WASHINGTON, May 19 (Reuters) - Global warming's effects this century could be twice as extreme as estimated just six years ago, scientists reported on Tuesday.

Earth's median surface temperature could rise 9.3 degrees F (5.2 degrees C) by 2100, the scientists at the Massachusetts Institute of Technology found, compared to a 2003 study that projected a median temperature increase of 4.3 degrees F (2.4 degrees C).

The new study, published in the American Meteorological Society's Journal of Climate, said the difference in projection was due to improved economic modeling and newer economic data than in previous scenarios.

Earlier climate warming may also have been masked by the global cooling effect of 20th-century volcanoes and by the emission of soot, which can add to warming, the scientists said in a statement.

To reach their conclusions, the MIT team used computer simulations that took world economic activity as well as climate processes into account, they said in a statement.

These projections indicate that "without rapid and massive action," this dramatic warming will take place this century, the statement said.

The outcome looks much worse if nothing is done to combat climate change, compared to earlier projections. But there is less change if strong policies are put in place now to cut greenhouse gas emissions.

Without action, said study co-author Ronald Prinn, "there is significantly more risk than we previously estimated. This increases the urgency for significant policy action."

The study was released as U.S. President Barack Obama announced a plan to set national emissions standards for cars and trucks to cut climate-warming pollution [nN19374415] and as a bill to institute a cap-and-trade system to curb greenhouse gases was debated in the House Energy and Commerce Committee. [nN19402140]

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