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Will the Earth return to ice age again? 

As Winston Churchill said, “The longer you look back, the farther you can look forward”, climate change research relies on studying past climate in order to project future climate. Through reconstructing paleoclimate, we can examine how climate changed over longer time scales. Paleoclimate data can also be used to compare with simulations of climate models, hence aiding to improve model capability. Through modelling paleoclimate with climate models, we can investigate driving forces of past climate change, providing insights into future climate change. Since the 1950s, climate models have advanced substantially and have become a key tool in climate change research.

Interglacial periods 

Figure 1   Temperature reconstruction over the last 800 thousand years. Cooler temperatures indicate glacial periods; warmer temperatures indicate interglacial periods. (Image source: The US National Aeronautics and Space Administration )

Paleoclimate reconstructions indicate that the Earth’s temperature had been like a roller coaster going through cold (glacial) and warm (interglacial) periods (Figure 1). Scientists generally believe that the wax and wane of glacial and interglacial periods were due to changes in the Earth’s orbital geometry (Milankovitch Cycles), which led to variations in intensity and latitudinal distribution of incoming solar radiation. During the previous interglacial period, summer incoming solar radiation at Northern Hemisphere was 10% higher than present-day level and climate model simulations suggested that the Arctic summer was 5℃ warmer. As we are currently in an interglacial period, one may think that global warming of the last century is natural. However, can the warming of the 20th century be explained by natural factors?

According to the IPCC Fifth Assessment Report, the mean global warming rate was about 0.3℃ to 0.8℃ per thousand years during the transition from the previous glacial period to the present interglacial period. However, instrumental data shows that the average rate of global temperature increase from 1880 to 2020 is 0.08℃ per decade. In other words, the warming rate over the last century is at least ten times faster than that during the inception of the present interglacial period. If the world continues on the high greenhouse gas concentration pathway, global temperature will be 3.7℃ higher than the average of 1986-2005 by the end of the 21st century, implying a temperature increasing rate about 50 times faster than that during the inception of the present interglacial period. Human influence on the climate is clear. Atmospheric greenhouse gases concentrations have increased rapidly since industrialisation began. As the single most important greenhouse gas in the atmosphere, carbon dioxide has reached concentration about 50% higher than preindustrial levels. A study even suggests that present-day atmospheric carbon dioxide level is unprecedented in the past 23 million years. Will the Earth enter a glacial period again and repeat the climate history? In fact, human-caused carbon emissions have postponed the next ice age by 100,000 years as pointed out by a study. Based on the above information, it is more likely that human is making new climate history. “Anthropocene” has become a popular scientific term to indicate the significant human impact on the establishment of a geological epoch.