Sammendrag
The Earth has warmed in the last century with the most rapid warming
occurring near the surface in the arctic. This enhanced surface warming in the
Arctic is partly because the extra heat is trapped in a thin layer of air near
the surface due to the persistent stable-stratification found in this region.
The warming of the surface air due to the extra heat depends upon the amount of
turbulent mixing in the atmosphere, which is described by the depth of the
atmospheric boundary layer (ABL). In this way the depth of the ABL determines
the effective response of the surface air temperature to perturbations in the
climate forcing. The ABL depth can vary from tens of meters to a few kilometers
which presents a challenge for global climate models which cannot resolve the
shallower layers. Here we show that the uncertainties in the depth of the ABL
can explain up to 60 percent of the difference between the simulated and
observed surface air temperature trends and 50 percent of the difference in
temperature variability for the Climate Model Intercomparison Project Phase 5
(CMIP5) ensemble mean. Previously the difference between observed and modeled
temperature was thought to be largely due to differences in individual models
treatment of large-scale circulation and other factors related to the forcing,
such as sea-ice extent. While this can be an important source of uncertainty in
climate projections, our results show that it is the representation of the ABL
in these models which is the main reason global climate models cannot reproduce
the observed spatial and temporal pattern of climate change. This highlights
the need for a better description of the stably-stratified ABL in global
climate models in order to constrain the current uncertainty in climate
variability and projections of climate change in the surface layer.
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