Talking about global warming can be tricky. Everyone has an opinion, some of them more informed than yours. But what information is forming those opinions, and where does the truth lie? We looked at the various arguments for either side of the debate.

Arguments against the existence of man-made climate change:

1. Climate changes all the time. It has changed before and will change again.

Yes, climate changes are usually a natural occurrence, caused by changes in the sun, volcanoes and other natural factors. But historic shifts show us how sensitive the planet is to greenhouse warming from carbon dioxide in the atmosphere, and hint at how costly our modern CO2 surplus can become. Current atmospheric levels of CO2 are around 380 parts per million, up from about 320 ppm in 1945, while global surface temperatures during that time have risen 1.2 degrees.

Humans continue to pump CO2 skyward at an ever-increasing rate. According to the U.N. Intergovernmental Panel on Climate Change, CO2 levels are projected to rise beyond 400 ppm in just the next five years.

2. Scientists do not have a consensus about climate change.

Climate skeptics point to the Petition Project, where 31,000 scientists signed a petition saying there is no evidence that human-released carbon dioxide will result in a warmer atmosphere. Climate Depot has published another list of 1,000 scientists who disagree with man-made global warming claims.

But the peer-reviewed science does not support this. A study of papers mentioning global warming published between 1993 and 2003 revealed that 75 percent agreed humans were causing climate change, and the other 25 percent made no comment on the issue.

A later survey of more than 3,000 earth scientists — 97 percent of whom have Ph.D.s or master's degrees, compared with 28 percent of those signing the Petition Project — found that 97.5 percent of the scientists who had actively published research on climate change agreed that human activity was a significant factor in rising global temperatures.

And as the website Skeptical Science points out, "There are no national or major scientific institutions anywhere in the world that dispute the theory of anthropogenic climate change."

3. Scientists talking about climate change are just looking for grant money.

A common complaint levied against scientists who publish studies about climate change is that they are only in it for funding and are therefore creating a scare among the public. But as the website Logical Science points out, there really isn't a lot of money in science. In addition, published climate science is peer-reviewed, with scientists around the world constantly checking each others' work both before and after publication.

4. The sun is causing global temperature rises.

In 2004, scientists with the Zurich-based Institute for Astronomy presented a paper at a conference saying the sun had been more active in the prior 60 years than in the entire 1,000 years before.

Yet the study also concluded that after 1975, solar activity did not have a correlative effect on global temperature. In fact, the study says, "at least this most recent warming episode must have another source."

Numerous other studies have shown that solar activity in the past 50 years has declined while global temperatures have risen.

5. Global warming is good for the economy and for civilization.

As the Heartland Institute wrote in 2003, previous warming periods allowed humanity to build its first civilizations and made it possible for the Vikings to settle in Greenland.

In fact, climate change may create a few economic benefits. For example, the Northwest Passage is now ice-free a few weeks a year. This may allow for greater flexibility and speed (not to mention reduced costs) in shipping, allowing cargo ships to travel through the Arctic Ocean from Asia to Europe, rather than going south through the Panama Canal.

But a 2008 study published by the Organisation for Economic Co-operation and Development found that climate change "poses a serious challenge to social and economic development." Water resources will change, farming practices will need to be adapted, building codes will need to be rewritten, sea walls will need to be built and energy costs will rise, according to the report.

Arguments for the existence of man-made climate change:

1. Humans have caused the global rise in CO2 and other [skipwords]greenhouse gases[/skipwords].

Carbon dioxide levels are currently "25% more than the highest natural levels over the past 800,000 years," according to the Environmental Defense Fund. Deforestation caused part of that, with the rest coming from the burning of fossil fuels.

How can we know that oil and coal have contributed to this rise in CO2? Simple: Fossil fuel emissions have a different "fingerprint" than the CO2 released by plants. According to a study published in the Journal of Mass Spectrometry, you can identify the source of carbon emissions by the ratio of carbon-12 and carbon-13 isotopes. The atmospheric level of these isotopes indicates that a greater ratio of CO2 is now coming from fossil fuels than from plants.

2. Climate change computing models are good enough to trust and take action.

While no computer model is perfect, they are constantly getting better, and as Skeptical Science points out, they are intended to predict trends, not actual events. Each model must be tested to be proven.

One of the classic cases of a model proving to be correct was seen following the 1991 eruption of Mount Pinatubo, which proved James Hansen's model that an increase in atmospheric sulfate aerosols would actually decrease global temperatures by 0.5 degrees Celsius in the short term. The IPCC's models for Arctic sea-ice loss have actually been too optimistic, and ice loss has been more dramatic than predicted in the IPCC's "worst-case scenario."

3. Arctic sea ice is melting.

According to the National Snow and Ice Data Center, Arctic sea ice in February 2011 tied with February 2005 for the lowest level in the satellite record. Sea ice those months covered 5.54 million square miles, down from the 1979-2000 average of 6.04 million square miles. Meanwhile, temperatures were between 4 and 7 degrees higher than normal.

This doesn't mean that all ice is melting. Ice area in Antarctica has increased over the past three decades, but according to a study published last year in the Proceedings of the National Academy of Sciences, this is due to increased precipitation, mostly snow, itself brought about by greater levels of moisture in the air due to climate change. This stabilized the ice shelf, reducing the amount of melting it would have otherwise experienced from warmer ocean temperatures.

4. Ocean acidification is rising, caused by rising CO2 levels.

The oceans are a natural carbon "sink," meaning they absorb CO2 from the atmosphere. But as CO2 rises in the atmosphere, it also rises in the oceans, increasing their acid levels (pH) to a point that will be harmful to marine life. According to data presented at the second symposium on the Ocean in a High-CO2 World in 2008, ocean acidity has increased 30 percent since the Industrial Revolution, 100 times faster than any change in the past 20 million years.

As for the future, a 2003 study published in Nature found that "oceanic absorption of CO2 from fossil fuels may result in larger pH changes over the next several centuries than any inferred from the geological record of the past 300 million years, with the possible exception of those resulting from rare, extreme events such as bolide impacts or catastrophic methane hydrate degassing."

5. Ten of the last 12 years were the hottest years on record.

Skeptics say the hottest year on record was 1998, but as Skeptical Science points out, an "abnormally strong El Niño" transferred heat from the Pacific Ocean to the atmosphere. Meanwhile, only one of the three temperature records (HadCRUT3) showed 1998 as the hottest year, and that has since been found to have been a sampling error. More recently, 2005 and 2010 were tied for the hottest years since 1850, according to the U.S. National Oceanic and Atmospheric Administration, and all 10 of the hottest years on record have occurred since 1997.

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