科学家估计，如果不减少二氧化碳排放，2100年地球平均气温将至少上升4摄氏度，到2200年会上升8摄氏度，无疑是地球“灾难性的噩梦”。

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.