Q+A: What are the risks from a warming world?

LONDON (Reuters) - Rising greenhouse gas levels mean the planet is very likely to become warmer, threatening more disasters such as Russia's record drought and Pakistan's deadly floods.

A warmer world is expected to place more risks on food supplies for a growing population. Changes in rainfall patterns could disrupt riverflows, water supplies and crops.

The U.N. panel of climate scientists says it is more than 90 percent probable that human activities, led by burning fossil fuels, are the main cause of warming in the past half-century. It says the world needs to dramatically reduce the amount of carbon emissions from burning fossil fuels and deforestation to avoid the worst impacts of climate change.

Following are some key questions about rising temperatures.

THE HEAT IS ON. BUT HOW MUCH?

Average surface air temperatures have warmed more than 0.7 degrees Celsius over the past century. But the rise has not been continuous, scientists say.

The warming trend over the past 50 years of 0.13 deg C per decade is nearly twice that for the past 100 years and the past decade is the warmest yet recorded.

The U.S. National Oceanic and Atmospheric Administration says the first nine months of 2010 tied the same period in 1998 for the warmest combined land and ocean surface temperature since records began in the 19th century.

The U.N.'s World Meteorological Organization says 1998 is the warmest year on record, followed by 2005, both years of powerful El Nino patterns that drove up temperatures globally.

For January to September 2010, the global combined land and ocean surface temperature was 14.75 deg C. This is 0.65 deg C above the 20th century average, NOAA says.

While this doesn't sound much, so sensitive is the climate system that the difference between an Ice Age and now is a rise of about 5 deg C.

WHAT ABOUT TEMPERATURES IN THE FUTURE?

In all depends on how nations tackle the rapid growth in greenhouse gas emissions. Computer models say the world could be between 1.1 and 6.4 deg C warmer by 2100 and that climate change would continue for centuries, the U.N's Intergovernmental Panel on Climate Change (IPCC) says.

Leaders of more than 120 nations agreed at the U.N.'s Copenhagen summit last December to limit warming to below 2 deg C to avoid the worst impacts of climate change, without giving a timeline. The IPCC says emissions would need to peak within 10 years.

WHAT ARE THE IMPACTS?

There are many. The IPCC says climate change, from heatwaves to rising temperatures, will put people, crops and livestock at greater risk.

Extreme droughts and floods are likely to be more frequent, hurricanes and cyclones are also likely to become more powerful, though not necessarily more frequent, and rising seas are expected to threaten river deltas, erode famed tourist beaches and damage coastal cities.

Crops such as wheat and rice will also face greater production uncertainty because of stronger droughts, floods and storms, while warmer seas and rising acidity threaten coral reefs that are nurseries for fish stocks crucial to millions of people.

Some areas might initially benefit -- longer growing seasons might boost crop growth in Siberia or northern Europe, for instance.

Sea levels have risen more than 20 centimeters since 1870 and the U.N. climate panel says sea levels could rise between 18 and 59 cm this century, and more if parts of Antarctica and Greenland melt faster.

WHAT ARE THE COSTS?

The IPCC said in its latest report in 2007 that it will cost less than 0.12 percent of world GDP a year until 2030 to avoid the worst impacts of climate change. By 2030, that would cut about 3 percent from the world economy, mostly to fund a shift to renewable energies such as solar and wind power from fossil fuels.

Insurer Swiss Re says the costs from hurricanes in southern Florida from winds, storm surges and floods could double over the next two decades to $33 billion in 2030, from an annual average loss of $17 billion in 2008.

Issued losses are also rising because more and more beachfront properties are being built in many coastal parts of the world from the Caribbean to Queensland in Australia and Bali.

In a September 2010 report, Swiss Re says climate change could exacerbate the death and destruction from droughts and floods, wiping out years of development gains and costing some countries up to 19 percent of their annual GDP by 2030.

(Sources: NOAA, WMO, Intergovernmental Panel on Climate Change, Australian Academy of Science)

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(Editing by Ed Lane)