agupubs.onlinelibrary.wiley.com

Abstract

Abstract

A region of the northern Atlantic–sometimes called the “cold blob”–has cooled since the 19th Century while the rest of the world has warmed. It is particularly the ocean which has cooled there. Scientists have been discussing whether this is because ocean currents bring less heat into this region, or because more heat is being lost through the sea surface there. An analysis of temperature data sets based on measurements show it is the former–changing ocean heat transport–which dominates heat content changes in the “cold blob.” This is of concern because a further weakening of Atlantic heat transport in future climate change could lead to serious impacts on climate and weather conditions in Europe and other parts of the world.

Worryingly, the observed energy imbalance is rising much faster than expected, reaching a level twice that predicted by climate models, having more than doubled within just two decades.

One proposed side effect of geoengineering with stratospheric sulfate aerosols is sky whitening during the day and afterglows near sunset, as is seen after large volcanic eruptions. Sulfate aerosols in the stratosphere would increase diffuse light received at the surface, but with a non-uniform spectral distribution. We use a radiative transfer model to calculate spectral irradiance for idealized size distributions of sulfate aerosols. A 2% reduction in total irradiance, approximately enough to offset anthropogenic warming for a doubling of CO2 concentrations, brightens the sky (increase in diffuse light) by 3 to 5 times, depending on the aerosol size distribution. The relative increase is less when optically thin cirrus clouds are included in our simulations. Particles with small radii have little influence on the shape of the spectra. Particles of radius ∼0.5 μm preferentially increase diffuse irradiance in red wavelengths, whereas large particles (∼0.9 μm) preferentially increase diffuse irradiance in blue wavelengths. Spectra show little change in dominant wavelength, indicating little change in sky hue, but all particle size distributions produce an increase in white light relative to clear sky conditions. Diffuse sky spectra in our simulations of geoengineering with stratospheric aerosols are similar to those of average conditions in urban areas today.