A new enigma is facing scientists studying climate change. Although some research points to an earth that has always been warming up, other research states the opposite: that the Earth was cooling down before humans starting affecting the climate.

When preparing for the annual report for the Intergovernmental Panel on Climate Change on worldwide temperature averages over the last 10,000 years, Zhengyu Liu of the University of Wisconsin-Madison, along with a team of his colleagues, recognized the disparity.

Last year, a study suggested that global cooling started around 7,000 years ago. After that, humans started affecting the environment, in turn affecting global temperatures and warming the Earth up. That study focused on data collected from samples of ice cores taken from around the world. However, some of that data was contradictory.

This year, computer models run by Liu and his team showed that the Earth consistently heated up over the last 10,000 years. Those computer simulations included the effect of sunlight and greenhouse gases on Earth and accounted for ice sheet changes. Each computer model showed that the average worldwide temperature was always warming.

"The question is, 'Who is right?'" says Liu. "Or, maybe none of us is completely right. It could be partly a data problem, since some of the data in last year's study contradicts itself. It could partly be a model problem because of some missing physical mechanisms."

Liu points out that last year's study didn't take into account an important factor: when scientists took the ice core samples. For example, a sample taken in the winter from a specific site would not be the same if taken in the summer. Also, his models, in this new study, did not take into account that volcanic activity also affects global temperatures.

With that being said, Liu and his team don't deny that humans played a huge part on the impact of the climate. Nothing in nature over the last 10,000 years could have created the warming caused by greenhouse gases and the rapid melting of the planet's ice sheets, even though the computer models suggest it could be possible, but unlikely.

Climate scientists plan to meet in the fall to discuss this puzzle and perhaps find out what they're missing. Understanding this problem may affect how scientists view climate change as well as how they run future climate change models.