Assessing two modeling challenges, Climate Sensitivity and Arctic Amplification

Climate models are tools used to understand how the climate system works and to tell us what might be in store as the climate system’s energy balance is perturbed by increasing concentrations of greenhouse gases (GHGs).  Two issues with which models have struggled will be examined in order to provide potential improvements particularly for projections of the future climate.  The first is the basic global temperature reaction to increasing GHGs often defined as the equilibrium climate sensitivity (ECS) or a related metric, the transient climate response (TCR).  Empirical evidence suggests the climate system shows lower sensitivity than models indicate, sometimes much lower.  However, direct calculations of ECS and TCR, which then are used to identify the direct impact of GHGs alone, is degraded due issues such as (a) the unknown influence of natural variations, (c) the relatively short time-scale of good observations, and (c) the uncertainty in the actual forcing of the GHGs themselves.  We will examine “Fast-Feedback Processes” which are not confounded by the difficulties just mentioned, so that a clearer relationship between a generic forcing and the associated response will be possible.  Two key variables we shall use are the tropospheric temperature and the heat flux

flux

The amount of some type of particle or energy crossing a unit area per unit time.

Source: NRC Library Source

leaving the planet because the heat that escapes to space (part of the sensitivity calculation) originates mostly from the troposphere, not the surface, hence the two metrics are directly connected.  We shall then relate these shorter-term metrics to ECS and TCR to better understand the magnitude of the longer-term climate response to perturbations in forcing.

The second issue is known as Arctic Amplification

arctic amplification

The amplified surface warming in the Arctic relative to the global mean.

Source: Pacific Northwest National Laboratory Source

(AA) in which the Arctic air temperatures are anticipated to warm faster than the globe as a whole.  Models have consistently underestimated AA compared with observations.  Our approach is to examine stable boundary layer (SBL) processes which are only crudely depicted in climate models.  Using a high-resolution model of the Arctic we shall be able to analyze the fluxes of energy in the SBL that are highly non-linear and dependent on subtle changes that impact the rate at which heat is mixed from the deeper atmosphere above to the cold surface layer.  The roles of aerosols,

aerosols

Small particles or droplets, such as wildfire smoke, volcanic gases, or salty sea spray, that float in the air. They are emitted by both natural events and human activities.

Source: NASA Global Climate Change Source

GHGs and surface roughness will be important as their presence can disrupt the delicate formation of the very cold surface layer, leading to higher surface temperatures through greater vertical mixing which we hypothesize is the reason for the observed extra warming at the surface.  These stability characteristics are only very crudely mimicked in climate models and we shall determine to what extent a better representation of the SBL might lead to a better match with the temperature changes observed.