Copenhagen: the Climate Summit

More than 190 countries will meet in the Danish capital from tomorrow through Dec. 18 to negotiate an international treaty to curb greenhouse gas emissions. The talks are the latest in a series of United Nations meetings dating back to 1992 to respond to man-made global warming, which scientists say is caused by the burning of fossil fuels. Here is a look at the science, the summit, and the stumbling blocks to reaching a deal there.

THE CHANGES

Hotter: Air temperatures have warmed about 1.3 degrees Fahrenheit over the last 100 years. They are expected to rise 3.1 to 7.2 degrees by the end of the century if worldwide emissions continue to grow unchecked.
Rising seas: Melting glaciers and the larger volume of warmer seawater have contributed to a sea level rise of about .12 inches a year since the 1990s, and sea level is projected to rise 7 to 23 inches, and perhaps more, by the end of the century.
Acidic oceans: Seawater’s absorption of carbon dioxide from the air is causing the ph of the world’s oceans to drop, threatening corals, shell-building animals, and possibly other marine life.
Extreme weather: The frequency and intensity of rainstorms and droughts are increasing in some places.
Changing seasons: Spring is lengthening in some places and starting earlier, in some instances changing bird migrations, egg laying, and flowering of plants.
Species shift: Ranges of plant and animal species are shifting toward the poles.

THE SCIENCE

Doesn’t the climate change naturally?

Yes. Changes in the earth’s orbit and the sun’s intensity, as well as volcanic eruptions, can cause climatic changes. Also, natural fluctuations in the ocean and atmosphere, such as El Niño and La Niña, can affect climate.

So how do we know humans are causing warming?

It's well accepted by scientists that greenhouse gases, such as carbon dioxide and methane, trap heat in the earth's atmosphere and warm the planet. Humans have released more of these gases in the last 100 years by the burning of fossil fuels -- chiefly coal and oil -- amplifying the natural greenhouse effect. Scientists can find no other explanation for the recent temperature rise without including humans' contribution. The Intergovernmental Panel on Climate Change, a United Nations body that is the scientific authority on global warming, says most of the global temperature increase since the mid-20th century is very likely due to man.

Aren’t there many uncertainties?

Yes. Scientists do not fully understand many aspects of climate change, including exactly how the natural system will respond to increased carbon dioxide; regional climatic changes; cooling effects of pollution particles that block the sun; how rapidly the Greenland and Antarctic ice sheets will melt; and whether hurricanes are gaining in frequency and intensity because of warmer ocean waters. As a result, scientists often give a range of projections about temperature increase, sea level rise, and other climatic changes.

Is New England experiencing man-made climate change?

New England winters have warmed about 4 degrees Fahrenheit over the last 40 years. While it is difficult to “see” the footprint of man-made global warming in smaller geographic regions, scientists strongly suspect some of New England’s warming is probably due to the worldwide phenomenon.
That’s because the temperature increase began accelerating around 1970 -- the same time overall global temperatures did; New England’s increase is lasting longer than previous warm stretches in the past century that were attributed to natural variability; and the entire region has been affected. Special report on global warming in New England.

How much do emissions need to be reduced?

In advance of the Copenhagen meeting, President Obama pledged to significantly lower emissions -- about 83 percent below 2005 levels by 2050. Some scientists believe even larger cuts are needed -- as much as 80 percent below 1990 levels -- to avoid the worst consequences of global warming. But opponents of government-mandated reductions say strict limits would boost energy prices and dramatically slow economic growth. The US Environmental Protection Agency, recently estimated the economic costs of pending Senate legislation seeking emissions cuts similar to those pledged by President Obama and passed by the House. The agency projected that average household consumption would be reduced by less than 1 percent and that it would cost households about $80 to $111 per year.

SUMMIT GOALS

Replace previous treaty

Countries are gathering to replace the Kyoto Protocol, the world’s first binding greenhouse gas treaty, which expires in 2012. It is widely expected that world leaders will create a blueprint in Copenhagen and sign a binding treaty next year.

Minimize warming

Limit the world’s warming by no more than 3.6 degrees Fahrenheit (2 degrees Celsius) compared to preindustrial levels and stabilize atmospheric carbon dioxide levels at no higher than 450 parts per million. Many scientists say an environmentally sounder goal would be 350 parts per million. (Levels today are at about 383 parts per million).

Ensure reductions from the United States and other developed countries

Win ambitious emission-reduction targets and timetables from the US, the developed world's largest greenhouse gas emitter, which never ratified the Kyoto Protocol, and other developed countries.

Help developing countries reduce emissions

Work with developing countries including China and India to ensure they reduce emissions below business as usual moving forward. China, now the largest emitter of greenhouse gases in the world, recently agreed to reduce its carbon intensity -- or the amount of carbon dioxide emitted per unit of production -- by 40 percent to 45 percent from 2005 levels by 2020, which would slow the increase of carbon emissions but not reduce them because of China's rapid growth.

Financial aid

Win committments from developed, rich countries to contribute billions of dollars to help developing countries adapt to climate change and switch to greener energy sources.

Keep trees from being cut

Hammer out an agreement to prevent deforestation in developing countries by protecting large tracts of forests that absorb carbon dioxide.

ACHIEVING REDUCTIONS

Most countries, including the United States, are expected to use a cap-and-trade system. This method places a gradually-falling cap on overall emissions from power plants and industry but allows polluters flexibility in how they comply, perhaps by buying credits from cleaner energy sources, investing in more energy-efficient equipment, or finding less expensive ways to offset their pollution -- such as planting or protecting trees that absorb carbon dioxide elsewhere.

OBSTACLES

The United States: It has not passed domestic legislation to lower greenhouse gases, action most other countries want to see before committing to their own goals.
Developing countries: Will become the biggest contributors to global emissions, but they say developed countries have a larger “historical footprint” and thus should make more aggressive reductions -- and give poor countries financial assistance -- more than $10 billion a year in the short term and far more in later years.

SOURCES: Intergovernmental Panel on Climate Change; Union of Concerned Scientists; NYTimes (for graphic); University Corporation for Atmospheric Research; United Nations Framework on Climate Change; National Oceanic and Atmospheric Administration; National Geographic