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.