Dec 3 (Reuters) - Following are findings of the U.N.'s Intergovernmental Panel on Climate Change in a 23-page summary about the risks of global warming issued on Nov. 17:

* OBSERVED CHANGES

"Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level."

* CAUSES OF CHANGE

"Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in ... greenhouse gas concentrations" from human activities.

Annual greenhouse gas emissions from human activities have risen by 70 percent since 1970. Concentrations of carbon dioxide, the main greenhouse gas, far exceed the natural range over the last 650,000 years.

* PROJECTED CLIMATE CHANGES

Temperatures are likely to rise by between 1.1 and 6.4 Celsius (2.0 and 11.5 Fahrenheit) and sea levels by between 18 cm and 59 cm (7 inches and 23 inches) this century.

Africa, the Arctic, small islands and Asian mega-deltas are likely to be especially affected by climate change. Sea level rise "would continue for centuries" because of the momentum of warming even if greenhouse gas levels are stabilised.

"Warming could lead to some impacts that are abrupt or irreversible". About 20-30 percent of species will be at increasing risk of extinction if future temperature rises exceed 1.5 to 2.5 Celsius.

* FIVE REASONS FOR CONCERN

-- Risks to unique and threatened systems, such as polar or high mountain ecosystems, coral reefs and small islands.

-- Risks of extreme weather events, such as floods, droughts and heatwaves.

-- Distribution of impacts -- the poor and the elderly are likely to be hit hardest, and countries near the equator, mostly the poor in Africa and Asia, generally face greater risks such as of desertification or floods.

-- Overall impacts -- there is evidence since 2001 that any benefits of warming would be at lower temperatures than previously forecast and that damages from larger temperature rises would be bigger.

-- Risks or "large-scale singularities", such as rising sea levels over centuries; contributions to sea level rise from Antarctica and Greenland could be larger than projected by ice sheet models.

* SOLUTIONS/COSTS

Governments have a wide range of tools -- higher taxes on emissions, regulations, tradeable permits and research. An effective carbon price could help cuts.

Emissions of greenhouse gases would have to peak by 2015 to limit global temperature rises to 2.0 to 2.4 Celsius over pre-industrial times, the strictest goal assessed.

The costs of fighting warming will range from less than 0.12 percent of global gross domestic product (GDP) per year for the most stringent scenarios until 2030 to less than 0.06 percent for a less tough goal. In the most costly case, that means a loss of GDP by 2030 of less than 3 percent.

-- For Reuters latest environment blogs click on: http://blogs.reuters.com/environment/ (Editing by Jeremy Laurence)

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