was placed with an outside Project Review Board prior to the experiment. Analysis showed there were fewer automobile accidents, fires, and crime in Jerusalem during the time of the experiment. Additionally, research results indicated that the Transcendental Meditation and TM-Sidhi group practice caused statistically measurable improvements in the quality of life in the country as a whole. The study measured reduction in crime, an increase in the stock market price, improved national mood (as measured by news content analysis) and a reduction of hostilities in the war in Lebanon (fewer war deaths and decreased war intensity as measured by news content analysis). According to the study, the effects of religious holidays, temperature, weekends, and other forms of seasonality were controlled for and did not account for the results. Additionally, according to the authors, all cross-correlations and transfer functions supported a causal interpretation.
Studies, Criticisms, Responses, and more studies
Most Buddhists accept the idea that meditation supports the local population's wellbeing, and it's cited as one rational for communities to support monks economically. If you accept the validity of this theory, then you should also accept the opposite, that thinking, worrying, anger, jealousy, greed and deception creates the opposite effect in the world. Enough to bring it all down?

Maybe. As an aside, it lends credence to all the critics of militaristic right-wing rhetoric in that it creates an indirect link between violence and rhetoric where we don't have a direct one.