Introduction

The Arctic is the region of the Earth where the strongest temperature rise has been observed. It is therefore an ideal and important region where the computational models that predict global climate change can be checked against reality. It is our intention to contribute to a better understanding of some of the processes involved in the temperature change in the Arctic. For this purpose, we will conduct airborne measurements in the Canadian Arctic in April and May 2012.

Difference (in degrees Celsius, or Kelvin) in mean springtime temperature between the years 2009-2013 and the years 1951-1980. Source: NASA GISS Surface Temperature Analysis

Arctic Climate Change

One of the reasons why the Arctic is so sensitive to climate change is the albedo effect. The surface in the Arctic is usually dark: sparsely vegetated soil, or the depths of the Arctic Ocean. However, the areas covered by snow and ice are very bright. Dark surfaces absorb a large fraction of the incoming solar energy, while bright surfaces reflect most of it back up into the atmosphere. Therefore, dark areas heat up more than bright areas when the Sun shines on them. So if a piece of ice melts and gives way to the underlying dark surface (soil or water), local heating occurs and more ice will melt. This is a positive feedback mechanism because the melting process is self-amplifying.

The amount of solar energy that reaches the surface is mostly controlled by two things: time and clouds. Time is crucial in the Arctic because in winter the Sun does not rise at all for months, while in summer there is no night. Also the high Sun around noon transports much more energy into a square meter of surface than a low Sun near the horizon.

Role of Arctic clouds

In the Arctic, clouds (in particular, boundary-layer clouds) are of special importance in the predictions of Arctic climate warming. On an annual average, the act similar to greenhouse gases: they keep thermal infrared radiation from escaping into space. This effect exceeds the cooling due to the reflection of incoming solar radiation by these clouds. In detail, this radiative effect is highly variable and depends on the surface albedo, aerosol properties, and cloud properties such as water content, cloud-droplet or ice-crystal size, and the thermodynamic phase (ice vs liquid water). Additionally, the long periods of permanent polar day and polar night have a strong impact on the competition between solar and terrestrial radiative effects. For example, the low surface albedo in summer (the dark ocean surface, instead of the bright ice) leads to a seasonal cooling effect of Arctic clouds.

The Earth's total energy budget is defined by the incoming and outgoing solar and the outgoing terrestrial (or thermal infrared) radiation. Solar radiation has wavelengths between 0.3 and 4 micrometers (one micrometer is one millionth of a meter, or one thousandth of a millimeter). Terrestrial radiation has wavelengths between 4 and 100 micrometers. Both are modified by scattering, absorption, and emission by atmospheric particles (including cloud droplets), but also by the Earth's surface. The energy budget of the Arctic differs significantly from that of the entire Earth's global and annual average. The Arctic areas are a major sink in the Earth's energy budget, especially because they receive so little solar energy.

The Arctic regions are highly sensitive to changes in the net solar radiation. Two circumstances increase the impact of atmospheric constituents in the Arctic: the Sun is always low (which increases the optical path of solar radiation, so there is more interaction with the atmosphere), and the surface is very bright (high surface albedo due to snow and ice; the reflected radiation has a second chance to interact with the atmosphere). This results in a high variability of the Arctic climate, as it has been reported in the Arctic Climate Impact Assessment (published by Corell, Cambridge University Press, U.K., 2004). Therefore, it is important to measure the components of the Arctic radiation budget (clouds and aerosols, surface properties, and their interaction with the radiation field), and to validate ground-based and space borne remote sensing of Arctic climate parameters, as well as climate models.