Cloud response to summer temperatures in Fennoscandia over the last thousand years

Abstract

Cloud cover is one of the most important factors controlling the radiation balance of the Earth. The response of cloud cover to increasing global temperatures represents the largest uncertainty in model estimates of future climate because the cloud response to temperature is not well-constrained. Here we present the first regional reconstruction of summer sunshine over the past millennium, based on the stable carbon isotope ratios of pine treerings from Fennoscandia. Comparison with the regional temperature evolution reveals the Little Ice Age (LIA) to have been sunny, with cloudy conditions in the warmest periods of the Medieval at this site. A negative shortwave cloud feedback is indicated at high latitude. A millennial climate simulation suggests that regionally low temperatures during the LIA were mostly maintained by a weaker greenhouse effect due to lower humidity. Simulations of future climate that display a negative shortwave cloud feedback for high-latitudes are consistent with our proxy interpretation.

Arctic tree rings as recorders of variations in light availability

“…33)33, but it cannot be excluded that undetected divergence is present or that divergence occurred at earlier times. Further analysis incorporating indications of changes in light availability using stable carbon isotopes5343536373839 or the covariance between light and moisture in water-limited trees40 would be useful. Finally, sensitivity to light variations complicates the interpretation of tree-ring chronologies but also suggests that tree-ring density and width records from low-light environments may aid in the reconstruction of past changes in surface radiation.…”

Tree ring celluloseδ18O ofFokienia hodginsiiin northern Laos: A promising proxy to reconstruct ENSO?

“…The number of samples per batch of the Brendel [ Brendel et al , 2000] and modified Brendel [ Evans and Schrag , 2004] methods is 56 and 80, respectively. Given that chemically treating hundreds or thousands of samples is time consuming, researchers [ Esper et al , 2010; Gagen et al , 2011b] have analyzed wood blocks of 5 or 10 years old together for a tree ring sample with a 1000 year length, in which interannual variations cannot be retained. Therefore, a method that can treat hundreds or thousands of rings simultaneously is necessary to save time.…”

Clouds and Aerosols

“…A few studies in the literature suggest negative feedbacks from Arctic clouds, based on spatial correlations of observed warming and cloudiness or tree-ring proxies of cloud shortwave effects over the last millenium (Gagen et al, 2011). However, the spatial correlations are not reliable indicators of feedback (Section 7.2.5.7), and the tree-ring evidence (assuming it is a good proxy) applies only to the shortwave effect of summertime cloud cover.…”