Carbon dioxide from the burning of fossil fuels - coal, oil and natural gas - is the chief cause of global warming.
The studies found there's a limit to how much manmade carbon dioxide can be added to the air before warming exceeds an increase of 2 degrees Celsius or 3.6 degrees Fahrenheit - the level that many governments have set as a goal. World average temperatures going higher than that may be dangerous, some scientists say.
To keep under that danger level, the world has to spew less than 1.1 trillion tons of carbon dioxide in the first half of the 21st Century, according to studies published in Thursday's edition of the journal Nature.
In the first nine years of the century, the world has already emitted one-third of that amount and is on pace to hit that trillion ton limit in just 20 years, said climate researcher Malte Meinshausen of Germany's Potsdam Institute for Climate Impact Research and lead author of one of the studies.
Even if the world ducks under that emissions limit, there is still a 25 percent chance of temperatures exceeding the dangerous mark, he said.
President Barack Obama said he wants to cut U.S. emissions of carbon dioxide by 80 percent. That is a "good start but it's not enough to limit warming," said Bill Hare, a study co-author who is also at the Potsdam Institute.
Assuming that other countries cut their per-person emission levels to match the United States, the United States has to cut its overall pollution by 90 to 95 percent to keep the world from exceeding the 1.1 trillion ton mark, Hare said.
Cutting emissions means not burning as much fossil fuels, leaving about three quarters of the known reserves in the ground, the study authors said.
"Not much at all of coal reserves can be burnt and still keep below" the 3.6 degrees of warming, Hare said.
World emissions must start dropping by 2015, otherwise cuts will have to be too draconian, Meinshausen said.
The studies, which used computer models, take a different approach than other research on figuring out how much carbon dioxide in the air is too much. Instead of the proportion of carbon dioxide in the air at any given time, they looked at the total amount spewed out over many decades to arrive at a tipping point of 1.1 trillion tons.
Stephen Schneider of Stanford University who paints a worst case scenario for global warming in a commentary in the journal, said the studies make it seem like scientists know where there's a solid danger line for emissions, when they don't. The papers acknowledge there is a 25 percent chance the limit should be lower. Schneider said that's a pretty big risk when the consequences of being wrong are severe.
"If you had a 25 percent chance that walking into a room would give you serious flu, would you?" Schneider asked.