Cloud of Doubt

Climate modelers continue to refine their assumptions about the sensitivity of global warming to greenhouse gases. Cloud cover is the latest refinement.

The rapidity and severity of climate change depends upon the degree Earth warms in response to increases in greenhouse gases; a concept climatologists refer to as “climate sensitivity.”

Water vapor has the greatest greenhouse effect. Clouds hoard warmth in the tropics. Contrastingly, less cloud cover in the extratropics (outside the tropics) warms by increasing incident sunlight.

Further, there is a positive feedback loop among greenhouse gases and water vapor that generates a warming gyre. By trapping extra heat, greenhouse gases affect the amount of water vapor in the air. Heating the air by adding CO2 means the air takes up more water vapor, which further warms the atmosphere. Thus, greenhouse gases engender a multiplier to their warming effect.

A crucial facet of climate model forecast accuracy is the correlation between global greenhouse gas levels and temperature. “Equilibrium climate sensitivity, the global surface temperature response to CO2 doubling, has been persistently uncertain. Recent consensus places it likely within 1.5–4.5K,” recently wrote American atmospheric scientist Mark Zelinka and colleagues. This estimated uncertainty is 300%: a wide variability.

That warming would aridify the air has long been thought. Climate modelers are increasingly appreciating this prognostication.

The latest generation of climate models revised their estimates of feedback effects from greenhouse gases: heightening the warming effect due to less water vapor in clouds and fewer clouds in higher latitudes. “This raises the possibility that climate sensitivity is indeed high in the real world,” realized Zelinka et al.

There has been a decided trend in the past decade that models underestimated climate sensitivity. The cloud of doubt has yet to clear that the hotting up won’t be even worse than expected.


Ishi Nobu, The Fruits of Civilization, BookBaby (2019).

Mark D. Zelinka et al, “Cause of higher climate sensitivity in CMIP6 models,” Geophysical Research Letters (December 2019).

Ishi Nobu, “Sea rise & ice melt,” (28 December 2019).

Ishi Nobu, “Climate model conservatism,” (21 December 2019).

Ishi Nobu, “Climate model accuracy,” (15 December 2019).

Ishi Nobu, “Climate models,” (13 December 2019).