Recent decadal trends in the spectral, textural and angular radiance of Earth observed from space

The earth's climate is unarguably changing. However, the definability of spatial and temporal scales of climate changes, as determined through satellite-retrieved products, remains a subject of debate and uncertainty, partially because actual spatial and temporal variability in atmospheric and surface properties cannot be fully decoupled from artificial variability introduced through the variability in the quality of the assumptions in satellite remote sensing algorithms.

Here we present robust evidence of the existence of recent decal trends in the earth's climate by directly examining the temporal variability in the spectral and textural radiance fields observed at different angles from the Multi-angle Image SpectraRadiometer (MISR) onboard the Terra spacecraft at regional and global scales. At a global scale, there exist significant trends of spectral difference in mean radiance fields between blue and other visible bands (up to 1.6% per decade) from both nadir and oblique views and trends of textural radiance fields (up to -2.7% per decade) only from oblique views, indicating the earth has been “visually” appearing bluer and smoother during the recent decade. The detected trends, which are free of algorithmic biases, cannot be explained either by uncertainties in radiometric calibration and degradation or variability in solar total and spectral irradiance, and hence reflect changes in the earth's climate. At a regional scale, significant trends in both spectral and textural radiance fields with a magnitude varying with viewing angles are most distinctive over polar regions and along the boundaries of subtropical highs. The spatial distribution of these trends is generally consistent with recent discovery of expansion of tropical belt and changes in polar dynamics. The hypotheses of causal attribution of these trends will also be discussed.