Approximately one third of all incident radiation from the sun is reflected back into space. This ratio between reflected and incoming solar radiation, the so called albedo, has to do with the surface of the earth, the clouds and small particles (aerosols) in the atmosphere. The northern hemisphere has much more land than the southern hemisphere that is mainly covered by sea. Land reflects the sun’s radiation to a much larger extent than the sea. The northern hemisphere also has much more reflecting aerosols. In spite of these differences, satellite data shows that the total albedo is the same for the northern and southern hemispheres.

− The beauty of this is that the clouds seem to compensate the asymmetry perfectly. There are several ways in which the clouds can compensate the reflection. For instance it seems like the clouds around the equator move south counteracting the cloud free difference in reflectivity, says Frida Bender at the Department of Meteorology.

Frida Bender, Department of Meteorology.

The climate system has several known ways to adjust for the albedo asymmetry between the hemispheres. Albedo symmetry is interesting to look at because we need to better understand clouds and how they are connected to the climate.

− If we can find better ways to study the clouds in our climate models we will enhance our ability to describe the current developments of the climate and also how it is affected by us humans and our activities. Cloud-climate feedback is the greatest uncertainty in climate models and the projections made from these models. We really need to be better at describing clouds in our models.

The mechanisms behind why the clouds behave in this compensating way are not known and this will be explored in this new project that has received a grant from the Swedish Research Council. To study this may provide further steps towards a better estimate of climate sensitivity and future climate changes.

− The albedo is the incident of solar energy that is reflected back to space. Thus it is directly controlling the radiative budget and global temperature. Poor model representation of the albedo symmetry may hence indicate failure to capture important cloud feedbacks and thereby misestimation of climate sensitivity.

The earth. Photo: NASA

For this research satellite observations will be used to investigate the stability of the cloud symmetry over time. Climate model experiments will also be used to isolate relevant adjustments in cloud properties and processes.