Severe droughts, floods and heat waves rocked the world last year as greenhouse gas levels climbed, likely boosting the odds of extreme weather events, international scientists said Tuesday.

The details are contained in the annual "State of the Climate in 2011" report, compiled by nearly 400 scientists from 48 countries and published in the peer-reviewed Bulletin of the American Meteorological Society.

The report itself remains "consciously conservative" when it comes to attributing the causes of certain weather events to climate change, and instead refers only to widely understood phenomena such as La Nina, it said.

However, it is accompanied for the first time by a separate analysis that explains how climate change may have influenced a selection of key events, from droughts in the United States and Africa to extreme cold and warm spells in Britain.

"2011 will be remembered as a year of extreme events, both in the United States and around the world," said Kathryn Sullivan, deputy administrator of the National Oceanic and Atmospheric Administration (NOAA).

"Every weather event that happens now takes place in the context of a changing global environment," she said, adding the reports shed light on "what has happened so we can all prepare for what is to come."