This winter, the drastic cold has battered not only China but many parts in the Northern Hemisphere, including Europe and North America. Shivering in the freezing weather, many now doubt the authenticity of global warming. We must realize, however, that particular weather events and long-term climate trends are two different matters.

Even if a region is struck by a strong cold wave with extremely low temperature that is unprecedented in decades, it might hardly impact the long-term global trend, since its effects could easily be neutralized by another warmer year or by higher temperatures in another region of the world. When we analyze the long-term trend of climate change, we have to examine data collected around the globe for at least 100 years.

The rising trend of global temperatures, however, is not as frightening as some scientists have suggested. According to reports from the Intergovernmental Panel on Climate Change (IPCC), global temperatures will soar by 1.4 to 5.8 C by the end of this century.

My estimation is much lower than the pessimistic views in the IPCC reports. Based on past data, global temperatures will rise by at most 0.6 degree in the 21st century, far less than the alarming 2 C threshold set in the Copenhagen climate conference.

The estimation is based on the temperature data since 1850 and long-term data from the 11th century. Since 1850, global temperatures have been indeed rising at a pace of 0.44 C per century. But the rising trend is not flat; it resembles more of a waveform. For instance, between the 1940s and 1970s, the global climate was cooling.

The pessimistic views are based on projections according to the rising trend from the last three decades. If we look at the bigger picture of the rise and fall of global temperatures, however, it should decline between this year and sometime around 2030, since we are now in another downhill course in the natural climate cycle.

If we examine an even longer period in the climate history, say for example since the start of the 11th century, a more fascinating story emerges. The curve of temperatures in the past millennium can be resolved into four cycles: 194.6 years, 116 years, 62.5 years and 21.2 years. The temperature curve formulated by adding the effects of the cycles together very much resembles the latest data provided by Michael Mann, a climatologist at Pennsylvania State University and one of the main creators of the famous “Hockey Stick Graph”, and other prominent climate experts. Hence, the hypothesis of climate cycles is on solid empirical ground.

The four cycles can help us understand the warming climate in recent decades, too. The wave crests of all four cycles appeared around the year 1998, a rare event that happened for the first time in a millennium. Hence, the years around 1998 were the hottest years in recorded climate history. The recent decades of higher temperatures can be well explained through natural cycles, while the effects of human activities remain ambiguous.

Another major flaw of climate change advocates is that they confuse correlation and causation. We admit that the global climate has been warming over the past three decades, and the concentration of carbon dioxide in the atmosphere has been surging over the same period. But they could only be called well-correlated, and do not necessarily imply carbon dioxide emissions have caused global warming. If the height of a tree planted in 1980 has also grown in the same period, can we conclude that its growth is caused by carbon dioxide emissions?

Other suspicious points remain to be clarified. The rise of carbon dioxide concentration is smooth, whereas the ascent of global temperature is circuitous and cyclic. Data show that carbon dioxide concentrations began rising steeply in the 1950s, while the upward inflection point of global temperatures was around the 1850s, according to the latest data by Mann and others. In the recent decade, carbon dioxide emissions have surged fourfold compared to the 1990s, whereas the global temperature did not increase between 1998 and 2008. If carbon dioxide is the culprit of climate change, how can we explain the disparities? We need further and deeper research into the details about climate change, rather than rush into a conclusion.

Nowadays, the view that human activities have caused climate change has dominated the scientific community, public opinion and political discussions. Why are we hardly listening to opposing voices?

In the scientific community, in fact, there are different views. Mainstream scientists who support the climate-change hypothesis, however, almost monopolize the discussion, and oppress the views of climate skeptics. Articles giving higher estimations for future temperatures are easier to publish, and the voices hyping the severity of climate change gain a wider audience. On the contrary, the works that doubt the authenticity of climate change are either rejected or neglected. The global scientific community behaves unfairly on this matter, and consequently it is not surprising that climate skeptics used “Climategate” prior to the Copenhagen conference as a counterstrike to the mainstream.

Climategate, however, was simply an event that stirred up trouble and should not be applauded. Non-mainstream scientists, despite all the unfairness they have suffered, should discuss with mainstream scientists, most of them reasonable and well-trained, on the scientific results and try to persuade public opinion. The scientific establishment as well as the general public should listen to the skeptics’ voices, too.

The author is a professor of atmospheric science at Peking University.

By Qian Weihong