CGD's Dr. Samuel Levis

Abstract

Simulations by the Community Atmosphere Model (CAM) and 15 other climate models suggest that climate sensitivity is linked to continental middle to high latitude present-day springtime albedo. We compare 1xCO₂ and 2xCO₂ CAM simulations against similar simulations with snow cover fraction purposely increased. Greater snow cover fraction leads to higher albedo and lower temperatures at 1xCO₂ but has less influence at 2xCO₂ when little snow remains due to global warming. This makes the simulation with higher albedo at 1xCO₂ more sensitive to increased CO₂, in agreement with past work. We show that the wide variation in simulated snow-albedo feedbacks and climate sensitivities among 15 other models correlates well with variations in the continental middle to high latitude present-day springtime albedo, in agreement with our CAM results. The development of more accurate snow and albedo parameterizations should improve model estimates of climate sensitivity.

Figure caption: Simulated (a) Δα_s/ΔT_s, (b) ΔT_s, and (c) ΔT _s^global versus simulated present-day α_s. Variables are averaged from 30 to 90°N over land in April. A corresponding observational estimate for α_s from years 2001-2004 is 0.32 (Lawrence & Chase 2007). ΔT _s^global is globally and annually averaged.

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Benjamin I. Cook, Gordon B. Bonan, Samuel Levis, Howard E. Epstein, Rapid vegetation responses and feedbacks amplify climate model response to snow cover changes, Clim Dyn, DOI 10.1007/s00382-007-0296-z, in press.

Abstract

We investigate the response of a climate system model to two different methods for estimating snow cover fraction. In the control case, snow cover fraction changes gradually with snow depth; in the alternative scenarios (one with prescribed vegetation and one with dynamic vegetation), snow cover fraction initially increases with snow depth almost twice as fast as the control method. In cases where the vegetation was fixed (prescribed), the choice of snow cover parameterization resulted in a limited model response. Increased albedo associated with the high snow caused some moderate localized cooling (3-5°C), mostly at very high latitudes (>70°N) and during the spring season. During the other seasons, however, the cooling was not very extensive. With dynamic vegetation the change is much more dramatic. The initial increases in snow cover fraction with the new parameterization lead to a large-scale southward retreat of boreal vegetation, widespread cooling, and persistent snow cover over much of the boreal region during the boreal summer. Large cold anomalies of up to 15°C cover much of northern Eurasia and North America and the cooling is geographically extensive in the northern hemisphere extratropics, especially during the spring and summer seasons. This study demonstrates the potential for dynamic vegetation within climate models to be quite sensitive to modest forcing. This highlights the importance of dynamic vegetation, both as an amplifier of feedbacks in the climate system and as an essential consideration when implementing adjustments to existing model parameters and algorithms.

Figure caption: Monthly observed climatologies for selected surface variabiles (2-m air temperature, precipitation, run-off, snow depth), compared to CTRL, our fixed vegetation control run using original CLM3 snow cover.