While we wait for the light – part 3
Various light sources in the sky above Ny-Ålesund. The total light consists of several sources, both natural and artificial. In this picture we see the dim sunlight behind the horizon in the south, the full moon in the north, the northern lights, the lights from the buildings and a laser light pointing upward towards the atmosphere from one of the buildings in Ny-Ålesund. For those who want to characterize the light in the polar night near Ny-Ålesund, one must be able to distinguish between all these sources, in addition to the light that one uses when one is in the “darkness” to make observations. Photo: Geir Johnsen.
While we wait for the light, we are still struggling to define how “dark” or “bright” it really is in the polar night. We know that organisms out there are able to respond to light we cannot see, and we know that polar night is not just a continuous darkness. But it is incredibly much more difficult to measure light in the dark than you might think.
23 February 2020
Text: Jørgen Berge (UiT The Arctic University of Norway, UNIS and NTNU), Geir Johnsen (NTNU and UNIS) and Dr. Jonathan Cohen (University of Delaware).
It is relatively easy to define from a physicist’s perspective what light is – Einstein taught us that light is both particles and waves. Simply put, sunlight is photons (particles) that flow like waves from the sun to the Earth. But it is far from trivial to define what light is for an organism, such as a krill in Kongsfjorden in Svalbard that detects and responds to these photons. To define what light is for this krill, we must know both the intensity of the light, the spectral composition and not least the time window the light shines. The water has optical properties that affect the light down in the water column, and the water has particles that scatter and reflect the light. In addition, there are an incredible number of different types of eyes. Light is simply so important that the ability to “see” has arisen and changed independently in many animal groups. It is therefore seldom sufficient, although this is precisely what most researchers do, to specify light climate as a simple quantity – the number of photons per unit of time and area.
But let’s just stick to quantity measurements of light for simplicity’s sake. Even this is not trivial in the polar night! That the light changes throughout the year should not be a shock to anyone, especially on our latitudes where we have the midnight sun in summer and polar night in winter. This is easy to measure, and of course there are countless series of measurements showing how the sunlight changes over a year. The vast majority of light sensors are designed to measure sunlight.
Figure A shows how the amount of light changes throughout the year outside Ny-Ålesund. These measurements are made with a standard light sensor that is very often used in biological experiments. But it fails to detect any light in the dark. According to this sensor, the polar night is dark. End of story. However, if we replace this sensor with a slightly more advanced sensor, we see (Figure B) that there is also light in the polar night. And the amount of photons changes significantly throughout the winter, as well as the obvious changes around the full moon. Especially in the darkest part of the polar night (December and January). To measure this, a rather advanced sensor is needed, which is capable of measuring light levels about 10 thousand times weaker than in the middle of the day in summer (Figure C). But even on the darkest day of the year (Figure D), there are 24-hour variations in the light, but these down to 100 million times weaker than the middle of the day in the summer! And if these measurements are all to be made within a certain wave band that is relevant to most living organisms, then it becomes extremely difficult to find suitable sensors. In fact, so difficult that these have to be specially designed for the task. Krill, on the other hand, obviously manages this very well!
Light pollution fastest growing pollution source
Another thing that obviously makes it difficult to make good measurements of light out in the field are problems related to light pollution. Even in the observatory outside Ny-Ålesund, where we now make continuous measurements throughout the year, there is light pollution that prevents us from making accurate measurements in the very darkest part of the polar night. In fact, we can see this well in the figure (panels B and C), where the lowest measurements through the polar night do not change, despite the angle between the sun and the horizon changing by more than 20 degrees during this period. The reason why we see no change in the “bottom line” is probably two-fold; the sensitivity of the sensors and background radiation from the settlement in Ny-Ålesund.
Removing them completely from light pollution is simply something that is becoming more and more difficult, not just in the Arctic. Globally, light pollution is the fastest growing source of pollution by about 6% annually, and almost 25% of all land area is today constantly affected by artificial light reflected from the sky. The Arctic is probably still virgin dark in that way, but here too we will probably see a big change in the future as climate change opens up for more and more activity. A good example of this is the ever-increasing tourist traffic we have seen to and from Longyearbyen also throughout the polar night. While not long ago it was very easy to get a flight to or from Svalbard in December and January, now the planes are mostly full. And virtually all human activity entails the use of light – light that will surely affect organisms adapted to exploit the extremely low natural light values that characterize polar night!
