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科學家估計，如果不減少二氧化碳排放，2100年地球平均氣溫將至少上升4攝氏度，到2200年會上升8攝氏度，無疑是地球“災難性的噩夢”。

此前有科學家用氣候模型分析稱，全球氣溫將受二氧化碳排放影響，上升1.5至5攝氏度。最新研究分析了云層的形成和氣候變化之間的關系，認定模型低估了氣候的敏感度，將氣溫上升的下限從1.5調整至3攝氏度，平均增幅達到4攝氏度。本項研究已經在學術雜志《自然》上發表。

According to a December 31 news release from the University of New South Wales, scientists estimate that by 2100, global average temperatures will rise at least 4 degrees Celsius if carbon dioxide emissions are not scaled back. Additionally, researchers say that the continued increase in global average temperatures will result in an additional 4 degrees Celsius by 2200.

The findings appear in a recent article in the journal Nature, and may explain one of the great unknowns of climate sensitivity: the role of cloud formation, and whether this will have a positive or negative influence on global climate change.

“Our research has shown climate models indicating a low temperature response to a doubling of carbon dioxide from preindustrial times are not reproducing the correct processes that lead to cloud formation,” said Steven Sherwood, a professor from the University of New South Wales’ Centre of Excellence for Climate System Science. “When the processes are correct in the climate models the level of climate sensitivity is far higher. Previously, estimates of the sensitivity of global temperature to a doubling of carbon dioxide ranged from 1.5°C to 5°C. This new research takes away the lower end of climate sensitivity estimates, meaning that global average temperatures will increase by 3°C to 5°C with a doubling of carbon dioxide.”

The key to this narrower – albeit much higher – estimate is found in the real world observations around the role of water vapor in cloud formation.

Observations show when water vapor is taken up by the atmosphere through evaporation, the updrafts can either rise to 15 km to form clouds that produce heavy rains or rise just a few kilometers before falling back to the surface without forming rain clouds.

The researchers discovered that climate models that exhibit a low global temperature response to carbon dioxide do not include enough of this lower-level water vapor process. In its place, they simulate nearly all updrafts as rising to 15 km and forming clouds.

However, when the procedures in climate models are adjusted to match the observations in the real world, the models produce cycles that take water vapor to a wider range of heights in the atmosphere, causing fewer clouds to form as the climate warms. Consequently, this increases the volume of sunlight and heat entering the atmosphere and increases the sensitivity of our climate to carbon dioxide or any other disturbance.

The result is such that, when water vapor processes are correctly represented, the sensitivity of the climate to a doubling of carbon dioxide – which will happen in the next 50 years – means that we can expect a temperature increase of at least 4 degrees Celsius by 2100.

（來源：中國日報網愛新聞iNews 編輯：丹妮）