A number of scientists say we are many years away from properly understanding global climate and that stopping global climate change through reducing greenhouse gases is an expensive, futile effort.

They say Earth has never had a stable climate; its only constant is change and, at times, quite rapidly. Many times in the past, temperatures were far higher than today, and occasionally, temperatures were colder. As recently as 6,000 years ago, it was about 12.7 degrees F warmer than now. Ten thousand years ago, while the world was coming out of the thousand-year-long "Younger Dryas" cold episode, evidence suggests temperatures rose as much as 14 degrees F in a decade -- 100 times faster than the past century's 1.4 degrees F warming.

Scientists agree that the greatest influence on the Earth’s temperature is the sun and its output varies. One theory suggests that when the sun's energy output is greater, the Earth warms due to direct solar heating and the sun’s strong solar wind blocks many cosmic rays from entering Earth’s atmosphere. Cloud cover decreases and the Earth warms still more. When the sun is less bright, more cosmic rays are able to get through to Earth's atmosphere, more clouds form, and the planet cools.

Solar scientists predict that, by 2020, the sun will start into its weakest Schwabe solar cycle (named for the scientist that observed the cycle of sunspots) of the past two centuries, likely leading to unusually cool conditions on Earth. They say governments should plan for adaptation to a cool period.

The methods employed by scientists investigating global warming have also been called into question. Critics believe the scientific evidence is skewed by scientists’ previous experience and expectations. They claim the environmental scientists interpret data based on what they believe to be true, and dismiss data that does not make sense to them as anomalies, oddities, irregularities, or simply flaws in their collection of evidence.