Abstract
Abstract. Shallow clouds covering vast areas of the world's middle- and high-latitude oceans play a key role in dampening the global temperature rise associated with CO2. These clouds, which contain both ice and supercooled water, respond to a warming world by transitioning to a state with more liquid water and a greater albedo, resulting in a negative “cloud-phase” climate feedback component. Here we argue that the magnitude of the negative cloud-phase feedback component depends on the amount and nature of the small fraction of aerosol particles that can nucleate ice crystals. We propose that a concerted research effort is required to reduce substantial uncertainties related to the poorly understood sources, concentration, seasonal cycles and nature of these ice-nucleating particles (INPs) and their rudimentary treatment in climate models. The topic is important because many climate models may have overestimated the magnitude of the cloud-phase feedback, and those with better representation of shallow oceanic clouds predict a substantially larger climate warming. We make the case that understanding the present-day INP population in shallow clouds in the cold sector of cyclone systems is particularly critical for defining present-day cloud phase and therefore how the clouds respond to warming. We also need to develop a predictive capability for future INP emissions and sinks in a warmer world with less ice and snow and potentially stronger INP sources.
Highlights
Projections of global warming due to increased anthropogenic greenhouse gas concentrations is of central importance for our society
It has been shown that the degree of supercooling correlates with the presence of specific aerosol species such as mineral dust (Tan et al, 2014; Choi et al, 2010). It has been shown using satellite data that there is a large contrast in the contribution of cloud phase changes to changes in cloud optical depth with temperature between land and ocean, which points to the importance of ice-nucleating particles (INPs) (Tan et al, 2019)
Modelling work has shown that cold-air outbreaks (CAOs) cloud systems are strongly impacted by INPs, with low INP concentrations leading to more extensive highly reflective stratus clouds, whereas high INP concentrations tends to lead to much patchier convective cloud with local albedos many hundreds of watts per square metre lower (Vergara-Temprado et al, 2018)
Summary
Projections of global warming due to increased anthropogenic greenhouse gas concentrations is of central importance for our society. Many of the CMIP6 models have a much more positive cloud feedback at latitudes poleward of 45◦, which correlates with higher ECS values (Fig. 1b) This illustrates the key role that clouds, shallow marine clouds in the middle and high latitudes, play in inter-model variations in ECS (Ceppi et al, 2017; Zelinka et al, 2020; Gettelman et al, 2019; Andrews et al, 2019). We argue that this issue has to be addressed urgently. We finish by outlining what research needs to be undertaken to reduce the uncertainty associated with the cloud-phase feedback
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